Summer Circulation Patterns Research Articles

Extreme large-scale atmospheric circulation associated with the “21·7” Henan flood

From July 19 to 21, 2021, Henan, a province in northern China (NC), was affected by severe flooding (referred to hereafter as “21·7”) caused by a prolonged record-breaking extreme precipitation (EP) event. Understanding the extremes of the large-scale circulation pattern during “21·7” is essential for predicting EP events and preventing future disasters. In this study, daily atmospheric large-scale circulations over NC in the summers from 1979 to 2021 were investigated using the circulation classification method of an obliquely rotated principal component analysis in T-mode (PCT). The geopotential height at 500 hPa and 925 hPa were applied successively in classification. Among the nine summer circulation patterns at 500 hPa were found, the 3 days of “21·7” belonged to the Type 8 pattern, which had the second highest probability of EP days among all patterns. It was characterized by a southeasterly wind toward North China Plain driven by a dipole geopotential height field, with the West Pacific subtropical high (WPSH) extending far north to 30°N and low to the south near NC. Tropical cyclones (TCs) occurred on 72.5% of EP days, in which larger amounts of precipitation and a longer duration of EP days were found along the mountains in NC, as compared with other patterns. The distribution of EP events under this pattern was mainly influenced by the location of the low at 925 hPa in the dipole. The subtype 8−3 circulation, with lows in the east of Taiwan Island, included “21·7” and accounted for 1.6% of all summer days. Typhoon In-fa, together with the WPSH, gave rise to intense column integrated moisture flux convergence (IMFC) via the southeasterly wind to Henan, which occurred continuously during the 3 days of “21·7”, resulting in the largest (second largest) mean IMFC among 3 consecutive EP days under type 8 (all types) during the past 43 summers in NC. Further analysis revealed that the large-scale dynamic process could not completely explain the record-breaking EP during “21·7”, indicating possible contributions of other dynamic processed related to meso-scale convective storms.

Response Process of Coastal Hypoxia to a Passing Typhoon in the East China Sea

Details of the development of coastal hypoxia in response to the passage of Typhoon Bavi (2020) in the East China Sea were reconstructed by numerical modeling using a three-dimensional coupled physical–biogeochemical model. The model was validated via repeated surveys along a transect across a submerged river valley off the Changjiang Estuary before and after the passage of Typhoon Bavi. Before Bavi’s arrival, survey data indicated that the coastal hypoxia had formed off the Changjiang Estuary. However, the hypoxia was not eliminated but instead migrated and aggravated along the observed transect after the typhoon passage. This phenomenon cannot be attributed to the typhoon-induced mixing. Simulation results reveal that the observed development and spatial migration of hypoxia was mainly controlled by typhoon-induced oceanic advection. Results show that Bavi stimulated a regional quasi-barotropic cyclonic loop circulation and coastal downwelling reversing general summer circulation patterns. The onshore transport of the warmer shelf water and subsequent downwelling resulted in a warming of coastal water. The southward coastal current and downwelling induced by the typhoon also led to a migration of the hypoxic zone. Meanwhile, a massive transport of resuspended planktonic detritus from the steep inner shelf and the shallow Changjiang bank toward the submerged river valley occurred. This study reveals that the typhoon-driven currents can play an important role in the development of hypoxia and redistribution of deposited organic matter in coastal shelf seas, which may have both short- and long-term effects on the regional marine biogeochemical environment.

A possible linkage of Eurasian heat wave and East Asian heavy rainfall in Relation to the Rapid Arctic warming

In June–July 2020 two remarkable weather events occurred in northern Eurasia. One is a severe heat wave that produced a record-breaking temperature of 38 °C at Verkhoyansk, eastern Siberia on 20 June. The other one is heavy rainfall events observed in East Asia (southern China and southwestern Japan) in early July, causing severe floods that brought about considerable damage to infrastructure and the economy, as well as the loss of human lives. Despite the accumulated evidence of stronger and more extreme heat waves and heavy rainfall as a result of global warming, little is known about the linkage between these phenomena. Given that the Arctic is warming twice as fast as the global mean, Arctic warming might be enhancing the increase of heavy rainfall events in East Asia.Here, we investigated the relationship between the Siberian heat wave and the East Asian heavy rainfall that occurred summer in 2020. An empirical orthogonal function (EOF) analysis applied to atmospheric reanalysis data of 1958–2020 period captures dominant summer circulation patterns reflecting atmospheric internal variability and externally forced anomalies. On the basis of these EOF patterns, operational forecasts of summer 2020 using the global model from the Japan Meteorological Agency (JMA) and a global climate model experiment based on 2-K warming future projection are utilized to examine roles of the internal variability and external forcing, respectively. Consistent results between them reveal that development of the blocking high over eastern Siberia has certain impacts on rainfall anomalies over East Asia. By a statistical technique applied to the ensemble forecast data, prediction of the East Asian precipitation is improved by 10–20% of its amplitude.Our research demonstrates possibility that East Asian rainfall is being enhanced by high-latitude atmospheric circulations due to the Arctic warming even in the current climate in which the tropical warming is not yet severe. Suggestions are given that continued Arctic warming and a future increase of tropical warming will lead to increases of the frequency and severity of heavy rainfall events in East Asia.

Three-dimensional numerical simulation of circulation and vertical temperature structure during summer in Cockburn Sound

Cockburn Sound is an important embayment close to a metropolitan area in Western Australia. In this paper, we study summer circulation patterns, volume balance, and mixing processes in Cockburn Sound and adjacent shelf using in-situ oceanographic surveys made during the 2016–2017 austral summer and numerical simulations using the Finite Volume Coastal Ocean Model (FVCOM). The model results revealed that the local wind variability played a primary role in regulating the water volume in the embayment, with decreasing volume associated with south-southeasterly winds. In Cockburn Sound, a diel stratification cycle during summer was caused by accumulation of heat in the surface layer due to net air–sea heat flux (mostly solar radiation) during the daytime, and wind mixing due to sea-breezes in the afternoon and convective mixing due to nightly heat losses. The regularity of well-mixed nature can be interrupted with intermittent periods of vertical stratification. During March 2017, very strong vertical stratification that coincided with low oxygen conditions was observed in the near bottom water. By analyzing the model results, we show that high air temperature, weak winds, and the southward compensating colder inflow at depth from the coastal waters to the north caused the sustained temperature difference between surface and bottom layers during March 2017. We conclude that this thermal stratification restricted ventilation of the bottom water, explaining the observed low oxygen content.Plain language SummaryLow oxygen conditions were observed in the bottom water of Cockburn Sound (a semi-enclosed coastal embayment) during the austral summer of 2017. This was unexpected in view of the shallow water depth and strong local wind conditions which should ensure that the water-column is well ventilated. However, modeled temperature profiles showed that thermal stratification lasting for up to a week can sometimes occur during late summer, potentially restricting oxygen ventilation. The stratification events are caused by a combination of high air temperature (especially when it remained high at night), low wind speed and inflow of cooler sub-surface water. Global warming is expected to enhance thermal stratification, reduce oxygen solubility and accelerate biological oxygen demand increasing the future risk of hypoxia in the area.

Evaluating boreal summer circulation patterns of CMIP6 climate models over the Asian region

Our confidence in future climate projection depends on the ability of climate models to simulate the current climate, and model performance in simulating atmospheric circulation affects its ability of simulating extreme events. In this study, the self-organizing map (SOM) method is used to evaluate the frequency, persistence, and transition characteristics of models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) for different ensembles of daily 500 hPa geopotential height (Z500) in Asia, and then all ensembles are ranked according to a comprehensive ranking metric (MR). Our results show that the SOM method is a powerful tool for assessing the daily-scale circulation simulation skills in Asia, and the results will not be significantly affected by different map sizes. Positive associations between each two of the performance in frequency, persistence and transition were found, indicating that a good ensemble of simulation for one metric is good for the others. The r10i1p1f1 ensemble of CanESM5 best simulates Z500 in Asia comprehensively, and it is also the best of simulating frequency characteristics. The MR simulation of the highest 10 ensembles for the Western North Pacific Subtropical High (WNPSH) and the South Asia High (SAH) are far better than those of the lowest 10. Such differences may lead to errors in the simulation of extreme events. This study will help future studies in the choice of ensembles with better circulation simulation skills to improve the credibility of their conclusions.

Investigating the effect of wearing glasses on the human eyes' temperature distribution in different ambient conditions.

The present study addressed the investigation of the effect of wearing glasses on the human eyes' temperature distribution in different ambient conditions. Besides, the effect of variations of ambient temperature, convection coefficient of ambient air, blinking, and body's temperature on the human eye's temperature distribution was investigated. Three scenarios (without glasses, wearing medical glasses, and wearing sunglasses) have been considered. The weather information for summer and winter for Divandarreh, Kurdistan province, Iran, has been used as the inputs. The sunlight intensity reaching the eye in three scenarios for both winter and summer was measured experimentally. In scenario 1 (without glasses), for the maximum radiation intensity, the Corneal temperature increased by 5°C, which can cause cataracts in the long term. The results show that by wearing sunglasses in winter, the Corneal temperature reduced by 4°C that may lead to blurred and diplopia visions. It was observed that by increasing the temperature difference between the sides of the Anterior chamber, the Aqueous Humor (AH) circulation speed increases. Also, it was found that the AH's circulation pattern in summer is clockwise, while in winter, it is counterclockwise. The results highlighted the significant effect of the convection coefficient of air ambient on the Corneal temperature. The results show that depending on the ambient temperature, blinking has a significant effect on eye temperature. Moreover, in the summer and scenario1, the difference in maximum temperature of the Cornea between normal and fever conditions is 1.01°C. On the other hand, the difference in maximum temperature of the Cornea between normal and hypothermia conditions is 1.51°C.

Coastal circulation and water transport properties of the Red Sea Project lagoon

The Red Sea Project (RSP) is based on a coastal lagoon with over 90 pristine islands. The project intends to transform the Red Sea coast into a world-class tourist destination. To better understand the regional dynamics and water exchange scenarios in the lagoon, a high-resolution numerical model is implemented. The general and tidal circulation dynamics are then investigated with a particular focus on the response of the lagoon to strong wind jets. Significant variations in winter and summer circulation patterns are identified. The tidal amplitude inside the lagoon is greater than that outside, with strong tidal currents passing over its surrounding coral reef banks. The lagoon rapidly responds to the strong easterly wind jets that occur mainly in winter; it develops a reverse flow at greater depths, and the coastal water elevation is instantly affected. Lagrangian particle simulations are conducted to study the residence time of water in the lagoon. The results suggest that water renewal is slow in winter. Analysis of the Lagrangian coherent structures (LCS) reveals that water renewal is largely linked to the circulation patterns in the lagoon. In winter, the water becomes restricted in the central lagoon with only moderate exchange, whereas in summer, more circulation is observed with a higher degree of interaction between the central lagoon and external water. The results of LCS also highlight the tidal contribution to stirring and mixing while identifying the hotspots of the phenomenon. Our analysis demonstrates an effective approach for studying regional water mixing and connectivity, which could support coastal management in data-limited regions.

A Later Onset of the Rainy Season in California

AbstractCalifornian hydroclimate is strongly seasonal and prone to severe water shortages. Recent changes in climate trends have induced shifts in seasonality, thus exacerbating droughts, wildfires, and adverse water shortage effects on the environment and economy. Previous studies have examined the timing of the seasonal cycle shifts mainly as temperature driven earlier onset of the spring season. In this paper, we address quantitative changes in the onset, amounts, and termination of the precipitation season over the past 6 decades, as well as the large‐scale atmospheric circulation underpinning the seasonal cycle changes. We discover that the onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California. The progressively later onset of the rainy season is shown to be related to the summer circulation pattern extending into autumn across the North Pacific, in particular, a delay in the strengthening of the Aleutian Low and later southward displacement of the North Pacific westerlies.

Seasonal predictability of primary East Asian summer circulation patterns by three operational climate prediction models

AbstractSeasonal predictability of the Primary East‐Asian Summer Circulation Patterns (PEASCPs), including the Western Pacific Subtropical High (WPSH), South Asian High (SAH), anomalous Philippine Sea AntiCyclone (PSAC) and East Asian Summer Monsoon (EASM), are investigated by using the hindcasts from the three operational climate prediction models, including BCC_CSM1.1(m), NCEP CFSv2 and ECMWF System 4. We show that prediction skills of the indices for representing these PEASCPs are sensitive to the initial calendar month of model prediction, and the ensemble mean of the three models provides relatively higher and more stable skills than forecasts from an individual model. In general, the indices of intensity and area have high prediction skills while the position indices have relatively low skills. Specifically, the skills of the WPSH intensity, area, SAH centre intensity, PSAC and EASM are higher, while the skills of the WPSH western boundary and SAH centre latitude are lower, and the skills of WPSH ridge line and SAH centre longitude are the lowest. Further analysis shows that the El Niño/Southern Oscillation (ENSO) has a large contribution to these prediction skills, and these patterns of atmospheric circulation anomalies in response to ENSO can be well captured by models, which is the major predictability source of the skills.

The Role of Synoptic Cyclones for the Formation of Arctic Summer Circulation Patterns as Clustered by Self-Organizing Maps

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.

Mean annual and seasonal circulation patterns and long-term variability of currents in the Baltic Sea

Abstract A hindcast modelling study conducted with the PM3D hydrodynamic model revealed some new effects. The study involved determination of seasonal characteristics of surface and subsurface currents. In addition, long-term relationships between the Baltic Sea currents and the North Atlantic Oscillation were explored. Analysis of the Baltic Sea current data spanning five decades has shown new, previously unknown, current characteristics, including higher stability (0.4–0.7) of subsurface current eddies relative to the stability of surface currents (0.2–0.5). Circulation anomaly patterns relative to the annual mean in spring and autumn present opposite flow directions, whereas winter and summer circulation patterns were similar in this respect. The spring circulation anomalies were dominated by the low sea-level outflow from the Baltic Sea, whereas the autumn circulation showed a strong inflow and infilling of the sea. In the analysed half-century (1958–2007), velocities of surface and bottom currents showed an increasing trend (0.9 and 0.1 cm s−1 per 50 years, respectively) and were related to the dominance of the positive NAO index phase. Current velocity fluctuations correlated positively with the NAO and BSI indices (correlation coefficients of 0.66 and 0.80, respectively).

The summertime circulation of the Bohai Sea simulated from a high-resolution wave-tide-circulation coupled model

The Bohai Sea is a shallow semi-enclosed inner sea with an average depth of 18 m and is located at the west of the northern Yellow Sea. The climatological circulation pattern in summer of the Bohai Sea is studied by using a wave-tide-circulation coupled model. The simulated temperature and the circulation agree with the observation well. The result shows that the circulation pattern of the Bohai Sea is jointly influenced by the tidal residual current, wind and baroclinic current. There exists an obvious density current along the temperature front from the west part of the Liaodong Bay to the offshore area of the Huanghe Estuary. In the Liaodong Bay there exists a clockwise gyre in the area north to the 40°N. While in the area south to the 40°N the circulation shows a two-gyre structure, the flow from the offshore area of the Huanghe Estuary to the Liaodong Bay splits into two branches in the area between 39°N and 40°N. The west branch turns into north-west and forms an anti-clockwise gyre with the south-westward density current off the west of the Liaodong Bay. The east branch turns to the east and forms a clockwise gyre with the flow along the east coast of the Liaodong Bay. The forming mechanism of the circulation is also discussed in this paper.

Circulation mechanisms responsible for wet or dry summers over Zimbabwe

Climate change has resulted in increased rainfall variability over many parts of the world including Southern Africa. As such, droughts and floods have become a frequent phenomenon in Zimbabwe and have potential to intensify socio-economic stressors. This study examined possible forcing factors behind the occurrence of extreme summer events using re-analysis datasets. Composite analysis and correlation methods were used to identify circulation mechanisms and their strength in determining rainfall patterns in Zimbabwe. Predominantly northerly airflow in the lower troposphere was found to favor wet while southerly airflow favors dry seasons. Negative geopotential anomalies (minimum of −20 hPa) to the west of Zimbabwe in the middle levels characterize wet summers which swing to positive anomalies (+24 hPa) during dry summers. Positive SST anomalies (maximum of 0.4) exist to the southwest of Madagascar extending to the western shore on the Angola-Namibian border characterize wet summers which swing to negative anomalies (−0.2 oC) during dry summer seasons. SST anomalies in the South western Indian and South eastern Atlantic oceans are crucial in the determination of the strength of both the South Indian and Atlantic Ocean high pressure systems which in turn control moisture advection and convergence into Zimbabwe during the summer period. If these SST anomalies at lag times of about 3 months can be used to predict the incoming summer circulation patterns then the accuracy of summer seasonal outlook forecasts can be improved. Studying the mechanisms behind drought and flood occurrence is important to the country which is in the process of downscaling regional prediction products to improve the accuracy of seasonal forecasts. These findings are useful in crafting relevant measures to maximize the benefits and minimize the risks of extreme rainfall events.

Summer atmospheric circulation anomalies over the Arctic Ocean and their influences on September sea ice extent: A cautionary tale

AbstractNumerous studies have addressed links between summer atmospheric circulation patterns and interannual variability and the downward trend in total September Arctic sea ice extent. In general, low extent is favored when the preceding summer is characterized by positive sea level pressure (SLP) anomalies over the central Arctic Ocean north of Alaska. High extent is favored when low pressure dominates. If such atmospheric patterns could be predicted several months out, these links provide an avenue for improved seasonal predictability of total September extent. We analyze detrended September extent time series (1979–2015), atmospheric reanalysis fields, ice age and motion, and Atmospheric Infrared Sounder data, to show that while there is merit to this summer circulation framework, it has limitations. Large departures in total September extent relative to the trend line are preceded by a wide range of summer circulation patterns. While patterns for the four years with the largest positive departures in September extent have below average SLP over the central Arctic Ocean, they differ markedly in the magnitude and location of pressure and air temperature anomalies. Differences in circulation for the four years with the largest negative departures are equally prominent. Circulation anomalies preceding Septembers with ice extent close to the trend also have a wide range of patterns. In turn, years (such as 2013 and 2014) with almost identical total September extent were preceded by very different summer circulation patterns. September ice conditions can also be strongly shaped by events as far back as the previous winter or spring.

Summer Arctic dipole wind pattern affects the winter Siberian High

This study investigates the relationship between the summer [June–July–August (JJA)] Arctic dipole wind pattern and the following winter [December–January–February (DJF)] Siberian High. It is found that the summer Arctic dipole wind pattern is not confined only to the Arctic region; it spans the large domain north of 20°N. The negative phase of this wind pattern depicts an anomalous anticyclone over the Arctic Ocean and its marginal seas, except for the Barents-Kara seas where an anomalous cyclone is dominant. This wind pattern is significantly correlated with the strength of the Siberian High during the following winter and with the frequency of extreme cold events over East Asia during the winters of 1979–2014. The relationship of this wind pattern with the winter Siberian High has strengthened over the past decades, particularly since the late 1980s. The more robust relationship coincides with significant changes in the winter atmospheric circulation and frequent occurrences of the negative phase of this wind pattern, which dynamically contributes to low September sea ice extent. The present study's results suggest that autumn Arctic sea ice provides a link between this wind pattern and climate variability over East Asia during the following winter. Results of simulation experiments suggest that (1) autumn sea ice loss favors the occurrence of a stronger East Asian winter monsoon; (2) the summer Arctic dipole wind pattern modulates winter atmospheric responses to sea ice loss, and the negative phase of this wind pattern enhances the negative feedback of Arctic sea ice loss on winter atmospheric variability over Eurasia and North America. These simulation experiments also imply that complex and varying summer circulation patterns obscure linkages between sea ice loss and large-scale circulation responses over Eurasia. Isolation of the summer Arctic dipole wind pattern, however, provides a potential precursor for the seasonal prediction of winter surface air temperature in a populous region of the world.

Formation of the modern current system in the East China Sea since the early Holocene and its relationship with sea level and the monsoon system

The Okinawa Trough is a natural laboratory for the study of air-sea interaction and paleoenvironmental change. It has been demonstrated that present offshore export of particles in the bottom nepheloid layer occur primarily with downwelling from the northeast winter monsoon, which is inhibited by a transverse circulation pattern in summer. This current system was very different during the Last Glacial Maximum owing to low sea level (-120 m) and exposure of a large shelf area. We collected sediment core Oki01 from the middle Okinawa Trough during 2012 using R/V Kexue No. 1 to elucidate the timing and cause of the current system transition in the East China Sea. Clay mineral, dry density, and elemental (Ti, Ca) composition of core Oki01 was analyzed. The results indicate that clay minerals derived mainly from the Huanghe (Yellow) and the Changjiang (Yangtze) Rivers during 16.0-11.6 ka, and the modern current system in the East China Sea formed beginning in the early Holocene. Therefore, mixing of East China Sea continental shelf, Changjiang River and partially Taiwan Island sediment are the major contributors. The decrease of log(Ti/Ca) and alternating provenance since the early Holocene indicate less sediment from the East China in summer because of resistance of the modern current system, i.e., a "water barrier" and upwelling. Conversely, sediment delivery persists in winter and log(Ti/Ca) indicates the winter monsoon signal since the early Holocene. Our evidence also suggests that sediment from Taiwan Island could be transported by the Kuroshio Current to the middle Okinawa Trough, where it mingles with winter monsoon-induced export of sediment from the Changjiang River and East China Sea continental shelf. Although the present research advances understanding of the evolutionary history of paleoenvironmental change in the Okinawa Trough, more sediment cores should be retrieved over wide areas to construct a larger scenario.

Evaluating the distribution of water resources in western Canada using synoptic climatology and selected teleconnections. Part 2: summer season

AbstractMinor changes to seasonal air temperature and precipitation can have a substantial impact on the availability of water resources within large watersheds. Two such watersheds, the north‐flowing Mackenzie and east‐flowing Saskatchewan Basins, have been identified as highly vulnerable to such changes and, therefore, selected for study as part of the Climatic Redistribution of western Canadian Water Resources project. This project aims to evaluate spatial and temporal changes to water resource distribution through the analysis of a suite of hydroclimatic and streamflow variables. As part of this analysis, dominant summer (May–October) circulation patterns at 500 hPa for 1950–2011 are identified using the method of self‐organizing maps. Surface climate variables associated with these patterns are then identified, including both daily air temperature and precipitation and seasonal Standardized Precipitation Evapotranspiration Index values. Statistical methods are applied to assess the relationships between dominant circulation patterns and the Southern Oscillation Index (SOI) and Pacific Decadal Oscillation (PDO). Results indicate that mid‐summer (July–August) is dominated by a split‐flow blocking pattern, resulting in cool (warm), wet (dry) conditions in the southern (northern) portion of the study area. By contrast, the shoulder season (May and October) is dominated by a trough of low pressure over the North Pacific Ocean. The frequency of weak split‐flow blocking is higher during positive SOI and negative PDO, whereas ridging over the western continent is more frequent during negative SOI and positive PDO. Results from this analysis increase our knowledge of processes, controlling the distribution of summer water resources in western Canada. Copyright © 2014 John Wiley & Sons, Ltd.

A numerical study of gyres, thermal fronts and seasonal circulation in austral semi-enclosed gulfs

This article analyses the results from a high resolution numerical model of the North Patagonian Gulfs (San Matías Gulf, SMG; Nuevo Gulf, NG; and San José Gulf, SJG), a region of the South Western Atlantic Shelf that has long been recognized for its high productivity and biodiversity. The aim of the study is to explore the physical processes that control the mean circulation and its seasonal variability with focus on the generation of recirculation features (gyres) and frontal structures. The numerical results showed that both tidal and wind forcing significantly contribute to delineate the frontal structures and the seasonal circulation in the North Patagonian Gulfs. The overall summer circulation pattern in SMG is dominated by two strong cyclonic subgyres in the northern and southern sectors while NG showed only one gulf-wide cyclonic gyre. The northern subgyre in SMG and the NG gyre are caused by the interaction of the tides and the evolving stratification driven by surface heat and freshwater fluxes. A series of sensitivity experiments showed that the formation and intensity of a summer zonal front in SMG is controlled by the wind-driven advection of cold waters from a homogenized pool generated by intense tidal mixing in the inner continental shelf (east of Valdés Península). From April to August, when winter erodes the stratification, the northern SMG subgyre and the NG gyre spin down and gradually shrink in size. At this time of the year, the western SMG and NG are occupied by an anticyclonic gyre driven by intense westerlies. In contrast, the mean circulation in SJG is dominated year-round by a pair of strong counter-rotating eddies produced by tidal rectification.

On the differences and climate impacts of early and late stratospheric polar vortex breakup

The stratospheric polar vortex breakup (SPVB) is an important phenomenon closely related to the seasonal transition of stratospheric circulation. In this paper, 62-year NCEP/NCAR reanalysis data were employed to investigate the distinction between early and late SPVB. The results showed that the anomalous circulation signals extending from the stratosphere to the troposphere were reversed before and after early SPVB, while the stratospheric signals were consistent before and after the onset of late SPVB. Arctic Oscillation (AO) evolution during the life cycle of SPVB also demonstrated that the negative AO signal can propagate downward after early SPVB. Such downward AO signals could be identified in both geopotential height and temperature anomalies. After the AO signal reached the lower troposphere, it influenced the Aleutian Low and Siberian High in the troposphere, leading to a weak winter monsoon and large-scale warming at mid latitudes in Asia. Compared to early SPVB, downward propagation was not evident in late SPVB. The high-latitude tropospheric circulation in the Northern Hemisphere was affected by early SPVB, causing it to enter a summer circulation pattern earlier than in late SPVB years.

A numerical study of the seasonal circulation in the Seto Inland Sea, Japan

The seasonal variation of water circulation in the Seto Inland Sea is investigated using a high resolution, three-dimensional numerical ocean model. The model results are assessed by comparison with long-term mean surface current and hydrographic data. The simulated model results are consistent with observations, showing a distinct summer and winter circulation patterns. In summer the sea water is highly stratified in basin regions, while it is well mixed near the straits due to strong tidal mixing there. During this period, a cold dome is formed in several basins, setting up stable cyclonic eddies. The cyclonic circulation associated with the cold dome develops from May and disappears in autumn when the surface cooling starts. The experiment without freshwater input shows that a basin-scale estuarine circulation coexists with cyclonic eddy in summer. The former becomes dominant in autumn circulation after the cold dome disappears. In winter the water is vertically well mixed, and the winter winds play a significant role in the circulation. The northwesterly winds induce upwind (downwind) currents over the deep (shallow) water, forming a “double-gyre pattern” in the Suo-Nada, two cyclonic eddies in Hiuchi-Nada, and anticyclonic circulation in Harima-Nada in vertically averaged current fields.