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1
Zelle, Hein, Gerrian Appeldoorn, Gerrit Burgers, and Geert Jan van Oldenborgh. "The Relationship between Sea Surface Temperature and Thermocline Depth in the Eastern Equatorial Pacific." Journal of Physical Oceanography 34, no. 3 (March 1, 2004): 643–55. http://dx.doi.org/10.1175/2523.1.
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Abstract The time dependence of the local relation between sea surface temperature (SST) and thermocline depth in the central and eastern equatorial Pacific Ocean is analyzed for the period 1990–99, using subsurface temperature measurements from the Tropical Atmosphere–Ocean Array/Triangle Trans-Ocean Buoy Network (TAO/TRITON) buoy array. Thermocline depth anomalies lead SST anomalies in time, with a longitude-dependent delay ranging from 2 weeks in the eastern Pacific to 1 year in the central Pacific. The lagged correlation between thermocline depth and SST is strong, ranging from r > 0.9 in the east to r ≈ 0.6 at 170°W. Time-lagged correlations between thermocline depth and subsurface temperature anomalies indicate vertical advection of temperature anomalies from the thermocline to the surface in the eastern Pacific. The measurements are compared with the results of forced OGCM and linear model experiments. Using model results, it is shown that the delay between thermocline depth and SST is caused mainly by upwelling and mixing between 140° and 90°W. Between 170°E and 140°W the delay has a different explanation: thermocline depth anomalies travel to the eastern Pacific, where upwelling creates SST anomalies that in turn cause anomalous wind in the central Pacific. SST is then influenced by these wind anomalies.
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Gemmrich, Johannes, and Adam Monahan. "Covariability of Near-Surface Wind Speed Statistics and Mesoscale Sea Surface Temperature Fluctuations." Journal of Physical Oceanography 48, no. 3 (March 2018): 465–78. http://dx.doi.org/10.1175/jpo-d-17-0177.1.
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AbstractThe atmospheric (ABL) and ocean (OBL) boundary layers are intimately linked via mechanical and thermal coupling processes. In many regions over the world’s oceans, this results in a strong covariability between anomalies in wind speed and SST. At oceanic mesoscale, this coupling can be driven either from the atmosphere or the ocean. Gridded SST and wind speed data at 0.25° resolution show that over the western North Atlantic, the ABL mainly responds to the OBL, whereas in the eastern North Pacific and in the Southern Ocean, the OBL largely responds to wind speed anomalies. This general behavior is also verified by in situ buoy observations in the Atlantic and Pacific. A stochastic, nondimensional, 1D coupled air–sea boundary layer model is utilized to assess the relative importance of the coupling processes. For regions of little intrinsic SST fluctuations (i.e., most regions of the world’s oceans away from strong temperature fronts), the inclusion of cold water entrainment at the thermocline is crucial. In regions with strong frontal activities (e.g., the western boundary regions), the coupling is dominated by the SST fluctuations, and the frontal variability needs to be included in models. Generally, atmospheric and ocean-driven coupling lead to an opposite relationship between SST and wind speed fluctuations. This effect can be especially important for higher wind speed quantiles.
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Chowdary, J. S., Anant Parekh, G. Srinivas, C. Gnanaseelan, T. S. Fousiya, Rashmi Khandekar, and M. K. Roxy. "Processes Associated with the Tropical Indian Ocean Subsurface Temperature Bias in a Coupled Model." Journal of Physical Oceanography 46, no. 9 (September 2016): 2863–75. http://dx.doi.org/10.1175/jpo-d-15-0245.1.
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AbstractSubsurface temperature biases in coupled models can seriously impair their capability in generating skillful seasonal forecasts. The National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2), coupled model, which is used for seasonal forecast in several countries including India, displays warm (cold) subsurface (surface) temperature bias in the tropical Indian Ocean (TIO), with deeper than observed mixed layer and thermocline. In the model, the maximum warm bias is reported between 150- and 200-m depth. Detailed analysis reveals that the enhanced vertical mixing by strong vertical shear of horizontal currents is primarily responsible for TIO subsurface warming. Weak upper-ocean stability corroborated by surface cold and subsurface warm bias further strengthens the subsurface warm bias in the model. Excess inflow of warm subsurface water from Indonesian Throughflow to the TIO region is partially contributing to the warm bias mainly over the southern TIO region. Over the north Indian Ocean, Ekman convergence and downwelling due to wind stress bias deepen the thermocline, which do favor subsurface warming. Further, upper-ocean meridional and zonal cells are deeper in CFSv2 compared to the Ocean Reanalysis System data manifesting the deeper mixing. This study outlines the need for accurate representation of vertical structure in horizontal currents and associated vertical gradients to simulate subsurface temperatures for skillful seasonal forecasts.
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Wilson, Earle A., Stephen C. Riser, Ethan C. Campbell, and Annie P. S. Wong. "Winter Upper-Ocean Stability and Ice–Ocean Feedbacks in the Sea Ice–Covered Southern Ocean." Journal of Physical Oceanography 49, no. 4 (April 2019): 1099–117. http://dx.doi.org/10.1175/jpo-d-18-0184.1.
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AbstractIn this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.
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Doney, Scott C., Steve Yeager, Gokhan Danabasoglu, William G. Large, and James C. McWilliams. "Mechanisms Governing Interannual Variability of Upper-Ocean Temperature in a Global Ocean Hindcast Simulation." Journal of Physical Oceanography 37, no. 7 (July 1, 2007): 1918–38. http://dx.doi.org/10.1175/jpo3089.1.
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Abstract The interannual variability in upper-ocean (0–400 m) temperature and governing mechanisms for the period 1968–97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño–Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air–sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.
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Zhang, Yu, and Geoffrey K. Vallis. "Ocean Heat Uptake in Eddying and Non-Eddying Ocean Circulation Models in a Warming Climate." Journal of Physical Oceanography 43, no. 10 (October 1, 2013): 2211–29. http://dx.doi.org/10.1175/jpo-d-12-078.1.
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Abstract Ocean heat uptake is explored with non-eddying (2°), eddy-permitting (0.25°), and eddy-resolving (0.125°) ocean circulation models in a domain representing the Atlantic basin connected to a southern circumpolar channel with a flat bottom. The model is forced with a wind stress and a restoring condition for surface buoyancy that is linearly dependent on temperature, both being constant in time in the control climate. When the restore temperature is instantly enhanced regionally, two distinct processes are found relevant for the ensuing heat uptake: heat uptake into the ventilated thermocline forced by Ekman pumping and heat absorption in the deep ocean through meridional overturning circulation (MOC). Temperature increases in the thermocline occur on the decadal time scale whereas, over most of the abyss, it is the millennial time scale that is relevant, and the strength of MOC in the channel matters for the intensity of heat uptake. Under global, uniform warming, the rate of increase of total heat content increases with both diapycnal diffusivity and strengthening Southern Ocean westerlies. In models with different resolutions, ocean responses to uniform warming share similar patterns with important differences. The transfer by mesoscale eddies is insufficiently resolved in the eddy-permitting model, resulting in steep isopycnals in the channel and weak lower MOC, and this in turn leads to weaker heat uptake in the abyssal ocean. Also, the reduction of the Northern Hemisphere meridional heat flux that occurs in a warmer world because of a weakening MOC increases with resolution. Consequently, the cooling tendency near the polar edge of the subtropical gyre is most significant in the eddy-resolving model.
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Carpenter, J. R., and M. L. Timmermans. "Does Rotation Influence Double-Diffusive Fluxes in Polar Oceans?" Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 289–96. http://dx.doi.org/10.1175/jpo-d-13-098.1.
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Abstract The diffusive (or semiconvection) regime of double-diffusive convection (DDC) is widespread in the polar oceans, generating “staircases” consisting of high-gradient interfaces of temperature and salinity separated by convectively mixed layers. Using two-dimensional direct numerical simulations, support is provided for a previous theory that rotation can influence DDC heat fluxes when the thickness of the thermal interface sufficiently exceeds that of the Ekman layer. This study finds, therefore, that the earth’s rotation places constraints on small-scale vertical heat fluxes through double-diffusive layers. This leads to departures from laboratory-based parameterizations that can significantly change estimates of Arctic Ocean heat fluxes in certain regions, although most of the upper Arctic Ocean thermocline is not expected to be dominated by rotation.
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Di Lorenzo, Emanuele, Arthur J. Miller, Niklas Schneider, and James C. McWilliams. "The Warming of the California Current System: Dynamics and Ecosystem Implications." Journal of Physical Oceanography 35, no. 3 (March 1, 2005): 336–62. http://dx.doi.org/10.1175/jpo-2690.1.
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Abstract Long-term changes in the observed temperature and salinity along the southern California coast are studied using a four-dimensional space–time analysis of the 52-yr (1949–2000) California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrography combined with a sensitivity analysis of an eddy-permitting primitive equation ocean model under various forcing scenarios. An overall warming trend of 1.3°C in the ocean surface, a deepening in the depth of the mean thermocline (18 m), and increased stratification between 1950 and 1999 are found to be primarily forced by large-scale decadal fluctuations in surface heat fluxes combined with horizontal advection by the mean currents. After 1998 the surface heat fluxes suggest the beginning of a period of cooling, consistent with colder observed ocean temperatures. Salinity changes are decoupled from temperature and appear to be controlled locally in the coastal ocean by horizontal advection by anomalous currents. A cooling trend of –0.5°C in SST is driven in the ocean model by the 50-yr NCEP wind reanalysis, which contains a positive trend in upwelling-favorable winds along the southern California coast. A net warming trend of +1°C in SST occurs, however, when the effects of observed surface heat fluxes are included as forcing functions in the model. Within 50–100 km of the coast, the ocean model simulations show that increased stratification/deepening of the thermocline associated with the warming reduces the efficiency of coastal upwelling in advecting subsurface waters to the ocean surface, counteracting any effects of the increased strength of the upwelling winds. Such a reduction in upwelling efficiency leads in the model to a freshening of surface coastal waters. Because salinity and nutrients at the coast have similar distributions this must reflect a reduction of the nutrient supply at the coast, which is manifestly important in explaining the observed decline in zooplankton concentration. The increased winds also drive an intensification of the mean currents of the southern California Current System (SCCS). Model mesoscale eddy variance significantly increases in recent decades in response to both the stronger upwelling winds and the warmer upper-ocean temperatures, suggesting that the stability properties of the SCCS have also changed.
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Kako, Shin’ichiro, Tomofumi Nakagawa, Katsumi Takayama, Naoki Hirose, and Atsuhiko Isobe. "Impact of Changjiang River Discharge on Sea Surface Temperature in the East China Sea." Journal of Physical Oceanography 46, no. 6 (June 2016): 1735–50. http://dx.doi.org/10.1175/jpo-d-15-0167.1.
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AbstractThis study investigated how the Changjiang River discharge (CRD) emptying into the East China Sea (ECS) affects the upper-ocean stratification [hence, sea surface temperature (SST) changes], based on ocean general circulation modeling with and without CRD. A new finding in this study is that CRD contributes significantly to a reduction in summer SST in the ECS. Comparison between the two model runs revealed that vertical one-dimensional processes contribute considerably to SST warming in the ECS, while horizontal advection plays an important role in lowering SST in summer. The results of a particle-tracking experiment suggested that the cold water mass formed along the Chinese coast during the previous winter contributes to the SST reduction in the following summer. From the end of the summer monsoon season, the less saline CRD advected toward the Chinese coast generates a shallow mixed layer (ML), which inhibits heat exchange between the ML and thermocline. In winter, heat loss of the ML through the sea surface results in a reduction in SST over a broad region. Water exchange through the bottom of the ML is relatively suppressed by robust stratification, which prevents cooling of the thermocline and leads to a temperature inversion. The northeastward ocean current associated with the summer monsoon carries the cold water mass in the ML across the ECS; therefore, SST decreases during the following season. These results suggest that CRD has a critical role on both the ocean circulation system and the coupled air–sea interactions in the ECS.
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Laurian, Audine, Alban Lazar, and Gilles Reverdin. "Generation Mechanism of Spiciness Anomalies: An OGCM Analysis in the North Atlantic Subtropical Gyre." Journal of Physical Oceanography 39, no. 4 (April 1, 2009): 1003–18. http://dx.doi.org/10.1175/2008jpo3896.1.
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Abstract Oceanic teleconnections between the low and midlatitudes are a key mechanism to understanding the climate variability. Spiciness anomalies (density-compensated anomalies) have been shown to transport temperature and salinity signals when propagating along current streamlines in the subtropical gyres of the Atlantic and Pacific Oceans. The generation mechanism of spiciness anomalies in the North Atlantic subtropical gyre is investigated using an analytical model based on the late-winter subduction of salinity and temperature anomalies along isopycnal surfaces. The keystone of this approach is the change of the coordinates frame from isobaric to isopycnic surfaces, suited for subduction problems. The isopycnal nature of spiciness anomalies and the use of a linear density equation allows for the analytical model to depend only upon surface temperature and salinity anomalies, the mean thermocline currents, and the surface density ratio. This model clarifies and above all quantifies the mechanism by which surface temperature and salinity anomalies are modulated by density ratios to produce fully different isopycnal temperature and salinity anomalies. A global run from the ocean GCM (OGCM) Océan Parallélisé (OPA) over the period 1948–2002 provides the reference data in which the North Atlantic subtropical thermocline spiciness variability is analyzed. Two EOF modes are sufficient to explain half of the low-frequency variability in the OGCM: one is maximum over the northeastern subtropics, and the other is in the central basin. The analytical model reproduces well the spatial pattern, amplitude, and sign of these two main modes. It confirms that the two centers of action of the anomalies are conditioned by the surface density ratio, the first corresponding to null salinity gradients and the second to near-density-compensated temperature gradients. Considering that the analytical model has good skills at reproducing the decadal variability of the OGCM spiciness anomalies in the permanent thermocline, it is believed that this is an interesting tool to understand and forecast the ventilation of the North Atlantic subtropical gyre at this time scale.
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Arzel, Olivier, and Alain Colin de Verdière. "Can We Infer Diapycnal Mixing Rates from the World Ocean Temperature–Salinity Distribution?" Journal of Physical Oceanography 46, no. 12 (December 2016): 3751–75. http://dx.doi.org/10.1175/jpo-d-16-0152.1.
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AbstractThe turbulent diapycnal mixing in the ocean is currently obtained from microstructure and finestructure measurements, dye experiments, and inverse models. This study presents a new method that infers the diapycnal mixing from low-resolution numerical calculations of the World Ocean whose temperatures and salinities are restored to the climatology. At the difference of robust general circulation ocean models, diapycnal diffusion is not prescribed but inferred. At steady state the buoyancy equation shows an equilibrium between the large-scale diapycnal advection and the restoring terms that take the place of the divergence of eddy buoyancy fluxes. The geography of the diapycnal flow reveals a strong regional variability of water mass transformations. Positive values of the diapycnal flow indicate an erosion of a deep-water mass and negative values indicate a creation. When the diapycnal flow is upward, a diffusion law can be fitted in the vertical and the diapycnal eddy diffusivity is obtained throughout the water column. The basin averages of diapycnal diffusivities are small in the first 1500 m [O(10−5) m2 s−1] and increase downward with bottom values of about 2.5 × 10−4 m2 s−1 in all ocean basins, with the exception of the Southern Ocean (50°–30°S), where they reach 12 × 10−4 m2 s−1. This study confirms the small diffusivity in the thermocline and the robustness of the higher canonical Munk’s value in the abyssal ocean. It indicates that the upward dianeutral transport in the Atlantic mostly takes place in the abyss and the upper ocean, supporting the quasi-adiabatic character of the middepth overturning.
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Chen, Gengxin, Weiqing Han, Yuanlong Li, and Dongxiao Wang. "Interannual Variability of Equatorial Eastern Indian Ocean Upwelling: Local versus Remote Forcing." Journal of Physical Oceanography 46, no. 3 (March 2016): 789–807. http://dx.doi.org/10.1175/jpo-d-15-0117.1.
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AbstractThe equatorial eastern Indian Ocean (EIO) upwelling occurs in the Indian Ocean warm pool, differing from the equatorial Pacific and Atlantic upwelling that occurs in the cold tongue. By analyzing observations and performing ocean model experiments, this paper quantifies the remote versus local forcing in causing interannual variability of the equatorial EIO upwelling from 2001 to 2011 and elucidates the associated processes. For all seasons, interannual variability of thermocline depth in the EIO, as an indicator of upwelling, is dominated by remote forcing from equatorial Indian Ocean winds, which drive Kelvin waves that propagate along the equator and subsequently along the Sumatra–Java coasts. Upwelling has prominent signatures in sea surface temperature (SST) and chlorophyll-a concentration but only in boreal summer–fall (May–October). Local forcing plays a larger role than remote forcing in producing interannual SST anomaly (SSTA). During boreal summer–fall, when the mean thermocline is relatively shallow, SSTA is primarily driven by the upwelling process, with comparable contributions from remote and local forcing effects. In contrast, during boreal winter–spring (November–April), when the mean thermocline is relatively deep, SSTA is controlled by surface heat flux and decoupled from thermocline variability. Advection affects interannual SSTA in all cases. The remote and local winds that drive the interannual variability of the equatorial EIO upwelling are closely associated with Indian Ocean dipole events and to a lesser degree with El Niño–Southern Oscillation.
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Nagura, Motoki, Yukio Masumoto, and Takanori Horii. "Meridional Heat Advection due to Mixed Rossby Gravity Waves in the Equatorial Indian Ocean." Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 343–58. http://dx.doi.org/10.1175/jpo-d-13-0141.1.
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Abstract This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the x–z plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.
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Jones, Daniel C., Emma Boland, Andrew J. S. Meijers, Gael Forget, Simon Josey, Jean-Baptiste Sallée, and Emily Shuckburgh. "The Sensitivity of Southeast Pacific Heat Distribution to Local and Remote Changes in Ocean Properties." Journal of Physical Oceanography 50, no. 3 (March 2020): 773–90. http://dx.doi.org/10.1175/jpo-d-19-0155.1.
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AbstractThe Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on time scales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally constrained, physically consistent global ocean model to examine the sensitivity of heat distribution in the recently ventilated subsurface Pacific (RVP) sector of the Southern Ocean to changes in ocean temperature and salinity. First, we define the RVP using numerical passive tracer release experiments that highlight the ventilation pathways. Next, we use an ensemble of adjoint sensitivity experiments to quantify the sensitivity of the RVP heat content to changes in ocean temperature and salinity. In terms of sensitivities to surface ocean properties, we find that RVP heat content is most sensitive to anomalies along the Antarctic Circumpolar Current (ACC), upstream of the subduction hotspots. In terms of sensitivities to subsurface ocean properties, we find that RVP heat content is most sensitive to basin-scale changes in the subtropical Pacific Ocean, around the same latitudes as the RVP. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the southeast Pacific.
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Nonaka, Masami, Julian P. McCreary, and Shang-Ping Xie. "Influence of Midlatitude Winds on the Stratification of the Equatorial Thermocline*." Journal of Physical Oceanography 36, no. 2 (February 1, 2006): 222–37. http://dx.doi.org/10.1175/jpo2845.1.
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Abstract The stratification of the equatorial thermocline is a key variable for tropical climate dynamics, through its influence on the temperature of the water that upwells in the eastern equatorial ocean. In this study, two types of ocean models are used, an ocean general circulation model (GCM) and a 1½-layer model, to investigate processes by which changes in the midlatitude winds affect the equatorial stratification. Specifically, the influences of anomalous mode-water formation, Ekman pumping, and entrainment in the subpolar ocean are examined. The effects of a “sponge layer” adjacent to the northern boundary of the basin are also assessed. Solutions are forced by idealized zonal winds with strong or weak midlatitude westerlies, and they are found in rectangular basins that extend from the equator to 36°N (small basin) or to 60°N (large basin). In the GCM solutions, a prominent response to reduced winds is the thinning of the mixed layer in the northwestern region of the subtropical gyre, leading to less subduction of low-potential-vorticity mode water and hence thinning of the upper thermocline in the central-to-eastern subtropics. Almost all of this thinning signal, however, recirculates within the subtropics, and does not extend to the equator. Another midlatitude response is shallowing (deepening) of the thermocline in the subtropical (subpolar) ocean in response to Ekman pumping. This, primarily, first-baroclinic-mode (n = 1) response has the most influence on the equatorial thermocline. First-baroclinic-mode Rossby waves propagate to the western boundary of the basin where they reflect as packets of coastal Kelvin and short-wavelength Rossby waves that carry the midlatitude signal to the equator. Subsequently, equatorial Kelvin waves spread it along the equator, leading to a shoaling and thinning of the equatorial thermocline. The layer-thickness field h in the 1½-layer model corresponds to thermocline depth in the GCM. Both the sponge layer and subpolar Ekman suction are important factors for the 1½-layer model solutions, requiring water upwelled in the interior ocean to be transported into the sponge layer via the western boundary layer. In the small basin, equatorial h thins in response to weakened westerlies when there is a sponge layer, but it thickens when there is not. In the large basin, equatorial h is unaffected by weakened westerlies when there is a sponge layer, but it thins when water is allowed to entrain into the layer in the subpolar gyre. It is concluded that the thinning of the equatorial thermocline in the GCM solutions is caused by the sponge layer in the small basin and by entrainment in the subpolar ocean in the large one.
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Cessi, Paola, and Maurizio Fantini. "The Eddy-Driven Thermocline." Journal of Physical Oceanography 34, no. 12 (December 1, 2004): 2642–58. http://dx.doi.org/10.1175/jpo2657.1.
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Abstract The role of baroclinic eddies in transferring thermal gradients laterally, and thus determining the stratification of the ocean, is examined. The hypothesis is that the density differences imposed at the surface by differential heating are a source of available potential energy that can be partially released by mesocale eddies with horizontal scales on the order of 100 km. Eddy fluxes balance the diapycnal mixing of heat and thus determine the vertical scale of penetration of horizontal thermal gradients (i.e., the depth of the thermocline). This conjecture is in contrast with the current thinking that the deep stratification is determined by a balance between diapycnal mixing and the large-scale thermohaline circulation. Eddy processes are analyzed in the context of a rapidly rotating primitive equation flow driven by specified surface temperature, with isotropic diffusion and viscosity. The barotropic component of the eddies is found to be responsible for most of the heat flux, and so the eddy transport is horizontal rather than isopycnal. This eddy transport takes place in the shallow surface layer where eddies, as well as the mean temperature, undergo diabatic, irreversible mixing. Scaling laws for the depth of the thermocline as a function of the external parameters are proposed. In the classical thermocline theory, the depth of the thermocline depends on the diffusivity, the rotation rate, and the imposed temperature gradient. In this study the authors find an additional dependence on the viscosity and on the domain width.
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Fedorov, Alexey, Marcelo Barreiro, Giulio Boccaletti, Ronald Pacanowski, and S. George Philander. "The Freshening of Surface Waters in High Latitudes: Effects on the Thermohaline and Wind-Driven Circulations." Journal of Physical Oceanography 37, no. 4 (April 1, 2007): 896–907. http://dx.doi.org/10.1175/jpo3033.1.
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Abstract The impacts of a freshening of surface waters in high latitudes on the deep, slow, thermohaline circulation have received enormous attention, especially the possibility of a shutdown in the meridional overturning that involves sinking of surface waters in the northern Atlantic Ocean. A recent study by Fedorov et al. has drawn attention to the effects of a freshening on the other main component of the oceanic circulation—the swift, shallow, wind-driven circulation that varies on decadal time scales and is closely associated with the ventilated thermocline. That circulation too involves meridional overturning, but its variations and critical transitions affect mainly the Tropics. A surface freshening in mid- to high latitudes can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west, the tropical zonal sea surface temperature gradient virtually disappears, and permanently warm conditions prevail in the Tropics. In a model that has both the wind-driven and thermohaline components of the circulation, which factors determine the relative effects of a freshening on the two components and its impact on climate? Studies with an idealized ocean general circulation model find that vertical diffusivity is one of the critical parameters that affect the relative strength of the two circulation components and hence their response to a freshening. The spatial structure of the freshening and imposed meridional temperature gradients are other important factors.
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Johnson, Gregory C., John M. Toole, and Nordeen G. Larson. "Sensor Corrections for Sea-Bird SBE-41CP and SBE-41 CTDs." Journal of Atmospheric and Oceanic Technology 24, no. 6 (June 2007): 1117–30. http://dx.doi.org/10.1175/jtech2016.1.
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Sensor response corrections for two models of Sea-Bird Electronics, Inc., conductivity–temperature–depth (CTD) instruments (the SBE-41CP and SBE-41) designed for low-energy profiling applications were estimated and applied to oceanographic data. Three SBE-41CP CTDs mounted on prototype ice-tethered profilers deployed in the Arctic Ocean sampled diffusive thermohaline staircases and telemetered data to shore at their full 1-Hz resolution. Estimations of and corrections for finite thermistor time response, time shifts between when a parcel of water was sampled by the thermistor and when it was sampled by the conductivity cell, and the errors in salinity induced by the thermal inertia of the conductivity cell are developed with these data. In addition, thousands of profiles from Argo profiling floats equipped with SBE-41 CTDs were screened to select examples where thermally well-mixed surface layers overlaid strong thermoclines for which standard processing often yields spuriously fresh salinity estimates. Hundreds of profiles so identified are used to estimate and correct for the conductivity cell thermal mass error in SBE-41 CTDs.
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Nagura, Motoki, and Shinya Kouketsu. "Spiciness Anomalies in the Upper South Indian Ocean." Journal of Physical Oceanography 48, no. 9 (September 2018): 2081–101. http://dx.doi.org/10.1175/jpo-d-18-0050.1.
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AbstractThis study investigates an isopycnal temperature/salinity T/S, or spiciness, anomaly in the upper south Indian Ocean for the period from 2004 to 2015 using observations and reanalyses. Spiciness anomalies at about 15°S on 24–26σθ are focused on, whose standard deviation is about 0.1 psu in salinity and 0.25°C in temperature, and they have a contribution to isobaric temperature variability comparable to thermocline heave. A plausible generation region of these anomalies is the southeastern Indian Ocean, where the 25σθ surface outcrops in southern winter, and the anticyclonic subtropical gyre advects subducted water equatorward. Unlike the Pacific and Atlantic, spiciness anomalies in the upper south Indian Ocean are not T/S changes in mode water, and meridional variations in SST and sea surface salinity in their generation region are not density compensating. It is possible that this peculiarity is owing to freshwater originating from the Indonesian Seas. The production of spiciness anomalies is estimated from surface heat and freshwater fluxes and the surface T/S relationship in the outcrop region, based on several assumptions including the dominance of surface fluxes in the surface T/S budget and effective mixed layer depth proposed by Deser et al. The result agrees well with isopycnal salinity anomalies at the outcrop line, which indicates that spiciness anomalies are generated by local surface fluxes. It is suggested that the Ningaloo Niño and El Niño–Southern Oscillation lead to interannual variability in surface heat flux in the southeastern Indian Ocean and contribute to the generation of spiciness anomalies.
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Grad, Marek, Rolf Mjelde, Wojciech Czuba, Aleksander Guterch, and Johannes Schweitzer. "Modelling of seafloor multiples observed in OBS data from the North Atlantic - new seismic tool for oceanography?" Polish Polar Research 32, no. 4 (January 1, 2011): 375–92. http://dx.doi.org/10.2478/v10183-011-0027-3.
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Modelling of seafloor multiples observed in OBS data from the North Atlantic - new seismic tool for oceanography?In marine seismic wide-angle profiling the recorded wave field is dominated by waves propagating in the water. These strong direct and multiple water waves are generally treated as noise, and considerable processing efforts are employed in order minimize their influences. In this paper we demonstrate how the water arrivals can be used to determine the water velocity beneath the seismic wide-angle profile acquired in the Northern Atlantic. The pattern of water multiples generated by air-guns and recorded by Ocean Bottom Seismometers (OBS) changes with ocean depth and allows determination of 2D model of velocity. Along the profile, the water velocity is found to change from about 1450 to approximately 1490 m/s. In the uppermost 400 m the velocities are in the range of 1455-1475 m/s, corresponding to the oceanic thermocline. In the deep ocean there is a velocity decrease with depth, and a minimum velocity of about 1450 m/s is reached at about 1.5 km depth. Below that, the velocity increases to about 1495 m/s at approximately 2.5 km depth. Our model compares well with estimates from CTD (Conductivity, Temperature, Depth) data collected nearby, suggesting that the modelling of water multiples from OBS data might become an important oceanographic tool.
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Li, Yuanlong, Weiqing Han, Toshiaki Shinoda, Chunzai Wang, M. Ravichandran, and Jih-Wang Wang. "Revisiting the Wintertime Intraseasonal SST Variability in the Tropical South Indian Ocean: Impact of the Ocean Interannual Variation*." Journal of Physical Oceanography 44, no. 7 (July 1, 2014): 1886–907. http://dx.doi.org/10.1175/jpo-d-13-0238.1.
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Abstract Intraseasonal sea surface temperature (SST) variability over the Seychelles–Chagos thermocline ridge (SCTR; 12°–4°S, 55°–85°E) induced by boreal wintertime Madden–Julian oscillations (MJOs) is investigated with a series of OGCM experiments forced by the best available atmospheric data. The impact of the ocean interannual variation (OIV), for example, the thermocline depth changes in the SCTR, is assessed. The results show that surface shortwave radiation (SWR), wind speed–controlled turbulent heat fluxes, and wind stress–driven ocean processes are all important in causing the MJO-related intraseasonal SST variability. The effect of the OIV is significant in the eastern part of the SCTR (70°–85°E), where the intraseasonal SSTs are strengthened by about 20% during the 2001–11 period. In the western part (55°–70°E), such effect is relatively small and not significant. The relative importance of the three dominant forcing factors is adjusted by the OIV, with increased (decreased) contribution from wind stress (wind speed and SWR). The OIV also tends to intensify the year-to-year variability of the intraseasonal SST amplitude. In general, a stronger (weaker) SCTR favors larger (smaller) SST responses to the MJO forcing. Because of the nonlinearity of the upper-ocean thermal stratification, especially the mixed layer depth (MLD), the OIV imposes an asymmetric impact on the intraseasonal SSTs between the strong and weak SCTR conditions. In the eastern SCTR, both the heat flux forcing and entrainment are greatly amplified under the strong SCTR condition, but only slightly suppressed under the weak SCTR condition, leading to an overall strengthening effect by the OIV.
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Vega, L. A. "Ocean Thermal Energy Conversion Primer." Marine Technology Society Journal 36, no. 4 (December 1, 2002): 25–35. http://dx.doi.org/10.4031/002533202787908626.
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The vertical temperature distribution in the open ocean can be simplistically described as consisting of two layers separated by an interface. The upper layer is warmed by the sun and mixed to depths of about 100 m by wave motion. The bottom layer consists of colder water formed at high latitudes. The interface or thermocline is sometimes marked by an abrupt change in temperature but more often the change is gradual. The temperature difference between the upper (warm) and bottom (cold) layers ranges from 10°C to 25°C, with the higher values found in equatorial waters. This implies that there are two enormous reservoirs providing the heat source and the heat sink required for a heat engine. A practical application is found in a system (heat engine) designed to transform the thermal energy into electricity. This is referred to as OTEC for Ocean Thermal Energy Conversion. Several techniques have been proposed to use this ocean thermal resource; however, at present it appears that only the closed cycle (CC-OTEC) and the open cycle (OC-OTEC) schemes have a solid foundation of theoretical as well as experimental work. In the CC-OTEC system, warm surface seawater and cold seawater are used to vaporize and condense a working fluid, such as anhydrous ammonia, which drives a turbine-generator in a closed loop producing electricity. In the OC-OTEC system, seawater is flash-evaporated in a vacuum chamber. The resulting low-pressure steam is used to drive a turbine-generator. Gold seawater is used to condense the steam after it has passed through the turbine. The open-cycle can, therefore, be configured to produce desalinated water as well as electricity.
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Zhu, Yuchao, and Rong-Hua Zhang. "A Modified Vertical Mixing Parameterization for Its Improved Ocean and Coupled Simulations in the Tropical Pacific." Journal of Physical Oceanography 49, no. 1 (January 2019): 21–37. http://dx.doi.org/10.1175/jpo-d-18-0100.1.
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AbstractClimate models suffer from significant biases over the tropical Pacific Ocean, including a too-cold cold tongue and too-warm temperature at the depth of the thermocline. The emergence of model biases can be partly attributed to vertical mixing parameterizations, in which there are great uncertainties in selections of functional forms and empirical parameters. In this paper, the impacts of two different vertical mixing schemes on the tropical Pacific temperature simulations are investigated using version 5 of the Modular Ocean Model (MOM5). One vertical mixing scheme is the widely used K-profile parameterization (KPP) scheme, and the other is a hybrid mixing scheme (the Chen scheme) by combining a Kraus–Turner-type bulk mixed layer (ML) model with Peters et al.’s shear instability mixing model (PGT model). It is shown that the Chen scheme works better than the KPP scheme for SST simulation but produces an exaggerated subsurface warm bias simultaneously. The better SST simulation can be attributed to the employment of the PGT model, which produces lower levels of shear instability mixing than its counterpart in the KPP scheme. Furthermore, a modified KPP scheme is presented in which its shear instability mixing model and constant background diffusivity are replaced by the PGT model and the Argo-derived background diffusivity, respectively. This new scheme is then employed into MOM5-based ocean-only and coupled simulations, demonstrating substantial improvements in temperature simulations over the tropical Pacific. The modified KPP scheme can be easily employed into other ocean models, offering an effective way to improve ocean simulations.
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Drushka, Kyla, Janet Sprintall, Sarah T. Gille, and Irsan Brodjonegoro. "Vertical Structure of Kelvin Waves in the Indonesian Throughflow Exit Passages." Journal of Physical Oceanography 40, no. 9 (September 1, 2010): 1965–87. http://dx.doi.org/10.1175/2010jpo4380.1.
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Abstract The subsurface structure of intraseasonal Kelvin waves in two Indonesian Throughflow (ITF) exit passages is observed and characterized using velocity and temperature data from the 2004–06 International Nusantara Stratification and Transport (INSTANT) project. Scatterometer winds are used to characterize forcing, and altimetric sea level anomaly (SLA) data are used to trace the pathways of Kelvin waves east from their generation region in the equatorial Indian Ocean to Sumatra, south along the Indonesian coast, and into the ITF region. During the 3-yr INSTANT period, 40 intraseasonal Kelvin waves forced by winds over the central equatorial Indian Ocean caused strong transport anomalies in the ITF outflow passages. Of these events, 21 are classed as “downwelling” Kelvin waves, forced by westerly winds and linked to depressions in the thermocline and warm temperature anomalies in the ITF outflow passages; 19 were “upwelling” Kelvin waves, generated by easterly wind events and linked to shoaling of the thermocline and cool temperature anomalies in the ITF. Both downwelling and upwelling Kelvin waves have similar vertical structures in the ITF outflow passages, with strong transport anomalies over all depths and a distinctive upward tilt to the phase that indicates downward energy propagation. A linear wind-forced model shows that the first two baroclinic modes account for most of the intraseasonal variance in the ITF outflow passages associated with Kelvin waves and highlights the importance of winds both in the eastern equatorial Indian Ocean and along the coast of Sumatra and Java for exciting Kelvin waves. Using SLA as a proxy for Kelvin wave energy shows that 37% ± 9% of the incoming Kelvin wave energy from the Indian Ocean bypasses the gap in the coastal waveguide at Lombok Strait and continues eastward. Of the energy that continues eastward downstream of Lombok Strait, the Kelvin waves are split by Sumba Island, with roughly equal energy going north and south to enter the Savu Sea.
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Jochum, Markus, and Raghu Murtugudde. "Temperature Advection by Tropical Instability Waves." Journal of Physical Oceanography 36, no. 4 (April 1, 2006): 592–605. http://dx.doi.org/10.1175/jpo2870.1.
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Abstract A numerical model of the tropical Pacific Ocean is used to investigate the processes that cause the horizontal temperature advection of tropical instability waves (TIWs). It is found that their temperature advection cannot be explained by the processes on which the mixing length paradigm is based. Horizontal mixing of temperature across the equatorial SST front does happen, but it is small relative to the “oscillatory” temperature advection of TIWs. The basic mechanism is that TIWs move water back and forth across a patch of large vertical entrainment. Outside this patch, the atmosphere heats the water and this heat is then transferred into the thermocline inside the patch. These patches of strong localized entrainment are due to equatorial Ekman divergence and due to thinning of the mixed layer in the TIW cyclones. The latter process is responsible for the zonal temperature advection, which is as large as the meridional temperature advection but has not yet been observed. Thus, in the previous observational literature the TIW contribution to the mixed layer heat budget may have been underestimated significantly.
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Lambert, Erwin, Tor Eldevik, and Michael A. Spall. "On the Dynamics and Water Mass Transformation of a Boundary Current Connecting Alpha and Beta Oceans." Journal of Physical Oceanography 48, no. 10 (October 2018): 2457–75. http://dx.doi.org/10.1175/jpo-d-17-0186.1.
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AbstractA subpolar marginal sea, like the Nordic seas, is a transition zone between the temperature-stratified subtropics (the alpha ocean) and the salinity-stratified polar regions (the beta ocean). An inflow of Atlantic Water circulates these seas as a boundary current that is cooled and freshened downstream, eventually to outflow as Deep and Polar Water. Stratification in the boundary region is dominated by a thermocline over the continental slope and a halocline over the continental shelves, separating Atlantic Water from Deep and Polar Water, respectively. A conceptual model is introduced for the circulation and water mass transformation in a subpolar marginal sea to explore the potential interaction between the alpha and beta oceans. Freshwater input into the shelf regions has a slight strengthening effect on the Atlantic inflow, but more prominently impacts the water mass composition of the outflow. This impact of freshwater, characterized by enhancing Polar Water outflow and suppressing Deep Water outflow, is strongly determined by the source location of freshwater. Concretely, perturbations in upstream freshwater sources, like the Baltic freshwater outflow into the Nordic seas, have an order of magnitude larger potential to impact water mass transports than perturbations in downstream sources like the Arctic freshwater outflow. These boundary current dynamics are directly related to the qualitative stratification in transition zones and illustrate the interaction between the alpha and beta oceans.
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Liu, Lingling, Yuanlong Li, and Fan Wang. "MJO-Induced Intraseasonal Mixed Layer Depth Variability in the Equatorial Indian Ocean and Impacts on Subsurface Water Obduction." Journal of Physical Oceanography 51, no. 4 (April 2021): 1247–63. http://dx.doi.org/10.1175/jpo-d-20-0179.1.
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AbstractChange of oceanic surface mixed layer depth (MLD) is critical for vertical exchanges between the surface and subsurface oceans and modulates surface temperature variabilities on various time scales. In situ observations have documented prominent intraseasonal variability (ISV) of MLD with 30–105-day periods in the equatorial Indian Ocean (EIO) where the Madden–Julian oscillation (MJO) initiates. Simulation of Hybrid Coordinate Ocean Model (HYCOM) reveals a regional maximum of intraseasonal MLD variability in the EIO (70°–95°E, 3°S–3°N) with a standard deviation of ~14 m. Sensitivity experiments of HYCOM demonstrate that, among all of the MJO-related forcing effects, the wind-driven downwelling and mixing are primary causes for intraseasonal MLD deepening and explain 83.7% of the total ISV. The ISV of MLD gives rise to high-frequency entrainments of subsurface water, leading to an enhancement of the annual entrainment rate by 34%. However, only a small fraction of these entrainment events (<20%) can effectively contribute to the annual obduction rate of 1.36 Sv, a quantification for the amount of resurfacing thermocline water throughout a year that mainly (84.6%) occurs in the summer monsoon season (May–October). The ISV of MLD achieves the maximal intensity in April–May and greatly affects the subsequent obduction. Estimation based on our HYCOM simulations suggests that MJOs overall reduce the obduction rate in the summer monsoon season by as much as 53%. A conceptual schematic is proposed to demonstrate how springtime intraseasonal MLD deepening events caused by MJO winds narrow down the time window for effective entrainment and thereby suppress the obduction of thermocline water.
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Tozuka, Tomoki, Motoki Nagura, and Toshio Yamagata. "Influence of the Reflected Rossby Waves on the Western Arabian Sea Upwelling Region." Journal of Physical Oceanography 44, no. 5 (April 24, 2014): 1424–38. http://dx.doi.org/10.1175/jpo-d-13-0127.1.
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Abstract The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.
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Pauthenet, Etienne, Fabien Roquet, Gurvan Madec, Jean-Baptiste Sallée, and David Nerini. "The Thermohaline Modes of the Global Ocean." Journal of Physical Oceanography 49, no. 10 (October 2019): 2535–52. http://dx.doi.org/10.1175/jpo-d-19-0120.1.
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AbstractThe first 2000 m of the global thermohaline structure of the ocean are statistically decomposed into vertical thermohaline modes, using a multivariate functional principal component analysis (FPCA). This method is applied on the Monthly Isopycnal and Mixed-Layer Ocean Climatology (MIMOC). The first three modes account for 92% of the joint temperature and salinity (T–S) variance, which yields a surprisingly good reduction of dimensionality. The first mode (69% of the variance) is related to the thermocline depth and delineates the subtropical gyres. The second mode (18%) is mostly driven by salinity and mainly displays the asymmetry between the North Pacific and Atlantic basins and the salty circumpolar deep waters in the Southern Ocean. The third mode (5%) identifies the low- and high-salinity intermediate waters, covarying with the freshwater inputs of the upper ocean. The representation of the ocean in the space defined by the first three modes offers a simple visualization of the global thermohaline structure that strikingly emphasizes the role of the Southern Ocean in linking and distributing water masses to the other basins. The vertical thermohaline modes offer a convenient framework for model and observation data comparison. This is illustrated by projecting the repeated Pacific section P16 together with profiles from the Array for Real-Time Geostrophic Oceanography (ARGO) global array of profiling floats on the modes defined with the climatology MIMOC. These thermohaline modes have a potential for water mass identification and robust analysis of heat and salt content.
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Smith, K. Shafer, and Raffaele Ferrari. "The Production and Dissipation of Compensated Thermohaline Variance by Mesoscale Stirring." Journal of Physical Oceanography 39, no. 10 (October 1, 2009): 2477–501. http://dx.doi.org/10.1175/2009jpo4103.1.
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Abstract Temperature–salinity profiles from the region studied in the North Atlantic Tracer Release Experiment (NATRE) show large isopycnal excursions at depths just below the thermocline. It is proposed here that these thermohaline filaments result from the mesoscale stirring of large-scale temperature and salinity gradients by geostrophic turbulence, resulting in a direct cascade of thermohaline variance to small scales. This hypothesis is investigated as follows: Measurements from NATRE are used to generate mean temperature, salinity, and shear profiles. The mean stratification and shear are used as the background state in a high-resolution horizontally homogeneous quasigeostrophic model. The mean state is baroclinically unstable, and the model produces a vigorous eddy field. Temperature and salinity are stirred laterally in each density layer by the geostrophic velocity and vertical advection is by the ageostrophic velocity. The simulated temperature–salinity diagram exhibits fluctuations at depths just below the thermocline of similar magnitude to those found in the NATRE data. It is shown that vertical diffusion is sufficient to absorb the laterally driven cascade of tracer variance through an amplification of filamentary slopes by small-scale shear. These results suggest that there is a strong coupling between vertical mixing and horizontal stirring in the ocean at scales below the deformation radius.
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Prakash, Kumar Ravi, Tanuja Nigam, and Vimlesh Pant. "Estimation of oceanic subsurface mixing under a severe cyclonic storm using a coupled atmosphere–ocean–wave model." Ocean Science 14, no. 2 (April 3, 2018): 259–72. http://dx.doi.org/10.5194/os-14-259-2018.
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Abstract. A coupled atmosphere–ocean–wave model was used to examine mixing in the upper-oceanic layers under the influence of a very severe cyclonic storm Phailin over the Bay of Bengal (BoB) during 10–14 October 2013. The coupled model was found to improve the sea surface temperature over the uncoupled model. Model simulations highlight the prominent role of cyclone-induced near-inertial oscillations in subsurface mixing up to the thermocline depth. The inertial mixing introduced by the cyclone played a central role in the deepening of the thermocline and mixed layer depth by 40 and 15 m, respectively. For the first time over the BoB, a detailed analysis of inertial oscillation kinetic energy generation, propagation, and dissipation was carried out using an atmosphere–ocean–wave coupled model during a cyclone. A quantitative estimate of kinetic energy in the oceanic water column, its propagation, and its dissipation mechanisms were explained using the coupled atmosphere–ocean–wave model. The large shear generated by the inertial oscillations was found to overcome the stratification and initiate mixing at the base of the mixed layer. Greater mixing was found at the depths where the eddy kinetic diffusivity was large. The baroclinic current, holding a larger fraction of kinetic energy than the barotropic current, weakened rapidly after the passage of the cyclone. The shear induced by inertial oscillations was found to decrease rapidly with increasing depth below the thermocline. The dampening of the mixing process below the thermocline was explained through the enhanced dissipation rate of turbulent kinetic energy upon approaching the thermocline layer. The wave–current interaction and nonlinear wave–wave interaction were found to affect the process of downward mixing and cause the dissipation of inertial oscillations.
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Melet, Angélique, Jacques Verron, Lionel Gourdeau, and Ariane Koch-Larrouy. "Equatorward Pathways of Solomon Sea Water Masses and Their Modifications." Journal of Physical Oceanography 41, no. 4 (April 1, 2011): 810–26. http://dx.doi.org/10.1175/2010jpo4559.1.
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Abstract The Solomon Sea is a key region of the southwest Pacific Ocean, connecting the thermocline subtropics to the equator via western boundary currents (WBCs). Modifications to water masses are thought to occur in this region because of the significant mixing induced by internal tides, eddies, and the WBCs. Despite their potential influence on the equatorial Pacific thermocline temperature and salinity and their related impact on the low-frequency modulation of El Niño–Southern Oscillation, modifications to water masses in the Solomon Sea have never been analyzed to our knowledge. A high-resolution model incorporating a tidal mixing parameterization was implemented to depict and analyze water mass modifications and the Solomon Sea pathways to the equator in a Lagrangian quantitative framework. The main routes from the Solomon Sea to the equatorial Pacific occur through the Vitiaz and Solomon straits, in the thermocline and intermediate layers, and mainly originate from the Solomon Sea south inflow and from the Solomon Strait itself. Water mass modifications in the model are characterized by a reduction of the vertical temperature and salinity gradients over the water column: the high salinity of upper thermocline water [Subtropical Mode Water (STMW)] is eroded and exported toward surface and deeper layers, whereas a downward heat transfer occurs over the water column. Consequently, the thermocline water temperature is cooled by 0.15°–0.3°C from the Solomon Sea inflows to the equatorward outflows. This temperature modification could weaken the STMW anomalies advected by the subtropical cell and thereby diminish the potential influence of these anomalies on the tropical climate. The Solomon Sea water mass modifications can be partially explained (≈60%) by strong diapycnal mixing in the Solomon Sea. As for STMW, about a third of this mixing is due to tidal mixing.
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Yim, Bo Young, Yign Noh, Bo Qiu, Sung Hyup You, and Jong Hwan Yoon. "The Vertical Structure of Eddy Heat Transport Simulated by an Eddy-Resolving OGCM." Journal of Physical Oceanography 40, no. 2 (February 1, 2010): 340–53. http://dx.doi.org/10.1175/2009jpo4243.1.
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Abstract The vertical structure of meridional eddy heat transport (EHT) of the North Pacific was investigated by analyzing the results from an eddy-resolving ocean general circulation model (OGCM) with a horizontal resolution of , while comparing with previous simulation results and observation data. In particular, the spatial and temporal variation of the effective depth of EHT He was investigated, which is defined by the depth integrated EHT (D-EHT) divided by EHT at the surface. It was found that the annual mean value of He is proportional to the eddy kinetic energy (EKE) level at the surface in general. However, its seasonal variation is controlled by the mixed layer depth (MLD) in the extratropical ocean (>20°N). Examination of the simulated eddy structures reveals that the temperature associated with mesoscale eddies is radically modified by the surface forcing in the mixed layer, while the velocity field is not, and the consequent enhanced misalignment of temperature and velocity anomalies leads to the radical change of EHT across the seasonal thermocline.
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Fu, Lee-Lueng. "Intraseasonal Variability of the Equatorial Indian Ocean Observed from Sea Surface Height, Wind, and Temperature Data." Journal of Physical Oceanography 37, no. 2 (February 1, 2007): 188–202. http://dx.doi.org/10.1175/jpo3006.1.
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Abstract The forcing of the equatorial Indian Ocean by the highly periodic monsoon wind cycle creates many interesting intraseasonal variabilities. The frequency spectrum of the wind stress observations from the European Remote Sensing Satellite scatterometers reveals peaks at the seasonal cycle and its higher harmonics at 180, 120, 90, and 75 days. The observations of sea surface height (SSH) from the Jason and Ocean Topography Experiment (TOPEX)/Poseidon radar altimeters are analyzed to study the ocean’s response. The focus of the study is on the intraseasonal periods shorter than the annual period. The semiannual SSH variability is characterized by a basin mode involving Rossby waves and Kelvin waves traveling back and forth in the equatorial Indian Ocean between 10°S and 10°N. However, the interference of these waves with each other masks the appearance of individual Kelvin and Rossby waves, leading to a nodal point (amphidrome) of phase propagation on the equator at the center of the basin. The characteristics of the mode correspond to a resonance of the basin according to theoretical models. For the semiannual period and the size of the basin, the resonance involves the second baroclinic vertical mode of the ocean. The theory also calls for similar modes at 90 and 60 days. These modes are found only in the eastern part of the basin, where the wind forcing at these periods is primarily located. The western parts of the theoretical modal patterns are not observed, probably because of the lack of wind forcing. There is also similar SSH variability at 120 and 75 days. The 120-day variability, with spatial patterns resembling the semiannual mode, is close to a resonance involving the first baroclinic vertical mode. The 75-day variability, although not a resonant basin mode in theory, exhibits properties similar to the 60- and 90-day variabilities with energy confined to the eastern basin, where the SSH variability seems in resonance with the local wind forcing. The time it takes an oceanic signal to travel eastward as Kelvin waves from the forcing location along the equator and back as Rossby waves off the equator roughly corresponds to the period of the wind forcing. The SSH variability at 60–90 days is coherent with sea surface temperature (SST) with a near-zero phase difference, showing the effects of the time-varying thermocline depth on SST, which may affect the wind in an ocean–atmosphere coupled process governing the intraseasonal variability.
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Lyman, John M., Gregory C. Johnson, and William S. Kessler. "Distinct 17- and 33-Day Tropical Instability Waves in Subsurface Observations*." Journal of Physical Oceanography 37, no. 4 (April 1, 2007): 855–72. http://dx.doi.org/10.1175/jpo3023.1.
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Abstract Tropical instability waves (TIWs) within a half-degree of the equator in the Pacific Ocean have been consistently observed in meridional velocity with periods of around 20 days. On the other hand, near 5°N, TIWs have been observed in sea surface height (SSH), thermocline depth, and velocity to have periods near 30 days. Tropical Atmosphere–Ocean (TAO) Project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of TIWs during 3 years of La Niña conditions from 1998 through 2001. Along 140°W, where the TIW temperature and velocity variabilities are at their maxima, these variabilities include two distinct TIWs with periods of 17 and 33 days, rather than one broadbanded process. As predicted by modeling studies, the 17-day TIW variability is shown to occur not only in meridional velocity at the equator, but also in subsurface temperature at 2°N and 2°S, while the 33-day TIW variability is observed primarily in subsurface temperature at 5°N. These two TIWs, respectively, are shown to have characteristics similar to a Yanai wave/surface-trapped instability and an unstable first meridional mode Rossby wave. One implication of such a description is that the velocity variability on the equator is not directly associated with the dominant 33-day variability along 5°N.
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Deppenmeier, Anna-Lena, Frank O. Bryan, William S. Kessler, and LuAnne Thompson. "Modulation of Cross-Isothermal Velocities with ENSO in the Tropical Pacific Cold Tongue." Journal of Physical Oceanography 51, no. 5 (May 2021): 1559–74. http://dx.doi.org/10.1175/jpo-d-20-0217.1.
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AbstractThe tropical Pacific Ocean cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño–Southern Oscillation (ENSO). The CT is supplied with cold water by the Equatorial Undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water toward the surface. As the thermocline shoals, its water is transformed through diabatic processes, producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold-tongue region from a global high-resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, and solar penetration) and find that vertical mixing is the single most important contribution in the thermocline and solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, are much reduced, whereas, during La Niña periods, strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.
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Lan, Kuo-Wei, Ming-An Lee, Hsueh-Jung Lu, Wei-Juan Shieh, Wei-Kuan Lin, and Szu-Chia Kao. "Ocean variations associated with fishing conditions for yellowfin tuna (Thunnus albacares) in the equatorial Atlantic Ocean." ICES Journal of Marine Science 68, no. 6 (May 17, 2011): 1063–71. http://dx.doi.org/10.1093/icesjms/fsr045.
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Abstract Lan, K-W., Lee, M-A., Lu, H-J., Shieh, W-J., Lin, W-K., and Kao, S-C. 2011. Ocean variations associated with fishing conditions for yellowfin tuna (Thunnus albacares) in the equatorial Atlantic Ocean. – ICES Journal of Marine Science, 68: 1063–1071. In this study, the Taiwanese longline (LL) fishery data were divided into two types: regular LL and deep LL. Furthermore, we collected environmental variables, such as sea surface temperature (SST), subsurface temperature, chlorophyll a concentration, net primary productivity, windspeed, and the north tropical Atlantic SST index (NTA) during the period 1998–2007 to investigate the relationship between LL catch data and oceanic environmental factors using principal component analysis (PCA). After the daily LL was separated into two types of LL, the results indicated that the deep LL was the major fishery catching yellowfin tuna (YFT) in the equatorial Atlantic Ocean. In 2003–2005, especially in 2005, the monthly catch by deep LL was double those of other years. The spatial distribution of the nominal catch per unit effort (cpue) by deep LL showed the maximum aggregation of YFT in waters with temperature above 24–25°C. The YFT mainly aggregated in the equatorial Atlantic, extending east in the first and second quarters of the year. In the third quarter of the year, the SST decreased off West Africa and the YFT migrated westwards to 15°W. Results of PCA indicated that higher subsurface water temperatures resulted in a deeper thermocline and caused a higher cpue of YFT, but the influence of NTA on the cpue of YFT seemed to be insignificant.
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Hodges, Benjamin A., and David M. Fratantoni. "AUV Observations of the Diurnal Surface Layer in the North Atlantic Salinity Maximum." Journal of Physical Oceanography 44, no. 6 (May 28, 2014): 1595–604. http://dx.doi.org/10.1175/jpo-d-13-0140.1.
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Abstract Autonomous underwater vehicle (AUV) surveys of temperature, salinity, and velocity in the upper 10 m of the ocean were carried out in low-wind conditions near the North Atlantic surface salinity maximum as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS) project. Starting from a well-mixed state, the development, deepening, and decay of a warm salty diurnal surface layer was observed at <1-h resolution. The evaporation rate deduced from the freshwater anomaly of the layer corroborates measurements at a nearby flux mooring. Profiles within a few hundred meters of the stationary research vessel showed evidence of mixing, highlighting the effectiveness of AUVs for collecting uncontaminated time series of near-surface thermohaline structure. A two-dimensional horizontal subsurface survey within the diurnal warm layer revealed coherent warm and cool bands, which are interpreted as internal waves on the diurnal thermocline.
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Qiu, Bo, and Shuiming Chen. "Eddy-Induced Heat Transport in the Subtropical North Pacific from Argo, TMI, and Altimetry Measurements." Journal of Physical Oceanography 35, no. 4 (April 1, 2005): 458–73. http://dx.doi.org/10.1175/jpo2696.1.
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Abstract Basin-scale heat transport induced by mesoscale oceanic eddies is estimated by combining satellite-derived sea surface height and temperature [temperature data are from the TRMM Microwave Imager (TMI)] data with Argo float temperature–salinity data. In the North Pacific Ocean subtropical gyre, warm (cold) temperature anomalies of mesoscale eddies are found to be consistently located to the west of high (low) SSH anomalies. The phase misalignment between the temperature and velocity anomalies, however, is largely confined to the seasonal thermocline, causing most of the eddy-induced heat transport to be carried in the surface 200-m layer. By establishing a statistical relationship between the surface and depth-integrated values of the eddy heat transport, the basin-scale eddy heat transport is derived from the concurrent satellite SSH/SST data of the past six years. In the Kuroshio Extension region, the meandering zonal jet is found to generate oppositely signed eddy heat fluxes. As a result, the zonally integrated poleward heat transport associated with the Kuroshio Extension is at a level O(0.1 PW), smaller than the previous estimates based on turbulent closure schemes. Large poleward eddy heat transport is also found in the subtropical North Pacific along a southwest–northeast-tilting band between Taiwan and the Midway Islands. This band corresponds to the region of the subtropical front, and it is argued that the relevant temperature field for identifying this band in the turbulent closure scheme models should be that averaged over the seasonal thermocline.
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Archer, Matthew, Amandine Schaeffer, Shane Keating, Moninya Roughan, Ryan Holmes, and Lia Siegelman. "Observations of Submesoscale Variability and Frontal Subduction within the Mesoscale Eddy Field of the Tasman Sea." Journal of Physical Oceanography 50, no. 5 (May 2020): 1509–29. http://dx.doi.org/10.1175/jpo-d-19-0131.1.
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AbstractSubmesoscale lenses of water with anomalous hydrographic properties have previously been observed in the East Australian Current (EAC) system, embedded within the thermocline of mesoscale anticyclonic eddies. The waters within these lenses have high oxygen content and temperature–salinity properties that signify a surface origin. However, it is not known how these lenses form. This study presents field observations that provide insight into a possible generation mechanism via subduction at upper-ocean fronts. High-resolution hydrographic and velocity measurements of submesoscale activity were taken across a front between a mesoscale eddy dipole downstream of the EAC separation point. The front had O(1) Rossby number, strong vertical shear, and flow conducive to symmetric instability. Frontogenesis was measured in conjunction with subduction of an anticyclonic water parcel, indicative of intrathermocline eddy formation. Twenty-five years of satellite imagery reveals the existence of strong mesoscale strain coupled with strong temperature fronts in this region and indicates the conditions that led to frontal subduction observed here are a persistent feature. These processes impact the vertical export of tracers from the surface and dissipation of mesoscale kinetic energy, implicating their importance for understanding regional ocean circulation and biological productivity.
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Ogata, Tomomichi, Motoki Nagura, and Yukio Masumoto. "Mean Subsurface Upwelling Induced by Intraseasonal Variability over the Equatorial Indian Ocean." Journal of Physical Oceanography 47, no. 6 (June 2017): 1347–65. http://dx.doi.org/10.1175/jpo-d-16-0257.1.
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AbstractA possible formation mechanism of mean subsurface upwelling along the equator in the Indian Ocean is investigated using a series of hierarchical ocean general circulation model (OGCM) integrations and analytical considerations. In an eddy-resolving OGCM with realistic forcing, mean vertical velocity in the tropical Indian Ocean shows rather strong upwelling, with its maximum on the equator in the subsurface layer below the thermocline. Heat budget analysis exhibits that horizontal and vertical heat advection by deviations (i.e., due to deviations of velocity and temperature from the mean) balances with vertical advection caused by mean equatorial upwelling. Horizontal heat advection is mostly associated with intraseasonal variability with periods of 3–91 days, while contributions from longer periods (>91 days) are small. Sensitivity experiments with a coarse-resolution OGCM further demonstrate that such mean equatorial upwelling cannot be reproduced by seasonal forcing only. Adding the intraseasonal wind forcing, especially meridional wind variability with a period of 15 days, generates significant mean subsurface upwelling on the equator. Further experiments with idealized settings confirm the importance of intraseasonal mixed Rossby–gravity (MRG) waves to generate mean upwelling, which appears along the energy “beam” of the MRG wave. An analytical solution of the MRG waves indicates that wave-induced temperature advection caused by the MRG waves with upward (downward) phase propagation results in warming (cooling) on the equator. This wave-induced warming (cooling) is shown to balance with the mean equatorial upwelling (downwelling), which is consistent with simulated characteristics in the OGCM experiments.
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Nagura, Motoki, and Michael J. McPhaden. "The Shallow Overturning Circulation in the Indian Ocean." Journal of Physical Oceanography 48, no. 2 (February 2018): 413–34. http://dx.doi.org/10.1175/jpo-d-17-0127.1.
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AbstractThe number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 106 m3 s−1) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.
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Pinardi, N., I. Allen, E. Demirov, P. De Mey, G. Korres, A. Lascaratos, P. Y. Le Traon, C. Maillard, G. Manzella, and C. Tziavos. "The Mediterranean ocean forecasting system: first phase of implementation (1998–2001)." Annales Geophysicae 21, no. 1 (January 31, 2003): 3–20. http://dx.doi.org/10.5194/angeo-21-3-2003.
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Abstract. The Mediterranean Forecasting system Pilot Project has concluded its activities in 2001, achieving the following goals: 1. Realization of the first high-frequency (twice a month) Voluntary Observing Ship (VOS) system for the Mediterranean Sea with XBT profiles for the upper thermocline (0–700 m) and 12 n.m. along track nominal resolution; 2. Realization of the first Mediterranean Multidisciplinary Moored Array (M3A) system for the Near-Real-Time (NRT) acquisition of physical and biochemical observations. The actual observations consists of: air-sea interaction parameters, upper thermocline (0–500 m) temperature, salinity, oxygen and currents, euphotic zone (0–100 m) chlorophyll, nutrients, Photosinthetically Available Radiation (PAR) and turbidity; 3. Analysis and NRT dissemination of high quality along track Sea Level Anomaly (SLA), Sea Surface Temperature (SST) data from satellite sensors to be assimilated into the forecasting model; 4. Assembly and implementation of a multivariate Reduced Order Optimal Interpolation scheme (ROOI) for assimilation in NRT of all available data, in particular, SLA and VOS-XBT profiles; 5. Demonstration of the practical feasibility of NRT ten day forecasts at the Mediterranean basin scale with resolution of 0.125° in latitude and longitude. The analysis or nowcast is done once a week; 6. Development and implementation of nested regional (5 km) and shelf (2–3 km) models to simulate the seasonal variability. Four regional and nine shelf models were implemented successfully, nested within the forecasting model. The implementation exercise was carried out in different region/shelf dynamical regimes and it was demonstrated that one-way nesting is practical and accurate; 7. Validation and calibration of a complex ecosystem model in data reach shelf areas, to prepare for forecasting in a future phase. The same ecosystem model is capable of reproducing the major features of the primary producers’ carbon cycle in different regions and shelf areas. The model simulations were compared with the multidisciplinary M3A buoy observations and assimilation techniques were developed for the biochemical data. This paper overviews the methodological aspects of the research done, from the NRT observing system to the forecasting/modelling components and to the extensive validation/calibration experiments carried out with regional/shelf and ecosystem models. Key words. Oceanography: general (ocean prediction; instruments and techniques) Oceanography: physical (currents)
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He, Hailun, Yuan Wang, Xiqiu Han, Yanzhou Wei, Pengfei Lin, Zhongyan Qiu, and Yejian Wang. "Anomalous distribution of distinctive water masses over the Carlsberg Ridge in May 2012." Ocean Science 16, no. 4 (July 27, 2020): 895–906. http://dx.doi.org/10.5194/os-16-895-2020.
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Abstract. In May 2012, we conducted a hydrographic survey over the Carlsberg Ridge in the northwest Indian Ocean. In this paper, we use these station data, in combination with some free-floating Argo profiles, to obtain the sectional temperature and salinity fields, and subsequently, the hydrographic characteristics are comprehensively analyzed. Through the basic T–S diagram, three salty water masses, Arabian Sea High-Salinity Water, Persian Gulf Water, and Red Sea Water, are identified. The sectional data show a clear ventilation structure associated with Arabian Sea High-Salinity Water. The 35.8 psu salty water sinks at 6.9∘ N and extends southward to 4.4∘ N at depths around the thermocline, where the thermocline depth is in the range of 100 to 150 m. This salty thermocline extends much further south than climatology indicates. Furthermore, the temperature and salinity data are used to compute the absolute geostrophic current over the specific section, and the results show mesoscale eddy vertical structure different from some widely used oceanic reanalysis data. We also find a west-propagating planetary wave at 6∘ N, and the related features are described in terms of phase speed and horizontal and vertical structures.
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Tracey, Karen L., Kathleen A. Donohue, and D. Randolph Watts. "Bottom Temperatures in Drake Passage." Journal of Physical Oceanography 47, no. 1 (January 2017): 101–22. http://dx.doi.org/10.1175/jpo-d-16-0124.1.
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AbstractLong time series of bottom temperatures in the Southern Ocean are rare. The cDrake array with over 40 current- and pressure-recording inverted echo sounders, moored across Drake Passage to monitor the Antarctic Circumpolar Current (ACC) variability and transport, measured temperature at 1 and 50 m above the seafloor at depths ≥ 3500 m and at the southern continental margin. The 4-yr dataset provided an opportunity to examine the temporal and spatial scales of bottom temperature variability. High variability was observed; ranges were 0.5°–0.9°C in the northern passage and 0.3°–0.6°C in the southern passage. Standard deviations in the two regions were 0.1°–0.15°C and <0.05°C, respectively. Meandering of the ACC with its deep-reaching thermocline accounted for up to 50% of the observed bottom temperature variance. Northern passage temperatures, spaced less than 40 km apart, were correlated with each other, while those in the southern portion, separated by 60–70 km, were not. A gap in the West Scotia Ridge provided a deep passageway for cold water to reach the northern passage from the southern basin; an extreme event during February 2008 brought bottom waters with in situ temperatures below 0.38°C as far north as 57°S. Strong vertical temperature gradients between 1 and 50 m above the bottom occurred intermittently due to intrusions associated with deep eddy circulations arising beneath the meandering jet and to flow over steep topography, permitting the generation of internal waves. High variability in temperature on interannual time scales requires record lengths of 13–17 yr to estimate long-term trends reliably.
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Schneider, Niklas, Emanuele Di Lorenzo, and Pearn P. Niiler. "Salinity Variations in the Southern California Current*." Journal of Physical Oceanography 35, no. 8 (August 1, 2005): 1421–36. http://dx.doi.org/10.1175/jpo2759.1.
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Abstract Hydrographic observations southwestward of the Southern California Bight in the period 1937–99 show that temperature and salinity variations have very different interannual variability. Temperature varies within and above the thermocline and is correlated with climate indices of El Niño, the Pacific decadal oscillation, and local upwelling. Salinity variability is largest in the surface layers of the offshore salinity minimum and is characterized by decadal-time-scale changes. The salinity anomalies are independent of temperature, of heave of the pycnocline, and of the climate indices. Calculations demonstrate that long-shore anomalous geostrophic advection of the mean salinity gradient accumulates along the mean southward trajectory along the California Current and produces the observed salinity variations. The flow anomalies for this advective process are independent of large-scale climate indices. It is hypothesized that low-frequency variability of the California Current system results from unresolved, small-scale atmospheric forcing or from the ocean mesoscale upstream of the Southern California Bight.
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Kilpatrick, Thomas, Niklas Schneider, and Emanuele Di Lorenzo. "Generation of Low-Frequency Spiciness Variability in the Thermocline*." Journal of Physical Oceanography 41, no. 2 (February 1, 2011): 365–77. http://dx.doi.org/10.1175/2010jpo4443.1.
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Abstract The generation of variance by anomalous advection of a passive tracer in the thermocline is investigated using the example of density-compensated temperature and salinity anomalies, or spiciness. A coupled Markov model is developed in which wind stress curl forces the large-scale baroclinic ocean pressure that in turn controls the anomalous geostrophic advection of spiciness. The “double integration” of white noise atmospheric forcing by this Markov model results in a frequency (ω) spectrum of large-scale spiciness proportional to ω−4, so that spiciness variability is concentrated at low frequencies. An eddy-permitting regional model hindcast of the northeast Pacific (1950–2007) confirms that time series of large-scale spiciness variability are exceptionally smooth, with frequency spectra ∝ ω−4 for frequencies greater than 0.2 cpy. At shorter spatial scales (wavelengths less than ∼500 km), the spiciness frequency spectrum is whitened by mesoscale eddies, but this eddy-forced variability can be filtered out by spatially averaging. Large-scale and long-term measurements are needed to observe the variance of spiciness or any other passive tracer subject to anomalous advection in the thermocline.
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Fan, Yalin, Isaac Ginis, and Tetsu Hara. "The Effect of Wind–Wave–Current Interaction on Air–Sea Momentum Fluxes and Ocean Response in Tropical Cyclones." Journal of Physical Oceanography 39, no. 4 (April 1, 2009): 1019–34. http://dx.doi.org/10.1175/2008jpo4066.1.
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Abstract In this paper, the wind–wave–current interaction mechanisms in tropical cyclones and their effect on the surface wave and ocean responses are investigated through a set of numerical experiments. The key element of the authors’ modeling approach is the air–sea interface model, which consists of a wave boundary layer model and an air–sea momentum flux budget model. The results show that the time and spatial variations in the surface wave field, as well as the wave–current interaction, significantly reduce momentum flux into the currents in the right rear quadrant of the hurricane. The reduction of the momentum flux into the ocean consequently reduces the magnitude of the subsurface current and sea surface temperature cooling to the right of the hurricane track and the rate of upwelling/downwelling in the thermocline. During wind–wave–current interaction, the momentum flux into the ocean is mainly affected by reducing the wind speed relative to currents, whereas the wave field is mostly affected by refraction due to the spatially varying currents. In the area where the current is strongly and roughly aligned with wave propagation direction, the wave spectrum of longer waves is reduced, the peak frequency is shifted to a higher frequency, and the angular distribution of the wave energy is widened.
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Emile-Geay, Julien, and Mark A. Cane. "Pacific Decadal Variability in the View of Linear Equatorial Wave Theory*." Journal of Physical Oceanography 39, no. 1 (January 1, 2009): 203–19. http://dx.doi.org/10.1175/2008jpo3794.1.
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Abstract It has recently been proposed, within the framework of the linear shallow-water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 yr, amplifying a midlatitude wind forcing with an essentially white spectrum. Here the authors use a different formalism of linear equatorial wave theory. The Green’s function is computed for the wind-forced response of a linear equatorial shallow-water ocean and use the earlier results of Cane and Moore to obtain a compact, closed form expression for the motion of the equatorial thermocline, which applies to all frequencies lower than seasonal. This expression is new and allows a systematic comparison of the effect of low- and high-latitude winds on the equatorial thermocline. At very low frequencies (decadal time scales), the planetary geostrophic solution used by Cessi and Louazel is recovered, as well as the equatorial wave solution of Liu, and a formal explanation for this convergence is given. Nonetheless, this more general solution leads one to a different interpretation of the results. In contrast to the aforementioned studies, the authors find that the equatorial thermocline is inherently more sensitive to local than to remote wind forcing and that planetary Rossby modes only weakly alter the spectral characteristics of the response. Tropical winds are able to generate a strong equatorial response with periods of 10 to 20 yr, while midlatitude winds can only do so for periods longer than about 50 yr. The results suggest that ocean basin modes are an unlikely explanation of decadal fluctuations in tropical Pacific sea surface temperature.
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Firing, Yvonne L., Mark A. Merrifield, Thomas A. Schroeder, and Bo Qiu. "Interdecadal Sea Level Fluctuations at Hawaii." Journal of Physical Oceanography 34, no. 11 (November 1, 2004): 2514–24. http://dx.doi.org/10.1175/jpo2636.1.
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Abstract Over the past century, tide gauges in Hawaii have recorded interdecadal sea level variations that are coherent along the island chain. The generation of this signal and its relationship to other interdecadal variability are investigated, with a focus on the last decade. Hawaii sea level is correlated with sea surface height (SSH) over a significant portion of the North Pacific Ocean, and with the Pacific–North America (PNA) index, which represents teleconnections between tropical and midlatitude atmospheric variations. Similar variations extend well below the thermocline in World Ocean Atlas temperature. Comparison with NCEP reanalysis wind and pressure shows that high (low) sea level phases around Hawaii are associated with an increase (decrease) in the strength of the Aleutian low. The associated wind stress curl pattern is dynamically consistent with observed sea level anomalies, suggesting that sea level at Hawaii represents large-scale changes that are directly wind-forced in concert with the PNA. Atmospheric modulation, as opposed to Rossby wave propagation, may explain the linkage of Hawaii sea level to North American sea level and ENSO events. A wind-forced, baroclinic Rossby wave model replicates some aspects of the interdecadal SSH variations and their spatial structure but fails to predict them in detail near Hawaii. The accuracy of wind products in this region and over this time period may be a limiting factor. Variations in mixed layer temperature due to surface heat flux anomalies may also contribute to the interdecadal sea level signal at Hawaii.