Thèses : « Convection (Oceanography) »

1

Pierce, David W. "Rotating convection and the oceanic general circulation /." Thesis, Connect to this title online; UW restricted, 1993. http://hdl.handle.net/1773/10993.

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Bhushan, Vikas. "Modeling convection in the Greenland Sea." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/58537.

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Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1998. Includes bibliographical references (leaves 155-161). A detailed examination of the development of a deep convection event observed in the Greenland Sea in 1988-89 is carried out through a combination of modeling, scale estimates, and data analysis. We develop a prognostic one-dimensional mixed layer model which is coupled to a thermodynamic ice model. Our model contains a representation of the lowest order boundary layer dynamics and adjustable coupling strengths between the mixed layer, ice, and atmosphere. We find that the model evolution is not very sensitive to the strength of the coupling between the ice and the mixed layer sufficiently far away from the limits of zero and infinite coupling; we interpret this result in physical terms. Further, we derive an analytical expression which provides a scale estimate of the rate of salinification of the mixed layer during the ice-covered preconditioning period as a function of the rate of ice advection. We also derive an estimate for the rate of the mixed layer deepening which includes ice effects. Based on these scale estimates and model simulations, we confirm that brine rejection and advection of ice out of the convection area were essential ingredients during the preconditioning process. We also demonstrate that an observed rise in the air temperature starting in late December 1988 followed by a period of moderately cold ~ -10*C temperatures was key to the development of the observed convection event. Finally, we show that haline driven deep convection underneath an ice cover is possible, but unlikely to occur in the Greenland Sea. On the basis of these results, we develop a coherent picture of the evolution of the convection process which is more detailed than that presented in any previous work. We also comment on the likelihood that deep convection occurred in the Greenland Sea in the past two decades from an examination of historical data, and relate these findings to what is known about the inter-annual variability of convective activity in the Greenland Sea by Vikas Bhushan. S.M.

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3

Steffen, Elizabeth Laird. "Observations of vertical and horizontal aspects of deep convection in the Labrador Sea by fully Lagrangian floats /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/11028.

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Wells, Mathew Graeme. "Convection, turbulent mixing and salt fingers." View thesis entry in Australian Digital Theses Program, 2001. http://thesis.anu.edu.au/public/adt-ANU20011212.103012/index.html.

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5

Wilkinson, Jeremy. "Sea ice, convection and the Greenland Sea." Thesis, University of Southampton, 2005. https://eprints.soton.ac.uk/25132/.

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The region where deep open-ocean convection occurs in the Greenland Sea corresponds to that where a sea ice winter feature, the Odden, usually forms. The role of sea ice in modifying the surface waters to overturn to depth is evaluated through the combination of in siu measurements, satellite imagery, meteorological measurements and drifting buoy data. Results suggest local meteorological and oceanographic conditions govern the ice conditions over the region. The high ambient wave energy precludes the formation of ice beyond the frazil-pancake stage; the changing surface pressure field, due to passing storm systems, influences the daily shape and extent of the Odden and enables pancake ice to expel brine at an increased rate. Finally, the analysis of drifting buoy data reveal that the ice is in free drift. t These characteristics suggests the Odden may be regarded as a large scale latent heat polynya, with the predominately northerly winds blowing newly formed sea-ice constantly southward such that it melts in a different area from that of its formation. This salt separation process whereby the majority of brine is deposited where the ice was formed, and a smaller amount being released, through brine drainage, as the ice drifts with the prevailing wind has important consequences for the spatial and temporal distribution of the salt flux and hence surrounding hydrography. This is clearly demonstrated through the development of a salt flux model, which involves brine drainage and drift. A simple one-dimensional mixed layer model, driven by results of the salt flux model, predicts a strong density enhancement and deepening of the mixed layer over time. It is therefore envisaged that the formation of sea ice, brine drainage and drift are fundamental in eroding the pycnocline between the surface waters and those below. Sea ice should therefore be viewed as a preconditioning activity to deep overturning of the waters of the central Greenland Sea.

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Pruis, Matthew J. "Energy and volume flux into the deep ocean : examining diffuse hydrothermal systems /." Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/10990.

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7

Grignon, Laure. "Causes of the interannual variability of deep convection." Thesis, University of Southampton, 2009. https://eprints.soton.ac.uk/72147/.

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Deep water formation in the Labrador Sea and the Gulf of Lion, for example, results from convection. A cyclonic circulation leads to a doming of the isopycnals at its centre, where stratification is then completely eroded by high surface winter buoyancy loss. This thesis assesses the causes of the interannual variability of deep convection. We first aim to quantify the relative importance of preconditioning, defined as the temperature and salinity structures and contents of the water column before the onset of convection, and of the buoyancy forcing (averaged over one winter) on the final convective mixed layer depth and on the temperature and salinity of the water mass formed. This study focuses on the Mediterranean and uses data from the Medar/Medatlas and Dyfamed data sets. The heat fluxes are studied and characterised. It is shown the the preconditioning is as important as the winter buoyancy fluxes in setting the final depth of convection. At the Dyfamed site (Corsica Strait), the seasonal cycle shows that the stratification frequency reaches a maximum in the intermediate layer in winter. This winter maximum is thought to be of critical importance. The second (and main) part focuses on the effect of the short-term (O(day)) variability of the surface forcing on convection, using an idealised model. The MIT model is integrated over a square box of size 64km x 64 km x 2km initialised with homogeneous salinity and a linear vertical temperature gradient. The configuration of the model is described and validated. A time-periodic cooling is then applied over a disc of radius 20km at the centre of the surface of the box. It is shown that the final mixed layer depth depends little on this short-term time variability because the lateral buoyancy fluxes are very responsive to the surface ones. Our results are compared with traditional parameterisation of the lateral buoyancy fluxes. General characteristics of the patch are also looked at, such as the rim current, the location of the angular momentum surfaces, the potential vorticity and the residual stratification in the mixed layer. The characteristics of the final water mass in each experiment are studied, showing that the short-term time variability of the forcing has an impact on the characteristics of the water mass formed. The last part compares the modelling study to gliders data for the Labrador Sea obtained by Peter Rhines and Charlie Eriksen of the University of Washington, WA, USA, in winter 2004-05. In that part of the real ocean, the variability of the boundary current seems more important than the variability in the surface forcing.

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Mpeta, Emmanuel Jonathan. "Intra-seasonal convection dynamics over Southwest and Northeast Tanzania : an observational study." Master's thesis, University of Cape Town, 1997. http://hdl.handle.net/11427/19650.

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Intraseasonal convection oscillation over the northeastern and southwestern Tanzania during MAM and DJF seasons respectively are examined using December, 1979 to May, 1994 pentad (5-day mean) Outgoing Longwave Radiation (OLR) as an indicator of convective cloud distribution. Area-averaged OLR indices are derived for the two areas. Time series of OLR indices for MAM and DJF indicate large quasi-periodic OLR fluctuations in some years and small fluctuations in other years. Periodogram analyses results reveal that dominant periodogram values for the oscillations were different in different years over both areas. Dominant periodogram peaks with periods more than 6 pentads (30 days) occurred 40% of the time on the average. Based on the pentad OLR time series plots deep convection and their precursors are composited. The time evolution of composite OLR maps reveal that patterns of low OLR values (indicating deep connection) shift north-eastwards coupled with low OLR values associated with mid-latitude troughs and linked to the ITCZ. Composite of kinematic and thermodynamic parameters associated with deep conJection and precursors are composited.

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9

Cuny, Jerome. "Labrador Sea boundary currents /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/10959.

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10

Straneo, Fiammetta. "Dynamics of rotating convection including a horizontal stratification and wind /." Thesis, Connect to this title online; UW restricted, 1999. http://hdl.handle.net/1773/10996.

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de, Lavaissiere de Lavergne Casimir. "Cessation of southern ocean deep convection under anthropogenic climate change." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119766.

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In 1974, newly available satellite observations unveiled the presence of a giant ice-free area within the Antarctic ice pack, which persisted throughout the winter, and formed again in the next two winters. Subsequent research showed that deep convective overturning kept the waters ice-free, through the massive release of heat rising from the deep sea. While the polynya has aroused continued interest among climate scientists, it has not reappeared since 1976. Here we use model experiments to show that deep convection in the Southern Ocean, common in current generation climate models, is highly sensitive to anthropogenic forcing, and ceases in many models when forced by a high emissions climate change scenario. The slowdown in deep ventilation follows from the gradual freshening of polar surface waters, a trend which is borne out by observations over recent decades. Our results suggest that deep convection in the Southern Ocean will be less common in future, and may have already been significantly reduced compared to the pre-industrial period, with important consequences for ocean circulation and climate. En 1974, des observations satellite nouvellement disponibles révélèrent la présence d'une géante surface d'eau libre au sein de la glace de mer entourant l'Antarctique, qui persista tout au long de l'hiver et réapparut les deux hivers suivants. Les recherches qui suivirent montrèrent que les eaux étaient maintenues libres de glace par la convection profonde, permettant à une grande quantité de chaleur de remonter des profondeurs pour être ensuite libérée dans l'atmosphère. Si la polynya continue de susciter l'intérêt des climatologues, elle n'est cependant pas réapparue depuis 1976. Nous utilisons ici des expériences de modélisation pour montrer que la convection profonde dans l'Océan Austral, commune dans les modèles de climat actuels, est fortement sensible au forçage anthropique, et cesse dans beaucoup de modèles quand ceux-ci sont forcés par un scénario de fortes émissions. Le ralentissement de la ventilation profonde résulte de la baisse progressive de la salinité des eaux de surface, une tendance corroborée par les observations des dernières décennies. Nos résultats suggèrent que la convection profonde dans l'Océan Austral sera moins fréquente dans le futur, et a peut-être déjà été significativement affaiblie relativement à la période préindustrielle, avec d'importantes conséquences pour la circulation océanique et le climat.

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Våge, Kjetil. "Circulation and convection in the Irminger Sea." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/58395.

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Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010. Cataloged from PDF version of thesis. Includes bibliographical references (p. 131-149). Aspects of the circulation and convection in the Irminger Sea are investigated using a variety of in-situ, satellite, and atmospheric reanalysis products. Westerly Greenland tip jet events are intense, small-scale wind phenomena located east of Cape Farewell, and are important to circulation and convection in the Irminger Sea. A climatology of such events was used to investigate their evolution and mechanism of generation. The air parcels constituting the tip jet are shown to have a continental origin, and to exhibit a characteristic deflection and acceleration around southern Greenland. The events are almost invariably accompanied both by a notable coherence of the lower-level tip jet with an overlying upper-level jet stream, and by a surface cyclone located in the lee (east) of Greenland. It is argued that the tip jet arises from the interplay of the synopticscale flow evolution and the perturbing effects of Greenland's topography upon the flow. The Irminger Gyre is a narrow, cyclonic recirculation confined to the southwest Irminger Sea. While the gyre's existence has been previously documented, relatively little is known about its specific features or variability. The mean strength of the gyre's circulation between 1991 and 2007 was 6.8 ± 1.8 Sv. It intensified at a rate of 4.3 Sv per decade over the observed period despite declining atmospheric forcing. Examination of the temporal evolution of the LSW layer thickness across the Irminger Basin suggests that local convection formed LSW during the early 1990s within the Irminger Gyre. In contrast, LSW appeared outside of the gyre in the eastern part of the Irminger Sea with a time lag of 2-3 years, consistent with transit from a remote source in the Labrador Sea. In the winter of 2007-08 deep convection returned to both the Labrador and Irminger seas following years of shallow overturning. The transition to a convective state took place abruptly, without going through a preconditioning phase, which is contrary to general expectations. Changes in the hemispheric air temperature, tracks of storms, flux of freshwater to the Labrador Sea, and distribution of pack ice all conspired to enhance the air-sea heat flux, resulting in the deep overturning. Kjetil Våge. Ph.D.

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Von, Eye Maxine Jutta Erika. "Sea ice and convection in the Greenland Sea." Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648527.

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Roberts, Zoe Louise. "The application of adaptive mesh modelling techniques to the study of open ocean deep convection." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/65672/.

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The rapid cooling of the waters at high latitudes creates an unstable strati cation which in turn leads to localised overturning (sinking) of the water column. This process is called open ocean deep convection (OODC). The process of OODC occurs in stages. Initially, individual convective elements known as plumes form and cold, dense water descends from the surface. Over time these plumes build up to produce a well-mixed `chimney' of cold dense uid. This chimney then slumps and sinks, and restratication (the return to a stable state throughout the water column) occurs. It is widely accepted that OODC plays a main role in driving the thermohaline circulation (THC) and hence has a potentially major role in climate. However, the mechanisms of OODC itself are not fully understood, and there is much debate surrounding how it contributes to THC. One di- culty is that OODC tends to occur sporadically in only a few isolated regions around the globe, making direct observations dicult. As a result, theoretical and numerical investigations have become key to the development of our understanding of OODC. The scale on which OODC occurs presents a further issue, with traditional numerical representations (parameterisations) of OODC in global circulation models (GCMs) omitting convective detail due to resolution. Due to the scales on which OODC occurs, it has been dicult to numerically investigate the nature of OODC in the small scale at the same time as resolving basin scale circulation. With the advent of nite element methods and adaptive meshing techniques, it is now possible to study OODC in regional models without the need to parameterise. One such model, the Imperial College Ocean Model (ICOM) is employed in this thesis for these purposes. ICOM is a 3D nite element, non-hydrostatic model with an adaptive, unstructured mesh and non-uniform resolution, allowing modelling of i the gyre circulation and resolution of OODC simultaneously. As the use of an adaptive, unstructured mesh model is novel in investigating Greenland Sea open ocean deep convection, it is of interest to assess the accuracy of the ICOM model, and the amount of numerical diusion present. The classical uid dynamics problem of parallel plate convection provides a simple test problem for this purpose. A series of tests investigating the linear stability of various temperature gradients were performed in order to diagnose the amount of numerical diusivity associated with hexahedral, tetrahedral and adaptive meshes within ICOM, and ICOM was further compared with a leading GCM (MITgcm). The use of the linear instability problem was found to be a useful case against which to test numerical models in an attempt to diagnose implicit diusivity and viscosity. A series of experiments were conducted in order to identify any prevailing dierences between model convection in xed and adaptive mesh congurations, under varying durations of applied cooling, and using varying extents of horizontal cooling. The adaptive mesh proved to be highly suitable for studying the convective problem, it was less computationally expensive and free from the numerical instability observed on the xed mesh. The sensitivity of model convection to the introduction of stratication was investigated. Uniform cooling was applied across the surface of a domain initialised with a weak stratication over the surface 1500m and a more strongly stratied region below, and the development of a convective layer was observed within the initial upper layer. Convection was constrained to the upper layer of stratication, and some penetrative convection was identied in the early stages of the model run.

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Lavender, Kara L. "The general circulation and open-ocean deep convection in the Labrador Sea : a study using subsurface floats /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2001. http://wwwlib.umi.com/cr/ucsd/fullcit?p3035893.

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Martin, Jeffrey T. "The influence of silica precipitation and thermoelastic stresses on the evolution of a ridge crest seafloor hydrothermal system." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/28026.

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Zhang, Fan. "Changing seasonality of convective events in the Labrador Sea." Thesis, Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/51896.

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The representation of deep convection in ocean models is a fundamental challenge for climate science. Here a regional simulation of the Labrador Sea circulation and convective activity obtained with the Regional Oceanic Modeling System (ROMS) over the period 1980-2009 is used to characterize the response of convection to atmospheric forcing and the variability in its seasonal cycle. This integration compares well with the sparse in time and space hydrographic surveys and ARGO data (Luo et al. 2012). It is found that convection in the convective region of the Labrador Sea has experienced variability in three key aspects over the 30 years considered. First, the magnitude of convection varies greatly at decadal scales. This aspect is supported by the in-situ observations. Second, the initiation and peak of convection (i.e. initiation and maximum) shift by two to three weeks between strong and weak convective years. Third, the duration of convection varies by approximately one month between strong and weak years. The last two changes are associated to the variability of winter and spring time heat fluxes in the Labrador Sea, while the first results from changes in both atmospheric heat fluxes and oceanic conditions through the inflow of warm Irminger Water from the boundary current system to the basin interior. Changes in heat fluxes over the Labrador Sea convective region are strongly linked to large scale modes of variability, the North Atlantic Oscillation and Arctic Oscillation. Correlations between the mode indices and the local heat fluxes in the convective area are largest in winter during strong, deep events and in spring whenever convection is shallow.

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Cordery, Matthew Jean. "Mantle convection, melt migration and the generation of basalts at mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/52936.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and Woods Hole Oceanographic Institute, 1991. Includes bibliographical references (leaves 183-190). by Matthew Jean Cordery. Ph.D.

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Walker, Carolyn Faye, and n/a. "Nutrient dynamics during winter convection in the North Atlantic Subtropical Gyre." University of Otago. Department of Chemistry, 2009. http://adt.otago.ac.nz./public/adt-NZDU20090825.142702.

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Storm-induced open-ocean convective mixing is one of the primary processes controlling the supply of nitrate to the sunlit layer of the oligotrophic North Atlantic Subtropical Gyre (NASG). Yet, the magnitude and timing of nitrate fluxes during winter convection is poorly understood due to an absence of targeted process studies. In the northwest NASG, multiple quasi-Lagrangian studies were conducted during the boreal winters of 2004 and 2005 in an effort to sample strong winter convection. During each of the time-series studies, inventories of vertically fluxed nitrate were quantified approximately every twelve hours using the distribution of helium isotopes ([delta]�He) and nitrate in the water column. This method is known as the Helium Flux Gauge Technique (HFGT). Large variability in surface forcing and density structure of the upper ocean was observed between the two years; however, only winter 2005 experienced convective mixing to depths greater than 150 m. In winter 2004, mild atmospheric conditions coincided with a positive phase in the winter North Atlantic Oscillation (NAO), consistent with the dominant regime experienced during the previous decade. On average 36 � 9 mmol m[-2] of fluxed nitrate was inferred by excess �He in the mixed layer of the ocean during the winter 2004 study period. This inventory of physically transported nitrate is attributed to the sampling of waters laterally advected from nearby eddy features. The sampling of multiple water masses is likely due to the inability of the drogue to persistently follow water masses efficiently. Although physical evidence indicates spatial variability within the time-series data, the length scales of convective mixing appear to be greater than those associated with spatial aliasing as a result of drogue performance. This observation provides us with increased confidence that the objectives for the present study are not compromised by spatial variability in the data. In contrast, winter 2005 experienced a negative NAO, strong physical forcing and convective mixing to depths > 250 m. Two convectively modified water masses, most likely resulting from a single storm event, were sampled at different stages of development. These two water masses exhibit large variability in the magnitude of nitrate entrained in the convective layer from the thermocline. An average inventory of 247 � 56 mmol NO₃[-]m[-2] was entrained in the rapidly expanding convective layer of the first water mass in the first few days following the storm approach. In contrast, ongoing entrainment of nitrate was absent from the second water mass, sampled two weeks later when the depth of the surface mixed layer was consistently ~ 300 m. These results indicate that surrounding fluid is entrained into the convective layer when it is actively expanding in the vertical. On the other hand, significant fluid entrainment does not occur at the base of the plume once sinking waters have reached a level of neutral buoyancy. The persistence of elevated nitrate stocks (~ 100 mmol m[-2]) in the convective layer two to three weeks after the inferred injection event, suggests sub-optimal nitrate uptake by resident phytoplankton. Phytoplankton growth was most likely resource limited by light or a micronutrient such as iron. Despite the implied biolimitation, changes in chlorophyll-a, a proxy for phytoplankton biomass, indicate net production within the convective layer. On average, the convective layer was observed to support an inventory of 62 � 6mg chlorophyll-a m[-2], increasing at an average rate of 3.4mg m[-2] d[-1]. This inventory indicates a slow build-up of phytoplankton biomass to near bloom levels, ahead of the main spring bloom that typically follows formation of the seasonal thermocline near Bermuda. Net production in the convective layer was likely due to transient periods of increased (weak) surface stability that were observed to support high phytoplankton biomass, following the cessation of thermocline fluid entrainment. When nitrate and excess �He in samples collected from the thermocline were regressed for the purpose of quantifying nitrate fluxes, the results showed that between 1.6 - 2.0 [mu]mol kg[-1] of dissolved nitrate was present during formation of the water mass. This suggests the source of this excess (above Redfield ratios) nitrate in the thermocline of the NASG is not local, and has ramifications for local nitrogen fixation budgets determined using geochemical approaches. Thesis supervisors: William J. Jenkins, Senior Scientist, WHOI (United States of America); Philip W. Boyd, Senior Scientist, NIWA (New Zealand); Michael W. Lomas, Senior Scientist, BIOS (Bermuda)

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Parnell-Turner, Ross Ernest. "Observations of transient mantle convection in the North Atlantic Ocean." Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648620.

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Kaspi, Yohai. "Turbulent convection in the anelastic rotating sphere : a model for the circulation on the giant planets." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45780.

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Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008. Includes bibliographical references (p. 207-221). This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Different from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to significant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility effects. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, contributing to the observed flat meridional emission. We show the transition from weak convection cases with symmetric spiraling columnar modes similar to those found in previous analytic linear theory, to more turbulent cases which exhibit similar, though less regular and solely cyclonic, convection columns which manifest on the surface in the form of waves embedded within the superrotation. We develop a mechanical understanding of this system and scaling laws by studying simpler configurations and the dependence on physical properties such as the rotation period, bottom boundary location and forcing structure. These columnar cyclonic structures propagate eastward, driven by dynamics similar to that of a Rossby wave except that the restoring planetary vorticity gradient is in the opposite direction, due to the spherical geometry in the interior. (cont.) We further study these interior dynamics using a simplified barotropic annulus model, which shows that the planetary vorticity radial variation causes the eddy angular momentum flux divergence, which drives the superrotating equatorial flow. In addition we study the interaction of the interior dynamics with a stable exterior weather layer, using a quasigeostrophic two layer channel model on a beta plane, where the columnar interior is therefore represented by a negative beta effect. We find that baroclinic instability of even a weak shear can drive strong, stable multiple zonal jets. For this model we find an analytic nonlinear solution, truncated to one growing mode, that exhibits a multiple jet meridional structure, driven by the nonlinear interaction between the eddies. Finally, given the density field from our 3D convection model we derive the high order gravitational spectra of Jupiter, which is a measurable quantity for the upcoming JUNO mission to Jupiter. by Yohai Kaspi. Ph.D.

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Hufford, Gwyneth Ellin. "Parameterization of convection in a rotating stratified ocean : comparison of numerical and laboratory experiments with theory." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/59632.

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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and Woods Hole Oceanographic Institution, 1995. Includes bibliographical references (leaves 70-73). by Gwyneth Ellin Hufford. M.S.

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Houssais, Marie-Noëlle. "Modelisation des interactions ocean-glace : application a la mer du groenland." Paris 6, 1987. http://www.theses.fr/1987PA066173.

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Les processus dynamiques et thermodynamiques du couplage glace-ocean sont etudies a l'aide d'une hierarchie de modeles numeriques. Un modele thermodynamique de glace de mer ou le flux de chaleur oceanique est impose revele l'influence de ce flux sur le cycle saisonnier d'epaisseur de la glace. L'influence de la derive de la glace sur les bilans locaux de fonte-congelation est examinee a l'aide d'un modele dynamique-thermodynamique de glace de mer

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Lherminier, Pascale. "Convection profonde en Mer du Groenland: Etude expérimentale des phases de préconditionnement et de mélange." Phd thesis, Université Pierre et Marie Curie - Paris VI, 1998. http://tel.archives-ouvertes.fr/tel-00881646.

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La convection profonde en Mer du Groenland, source importante des eaux profondes de l'Atlantique Nord, fait actuellement l'objet de recherches approfondies. Nous présentons ici les principaux résultats obtenus au cours des hivers de 1993 et 1994 à partir de mesures de flotteurs isobares dérivant entre 250 et 850 m de profondeur et d'un vaste ensemble de données hydrologiques, météorologiques et glaciologiques. L'hiver 1994, peu rigoureux, ne donne lieu qu'à une convection semi-profonde dans un bassin libre de glace, où un tourbillon anticyclonique d'environ 40 km de diamètre apparaît comme un site privilégié du mélange convectif à l'ouest du gyre du Groenland. Il semble provenir d'une interaction entre de l'Eau Atlantique Modifiée advectée en surface et une poche froide d'Eau Arctique Intermédiaire située en dessous et issue d'événements convectifs antérieurs pouvant remonter à l'hiver précédent. L'analyse de la physionomie et de la stabilité spatiale et temporelle de ce tourbillon n'a pas permis de relier son origine et son évolution à des scénarios de type cascade directe et cascade inverse habituellement invoqués dans les phénomènes de convection plus profonde. En 1994, l'évolution de la couche mélangée a été plutôt dominée par une dynamique turbulente tridimensionnelle très dépendante du forçage en surface et de la stratification du bassin, elle-même fortement influencée par la présence de structures de méso-échelle. Corrélée aux flux air-mer les plus intenses de l'hiver 1994, la phase de mélange active est identifiée par un changement de régime dans les amplitudes et la signature en fréquence des vitesses verticales de l'eau w mesurées par les flotteurs. A l'aide d'un modèle non hydrostatique à haute résolution simulant le comportement de flotteurs isobares, nous avons montré que, pendant la phase de mélange, l'apparition d'une composante basse fréquence observée sur les vitesses w est vraisemblablement associée à l'organisation de cellules turbulentes (panaches) engendrées par les fortes et soudaines pertes de flottabilité en surface.

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Liu, Lei. "The link between convection and crystallization in a sub-axial magma chamber and heat output in a seafloor hydrothermal system." Thesis, Available online, Georgia Institute of Technology, 2007, 2007. http://etd.gatech.edu/theses/available/etd-07072007-142506/.

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Farrar, J. Thomas (John Thomas) 1976. "Air-sea interaction at contrasting sites in the Eastern Tropical Pacific : mesoscale variability and atmospheric convection at 10°N." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/39009.

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Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. Includes bibliographical references (p. 153-166). The role of ocean dynamics in driving air-sea interaction is examined at two contrasting sites on 125°W in the eastern tropical Pacific Ocean using data from the Pan American Climate Study (PACS) field program. Analysis based on the PACS data set and satellite observations of sea surface temperature (SST) reveals marked differences in the role of ocean dynamics in modulating SST. At a near-equatorial site (3°S), the 1997-1998 El Nifio event dominated the evolution of SST and surface heat fluxes, and it is found that wind-driven southward Ekman transport was important in the local transition from El Nifio to La Nifia conditions. At a 10'N site near the summertime position of the Inter-tropical Convergence Zone, oceanic niesoscale motions played an important role in modulating SST at intraseasonal (50- to 100-day) timescales, and the buoy observations suggest that there are variations in surface solar radiation coupled to these mesoscale SST variations. This suggests that the mesoscale oceanic variability may influence the occurrence of clouds. The intraseasonal variability in currents, sea surface height, and SST at the northern site is examined within the broader spatial and temporal context afforded by satellite data. (cont.) The oscillations have zonal wavelengths of 550-1650 km and propagate westward in a manner consistent with the dispersion relation for first baroclinic mode, free Rossby waves in the presenice of a, mean westward flow. The hypothesis that the intraseasonal variability and its annual cycle are associated with baroclinic instability of the North Equatorial Current is supported by a spatio-temporal correlation between the amplitude of intraseasonal variability and the occurrence of westward zonal flows meeting an approximate necessary condition for baroclinic instability. Focusing on 100N in the eastern tropical Pacific, the hypothesis that mesoscale oceanic SST variability can systematically influence cloud properties is investigated using several satellite data products. A statistically significant relationship between SST and columnar cloud liquid water (CLW), cloud reflectivity, and surface solar radiation is identified within the wavenumber-frequency band corresponding to oceanic Rossby waves. Analysis of seven years of CLW data and 20 years surface solar radiation data indicates that 10-20% of the variance of these cloud-related properties at intraseasonal periods and wavelengths on the order of 100 longitude can be ascribed to SST signals driven by oceanic Rossby waves. by J. Thomas Farrar. Ph.D.

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Wood, Dylan M. "Solving Unsteady Convection-Diffusion Problems in One and More Dimensions with Local Discontinuous Galerkin Methods and Implicit-Explicit Runge-Kutta Time Stepping." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461181441.

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Lewis, Kayla Christine. "Numerical Modeling of Two-Phase Flow in the Sodium Chloride-Water System with Applications to Seafloor Hydrothermal Systems." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19810.

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In order to explain the observed time-dependent salinity variations in seafloor hydrothermal vent fluids, quasi-numerical and fully numerical fluid flow models of the NaCl-H2O system are constructed. For the quasi-numerical model, a simplified treatment of phase separation of seawater near an igneous dike is employed to obtain rough estimates of the thickness and duration of the two-phase zone, the amount of brine formed, and its distribution in the subsurface. For the fully numerical model, the equations governing fluid flow, the thermodynamic relations between various quantities employed, and the coupling of these elements together in a time marching scheme is discussed. The fully numerical model is benchmarked against previously published heat pipe and Elder problem simulation results, and is shown to be largely in agreement with those results. A number of simulation results are presented in the context of two-phase flow and phase separation within the framework of the single pass model. It is found that a quasi-stable two-phase (liquid + vapor) zone at depth below the hydrothermal discharge outlet gives rise to vent fluid with lower than normal seawater salinity. Additionally, it is shown that increasing the spatial extent of the two-phase zone can lower vent fluid salinity. The numerical approach used in this thesis is able to generate salinity patterns predicted by a widely held conceptual model of vent fluid salinity variation, and may be able to explain the vent fluid salinities and temperatures found at the Main Endeavour Vent Field on the Juan de Fuca Ridge, as this approach is able to produce simulated vent fluid salinities that match observed values from the Endeavour Field vents Dante and Hulk.

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MacTavish, Flora Pamela. "The application of adaptive mesh techniques to convective processes in oceanography." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11616.

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Idealised numerical simulations of two oceanographic processes, salt finger formation and the restratification phase of open ocean deep convection, are considered. These processes are modelled using the Imperial College Ocean Model (Fluidity-ICOM). This is a finite element code with the novel capability to perform mesh adaptivity. The mesh is triangular/tetrahedral and can be unstructured. If mesh adaptivity is used, all the fields are periodically interpolated onto a new mesh that has been optimised from the previous mesh. The new mesh is designed to represent one or more of the solution fields as accurately as possible by putting more nodes in regions where the Hessians of the fields being adapted to are higher. The first process to be presented is the formation of salt fingers in double diffusive convection. A secondary instability is observed to form. Due to the unstructured mesh, the fingers start off slightly different lengths from each other and this difference is observed to grow with time. A new set of simulations are run in which the secondary instability is initialised from a perturbation in the initial condition. These results are used to compare between fixed and adaptive mesh results. Evidence is obtained to show that adaptive mesh is able to produce the same results as the fixed mesh with fewer computational nodes because the resolution is used in the regions where is it most needed. The second process is the restratification after open ocean deep convection. In order to run ocean scale simulations the model must be able to accurately represent geostrophic and hydrostatic balance on a high aspect ratio domain. In order to do this with an unstructured mesh it is shown that it is necessary to constrain the nodes to be aligned in the vertical. This type of mesh is known as a 2+1 mesh and it can be adapted in both the vertical and the horizontal in order to resolve the solution fields more accurately. The model is able to reproduce previous results for a simple restratification test case using mesh adaptivity. The representation of balance is investigated using different types of mesh and different finite element shape functions. A more complex restratification test case in which baroclinic eddies form is then examined. The results obtained are compared to other models with different numerical schemes. Fixed and adaptive results are compared. These results demonstrate that Fluidity-ICOM is able to represent balance and model relatively complex processes on ocean scale, high aspect ratio domains whilst using mesh adaptivity.

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Blamey, Ross. "Mesoscale convective complexes over southern Africa." Doctoral thesis, University of Cape Town, 2012. http://hdl.handle.net/11427/10167.

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Includes bibliographical references. A combination of numerous factors, including geographic position, regional orography and local sea surface temperatures, ensures that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events.

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Beuvier, Jonathan. "Modélisation de la variabilité climatique de la circulation et des masses d'eau en Méditerranée : impacts des échanges océan-atmosphère." Phd thesis, Palaiseau, Ecole polytechnique, 2011. https://pastel.hal.science/docs/00/67/68/96/PDF/Beuvier_2011_PhD-thesis_1.pdf.

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Cette thèse a pour but de progresser sur des points essentiels concernant le réalisme de la représentation de la formation et du trajet des masses d'eau en Mer Méditerranée, ainsi que de leur variabilité. A cet effet, plusieurs modèles océaniques régionaux de la Méditerranée, de résolutions horizontales différentes, sont développés et utilisés. Une configuration réaliste permettant de représenter la variabilité interannuelle des conditions aux limites de ces modèles (atmosphère, océan Atlantique, fleuves, mer Noire) est utilisée pour réaliser des simulations à long terme des 50 dernières années en Méditerranée. Deux événements rares, caractérisant la variabilité décennale en Méditerranée, sont étudiés : l'Eastern Mediterranean Transient (EMT) et la Western Mediterranean Transition (WMT). L'EMT est la période, au début des années 1990, pendant laquelle le site principal de formation d'eau dense dans le bassin oriental méditerranéen est passé du sous-bassin Adriatique au sous-bassin Egée. La capacité des simulations à long terme à reproduire la séquence d'événements composant l'EMT est tout d'abord démontrée. Parmi les éléments de préconditionnement et de déclenchement de l'EMT suggérés dans la littérature, nous montrons que les facteurs essentiels sont les flux hivernaux intenses au-dessus du sous-bassin Egée pendant les hivers 1992 et 1993. Le réalisme de la formation et de la propagation de l'eau crétoise profonde (Cretan Deep Water, CDW) pendant l'EMT est ensuite analysé dans les simulations de référence et de sensibilité. La propagation de la CDW au fond de la Méditerranée orientale n'est reproduite qu'avec des conditions atmosphériques modifiées. La WMT a commencé à l'hiver 2005 dans le Golfe du Lion, pendant lequel a été formé un volume très important d'eau profonde ouest-méditerranéenne (Western Mediterranean Deep Water, WMDW), caractérisée par une température et une salinité inhabituellement élevées. Les simulations reproduisent l'intensité de la convection profonde dans le Golfe du Lion pendant l'hiver 2005, qui est est due à la forte perte de flottabilité en surface. Elles indiquent également que des tourbillons cycloniques profonds, d'une centaine de kilomètres de diamètre, sont responsables du transport rapide de la nouvelle WMDW vers le Sud. Puis, les simulations à long terme permettent de replacer la WMT dans la variabilité décennale de la Méditerranée nord-occidentale. Elles montrent que l'EMT a potentiellement doublé le volume de nouvelle WMDW formé en 2005 dans le Golfe du Lion, mais qu'il n'est pas responsable de la température et de la salinité élevées de la nouvelle WMDW. Ces caractéristiques inhabituelles sont dues à l'absence de convection intense dans le Golfe du Lion pendant les années 1990, ce qui a favorisé l'accumulation de sel et de chaleur dans la Méditerranée nord-occidentale This thesis aims at progressing on key points about the realistic reproduction of the formation and the paths of the Mediterranean water masses, and their variability. For that purpose, several regional oceanic models of the Mediterranean Sea, with different horizontal resolutions, are developped and used. A realistic configuration, representing the interannual variability of the boundary conditions of these models (atmosphere, Atlantic Ocean, rivers, Black Sea) is used to carry out long-term simulations of the Mediterranean for the last 50 years. Two rare events, characterising the decennial variability in the Mediterranean, are studied: the Eastern Mediterranean Transient (EMT) and the Western Mediterranean Transition (WMT). The EMT is a period, at the beginning of the 1990's, during which the main site of dense water formation in the eastern Mediterreanean basin switched from the Adriatic subbasin to the Aegean subbasin. The ability of the long-term simulations to reproduce the sequence of the EMT events is first proved. Among the preconditionning and triggering elements of the EMT suggested in the literature, we show that the main factors are the intense winter fluxes over the Aegean subbasin during winters 1992 and 1993. The realism of the Cretan Deep Water (CDW) formation and propagation during the EMT is then analysed in reference and sensitivity simulations. The spreading of the CDW on the bottom of the eastern Mediterranean is only reproduced with modified atmospheric conditions. The WMT has been starting during winter 2005 in the Gulf of Lions, during which a huge volume of Western Mediterranean Deep Water (WMDW) was formed with unusual high temperature and salinity. The simulations reproduce the intensity of the winter 2005 deep convection in the Gulf of Lions, which is due to the strong surface buoyancy loss. They also show that 100-km width deep cyclonic eddies are responsible for the fast southwards spreading of the new WMDW. Then, the long-term simulations allow to set back the WMT in the decennial variability of the north-western Mediterranean. They show that the EMT potentially doubled the volume of new WMDW formed in winter 2005 in the Gulf of Lions, but that it is not responsible for the high temperature and salinity of the new WMDW. These unusual characteristics are due to the absence of intense convection in the Gulf of Lions during the 1990's, which favours a salt and heat accumulation in the north-western Mediterranean

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Athanase, Marylou. "On the recent evolution of Atlantic Water at the entrance to the Arctic Ocean : observations and Mercator Ocean operational model." Electronic Thesis or Diss., Sorbonne université, 2020. http://www.theses.fr/2020SORUS263.

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L’analyse combinée d’observations et de sorties du modèle opérationnel Mercator Océan (appelé PSY4) a apporté un nouvel éclairage sur les récentes conditions océaniques du Bassin Eurasien Arctique occidental. Les mesures physiques collectées par deux plateformes autonomes dans le Bassin Eurasien occidental (0-350 m) ont indiqué que la couche de surface observée en 2017 était moins salée, et la couche chaude des eaux Atlantiques était moins profonde qu’en 2007-2012 (climatologie WOA13). PSY4 a apporté des informations complémentaires sur la nature des structures de méso-échelle documentées in-situ: dans le Bassin Nansen, une structure d’eaux Atlantiques issue du courant de bord a évolué en un tourbillon anticyclonique, tandis qu’une seconde structure à l’ouest du Plateau de Yermak concordait avec une branche d’eaux Atlantiques recirculant vers le Détroit de Fram. Les 14 années de sorties PSY4 ont montré qu’en hiver, les propriétés des eaux Atlantiques ont été considérablement modifiées par la convection hivernale profonde et par l’écoulement récurrent d’eaux moins chaudes et salées venant du plateau continental. Les régions du Plateau de Yermak et du talus continental sont devenues des « Zones Marginales de Convection » avec, à partir de 2011, l’absence occasionnelle de banquise en hiver ainsi que des couches de mélanges et flux océan-atmosphère fortement variables. Enfin, PSY4 a indiqué des changements de circulation dans le Bassin Nansen occidental, avec l’intensification de la Yermak Branch allant alimenter la Return Yermak Branch sur le flanc est du Plateau. PSY4 a également souligné la mise en place de nouveaux chemins empruntés par les eaux Atlantiques: dans le Sofia Deep, une circulation anticyclonique s’est développée. En aval du Plateau de Yermak, un courant s’est mis en place le long de l’isobathe 3800 m. A l’est de 20°E, ce courant a été alimenté par l’injection régulière de structures de méso-échelles issues du courant de bord Combination of observations and Mercator Ocean operational model (PSY4 hereafter) outputs provided insights on the recent oceanic conditions in the changing Western Eurasian and Nansen basins, entry region for the warm and salty Atlantic Water (AW) to the Arctic Ocean. Autonomous platforms showed that the Western Eurasian Basin exhibited a fresher surface layer and shallower warm AW layer in 2017 than in the 2005‐2012 World Ocean Atlas climatology. PSY4 brought insights on the mesoscale structures observed in the halocline and warm layer. In particular, in the Nansen Basin a large mesoscale structure of AW from the boundary current turned into an anticyclonic eddy after the platforms passage. A second AW structure northwest of the Yermak Plateau, was a branch of AW recirculating back toward Fram Strait. The performance of PSY4 was assessed using independent observations over 2007-2020 in the Western Nansen Basin (WNB). We took advantage of 14 years of fields from PSY4 to examine winter conditions in the WNB over 2007-2020. PSY4 showed that deep winter convection and recurrent outflows from troughs northeast of Svalbard dramatically modified the AW. The northern Yermak Plateau-Sofia Deep and continental slope areas became “Marginal Convection Zones" in 2011 with, from then on, occurrences of ice-free conditions and mixed layer depths deeper than 200m in winter, and highly variable ocean-to-atmosphere heat fluxes. PSY4 also showed changes in circulation in the WNB over 2008-2020, with the strengthening of the Yermak Branch, which fed the southward Return Yermak Branch along the eastern flank of the Plateau. PSY4 highlighted the onset of new AW pathways: a recurrent anticyclonic circulation established in Sofia Deep. An offshore AW circulation developed downstream of the Yermak Plateau (following the 3800m isobaths). East of 20°E, additional AW from boundary current was injected in this offshore circulation, via enhanced basin-ward mesoscale activity

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Beuvier, Jonathan. "Modélisation de la variabilité climatique de la circulation et des masses d'eau en Méditerranée : impacts des échanges océan-atmosphère." Phd thesis, Ecole Polytechnique X, 2011. http://pastel.archives-ouvertes.fr/pastel-00676896.

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Cette thèse a pour but de progresser sur des points essentiels concernant le réalisme de la représentation de la formation et du trajet des masses d'eau en Mer Méditerranée, ainsi que de leur variabilité. A cet effet, plusieurs modèles océaniques régionaux de la Méditerranée, de résolutions horizontales différentes, sont développés et utilisés. Une configuration réaliste permettant de représenter la variabilité interannuelle des conditions aux limites de ces modèles (atmosphère, océan Atlantique, fleuves, mer Noire) est utilisée pour réaliser des simulations à long terme des 50 dernières années en Méditerranée. Deux événements rares, caractérisant la variabilité décennale en Méditerranée, sont étudiés : l'Eastern Mediterranean Transient (EMT) et la Western Mediterranean Transition (WMT). L'EMT est la période, au début des années 1990, pendant laquelle le site principal de formation d'eau dense dans le bassin oriental méditerranéen est passé du sous-bassin Adriatique au sous-bassin Egée. La capacité des simulations à long terme à reproduire la séquence d'événements composant l'EMT est tout d'abord démontrée. Parmi les éléments de préconditionnement et de déclenchement de l'EMT suggérés dans la littérature, nous montrons que les facteurs essentiels sont les flux hivernaux intenses au-dessus du sous-bassin Egée pendant les hivers 1992 et 1993. Le réalisme de la formation et de la propagation de l'eau crétoise profonde (Cretan Deep Water, CDW) pendant l'EMT est ensuite analysé dans les simulations de référence et de sensibilité. La propagation de la CDW au fond de la Méditerranée orientale n'est reproduite qu'avec des conditions atmosphériques modifiées. La WMT a commencé à l'hiver 2005 dans le Golfe du Lion, pendant lequel a été formé un volume très important d'eau profonde ouest-méditerranéenne (Western Mediterranean Deep Water, WMDW), caractérisée par une température et une salinité inhabituellement élevées. Les simulations reproduisent l'intensité de la convection profonde dans le Golfe du Lion pendant l'hiver 2005, qui est est due à la forte perte de flottabilité en surface. Elles indiquent également que des tourbillons cycloniques profonds, d'une centaine de kilomètres de diamètre, sont responsables du transport rapide de la nouvelle WMDW vers le Sud. Puis, les simulations à long terme permettent de replacer la WMT dans la variabilité décennale de la Méditerranée nord-occidentale. Elles montrent que l'EMT a potentiellement doublé le volume de nouvelle WMDW formé en 2005 dans le Golfe du Lion, mais qu'il n'est pas responsable de la température et de la salinité élevées de la nouvelle WMDW. Ces caractéristiques inhabituelles sont dues à l'absence de convection intense dans le Golfe du Lion pendant les années 1990, ce qui a favorisé l'accumulation de sel et de chaleur dans la Méditerranée nord-occidentale.

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Blamey, Ross. "Numerical simulation of a mesoscale convective system over the east coast of South Africa." Master's thesis, University of Cape Town, 2007. http://hdl.handle.net/11427/6463.

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Includes bibliographical references (leaves 189-200). Weather stations across the northern KwaZulu-Natal coastline recorded over 100 mm of rainfall over the 11112 February 2005, with Cape St. Lucia and Richards Bay measuring 111 mm and 96.8 mm, respectively. This heavy rainfall was associated with a mesoscale convective system (MCS) that initiated through small convective storms beginning early in the afternoon on 11 February 2005 and eventually decayed in the early morning hours on the 12th. The high-lying topography of the eastern escarpment and high diurnal surface heating possibly provided the trigger for the event. It was also identified that a combination of synoptic features in and around South Africa contributed to the evolution of the system. This particular MCS is investigated with a non-hydrostatic numerical model (MM5) to help determine which processes were important in its initiation and development, as well as what factors contributed to the associated heavy rainfall. The model is also used to conduct sensitivity tests to determine the role that local features, such as the regional topography and sea surface temperature, played in the evolution of the system. Through the various MM5 simulations, it was evident that the eastern escarpment played a key role in triggering the convective event, while it also had an influence on the low level winds that advected moisture into the region. A sea surface temperature sensitivity simulation highlighted the important role that the Agulhas Current plays in supplying moisture to fuel extreme precipitation events in South Africa. The significance of resolving large-scale features in the mid-latitudes in numerical simulations of weather events in South Africa was identified when excluding these features from the simulation. Through these simulations it was identified that the development of the MCS and the heavy nocturnal precipitation was due to a combination of the continuous moisture supply into the region, a conditionally unstable atmosphere, and uplift due to low level convergence and the local topography.

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Jeffery, Christopher D. "Diurnal warming and convective CO2 exchange in the Tropical Atlantic." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/63292/.

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Andrié, Chantal. "Utilisation des traceurs helium-3 et tritium en oceanographie." Paris 6, 1987. http://www.theses.fr/1987PA066241.

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Le traceur transitoire que constitue le tritium est devenu l'un des outils les plus prometteurs dens l'etude de la circulation oceanique generale et de la capacite de l'ocean a absorber le gaz carbonique anthropogene. L'utilisation simultanee du tritium et de son descendant par voie radioactive l'helium-3 ajoute une information supplementaire: la paire de traceurs tritium et helium-3 constitue une horloge dans l'etude des masses d'eau. En plus de son origine "tritiumgenique", l'helium-3 est emis au niveau des dorsales oceaniques et il constitue un traceur de choix dans l'etude de la circulation oceanique profonde. Toutes les mesures d'helium-3 et de tritium ont ete faites par spectrometrie de masse. Protocole analytique, limite de detection et reproductibilite de la methode sont reportes dans ce travail. Sont abordes au niveau de l'interpretation des resultats: -1) les donnees d'helium-3 a la campagne merou a (ete 1982) ont permis de localiser une source active et de mettre en evidence un courant de retour intermediaire (centre vers 1000 m de profondeur), prolongement du courant profond upwelle dans la partie meridionale du bassin. -2) l'utilisation des donnees de tritium concernant la mission phycemed 1 (avril 1981) a permis de decrire la grande variabilite spatio-temporelle des processus de convection ayant lieu dans le bassin nord; le temps de renouvellement des eaux profonbdes du golfe du lion est evalue a 11 +ou- 2 ans. Les circulations profondes et de subsurface au niveau des detroits de sardaigne et de gibraltar sont precisees. -3) l'etude simultanee, suivant des niveaux isopycnaux, des donnees tritium et helium-3 de la mission topogulf (ete 1983) a permis de localiser les zones ou les processus de convection sont actifs. Une approche theorique relative a l'utilisation simultanee du tritium et de l'helium-3 est abordee. Elle utilise un modele de melange distinguant, pour une masse d'eau consideree, le temps de transit du temps de ventilation. La validite de "age tritium-helium" est testee par comparaison avec les temps de transit et de ventilation determines par le modele

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Hogancamp, Kyle J. "Characterizing South American Mesoscale Convective Complexes Using Isotope Hydrology." TopSCHOLAR®, 2017. http://digitalcommons.wku.edu/theses/1937.

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Mesoscale convective complexes (MCCs) over subtropical South America contribute an average annual volume of precipitation equal to approximately seven km3 and occur with an average regularity in the region, with more than 30 per warm season. Isotopic characteristics of precipitation, such as δ2H and δ18O values, provide information that can be used to identify unique processes and sources related to precipitation events. The largest database of isotope characteristics of precipitation within the region is the Global Network of Isotopes in Precipitation (GNIP), which provides varying temporal resolution data from stations around the world, including subtropical South America. Using this database, isotope characteristics of precipitation samples within the study area of Brazil were examined to identify patterns in storm characteristics, the isotope characteristics in MCC events, and to assess the use of event (daily) resolution data for storm events that lasted 14 hours, on average. This research resulted in Local Meteoric Water Lines (LMWL) that describe the isotopic composition of precipitation and rivers at various points throughout the year and found precipitation within the study region much closer to the Global Meteoric Water Line (GMWL) than river water. While event (daily) resolution is useful, a greater number of samples at higher-resolution would provide better descriptions for specific storm events, such as MCCs, as well as to differentiate between MCC and non-MCC events more effectively. Differences in source waters and processes were evident in the data, meaning future research at higher resolutions could benefit from identifying the contribution of each source and process to any distinct MCC event in the region.

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Robinson, Elizabeth M. "The effect of a shallow low viscosity zone on mantle convection and its expression at the surface of the earth." Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/58496.

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Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. Includes bibliographical references (v.2, leaves 309-317). Many features of the oceanic plates cannot be explained by conductive cooling with age. A number of these anomalies require additional convective thermal sources at depths below the plate: mid-plate swells, the evolution of fracture zones, the mean depth and heat flow relationships with age and the observation of small scale (150-250 km) geoid and topography anomalies in the Central Pacific and Indian oceans. Convective models are presented of the formation and evolution of these features. In particular, the effect of a shallow low viscosity layer in the uppermost mantle on mantle flow and its geoid, topography, gravity and heat flow expression is explored. A simple numerical model is employed of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of (1) the viscosity contrast between the two fluid layers, (2) the thickness of the low viscosity zone, (3) the thickness of the conducting lid, and (4) the Rayleigh number of the fluid based on the viscosity of the lower layer. A model simple for mid-plate swells is that they are the surface expression of a convection cell driven by a heat flux from below. The low viscosity zone causes the top boundary layer of the convection cell to thin and, at high viscosity contrasts and Rayleigh numbers, it can cause the boundary layer to go unstable. The low viscosity zone also mitigates the transmission of normal stress to the conducting lid so that the topography and geoid anomalies decrease. The geoid anomaly decreases faster than the topography anomaly, however, so that the depth of compensation can appear to be well within the conducting lid. Because the boundary layer is thinned, the elastic plate thickness also decreases and, since the low viscosity allows the fluid to flow faster in the top layer, the uplift time decreases as well. We have compared the results of this modeling to data at the Hawaii, Bermuda, Cape Verde and Marquesas swells, and have found that it can reproduce their observed anomalies. The viscosity contrasts that are required range from 0.2-0.01, which are in agreement with other estimates of shallow viscosity variation in the upper mantle. Also, the estimated viscosity contrast decreases as the age of the swell increases. This trend is consistent with theoretical estimates of the variation of such a low viscosity zone with age. Fracture zones juxtapose segments of the oceanic plates of different ages and thermal structures. The flow induced by the horizontal temperature gradient at the fracture zone initially downwells immediately adjacent to the fracture zone on the older side, generating cells on either side of the plume. The time scale and characteristic wavelength of this flow depends initially on the viscosity near the largest temperature gradient in the fluid which, in our model, is the viscosity of the low viscosity layer. They therefore depend on both the Rayleigh number and the viscosity contrast between the layers. Eventually the flow extends throughout the box, and the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers. When the Rayleigh number based on the viscosity of the top la er, and the depth of both fluid layers, is less than 10 , the geoid anomalies of these flows are dominated by the convective signal. When this Rayleigh number exceeds 106, the geoid anomalies retain a step across the fracture zone out to large ages. We have compared our results to geoid anomalies over the Udintsev fracture zone, and have found that the predicted geoid anomalies, with high effective Rayleigh numbers, agree at longer wavelengths with the observed anomalies and can produce the observed geoid slope-age behaviour. We have also compared the calculated topographic steps to those predicted by the average depth-age relationships observed in the oceans. We have found that only with a low viscosity zone will the flow due to fracture zones not disturb the average depth versus age relationships. We have also applied the model to a numerical study of the effect of a low viscosity zone in the uppermost mantle on the onset and surface expression of convective instabilities in the cooling oceanic plates. We find that the onset and magnitude of the geoid, topography and heat flow anomalies produced by these instabilities are very sensitive to the viscosity contrast and the Rayleigh number, and that the thickness of the low viscosity zone is constrained by the wavelength of the observables. If the Rayleigh number of the low viscosity zone exceeds a critical value then the convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the Rayleigh number. The small scale convection will eventually decay into longer wavelength convection which extends throughout the upper mantle, so that the small scale convective signal will eventually be succeeded by a longer wavelength signal. We compare our model to the small scale geoid and topography anomalies observed in the Southeast Pacific. The magnitude (0.50-0.80 m in geoid and 250 m in topography), early onset time (5-10 m.y.) and lifetime (over 40 m.y.) of these anomalies suggest a large viscosity contrast of greater than two orders of magnitude. The trend to longer wavelengths also suggests a high Rayleigh number of near or over 10 and their original 150-250 km wavelength indicates a low viscosity zone of 75- 125 km thickness. We have found that the presence of such small scale convection does not disturb the slope of the depth-age curve but elevates it by up to 250 m, and it is not until the onset of long wavelength convection that the depth-age curves radically depart from a cooling halfspace model. In the Pacific, the depth-age curve is slightly elevated in the region where small scale convection is observed and it does not depart from a halfspace cooling model until an age of 70 m.y.. Models that produce the small scale anomalies predict a departure time between 55 and 65 m.y. by Elizabeth M. Robinson. Ph.D.

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Shearman, R. Kipp. "Dynamics of mesoscale motion in the California current." Thesis, 1999. http://hdl.handle.net/1957/27789.

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Li, Guoqing. "Simulating interdecadal variation of the thermohaline circulation by assimilating time-dependent surface data into an ocean climate model /." 1994. http://collections.mun.ca/u?/theses,75354.

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Mullarney, Julia C. "Thermal and thermohaline convection models for the meridional overturning circulation of the oceans." Phd thesis, 2004. http://hdl.handle.net/1885/146265.

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Tesdal, Jan-Erik. "Circulation changes associated with freshwater and heat content variability and implications for biological productivity in the subpolar North Atlantic Ocean." Thesis, 2020. https://doi.org/10.7916/d8-36h5-xz52.

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Large-scale circulation in the northern North Atlantic plays a crucial role in the global climate by influencing ocean storage of atmospheric heat and carbon. Temperature and salinity changes in this region can have important consequences on ocean circulation due to density stratification at sites of deep water formation. Such influences can involve feedback mechanisms related to the Atlantic Meridional Overturning Circulation, which has been shown to influence the hydrography of the northern North Atlantic on decadal timescales. Current expectations are that through increasing sea-ice melting, river discharge, an intensifying hydrological cycle and glacial melt anomalies, future climate change could disrupt North Atlantic circulation patterns with cascading effects on carbon cycling and global climate. These interactions were investigated through circulation changes associated with salinity and freshwater variability, as well as variability in temperature and heat content. Recent changes in phytoplankton concentration and biological productivity in the Labrador Sea were also examined as part of this study. Spatial and temporal patterns of salinity in the North Atlantic were examined with the help of objective analysis and reanalysis salinity products using Argo observations of the recent decade (2005 to 2015). An overall freshening trend was evident, but with clear regional differences, particularly between the western subpolar gyre and the central North Atlantic. In general, the western subpolar region exhibited high interannual variability in surface salinity compared to the central North Atlantic. The western subpolar region also revealed a seasonal pattern of salinity fluctuation related to sea ice retreat and accretion, but with some years (i.e., 2008, 2012 and 2015) showing unusually large and negative salinity anomalies which were not present in the central or eastern North Atlantic. To understand the dominant factors influencing salinity and freshwater in the northern North Atlantic, budgets for liquid freshwater content over the northern North Atlantic were derived using a state-of-the-art ocean state estimate (ECCOv4). Here the subpolar North Atlantic (between $\sim$45\oN and the Greenland Scotland ridge) is distinguished from the Nordic Seas (north of the Greenland Scotland ridge). In a separate investigation ECCOv4 was used to describe global ocean heat budgets at varying spatial and temporal resolutions. This analysis showed that anomalies in temperature tendency are driven by atmospheric forcing at short time scales, while advection is the principle term at long time scales. ECCOv4 budget analysis was then used to investigate mechanisms behind interannual freshwater content variability in the northern North Atlantic over the time period 1992-2015. From the mid-1990s to the mid-2000s warming and salinification occurred in the subpolar North Atlantic. Consistent with the upper layer analysis with Argo-observations, ECCOv4 confirmed an overall freshening since about 2005. This freshening occurs simultaneously with an overall cooling in the subpolar North Atlantic. Advective convergence has been identified as the dominant driver of liquid freshwater content and ocean heat content variability in the subpolar North Atlantic, with liquid freshwater and heat content being anti-correlated. Consistent with the global heat analysis in ECCOv4, our results revealed that forcing is only important for establishing anomalies over shorter time scales (i.e., seasonal to interannual), but advective convergence becomes more important at longer (i.e., decadal) scales. Advection is the dominant term due to changes across the southern boundary on the decadal time scale, while exchanges with the Arctic Ocean have minor impact. Changes in freshwater and heat content in the subpolar North Atlantic due to advection occur through anomalies in the circulation itself, and not by the advection of anomalies in either liquid freshwater or heat content. In contrast to the subpolar North Atlantic, in the Nordic Seas interannual changes in liquid freshwater content are predominantly driven by forcing due to sea ice melting, which is in turn strongly correlated with Arctic sea ice export through Fram Strait. The overall concurrent warming and salinification followed by cooling and freshening in the subpolar North Atlantic suggests a relationship with changes in northward heat and salt transport through the Atlantic Meridional Overturning Circulation. This is consistent with decadal variability in deep convection in the Labrador Sea. It is evident that another consequence of changes in the Labrador Sea deep convection is the potential effects on nutrient availability and thus biological productivity. The Labrador Sea has become more productive in recent years, with mean chlorophyll-a concentrations closely correlated with silicate concentrations in the upper waters, which in turn are strongly correlated with wintertime convection depth. Thus annual production in the Labrador Sea appears to be influenced by the extent of deep winter mixing, thereby linking the Atlantic Meridional Overturning Circulation and deep convection to nutrient availability and ocean productivity in the subpolar North Atlantic.

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Marsland, SJ. "Coupled ocean/sea-ice modelling in the Southern Ocean." Thesis, 1999. https://eprints.utas.edu.au/20449/1/whole_MarslandSimonJames1999_thesis.pdf.

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The ocean/sea-ice interaction in the Southern Ocean is investigated using the Hamburg Ocean Primitive Equation Model (HOPE). The ocean model is three dimensional and prognostically calculates velocity, temperature, salinity and sea-surface elevation on a multi-level staggered Arakawa E-grid. Coupled to this is a thermodynamic model of sea-ice growth and melt, and a dynamic model with viscous-plastic rheology. Two versions have been formulated: a high-resolution re-entrant channel model of the East Antarctic coastline; and a medium-resolution southern hemisphere model. In the high-resolution model the mean annual oceanic heat flux (OHF) to the sea-ice (8 W m -2 ) is dominated by convection. There is considerable seasonal variability in the OHF, with areal-averaged values approaching 20 W m-2in winter, and falling below 5 W M-2 in summer. There is also considerable spatial variability in the OHF: near the sea-ice edge mean monthly values can be above 100 W m -2; within a coastal polynya, where the maximum annual in situ sea-ice growth is 15 m, values approach 50 W m-2; in the seasonal zone (62-64°S) an OHF in the range 5-20 W m -2 is common in the winter months; but for most of the year at most other locations the oceanic heat flux lies in the range of 0-5 W m-2 . Sensitivities to the magnitudes of sea-ice salinity and precipitation minus evaporation (P-E) are investigated. The model is found to be very sensitive to processes that affect the sea surface salinity, which determines the vertical stability of the ocean. In turn this controls the rate of convection, and consequently the sea-ice thickness. For the extreme case of zero P-E the model enters into a mode of temperature-driven deep convection, characterised by relatively warm sea surface temperature and a total collapse of the seasonal sea-ice cycle. The medium-resolution version is used to investigate the response of the coupled ocean/sea-ice system to a number of surface fresh water flux (SFWF) climatologies, and to changes in the mean surface air temperature in the southern hemisphere. As with the high-resolution model the seaice is very sensitive to the SFWF. In particular, the occurrence of a large scale polynya in the Weddell Sea is found to depend critically on there being a sufficient SFWF. The mean annual OHF for the Southern Ocean is found to be around 25 W m -2 , also with considerable seasonal and spatial variability. Increasing the SFWF by 10 cm a-1results in a decrease of 10% in the OHF, while increasing the surface air temperature by 2°C increases the OHF by 10%. The increase in surface air temperature results in a decrease in mean annual sea-ice extent and volume of around 20%, while the increase in SFWF results in an increase in mean annual sea-ice extent of around 5%, and an increase in mean annual sea-ice thickness of around 12%.

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(8066834), David J. Cannon. "Hypolimnetic Mixing in Lake Michigan." Thesis, 2019.

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Little work has been done to estimate turbulence characteristics in the hypolimnetic waters of large lakes, where the magnitude and vertical structure of turbulent parameters have important implications for nutrient cycling and benthic exchange. In this thesis, hypolimnetic mixing is investigated over the annual stratification cycle in a large lake using a series of experiments in Lake Michigan that utilize acoustic Doppler velocimeters, thermistors, and microstructure profilers to characterize mean flow and turbulence throughout the water column. More than 500 days of physical limnological data were collected and analyzed over the course of this study, creating the most comprehensive data set of its kind in the Laurentian Great Lakes. While we found that bottom boundary layer turbulence and mean flow follow law-of-the-wall predictions in the mean, individual estimates were shown to deviate significantly from canonical expectations, with deviations linked to weakly energetic flow conditions (i.e. low speeds) and seiche-scale flow unsteadiness. Bottom boundary layer characteristics, including the mean current speed (U50=3 cm/s), drag coefficient (Cd50=0.0052), and turbulent kinetic energy dissipation (ϵ50 =10-8 W/kg), showed very little seasonal variation, despite highly variable surface forcing (e.g. stratification, wind speeds). Full water column turbulence profiles measured during the stratified summer were largely buoyancy suppressed, with internal Poincaré waves driving enhanced turbulent kinetic energy dissipation (ϵ= 10-7 W/kg) in the relatively compact thermocline and weak hypolimnetic mixing (turbulent scalar diffusivity: Kz=10-6 m2/s) limiting benthic nutrient delivery. Although small temperature gradients drove strong mixing over the isothermal period (Kz=10-3 m2/s), velocity shear was overwhelmed by weakly stable stratification (Richardson number:Ri≈0.2), limiting the development of the surface mixed layer and suppressing hypolimnetic turbulence (ϵ=10-9 W/kg; Kz=10-4 m2/s). When surface temperatures fell below the temperature of maximum density (TMD≈ 4℃), radiative convection played a major role in driving vertical transport, with energetic full water column mixing throughout the day followed by surface cooling and restratification overnight. During this “convective winter” period, daily temperature instabilities were directly correlated with elevated turbulence levels (ϵ=10-7 W/kg; Kz≈10-1 m2/s), and overnight turbulence characteristics were similar to those observed over the isothermal spring. Near surface dissipation and diffusivity measurements followed similarity scaling arguments, with wind shear and surface fluxes dominating production in the surface mixed layer during all three seasons. Together, these results are used to model the influence of invasive dreissenids over each forcing period, providing insight into the annual variability of effective filtration rates in the calm, hypolimnetic waters of Lake Michigan.

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Gamble, Rosevear MM. "Fine-scale ocean processes driving the basal melting of Antarctic ice shelves." Thesis, 2021. https://eprints.utas.edu.au/37966/1/Gamble_Rosevear_whole_thesis.pdf.

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The Antarctic Ice Sheet, which comprises the largest volume of ice on our planet, is losing mass due to ocean-driven melting of its fringing ice shelves. Efforts to represent basal melting in sea level projections are undermined by poor understanding of the turbulent ice shelf-ocean boundary layer (ISOBL), a meters-thick band of ocean that regulates heat, salt and momentum transfer between the far field ocean and the ice. Regional ocean models cannot resolve the ISOBL and instead rely on parameterisations to predict melting. However, observations suggest that these parameterisations only perform well for a subset of relevant ocean conditions, namely in cold, energetic environments. This thesis uses both observational data and turbulence-resolving model simulations to address this shortfall by characterising melting and ISOBL dynamics across a broad range of ocean states. Chapter 1 of this thesis outlines the motivation and context for the work that follows, highlighting the urgent knowledge gaps that will be addressed. In chapter 2, a unique set of observations from beneath the Amery Ice Shelf, including in situ basal melt rate, ocean velocity, temperature and salinity data are analysed and ocean conditions are characterised. The mean basal melt rate at the site (0.5 m yr$^1$) is a factor of 2-4 lower than predicted from common ice-ocean parameterisations. This result suggests an important role for stratification at this site, either through suppression of heat transport to the ice-ocean interface or a shoaling of the mixed layer depth. These processes cannot be unraveled from the available observational data, further motivating the need for turbulence resolving simulations. In chapter 3, large-eddy simulation is used to model the ISOBL. The model domain consists of a horizontal ice-ocean interface with a melting boundary condition at the upper surface, underlain by a stratified ocean. The domain is periodic in both horizontal directions, and is forced with a steady flow in geostrophic balance. At relatively warm, low velocity conditions a small-scale mixing process (double-diffusive convection) is shown to determine ice shelf melt rate and the properties of the mixed layer that forms beneath the ice. In double-diffusive regime, melting is found to be inherently unsteady in time and insensitive to shear from the imposed current. Simulated melt rates and water column structure are consistent with observations made near the grounding line of the Ross Ice Shelf. In chapter 4, model forcing conditions are expanded to encompass colder and more energetic cavity environments in which current shear controls melting. Two distinct mixing regimes emerge: a stratified regime in which boundary layer turbulence is strongly affected by the surface buoyancy flux due to melting and a well-mixed regime in which buoyancy has little effect. The stratified regime supports strong temperature and salinity gradients near the ice, decoupling the interface and far field conditions. The relative strength of the surface buoyancy flux and shear, characterised by the Obukhov length scale, is shown to be critical to both heat flux and boundary layer depth. Results from chapters 3 and 4 are used to develop a regime diagram for ISOBL dynamics beneath horizontal, melting ice in discussion chapter 5. This novel diagram provides new insight into the varied and nonlinear responses of basal melting and ISO BL dynamics to local conditions around Antarctica. Comparison to observed sub-ice shelf conditions and melt rates from chapter 2 and other published studies is favorable and demonstrates the relevance of these regimes over a broad range of realistic conditions. Insights from this thesis significantly extend the current understanding of the ISOBL and basal melting. The inclusion of the double-diffusive and stratified regimes in future parameterisations of ice-ocean interactions will significantly improve melt rate estimates, with consequences for predictions of ice sheet stability.

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Thèses sur le sujet « Convection (Oceanography) »

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