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1
Nastos, P. T., and I. T. Matsangouras. "Analysis of synoptic conditions for tornadic days over Western Greece." Natural Hazards and Earth System Sciences Discussions 2, no. 3 (March 31, 2014): 2215–43. http://dx.doi.org/10.5194/nhessd-2-2215-2014.
Анотація:
Abstract. Tornadoes have been reported in Greece during the last decades and recent studies have given evidence that west Greece is a vulnerable area for tornadoes, waterspouts and funnel clouds to occur. In this study, the composite means and anomalies of synoptic conditions for tornadic events (tornadoes, waterspouts and funnel clouds) over west Greece are analyzed and discussed. The daily composite means of synoptic conditions were based on National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, for the period 12 August 1953 to 31 December 2012. The daily composite anomalies were calculated with respect to 30 years climatology (1981–2010) of the synoptic conditions. The analysis was carried out in terms of seasonal and monthly variability of composite means and anomalies of synoptic conditions for specific isobaric levels of 500, 700, 850, 925 hPa and the sea level pressure (SLP). In addition, an analysis and discussion about the dynamic Lifted Index from NCEP/NCAR Reanalysis datasets is presented.
2
Wu, Renguang, Yuqi Wang, and Xi Cao. "What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?" Journal of Climate 34, no. 9 (May 2021): 3645–62. http://dx.doi.org/10.1175/jcli-d-20-0477.1.
Анотація:
AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.
3
Kaspar, M., and M. Müller. "Cyclogenesis in the Mediterranean basin: a diagnosis using synoptic-dynamic anomalies." Natural Hazards and Earth System Sciences 9, no. 3 (June 25, 2009): 957–65. http://dx.doi.org/10.5194/nhess-9-957-2009.
Анотація:
Abstract. This work deals with the hypothesis that synoptically-driven heavy rains in the Mediterranean region are connected with the occurrence of synoptic-dynamic anomalies. We investigate this hypothesis by conducting a diagnostic study of a cyclogenesis event from 18–21 July 2001. Cyclogenesis started over the western Mediterranean Sea and was triggered by the approach of a pre-existing upper trough over a lower frontal zone. The event was first accompanied by heavy convective rains in northern Italy and later by heavy, widespread and steady rains of non-convective character in Central Europe. Using re-analyses from the database ERA-40, we evaluate the synoptic-dynamic anomalies by the cumulative distribution function relatively to July–August area-related climatology over the reference period from 1958 to 2002. For the duration of cyclogenesis, we detect significant anomalies associated with the processes that support the completion of vertical coupling between the lower frontal zone and the upper vorticity maximum. The periods of heavy rains in both the Mediterranean region and Central Europe were characterised by the occurrence of anomalies that created synoptic-scale conditions favourable for triggering and sustaining heavy rains. Although these anomalies were somewhat weaker in the period of heavy rains in Central Europe than in the Mediterranean region, the analysis of their spatio-temporal correspondence over the reference rainfall events in the Czech Republic shows that they are typical of heavy rains there. Due to the relatively high variability of the causal synoptic-scale conditions, this study would benefit post-processing procedures aimed at improving warning about this hazardous weather phenomenon to further investigate which anomalies generally occur during synoptically-driven heavy rains in the Mediterranean region and the strengths of these anomalies.
4
Nastos, P. T., and I. T. Matsangouras. "Analysis of synoptic conditions for tornadic days over western Greece." Natural Hazards and Earth System Sciences 14, no. 9 (September 10, 2014): 2409–21. http://dx.doi.org/10.5194/nhess-14-2409-2014.
Анотація:
Abstract. Tornadoes have been reported in Greece during the last few decades and recent studies have given evidence that western Greece is an area vulnerable to tornadoes, waterspouts and funnel clouds In this study, the composite means and anomalies of synoptic conditions for tornadic events (tornadoes, waterspouts and funnel clouds) over western Greece are analyzed and discussed. The daily composite means of synoptic conditions were based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data sets, for the period 12 August 1953 to 31 December 2012. The daily composite anomalies were calculated with respect to 30 years of climatological study (1981–2010) of the synoptic conditions. The analysis was carried out in terms of seasonal and monthly variability of composite means and anomalies of synoptic conditions for specific isobaric levels of 500, 700, 850, 925 hPa and the sea level pressure (SLP). In addition, an analysis and discussion about the dynamic lifted index from NCEP–NCAR reanalysis data sets is presented. The daily composite mean analysis of 500 hPa revealed a trough line across the northern Adriatic Sea and central Italy, associated with a SW upper-air stream over western Greece. The maximum composite anomalies were depicted at the isobaric level of 500 hPa during autumn, spring and summer, against winter when the anomaly appeared at 925 hPa isobaric level. In addition, 48% of tornado events during the autumn season occurred in pre-frontal weather conditions (cold fronts) and 27% developed after the passage of the cold front. Furthermore, the main difference in synoptic patterns between tornado and waterspout days along western Greece during the autumn season is the maximum daily composite anomaly over the Gulf of Taranto.
5
Naumenko, M. A., and L. A. Timofeeva. "Synoptic-scale anomalies of Lake Ladoga surface temperature fields." Russian Meteorology and Hydrology 34, no. 12 (December 2009): 828–34. http://dx.doi.org/10.3103/s1068373909120085.
6
Jaffe, Sharon C., Jonathan E. Martin, Daniel J. Vimont, and David J. Lorenz. "A Synoptic Climatology of Episodic, Subseasonal Retractions of the Pacific Jet." Journal of Climate 24, no. 11 (June 1, 2011): 2846–60. http://dx.doi.org/10.1175/2010jcli3995.1.
Анотація:
Abstract Twenty-eight years of NCEP–NCAR reanalysis data are employed in a composite analysis of the structure and evolution of the large-scale circulation associated with rapid, subseasonal, westward retractions of the Northern Hemisphere Pacific jet. Nineteen Pacific jet retractions are identified in the dataset. The salient characteristics of these transitions are presented, emphasizing the structure and evolution of anomalies in the zonal wind, upper-tropospheric geopotential height, sea level pressure (SLP), and tropopause potential vorticity (PV). The composite analysis demonstrates that as the jet retracts, a transition from a dominant negative anomaly to a dominant positive anomaly across the North Pacific in both the 500-hPa geopotential height and SLP fields occurs in ~10 days. The resulting anticyclonic anomalies are sprawling and intense with SLP anomalies of nearly 20 hPa and 500-hPa geopotential height anomalies of more than 200 m. The vertical superposition of these upper- and lower-level anomalies indicates that these composite structures are equivalent barotropic. These results are also consistent with the composite 200–250-hPa Ertel PV anomaly field, in which a jet retraction event is characterized by the formation of a zonally elongated, meridionally oriented PV anomaly couplet in the central North Pacific that becomes increasingly isotropic in the jet exit region simultaneous with the retraction of the jet. Because of the large spatial scale of these events, the results are also discussed in the context of storm-track variability, the East Asian winter monsoon, and tropical diabatic heating anomalies.
7
Lee, Min-Hee, and Joo-Hong Kim. "The Role of Synoptic Cyclones for the Formation of Arctic Summer Circulation Patterns as Clustered by Self-Organizing Maps." Atmosphere 10, no. 8 (August 19, 2019): 474. http://dx.doi.org/10.3390/atmos10080474.
Анотація:
Contribution of extra-tropical synoptic cyclones to the formation of mean summer atmospheric circulation patterns in the Arctic domain (≥60° N) was investigated by clustering dominant Arctic circulation patterns based on daily mean sea-level pressure using self-organizing maps (SOMs). Three SOM patterns were identified; one pattern had prevalent low-pressure anomalies in the Arctic Circle (SOM1), while two exhibited opposite dipoles with primary high-pressure anomalies covering the Arctic Ocean (SOM2 and SOM3). The time series of their occurrence frequencies demonstrated the largest inter-annual variation in SOM1, a slight decreasing trend in SOM2, and the abrupt upswing after 2007 in SOM3. Analyses of synoptic cyclone activity using the cyclone track data confirmed the vital contribution of synoptic cyclones to the formation of large-scale patterns. Arctic cyclone activity was enhanced in the SOM1, which was consistent with the meridional temperature gradient increases over the land–Arctic ocean boundaries co-located with major cyclone pathways. The composite daily synoptic evolution of each SOM revealed that all three SOMs persisted for less than five days on average. These evolutionary short-term weather patterns have substantial variability at inter-annual and longer timescales. Therefore, the synoptic-scale activity is central to forming the seasonal-mean climate of the Arctic.
8
Udy, Danielle G., Tessa R. Vance, Anthony S. Kiem, Neil J. Holbrook, and Mark A. J. Curran. "Links between Large-Scale Modes of Climate Variability and Synoptic Weather Patterns in the Southern Indian Ocean." Journal of Climate 34, no. 3 (February 2021): 883–99. http://dx.doi.org/10.1175/jcli-d-20-0297.1.
Анотація:
AbstractWeather systems in the southern Indian Ocean (SIO) drive synoptic-scale precipitation variability in East Antarctica and southern Australia. Improved understanding of these dynamical linkages is beneficial to diagnose long-term climate changes from climate proxy records as well as informing regional weather and climate forecasts. Self-organizing maps (SOMs) are used to group daily 500-hPa geopotential height (z500; ERA-Interim) anomalies into nine regional synoptic types based on their dominant patterns over the SIO (30°–75°S, 40°–180°E) from January 1979 to October 2018. The pattern anomalies represented include four meridional, three mixed meridional–zonal, one zonal, and one transitional node. The frequency of the meridional nodes shows limited association with the phase of the southern annular mode (SAM), especially during September–November. The zonal and mixed patterns were nevertheless strongly and significantly correlated with SAM, although the regional synoptic representation of SAM+ conditions was not zonally symmetric and was represented by three separate nodes. We recommend consideration of how different synoptic conditions vary the atmospheric representation of SAM+ in any given season in the SIO. These different types of SAM+ mean a hemispheric index fails to capture the regional variability in surface weather conditions that is primarily driven by the synoptic variability rather than the absolute polarity of the SAM.
9
Kim, Jineun, Donghyuck Yoon, Dong-Hyun Cha, Yonghan Choi, Joowan Kim, and Seok-Woo Son. "Impacts of the East Asian Winter Monsoon and Local Sea Surface Temperature on Heavy Snowfall over the Yeongdong Region." Journal of Climate 32, no. 20 (September 12, 2019): 6783–802. http://dx.doi.org/10.1175/jcli-d-18-0411.1.
Анотація:
Abstract This research investigates the impact of local sea surface temperature (SST) on the 2-month (January and February) accumulated snowfall over the Yeongdong (YD) region. The YD region is strongly affected by synoptic-scale factors such as the East Asian winter monsoon (EAWM). The relationships of snowfall over the YD region to the EAWM and local SST are examined based on observational analyses and sensitivity experiments using a regional climate model. In the sensitivity experiments, local SST is replaced with the 33-yr mean winter SST (1982–2014). The observational analysis shows that both the synoptic environment and local SST are important factors for the occurrence of anomalous heavy snowfall over the YD region. The favorable synoptic environments can be characterized by eastward expansion of the Siberian high over Manchuria and corresponding enhancement of easterly anomalies over the YD region. These conditions are more frequently observed during the weak EAWM years than during the strong EAWM. Furthermore, warm SST over the East Sea contributes to heavy snowfall over the YD region by providing heat and moisture in the lower troposphere, which are important sources of energy for the formation of heavy snowfall. Warm SST anomalies over the East Sea enhance low-level moisture convergence over the YD region, while cold SST anomalies lead to reduced moisture convergence. Sensitivity experiments indicate that local SST can significantly affect snowfall amount over the YD region when the synoptic environments are favorable. However, without these synoptic conditions (expansion of the Siberian high and easterly inflow), the impact of local SST on the snowfall over the YD region is not significant.
10
Plu, Matthieu, Philippe Arbogast, and Alain Joly. "A Wavelet Representation of Synoptic-Scale Coherent Structures." Journal of the Atmospheric Sciences 65, no. 10 (October 2008): 3116–38. http://dx.doi.org/10.1175/2008jas2618.1.
Анотація:
Midlatitude cyclogenesis as interpreted in the framework of either baroclinic development or potential vorticity thinking heavily relies on the concept of synoptic-scale anomaly. Given the existence of potential vorticity inversion and attribution, what is at stake to provide a mathematical definition for this concept is a complete finite-amplitude alternative to the linear-based theory of cyclogenesis. The existence of a reasonably objective way to represent anomalies in both real and idealized flows would not only help understanding cyclogenesis, it would also have many other applications for both theory and in practical forecasts. Inspired by the recent theory of wavelet representation of coherent structures in two-dimensional fluid mechanics, a wavelet representation of three-dimensional potential vorticity anomalies is built. This algorithm relies on the selection of the appropriate two-dimensional wavelet coefficients from the stationary wavelet transform in order to guarantee the critical translation-invariance property. The sensitivity of the algorithm to the position, size, and shape of the structures is assessed. The wavelet extraction is then applied to the upper-level precursor of a real-case storm of December 1999 and is compared to a basic monopolar extraction. Using potential vorticity inversion and forecasts with a primitive-equation model, it is found that both anomalies have similar implications on the development of the surface cyclone. However, the coherence in time of the extracted wavelet structure in the forecast and analysis sequence is more satisfactory than the extracted monopole: this suggests that the underlying mathematical description of an anomaly proposed here does, indeed, point toward the direction of an actual physical reality of the concept.
11
Pfahl, S. "Characterising the relationship between weather extremes in Europe and synoptic circulation features." Natural Hazards and Earth System Sciences 14, no. 6 (June 6, 2014): 1461–75. http://dx.doi.org/10.5194/nhess-14-1461-2014.
Анотація:
Abstract. Extreme weather events in Europe are closely linked to anomalies of the atmospheric circulation and in particular to circulation features like cyclones and atmospheric blocking. In this study, this linkage is systematically characterised with the help of conditional cyclone and blocking frequencies during precipitation, wind gust and temperature extremes at various locations in Europe. Such conditional frequency fields can serve as a dynamical fingerprint of the extreme events and yield insights into their most important physical driving mechanisms. Precipitation extremes over the ocean and over flat terrain are shown to be closely related to cyclones in the vicinity and the associated dynamical lifting. For extreme precipitation over complex terrain, cyclone anomalies are found at more remote locations, favouring the flow of moist air towards the topography. Wind gust extremes are associated with cyclone and blocking anomalies in opposite directions, with the cyclones occurring mostly over the North and Baltic seas for extreme events in central Europe. This setting is associated with pronounced surface pressure gradients and thus high near-surface wind velocities. Hot temperature extremes in northern and central Europe typically occur in the vicinity of a blocking anticyclone, where subsidence and radiative forcing are strong. Over southern Europe, blocking anomalies are shifted more to the north or northeast, indicating a more important role of warm air advection. Large-scale flow conditions for cold extremes are similar at many locations in Europe, with blocking anomalies over the North Atlantic and northern Europe and cyclone anomalies southeast of the cold extreme, both contributing to the advection of cold air masses. This characterisation of synoptic-scale forcing mechanisms can be helpful for better understanding and anticipating weather extremes and their long-term changes.
12
Pfahl, S. "Characterising the relationship between weather extremes in Europe and synoptic circulation features." Natural Hazards and Earth System Sciences Discussions 2, no. 2 (February 26, 2014): 1867–911. http://dx.doi.org/10.5194/nhessd-2-1867-2014.
Анотація:
Abstract. Extreme weather events in Europe are closely linked to anomalies of the atmospheric circulation and in particular to circulation features like cyclones and atmospheric blocking. In this study, this linkage is systematically characterised with the help of conditional cyclone and blocking frequencies during precipitation, wind gust and temperature extremes at various locations in Europe. Such conditional frequency fields can serve as a dynamical fingerprint of the extreme events and yield insights into their most important physical driving mechanisms. Precipitation extremes over the ocean and over flat terrain are shown to be closely related to cyclones in the vicinity and the associated dynamical lifting. For extreme precipitation over complex terrain, cyclone anomalies are found at more remote locations, favouring the flow of moist air towards the topography. Wind gust extremes are associated with cyclone and blocking anomalies in opposite directions, with the cyclones occurring mostly over the North and Baltic Seas for extreme events in central Europe. This setting is associated with pronounced surface pressure gradients and thus high near-surface wind velocities. Hot temperature extremes in northern and central Europe typically occur in the vicinity of a blocking anticyclone, where subsidence and radiative forcing are strong. Over southern Europe, blocking anomalies are shifted more to the north or northeast, indicating a more important role of warm air advection. Large-scale flow conditions for cold extremes are similar at many locations in Europe, with blocking anomalies over the North Atlantic and northern Europe and cyclone anomalies southeast of the cold extreme, both contributing to the advection of cold air masses. This characterisation of synoptic-scale forcing mechanisms can be helpful for better understanding and anticipating weather extremes and their long-term changes.
13
KISLOV, A. V., U. I. ANTIPINA, and I. A. KORNEVA. "EXTREME PRECIPITATION IN THE EUROPEAN ARCTIC IN SUMMER: STATISTICS AND SYNOPTIC MODELS." Meteorologiya i Gidrologiya, no. 7 (2021): 20–34. http://dx.doi.org/10.52002/0130-2906-2021-7-20-34.
Анотація:
Extreme precipitation in summer is classified in terms of belonging to the certain baseline probability distribution. The Pareto distribution can be used as its approximation. Events deviating from the baseline distribution are represented by the largest daily total precipitation. For them, the compliance with the probability (or the average repetition time) is completely lost, that is, any anomalies can occur, but they do not exceed some limit values typical of the Arctic. Thus, for the entire set of extremes, a specific distribution law of random variables can be introduced, that describes anomalies not exceeding certain absolute values; the presence of the boundary is felt only when approaching it. For 50 analyzed years at each station in the European Arctic, approximately four such anomalies were recorded. Synoptic objects in which super-large precipitation anomalies occur are determined: these are cyclones or pressure troughs with high water vapor content in the air (exceeding ~25 kg/m2) and with mesoscale systems embedded into the fronts, which are characterized by the vertical wind shear.
14
Dione, Cheikh, Fabienne Lohou, Marjolaine Chiriaco, Marie Lothon, Sophie Bastin, Jean-Luc Baray, Pascal Yiou, and Aurélie Colomb. "The Influence of Synoptic Circulations and Local Processes on Temperature Anomalies at Three French Observatories." Journal of Applied Meteorology and Climatology 56, no. 1 (January 2017): 141–58. http://dx.doi.org/10.1175/jamc-d-16-0113.1.
Анотація:
AbstractThe relative contribution of the synoptic-scale circulations to local and mesoscale processes was quantified in terms of the variability of midlatitude temperature anomalies from 2003 to 2013 using meteorological variables collected from three French observatories and reanalyses. Four weather regimes were defined from sea level pressure anomalies using National Centers for Environmental Prediction reanalyses with a K-means algorithm. No correlation was found between daily temperature anomalies and weather regimes, and the variability of temperature anomalies within each regime was large. It was therefore not possible to evaluate the effect of large scales on temperature anomalies by this method. An alternative approach was found with the use of the analogs method: the principle being that for each day of the considered time series, a set of days that had a similar large-scale 500-hPa geopotential height field within a fixed domain was considered. The observed temperature anomalies were then compared with those observed during the analog days: the closer the two types of series are to each other, the greater is the influence of the large scale. This method highlights a widely predominant influence of the large-scale atmospheric circulation on the temperature anomalies. It showed a potentially larger influence of the Mediterranean Sea and orographic flow on the two southern observatories. Low-level cloud radiative effects substantially modulated the variability of the daily temperature anomalies.
15
Hart, Robert E., and Richard H. Grumm. "Using Normalized Climatological Anomalies to Rank Synoptic-Scale Events Objectively." Monthly Weather Review 129, no. 9 (September 2001): 2426–42. http://dx.doi.org/10.1175/1520-0493(2001)129<2426:uncatr>2.0.co;2.
16
Wang, B., and H. Rui. "Synoptic climatology of transient tropical intraseasonal convection anomalies: 1975?1985." Meteorology and Atmospheric Physics 44, no. 1-4 (1990): 43–61. http://dx.doi.org/10.1007/bf01026810.
17
Song, Jie. "Understanding Anomalous Synoptic Eddy Vorticity Forcing in Pacific–North American Teleconnection Pattern Events." Journal of the Atmospheric Sciences 75, no. 12 (November 28, 2018): 4287–312. http://dx.doi.org/10.1175/jas-d-18-0071.1.
Анотація:
Abstract Utilizing a decomposition of anomalous eddy vorticity forcing (EVF) proposed by Song in 2016 and a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model, this study provides a different understanding of physical mechanisms that are responsible for the formation of the anomalous synoptic EVF (SEVF) associated with Pacific–North American teleconnection pattern (PNA) events. A series of short-term control experiments (CEs) and initial-value modified experiments (IVMEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. IVMEs are exactly the same as CEs except that appropriate small perturbations are introduced into the initial-value fields of CEs. The modified initial-value fields led to a gradual development of the PNA-like flow anomalies in IVMEs. Based on these numerical results, deformations of the synoptic eddy due to the emergence of the PNA pattern can be easily acquired by subtracting the synoptic eddy in CEs from the synoptic eddy in IVMEs . The anomalous SEVF associated with the PNA events in the model can be decomposed into ensembles of two linear and interaction terms (EVF1 and EVF2) and a nonlinear self-interaction term (EVF3). It is demonstrated that the physical essence of the anomalous SEVF associated with the PNA events is a competition result between EVF1 plus EVF2 and EVF3. Results also indicate that the different signs of SEVF associated with the positive and negative PNA events are not necessarily related to the different tilts of the synoptic eddy.
18
Lemburg, Alexander, Jürgen Bader, and Martin Claussen. "Sahel Rainfall–Tropical Easterly Jet Relationship on Synoptic to Intraseasonal Time Scales." Monthly Weather Review 147, no. 5 (May 1, 2019): 1733–52. http://dx.doi.org/10.1175/mwr-d-18-0254.1.
Анотація:
Abstract The tropical easterly jet (TEJ) is a characteristic upper-level feature of the West African monsoon (WAM) circulation. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. In contrast, the relationship between Sahel rainfall and the regional TEJ on synoptic to intraseasonal time scales is unclear. Therefore, this relationship is investigated by means of multiple statistical analyses using temporally highly resolved measurement and reanalysis data. It is shown that average correlations between convective activity and regional TEJ intensity remain below 0.3 for all synoptic to intraseasonal time scales. Especially on the synoptic time scale, the TEJ significantly lags anomalies in convective activity by one or two days, which indicates that convection anomalies are more likely to drive changes in the regional TEJ than vice versa. To further shed light on the role of the TEJ for rainfall over West Africa, a previously proposed effect of TEJ-induced upper-level divergence on the development of mesoscale convective systems (MCSs) is examined more closely. An analysis of nearly 300 Sahelian MCSs shows that their initiation is generally not associated with significant TEJ anomalies or jet-induced upper-level divergence. Furthermore, no statistically significant evidence is found that preexisting TEJ-related upper-level divergence anomalies affect intensity, size, and lifetime of MCSs. A limiting factor of this study is the focus on TEJ-induced upper-level divergence. Therefore, a possible effect of the TEJ on Sahel rainfall via other mechanisms cannot be ruled out and should be subject to future studies.
19
Spensberger, Clemens, Joseph Egger, and Thomas Spengler. "Synoptic Systems Interacting with the Rocky Mountain Barrier: Observations and Theories." Monthly Weather Review 145, no. 3 (March 2017): 783–94. http://dx.doi.org/10.1175/mwr-d-16-0195.1.
Анотація:
Using a composite analysis for strong sea level pressure perturbations off the west coast of North America, the evolution of large-amplitude synoptic systems upstream of the Rocky Mountains is investigated for the winter season. Corresponding previous analyses are refined by avoiding multiple counting of events and extended by including potential vorticity, vertical motion, and deformation in the analysis. Cyclonic and anticyclonic anomalies behave similarly, with weak local extrema forming in the lee of the mountain range southeast of the parent systems. However, neither the geopotential anomaly nor the associated potential vorticity anomaly cross the mountain range. Nevertheless, these anomalies contribute to the sea level pressure anomaly in the lee. For both positive and negative anomalies, potential vorticity exhibits a bipolar structure with lobes over the reference point and over the Cordillera, respectively. The relevance of several theories describing the interaction between synoptic systems and mountains are discussed in the light of these findings. It is important to note that these findings differ considerably from results reported in an earlier study. Key differences are the previously reported passage of a wave train over the reference point and the movement of the anomalies over the Rocky Mountains. Both features are absent in the current analysis. However, these features can be recovered if a 6-day high-pass filter is applied before the events are selected or if the analysis is applied to predominantly zonal flow situations.
20
Kalashnikov, Dmitri A., Paul C. Loikith, Arielle J. Catalano, Duane E. Waliser, Huikyo Lee, and John T. Abatzoglou. "A 30-Yr Climatology of Meteorological Conditions Associated with Lightning Days in the Interior Western United States." Journal of Climate 33, no. 9 (May 1, 2020): 3771–85. http://dx.doi.org/10.1175/jcli-d-19-0564.1.
Анотація:
AbstractA 30-yr climatology of lightning days and associated synoptic meteorological patterns are characterized across the interior western United States (WUS). Locally centered composite analyses show preferred synoptic meteorological patterns with positive 500-hPa geopotential height anomalies located to the northeast and negative sea level pressure anomalies to the northwest and collocated with local lightning days. Variations in preferred patterns for local lightning days are seen across the interior WUS. Areas not commonly affected by the North American monsoon system including the western Great Basin and northern Rocky Mountains show higher-amplitude anomalies of geopotential height, moisture, and midtropospheric instability patterns suggesting the importance of episodic midlatitude dynamics to lightning days in such locations. By contrast, locations closer to the core of the North American monsoon show weaker anomalies, likely reflecting the prevalence of favorable mesoscale dynamics key to lightning production during warm-season months in locations in the interior Southwest. Meteorological patterns for select locations are explored in more detail and two case studies of notably active lightning events are presented. Results from this observational analysis provide a foundation for evaluating meteorological conditions on lightning days in climate model simulations for the interior WUS.
21
Rivière, Gwendal, and Marie Drouard. "Dynamics of the Northern Annular Mode at Weekly Time Scales." Journal of the Atmospheric Sciences 72, no. 12 (November 19, 2015): 4569–90. http://dx.doi.org/10.1175/jas-d-15-0069.1.
Анотація:
Abstract Rapid onsets of positive and negative tropospheric northern annular mode (NAM) events during boreal winters are studied using ERA-Interim datasets. The NAM anomalies first appear in the North Pacific from low-frequency Rossby wave propagation initiated by anomalous convection in the western tropical Pacific around 2 weeks before the peak of the events. For negative NAM, the enhanced convection leads to a zonal acceleration of the Pacific jet, while for positive NAM, the reduced convection leads to a poleward-deviated jet in its exit region. The North Atlantic anomalies, which correspond to North Atlantic Oscillation (NAO) anomalies, are formed in close connection with the North Pacific anomalies via downstream propagation of low-frequency planetary-scale and high-frequency synoptic waves, the latter playing a major role during the last onset week. Prior to positive NAM, the generation of synoptic waves in the North Pacific and their downstream propagation is strong. The poleward-deviated Pacific jet favors a southeastward propagation of the waves across North America and anticyclonic breaking in the North Atlantic. The associated strong poleward eddy momentum fluxes push the Atlantic jet poleward and form the positive NAO phase. Conversely, prior to negative NAM, synoptic wave propagation across North America is significantly reduced and more zonal because of the more zonally oriented Pacific jet. This, together with a strong eddy generation in the North Atlantic, leads to equatorward eddy momentum fluxes, cyclonic wave breaking, and the formation of the negative NAO phase. Even though the stratosphere may play a role in some individual cases, it is not the main driver of the composited tropospheric NAM events.
22
Newman, Matthew, George N. Kiladis, Klaus M. Weickmann, F. Martin Ralph, and Prashant D. Sardeshmukh. "Relative Contributions of Synoptic and Low-Frequency Eddies to Time-Mean Atmospheric Moisture Transport, Including the Role of Atmospheric Rivers." Journal of Climate 25, no. 21 (November 2012): 7341–61. http://dx.doi.org/10.1175/jcli-d-11-00665.1.
Анотація:
The relative contributions to mean global atmospheric moisture transport by both the time-mean circulation and by synoptic and low-frequency (periods greater than 10 days) anomalies are evaluated from the vertically integrated atmospheric moisture budget based on 40 yr of “chi corrected” NCEP–NCAR reanalysis data. In the extratropics, while the time-mean circulation primarily moves moisture zonally within ocean basins, low-frequency and synoptic anomalies drive much of the mean moisture transport both from ocean to land and toward the poles. In particular, during the cool-season low-frequency variability is the largest contributor to mean moisture transport into southwestern North America, Europe, and Australia. While some low-frequency transport originates in low latitudes, much is of extratropical origin due to large-scale atmospheric anomalies that extract moisture from the northeast Pacific and Atlantic Oceans. Low-frequency variability is also integral to the Arctic (latitudes > 70°N) mean moisture budget, especially during summer, when it drives mean poleward transport from relatively wet high-latitude continental regions. Synoptic variability drives about half of the mean poleward moisture transport in the midlatitudes of both hemispheres, consistent with simple “lateral mixing” arguments. Extratropical atmospheric transport is also particularly focused within “atmospheric rivers” (ARs), relatively narrow poleward-moving moisture plumes associated with frontal dynamics. AR moisture transport, defined by compositing fluxes over those locations and times where column-integrated water vapor and poleward low-level wind anomalies are both positive, represents most of the total extratropical meridional moisture transport. These results suggest that understanding potential anthropogenic changes in the earth ’s hydrological cycle may require understanding corresponding changes in atmospheric variability, especially on low-frequency time scales.
23
Attard, Hannah E., and Andrea L. Lang. "Troposphere–Stratosphere Coupling Following Tropospheric Blocking and Extratropical Cyclones." Monthly Weather Review 147, no. 5 (May 1, 2019): 1781–804. http://dx.doi.org/10.1175/mwr-d-18-0335.1.
Анотація:
Abstract A climatology of the 100- and 250-hPa 45°–75°N zonal-mean meridional eddy heat flux anomaly, hereafter heat flux anomaly, was created to examine its variability following cool-season (i.e., October–April) blocks and extratropical cyclones. The goal is to elucidate the dynamical and environmental differences between synoptic events followed by the most extreme heat flux anomalies. The analysis was conducted with the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications, version 2 reanalysis. The results show that, on average, European blocks and west Pacific cyclones are followed by positive heat flux anomalies while west Pacific blocks and Atlantic extratropical cyclones are followed by negative heat flux anomalies. However, there was a large range of the 11-day-average heat flux anomaly following the events. Events in each region were further partitioned by their 100-hPa heat flux anomaly for a temporal and spatial analysis of the top and bottom quartile of events. Top-quartile events exhibited a baroclinic wave structure with height from the troposphere through the stratosphere, whereas bottom-quartile events were associated with a barotropic wave structure with height; these structures are significant at the 5% level. The results suggest that the sign of the heat flux anomaly is not dependent on the location of the synoptic event alone, but that there are common climatological and anomalous wave patterns surrounding the synoptic events that result in positive or negative heat flux anomaly. Regardless of event region, the precursor stratospheric structure is a key indicator in whether an event is followed by positive or negative 100-hPa heat flux anomalies.
24
Barrett, Bradford S., Jorge F. Carrasco, and Anthony P. Testino. "Madden–Julian Oscillation (MJO) Modulation of Atmospheric Circulation and Chilean Winter Precipitation." Journal of Climate 25, no. 5 (March 2012): 1678–88. http://dx.doi.org/10.1175/jcli-d-11-00216.1.
Анотація:
The leading intraseasonal mode of tropical atmospheric variability, the Madden–Julian oscillation (MJO), has been shown to modulate precipitation and circulation on a global and regional scale. Winter precipitation in Chile has been connected to a variety of synoptic-scale forcing mechanisms. This study explored the links between the two, first examining the intraseasonal variability of Chilean precipitation from surface gauges and the Tropical Rainfall Measuring Mission (TRMM) and then examining the variability of synoptic-scale circulation. Composites of precipitation, precipitation intensity, and lower-, middle-, and upper-tropospheric circulation were created using the Real-Time Multivariate MJO index, which divides the MJO into eight longitudinally based phases. Precipitation was found to vary across MJO phases, with positive precipitation anomalies in central and south-central Chile (30°–45°S) for MJO phases 8, 1, and 2, and negative anomalies in phases 3–7. Circulation was also found to vary across phase, in good agreement with precipitation: low geopotential height and negative omega (corresponding to upward vertical motion) anomalies were found over and upstream of Chile during the rainier phases, and the anomalies reversed during the drier phases. Surface pressure and middle- and upper-tropospheric geopotential height anomalies showed a classic equivalent barotropic wave train, indicating a teleconnection response to deep convective activity in the Maritime Continent in agreement with numerous earlier observational, modeling, and theoretical studies.
25
Jin, F.-F., L.-L. Pan, and M. Watanabe. "Dynamics of Synoptic Eddy and Low-Frequency Flow Interaction. Part I: A Linear Closure." Journal of the Atmospheric Sciences 63, no. 7 (July 1, 2006): 1677–94. http://dx.doi.org/10.1175/jas3715.1.
Анотація:
Abstract The interaction between synoptic eddy and low-frequency flow (SELF) has been recognized for decades to play an important role in the dynamics of the low-frequency variability of the atmospheric circulation. In this three-part study a linear framework with a stochastic basic flow capturing both the climatological mean flow and climatological measures of the synoptic eddy flow is proposed. Based on this linear framework, a set of linear dynamic equations is derived for the ensemble-mean eddy forcing that is generated by anomalous time-mean flows. By assuming that such dynamically determined eddy-forcing anomalies approximately represent the time-mean anomalies of the synoptic eddy forcing and by using a quasi-equilibrium approximation, an analytical nonlocal dynamical closure is obtained for the two-way SELF feedback. This linear closure, directly relating time-mean anomalies of the synoptic eddy forcing to the anomalous time–mean flow, becomes an internal part of a new linear dynamic system for anomalous time–mean flow that is referred to as the low-frequency variability of the atmospheric circulation in this paper. In Part I, the basic approach for the SELF closure is illustrated using a barotropic model. The SELF closure is tested through the comparison of the observed eddy-forcing patterns associated with the leading low-frequency modes with those derived using the SELF feedback closure. Examples are also given to illustrate an important role played by the SELF feedback in regulating the atmospheric responses to remote forcing. Further applications of the closure for understanding the dynamics of low-frequency modes as well as the extension of the closure to a multilevel primitive equation model will be given in Parts II and III, respectively.
26
Křížová, Milada. "Air temperature anomalies in Czechia." Geografie 121, no. 1 (2016): 79–98. http://dx.doi.org/10.37040/geografie2016121010079.
Анотація:
Air temperature extremes and temperature records in various places are favorite topics of the mass media. The study of spatial distribution of temperature anomalies is an important part of regional climatology and physical geography. It is also a relevant base for synoptic meteorologists, who can make better predictions of the temperature regime in various regions during various meteorological situations. This paper offers a modus operandi for an evaluation of climate stations. The crucial element for the evaluation is the representative character of a climate station for a specific region. To find out the temperature differences in climate stations, it is necessary to compare them without any influence of their geographical position. This means analyzing the data without the vertical temperature gradients and the latitude and longitude temperature variations.
27
Liu, Hong-Bo, Jing Yang, Da-Lin Zhang, and Bin Wang. "Roles of Synoptic to Quasi-Biweekly Disturbances in Generating the Summer 2003 Heavy Rainfall in East China." Monthly Weather Review 142, no. 2 (January 24, 2014): 886–904. http://dx.doi.org/10.1175/mwr-d-13-00055.1.
Анотація:
Abstract During the mei-yu season of the summer of 2003, the Yangtze and Huai River basin (YHRB) encountered anomalously heavy rainfall, and the northern YHRB (nYHRB) suffered a severe flood because of five continuous extreme rainfall events. A spectral analysis of daily rainfall data over YHRB reveals two dominant frequency modes: one peak on day 14 and the other on day 4 (i.e., the quasi-biweekly and synoptic-scale mode, respectively). Results indicate that the two scales of disturbances contributed southwesterly and northeasterly anomalies, respectively, to the mei-yu frontal convergence over the southern YHRB (sYHRB) at the peak wet phase. An analysis of bandpass-filtered circulations shows that the lower and upper regions of the troposphere were fully coupled at the quasi-biweekly scale, and a lower-level cyclonic anomaly over sYHRB was phase locked with an anticyclonic anomaly over the Philippines. At the synoptic scale, the strong northeasterly components of an anticyclonic anomaly with a deep cold and dry layer helped generate the heavy rainfall over sYHRB. Results also indicate the passages of five synoptic-scale disturbances during the nYHRB rainfall. Like the sYHRB rainfall, these disturbances originated from the periodical generations of cyclonic and anticyclonic anomalies at the downstream of the Tibetan Plateau. The nYHRB rainfalls were generated as these disturbances moved northeastward under the influence of monsoonal flows and higher-latitude eastward-propagating Rossby wave trains. It is concluded that the sYHRB heavy rainfall resulted from the superposition of quasi-biweekly and synoptic-scale disturbances, whereas the intermittent passages of five synoptic-scale disturbances led to the flooding rainfall over nYHRB.
28
Tan, Xin, Ming Bao, Dennis L. Hartmann, and Paulo Ceppi. "The Role of Synoptic Waves in the Formation and Maintenance of the Western Hemisphere Circulation Pattern." Journal of Climate 30, no. 24 (December 2017): 10259–74. http://dx.doi.org/10.1175/jcli-d-17-0158.1.
Анотація:
Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the east Pacific–North American sector, respectively. A newly identified low-frequency atmospheric regime—the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North American–North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ECMWF reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North American–North Atlantic regions and by associated storm-track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the midlatitude jet and the deepening of the mid-to-high-latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the northeast Pacific ridge anomaly with the NAO are discussed.
29
Sun, Bo, Huijun Wang, Biwen Wu, Min Xu, Botao Zhou, Huixin Li, and Teng Wang. "Dynamic Control of the Dominant Modes of Interannual Variability of Snowfall Frequency in China." Journal of Climate 34, no. 7 (April 2021): 2777–90. http://dx.doi.org/10.1175/jcli-d-20-0705.1.
Анотація:
AbstractThis study investigates the first two leading modes of the interannual variability of frequency of snowfall events (FSE) over China in the winter during 1986–2018. The positive phase of the first leading mode (EOF1) is mainly characterized by positive FSE anomalies in northeastern–northwestern China and negative FSE anomalies in the three-river-source region. In contrast, the positive phase of the second leading mode (EOF2) is mainly characterized by positive FSE anomalies in central-eastern China (CEC). EOF1 is affected by the synoptic-scale wave activity over the midlatitudes of the East Asian continent, where active synoptic-scale wave activity over the midlatitudes may cause increased FSE over northeastern–northwestern China, and vice versa. In a winter of a negative phase of the North Atlantic Oscillation, an anomalous deep cold low may occur over Siberia, which may induce increased meridional air temperature gradient, increased atmospheric baroclinicity, and hence increased FSE over the midlatitudes of the East Asian continent. The EOF2 is affected by the interaction between anomalous northerly cold advection and anomalous southerly water vapor transport over CEC. The positive phase of EOF2 is associated with negative sea ice anomalies in the Barents Sea–Kara Sea region and negative sea surface temperature anomalies in the central-eastern tropical Pacific. Reduced sea ice in the Barents Sea–Kara Sea during January–February may cause increased northerly cold advection over CEC, while a La Niña–like condition during January may induce southerly water vapor transport anomalies over CEC.
30
KAMADA, Tetsuo. "Synoptic report of VLF sudden phase anomalies observed at Toyokawa, Japan." Journal of geomagnetism and geoelectricity 37, no. 7 (1985): 667–99. http://dx.doi.org/10.5636/jgg.37.667.
31
Khan, Ibrahim, M. Indira Devi, T. Arunamani, and D. N. Madhusudhana Rao. "A synoptic study of VLF sudden phase anomalies recorded at Visakhapatnam." Earth, Planets and Space 57, no. 11 (November 2005): 1073–81. http://dx.doi.org/10.1186/bf03351886.
32
Efimov, V. V., and M. V. Shokurov. "Bispectral analysis of geopotential-height anomalies for synoptic-intraseasonal time-scales." Quarterly Journal of the Royal Meteorological Society 127, no. 575 (July 2001): 1707–29. http://dx.doi.org/10.1002/qj.49712757513.
33
Moshonkin, S. N., and N. A. Diansky. "Upper mixed layer temperature anomalies at the North Atlantic storm-track zone." Annales Geophysicae 13, no. 10 (October 31, 1995): 1015–26. http://dx.doi.org/10.1007/s00585-995-1015-x.
Анотація:
Abstract. Synoptic sea surface temperature anomalies (SSTAs) were determined as a result of separation of time scales smaller than 183 days. The SSTAs were investigated using daily data of ocean weather station "C" (52.75°N; 35.5°W) from 1 January 1976 to 31 December 1980 (1827 days). There were 47 positive and 50 negative significant SSTAs (lifetime longer than 3 days, absolute value greater than 0.10 °C) with four main intervals of the lifetime repetitions: 1. 4–7 days (45% of all cases), 2. 9–13 days (20–25%), 3. 14–18 days (10–15%), and 4. 21–30 days (10–15%) and with a magnitude 1.5–2.0 °C. An upper layer balance model based on equations for temperature, salinity, mechanical energy (with advanced parametrization), state (density), and drift currents was used to simulate SSTA. The original method of modelling taking into account the mean observed temperature profiles proved to be very stable. The model SSTAs are in a good agreement with the observed amplitudes and phases of synoptic SSTAs during all 5 years. Surface heat flux anomalies are the main source of SSTAs. The influence of anomalous drift heat advection is about 30–50% of the SSTA, and the influence of salinity anomalies is about 10–25% and less. The influence of a large-scale ocean front was isolated only once in February-April 1978 during all 5 years. Synoptic SSTAs develop just in the upper half of the homogeneous layer at each winter. We suggest that there are two main causes of such active sublayer formation: 1. surface heat flux in the warm sectors of cyclones and 2. predominant heat transport by ocean currents from the south. All frequency functions of the ocean temperature synoptic response to heat and momentum surface fluxes are of integral character (red noise), though there is strong resonance with 20-days period of wind-driven horizontal heat advection with mixed layer temperature; there are some other peculiarities on the time scales from 5.5 to 13 days. Observed and modelled frequency functions seem to be in good agreement.
34
O’Hara, Brian F., Michael L. Kaplan, and S. Jeffrey Underwood. "Synoptic Climatological Analyses of Extreme Snowfalls in the Sierra Nevada." Weather and Forecasting 24, no. 6 (December 1, 2009): 1610–24. http://dx.doi.org/10.1175/2009waf2222249.1.
Анотація:
Abstract The Sierra Nevada of eastern California receives heavy snowfall each year. However, it is the snowstorms that deposit heavy snowfall in a relatively short period of time that can cause major inconveniences and even life-threatening situations for the residents and visitors to the region. Some of these snowstorms are so extreme as to become legendary, and with increased population in this region a synoptic climatology of these extreme snowstorms can be a useful tool for assessing snowfall potential by operational forecasters. Additionally, the hydrological and climatological implications of extreme Sierra Nevada snowfalls are important for state and local resource- and emergency-planning purposes. A climatology of these snowstorms will be presented. The period of study will include the snowfall seasons (October–May) 1949/50 through 2004/05. A total of 542 snowstorms occurred during these 56 snowfall seasons. These snowstorms were analyzed to determine any common synoptic features. The most intense snowstorms in the highest decile of snowfall totals were analyzed in more detail in order to determine the parameters associated with these strongest snowstorms. Upper-level synoptic and thermodynamic characteristics associated with each snowstorm were then diagnosed to determine what common synoptic hydrodynamic and thermodynamic parameters the snowstorms share. Synoptic patterns were studied using the National Centers for Environmental Prediction (NCEP) model reanalysis data. Wind speeds at 200 hPa, and height anomalies at 500 hPa, were analyzed for each snowstorm from 3 days prior to the start of snowfall and continuing through the end of the storm. Anomalies and the transport of precipitable water were studied in order to determine the relative amount of moisture that was available to each snowstorm. A conceptual model for forecasting the strongest snowstorms was developed. Key findings include the following: 1) the importance of a fetch of moisture from the subtropics with relatively large positive moisture anomalies, 2) the importance of the atmospheric moisture stream being normal to the Sierra, 3) the low static stability accompanying these snowstorms, and 4) the importance of relatively strong upper-level dynamics, which helped to intensify the systems as they approached the Sierra.
35
James, P. M., and D. Peters. "The Lagrangian structure of ozone mini-holes and potential vorticity anomalies in the Northern Hemisphere." Annales Geophysicae 20, no. 6 (June 30, 2002): 835–46. http://dx.doi.org/10.5194/angeo-20-835-2002.
Анотація:
Abstract. An ozone mini-hole is a synoptic-scale area of strongly reduced column total ozone, which undergoes a growth-decay cycle in association with baroclinic weather systems. The tracks of mini-hole events recorded during the TOMS observation period over the Northern Hemisphere provide a database for building anomaly fields of various meteorological parameters, following each mini-hole center in a Lagrangian sense. The resulting fields provide, for the first time, a complete mean Lagrangian picture of the three-dimensional structure of typical ozone mini-holes in the Northern Hemisphere. Mini-holes are shown to be associated with anomalous warm anticyclonic flow in the upper troposphere and cold cyclonic anomalies in the middle stratosphere. Ascending air columns occur upstream and descent downstream of the mini-hole centers. Band-pass filtering is used to reveal the transient synoptic nature of mini-holes embedded within larger scale circulation anomalies. Significant correlations between ozone and Ertel’s potential vorticity on isentropes (IPV) both near the tropopause and in the middle stratosphere are shown and then utilized by reconstructing the Lagrangian analysis to follow local IPV anomalies instead of ozone minima. By using IPV as a proxy for ozone, the geopotential anomaly dipolar structure in the vertical characteristic of mini-holes is shown to result from a superposition of two largely independent dynamical components, stratospheric and tropospheric, typically operating on different time scales. Hence, ozone mini-holes may be viewed primarily as phenomena of coincidence.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; synoptic-scale meteorology)
36
Pan, L.-L., F.-F. Jin, and M. Watanabe. "Dynamics of Synoptic Eddy and Low-Frequency Flow Interaction. Part III: Baroclinic Model Results." Journal of the Atmospheric Sciences 63, no. 7 (July 1, 2006): 1709–25. http://dx.doi.org/10.1175/jas3717.1.
Анотація:
Abstract In this three-part study, a linear closure has been developed for the synoptic eddy and low-frequency flow (SELF) interaction and demonstrated that internal dynamics plays an important role in generating the leading low-frequency modes in the extratropical circulation anomalies during cold seasons. In Part III, a new linearized primitive equation system is first derived for time-mean flow anomalies. The dynamical operator of the system includes a traditional part depending on the observed climatological mean state and an additional part from the SELF feedback closure utilizing the observed climatological properties of synoptic eddy activity. The latter part relates nonlocally all the anomalous eddy-forcing terms in equations of momentum, temperature, and surface pressure to the time-mean flow anomalies. Using the observational data, the closure was validated with reasonable success, and it was found that terms of the SELF feedback in the momentum and pressure equations tend to reinforce the low-frequency modes, whereas those in the thermodynamic equation tends to damp the temperature anomalies to make the leading modes equivalent barotropic. Through singular vector analysis of the linear dynamical operator, it is highlighted that the leading modes of the system resemble the observed patterns of the Arctic Oscillation, Antarctic Oscillation, and Pacific–North American pattern, in which the SELF feedback plays an essential role, consistent with the finding of the barotropic model study in Part II.
37
Li, Richard C. Y., Wen Zhou, and Tim Li. "Influences of the Pacific–Japan Teleconnection Pattern on Synoptic-Scale Variability in the Western North Pacific." Journal of Climate 27, no. 1 (January 1, 2014): 140–54. http://dx.doi.org/10.1175/jcli-d-13-00183.1.
Анотація:
Abstract This study investigates the influences of the Pacific–Japan (PJ) teleconnection pattern on synoptic-scale variability (SSV) in the western North Pacific (WNP). The PJ pattern exhibits salient intraseasonal variations, with a dominant peak at 10–50 days. During positive PJ phases, strengthened SSV is found in the WNP, with a much stronger and better organized synoptic wave train structure. Such a synoptic-scale wave train, however, is greatly weakened during negative PJ phases. Examination of the vertical profiles of the observational data suggests that environmental parameters are generally more (less) favorable for the growth of synoptic disturbances under positive (negative) PJ conditions. Observational results are further verified with an anomaly atmospheric general circulation model, which reveals faster (slower) growth of the synoptic-scale wave train when the environmental anomalies associated with positive (negative) PJ phases are incorporated into the summer mean state of the model. In addition, sensitivity experiments indicate that thermodynamic parameters of the planetary boundary layer (PBL) play a determining role in controlling the development of synoptic disturbances in the WNP. The increase (decrease) in background PBL moisture during positive (negative) PJ phases enhances (suppresses) perturbation moisture convergence and thus the convective heating associated with SSV, leading to strengthened (weakened) synoptic-scale activity in the WNP. Serving as potential seed disturbances for cyclogenesis, the strengthened (weakened) synoptic-scale activity may also contribute to the enhancement (suppression) in intraseasonal TC frequency during positive (negative) PJ phases.
38
Jin, F.-F. "Eddy-Induced Instability for Low-Frequency Variability." Journal of the Atmospheric Sciences 67, no. 6 (June 1, 2010): 1947–64. http://dx.doi.org/10.1175/2009jas3185.1.
Анотація:
Abstract Synoptic eddy–mean flow interaction has been recognized as one of the key sources for extratropical low-frequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and low-frequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability.
39
Dai, Panxi, and Ji Nie. "A Global Quasigeostrophic Diagnosis of Extratropical Extreme Precipitation." Journal of Climate 33, no. 22 (November 15, 2020): 9629–42. http://dx.doi.org/10.1175/jcli-d-20-0146.1.
Анотація:
AbstractThis paper presents a global picture of the dynamic processes and synoptic characteristics of extratropical extreme precipitation events (EPEs), defined as annual maximum daily precipitation averaged over 7.5° × 7.5° regional boxes. Based on the quasigeostrophic omega equation, extreme precipitation can be decomposed into components forced by large-scale adiabatic disturbances and amplified by diabatic heating feedback. The spatial distribution of the diabatic feedback parameter is largely controlled by atmospheric precipitable water and captured by a simple model. Most spatial heterogeneities of EPEs in the middle and high latitudes are due to the spatial variations of large-scale adiabatic forcing. The adiabatic component includes the processes of vorticity advection, in which the synoptic vorticity advection by background wind dominates; temperature advection, in which the total meridional temperature advection by synoptic wind dominates; and boundary forcing. The synoptic patterns of EPEs in all extratropical regions can be classified into six clusters using the self-organizing map method: two clusters in low latitudes and four clusters in middle and high latitudes. Synoptic disturbances are characterized by strong pressure anomalies throughout the troposphere over the coastal regions and oceans and feature upper-level shortwave disturbances and a large westward tilt with height over land. Synoptic configurations favor moisture transport from ocean to land over coastal regions.
40
Warner, Michael D., Clifford F. Mass, and Eric P. Salathé. "Wintertime Extreme Precipitation Events along the Pacific Northwest Coast: Climatology and Synoptic Evolution." Monthly Weather Review 140, no. 7 (July 1, 2012): 2021–43. http://dx.doi.org/10.1175/mwr-d-11-00197.1.
Анотація:
Abstract Extreme precipitation events impact the Pacific Northwest during winter months, causing flooding, landslides, extensive property damage, and loss of life. Outstanding questions about such events include whether there are a range of associated synoptic evolutions, whether such evolutions vary along the coast, and the associated rainfall duration and variability. To answer these questions, this study uses 60 years of National Climatic Data Center (NCDC) daily precipitation observations to identify the top 50 events in two-day precipitation at six coastal stations from northern California to northwest Washington. NCEP–NCAR reanalysis data were used to construct synoptic composite evolutions of these events for each coastal location. Most regional flooding events are associated with precipitation periods of 24 h or less, and two-day precipitation totals identify nearly all major events. Precipitation areas of major events are generally narrow, roughly 200 km in width, and most are associated with atmospheric rivers. Composite evolutions indicate negative anomalies in sea level pressure and upper-level height in the central Pacific, high pressure anomalies over the southwest United States, large positive 850-hPa temperature anomalies along the coast and offshore, and enhanced precipitable water and integrated water vapor fluxes over southwest to northeast swaths. A small subset of extreme precipitation events over the southern portion of the domain is associated with a very different synoptic evolution: a sharp trough in northwesterly flow and post-cold-frontal convection. High precipitable water values are more frequent during the summer, but are not associated with heavy precipitation due to upper-level ridging over the eastern Pacific and weak onshore flow that limit upward vertical velocities.
41
Bowley, Kevin A., Eyad H. Atallah, and John R. Gyakum. "Synoptic-Scale Zonal Available Potential Energy Increases in the Northern Hemisphere." Journal of the Atmospheric Sciences 75, no. 7 (July 1, 2018): 2385–403. http://dx.doi.org/10.1175/jas-d-17-0292.1.
Анотація:
Abstract Available potential energy (APE), a measure of the energy available for conversion to kinetic energy, has been previously applied to examine changes in baroclinic instability and seasonal changes in the general circulation. Here, pathways in which the troposphere can build the reservoir of zonal available potential energy AZ on synoptic (3–10 day) time scales are explored. A climatology of AZ and its generation GZ and conversion terms are calculated from the National Centers for Environmental Prediction–Department of Energy Reanalysis 2 dataset from 1979 to 2011 for 20°–85°N. A standardized anomaly-based identification technique identifies 183 AZ buildup events, which are grouped into two event types based upon their final AZ standardized anomaly (σ) value: 1) buildup anomalous (BA) events, which exceed 1.5σ, and 2) buildup neutral (BN) events, which do not exceed 1.5σ. Increases in GZ and reductions in baroclinic conversion CA, source and sink terms for AZ, are shown to equally contribute toward increasing AZ in most seasons. A synoptic analysis of composited mass fields for winter BA events (n = 18 events) and winter BN events (n = 28 events) is performed to identify contributions to anomalously low CA and high GZ. A process of high-latitude cooling near 160°E–120°W is found for both composite event types. The cooling processes are characterized by a period of poleward moisture flux and ascent followed by an isolation of the Arctic from the midlatitude flow, resulting in enhanced GZ. Negative anomalies in CA are also diagnosed, which generally occur in regions with northerly dynamic tropopause wind anomalies and neutral to positive thickness anomalies.
42
Poan, D. Emmanuel, Romain Roehrig, Fleur Couvreux, and Jean-Philippe Lafore. "West African Monsoon Intraseasonal Variability: A Precipitable Water Perspective." Journal of the Atmospheric Sciences 70, no. 4 (April 1, 2013): 1035–52. http://dx.doi.org/10.1175/jas-d-12-087.1.
Анотація:
Abstract West African monsoon intraseasonal variability has important implications for food security and drought early warnings. In the present study, intraseasonal variability over the Sahel is assessed from the perspective of precipitable water, as provided by model reanalyses and GPS measurements. In the eastern Sahel, precipitable water variability is dominated by time scales longer than 10 days, whereas synoptic scales dominate in the western Sahel, especially because of African easterly waves (AEWs). The present work then focuses on the moisture footprint of AEWs along the northern side of the African easterly jet, as detected and analyzed directly from the main synoptic disturbances associated with precipitable water. Composite wet and dry precipitable water anomalies within AEWs propagate westward with a 5–6-day period. Their robustness, consistency, and spatial footprint, as well as their significant modulation of the convective activity, imply potential skill for short- to medium-range forecasts of wet and dry events over the Sahel. A composite moisture budget points out the key processes involved in the evolution of moisture anomalies. Advection processes are shown to be dominant during their life cycle. A linear adiabatic analysis of the propagation and growth of AEW precipitable water anomalies captures the main observed properties well, even though a key role of diabatic processes such as rain evaporation is needed to fully understand the life cycle of such precipitable water anomalies, especially their growth over the continent.
43
Yang, Qiu, and Andrew J. Majda. "Upscale Impact of Mesoscale Disturbances of Tropical Convection on Synoptic-Scale Equatorial Waves in Two-Dimensional Flows." Journal of the Atmospheric Sciences 74, no. 9 (September 1, 2017): 3099–120. http://dx.doi.org/10.1175/jas-d-17-0068.1.
Анотація:
Abstract Superclusters on the synoptic scale containing mesoscale systems are frequently organized by convectively coupled equatorial waves (CCEWs). Present-day global models struggle to simulate multiscale tropical convection, and the upscale effects of mesoscale systems are not well understood. A simple two-dimensional multiscale model with prescribed two-scale heating and eddy transfer of momentum and temperature drives the synoptic-scale circulation, successfully reproduces key features of flow fields with a front-to-rear tilt, and compares well with results from a cloud-resolving model (CRM). In the scenario with an elevated upright mean heating, the tilted vertical structure of synoptic-scale circulation is still induced by the upscale impact of mesoscale disturbances. In a faster propagation scenario, the upscale impact becomes less important as a result of competing effects of eddy transfer of momentum and temperature, while the synoptic-scale circulation response to mean heating dominates, in agreement with cloud-resolving models. In the unrealistic scenario with upward–westward-tilted mesoscale heating, positive potential temperature anomalies are induced in the leading edge, which will suppress shallow convection in a moist environment.
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Batstone, Crispian P., Adrian J. Matthews, and David P. Stevens. "Coupled Ocean–Atmosphere Interactions between the Madden–Julian Oscillation and Synoptic-Scale Variability over the Warm Pool." Journal of Climate 18, no. 12 (June 15, 2005): 2004–20. http://dx.doi.org/10.1175/jcli3398.1.
Анотація:
Abstract A principal component analysis of the combined fields of sea surface temperature (SST) and surface zonal and meridional wind reveals that the dominant mode of intraseasonal (30 to 70 day) covariability during northern winter in the tropical Eastern Hemisphere is that of the Madden–Julian oscillation (MJO). Regression calculations show that the submonthly (30-day high-pass filtered) surface wind variability is significantly modulated during the MJO. Regions of increased (decreased) submonthly surface wind variability propagate eastward, approximately in phase with the intraseasonal surface westerly (easterly) anomalies of the MJO. Because of the dependence of the surface latent heat flux on the magnitude of the total wind speed, this systematic modulation of the submonthly surface wind variability produces a significant component in the intraseasonal latent heat flux anomalies, which partially cancels the latent heat flux anomalies due to the slowly varying intraseasonal wind anomalies, particularly south of 10°S. A method is derived that demodulates the submonthly surface wind variability from the slowly varying intraseasonal wind anomalies. This method is applied to the wind forcing fields of a one-dimensional ocean model. The model response to this modified forcing produces larger intraseasonal SST anomalies than when the model is forced with the observed forcing over large areas of the southwest Pacific Ocean and southeast Indian Ocean during both phases of the MJO. This result has implications for accurate coupled modeling of the MJO. A similar calculation is applied to the surface shortwave flux, but intraseasonal modulation of submonthly surface shortwave flux variability does not appear to be important to the dynamics of the MJO.
45
Görgen, K., J. Bareiss, A. Helbig, A. Rinke, and K. Dethloff. "An observational and modelling analysis of Laptev Sea (Arctic Ocean) ice variations during summer." Annals of Glaciology 33 (2001): 533–38. http://dx.doi.org/10.3189/172756401781818699.
Анотація:
AbstractIn this study we investigate the relationship of the atmospheric circulation and the sea-ice distribution in the Laptev Sea, Arctic Ocean, in the summers 1979−96. Sea-ice data from passive-microwave radiometers, global atmospheric data analyses, cyclone statistics and simulations of the regional climate model HIRHAM4 were analyzed to find out if periods of reduced or increased sea-ice concentrations are linked to synoptic patterns (circulation anomalies, cyclone activity). A canonical correlation analysis between Arctic sea-level pressure and sea-ice concentration anomalies confirms large-scale relationships among these variables. We did not find a simple relationship between sea-ice area anomalies and cyclone activity in the Laptev Sea area
46
Berkovic, Sigalit. "Wind regimes and their relation to synoptic variables using self-organizing maps." Advances in Science and Research 15 (January 25, 2018): 1–9. http://dx.doi.org/10.5194/asr-15-1-2018.
Анотація:
Abstract. This study exemplifies the ability of the self-organizing maps (SOM) method to directly define well known wind regimes over Israel during the entire year, except summer period, at 12:00 UTC. This procedure may be applied at other hours and is highly relevant to future automatic climatological analysis and applications. The investigation is performed by analysing surface wind measurements from 53 Israel Meteorological Service stations. The relation between the synoptic variables and the wind regimes is revealed from the averages of ECMWF ERA-INTERIM reanalysis variables for each SOM wind regime. The inspection of wind regimes and their average geopotential anomalies has shown that wind regimes relate to the gradient of the pressure anomalies, rather than to the specific isobars pattern. Two main wind regimes – strong western and the strong eastern or northern – are well known over this region. The frequencies of the regimes according to seasons is verified. Strong eastern regimes are dominant during winter, while strong western regimes are frequent in all seasons.
47
Xiao, Dong, and Hongli Ren. "Interdecadal changes in synoptic transient eddy activity over the Northeast Pacific and their role in tropospheric Arctic amplification." Climate Dynamics 57, no. 3-4 (April 3, 2021): 993–1008. http://dx.doi.org/10.1007/s00382-021-05752-6.
Анотація:
AbstractArctic amplification refers to the greater surface warming of the Arctic than of other regions during recent decades. A similar phenomenon occurs in the troposphere and is termed “tropospheric Arctic amplification” (TAA). The poleward eddy heat flux and eddy moisture flux are critical to Arctic warming. In this study, we investigate the synoptic transient eddy activity over the North Pacific associated with TAA and its relationship with the subtropical jet stream, and propose the following mechanism. A poleward shift of the subtropical jet axis results in anomalies of the meridional gradient of zonal wind over the North Pacific, which drive a meridional dipole pattern of synoptic transient wave intensity over the North Pacific, referred to as the North Pacific Synoptic Transient wave intensity Dipole (NPSTD). The NPSTD index underwent an interdecadal shift in the late 1990s accompanying that of the subtropical jet stream. During the positive phase of the NPSTD index, synoptic eddy heat flux transports more heat to the Arctic Circle, and the eddy heat flux diverges, increasing Arctic temperature. This mechanism highlights the need to consider synoptic transient eddy activity over the North Pacific as the link between the mean state of the North Pacific subtropical upper jet and TAA.
48
Nie, Yu, Hong-Li Ren, and Yang Zhang. "The Role of Extratropical Air–Sea Interaction in the Autumn Subseasonal Variability of the North Atlantic Oscillation." Journal of Climate 32, no. 22 (October 16, 2019): 7697–712. http://dx.doi.org/10.1175/jcli-d-19-0060.1.
Анотація:
Abstract Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.
49
Bowley, Kevin A., John R. Gyakum, and Eyad H. Atallah. "The Role of Dynamic Tropopause Rossby Wave Breaking for Synoptic-Scale Buildups in Northern Hemisphere Zonal Available Potential Energy." Monthly Weather Review 147, no. 2 (January 9, 2019): 433–55. http://dx.doi.org/10.1175/mwr-d-18-0143.1.
Анотація:
Abstract Zonal available potential energy AZ measures the magnitude of meridional temperature gradients and static stability of a domain. Here, the role of Northern Hemisphere dynamic tropopause (2.0-PVU surface) Rossby wave breaking (RWB) in supporting an environment facilitating buildups of AZ on synoptic time scales (3–10 days) is examined. RWB occurs when the phase speed of a Rossby wave slows to the advective speed of the atmosphere, resulting in a cyclonic or anticyclonic RWB event (CWB and AWB, respectively). These events have robust dynamic and thermodynamic feedbacks through the depth of the troposphere that can modulate AZ. Significant synoptic-scale buildups in AZ and RWB events are identified from the National Centers for Environmental Prediction Reanalysis-2 dataset from 1979 to 2011 for 20°–85°N. Anomalies in AWB and CWB are assessed seasonally for buildup periods of AZ. Positive anomalies in AWB and negative anomalies in CWB are found for most AZ buildup periods in the North Pacific and North Atlantic basins and attributed to localized poleward shifts in the jet stream. Less frequent west–east dipoles in wave breaking anomalies for each basin are attributed to elongated and contracted regional jet exit regions. Finally, an analysis of long-duration AWB events for winter AZ buildup periods to an anomalously high AZ state is performed using a quasi-Lagrangian grid-shifting technique. North Pacific AWB events are shown to diabatically intensify the North Pacific jet exit region (increasing Northern Hemisphere AZ) through latent heating equatorward of the jet exit and radiative and evaporative cooling poleward of the jet exit.
50
Pohl, Benjamin, Bastien Dieppois, Julien Crétat, Damian Lawler, and Mathieu Rouault. "From Synoptic to Interdecadal Variability in Southern African Rainfall: Toward a Unified View across Time Scales." Journal of Climate 31, no. 15 (August 2018): 5845–72. http://dx.doi.org/10.1175/jcli-d-17-0405.1.
Анотація:
During the austral summer season (November–February), southern African rainfall, south of 20°S, has been shown to vary over a range of time scales, from synoptic variability (3–7 days, mostly tropical temperate troughs) to interannual variability (2–8 years, reflecting the regional effects of El Niño–Southern Oscillation). There is also evidence for variability at quasi-decadal (8–13 years) and interdecadal (15–28 years) time scales, linked to the interdecadal Pacific oscillation and the Pacific decadal oscillation, respectively. This study aims to provide an overview of these ranges of variability and their influence on regional climate and large-scale atmospheric convection and quantify uncertainties associated with each time scale. We do this by applying k-means clustering onto long-term (1901–2011) daily outgoing longwave radiation anomalies derived from the 56 individual members of the Twentieth Century Reanalysis. Eight large-scale convective regimes are identified. Results show that 1) the seasonal occurrence of the regimes significantly varies at the low-frequency time scales mentioned above; 2) these modulations account for a significant fraction of seasonal rainfall variability over the region; 3) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and 4) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The short-lived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger.