Summer Circulation Research Articles

Role of the South Asian High in the Onset Process of the Asian Summer Monsoon during Spring-to-Summer Transition

The evolution of the South Asian high (SAH) and its role in the onset process of the Asian summer monsoon (ASM) during the spring-to-summer transition are investigated by using the NCEP-DOE reanalysis II dataset, with a focus on climatology and interannual time scales. Our results show four sudden changes of the SAH in its Northwestward evolution from the Western Pacific to the South China Sea (SCS), the Indochina Peninsula and the South Asian plateaus, coincide with the ASM onset over the Bay of Bengal, the SCS, and the Indian summer monsoon region. The physical process for the mutual promotion between the SAH and ASM rainfall is revealed. Accompanying the SAH evolution, the upper-level Easterly wind along the Southern flank and the upper-level divergence associated with the SAH shift Northwestward accordingly. The upper-level Easterly wind coordinates with the lower-level Southwesterly wind, and forms the summer circulation structure in the ASM regions gradually. Besides, the upper-level divergence associated with the SAH enhances ascending motion in ASM regions and increases the monsoon rainfall accordingly. Subsequently, the latent heat associated with the monsoon rainfall in the monsoon onset region excites an anticyclone to its Northwest in the upper level, which keeps strengthening the SAH and moving it Northwestward. This mutual promotion between the SAH and ASM rainfall can be affected by the sea surface temperatures (SSTs) in the Western Pacific and tropical Indian Ocean in the previous month. Colder (warmer) SSTs over the Western Pacific and inactive (active) convection over the Southern Philippines suppress (favor) the Northwestward development of the SAH in late April. In addition, the warmer (colder) SSTs in the tropical Indian Ocean excites anomalous anticyclone (cyclone) in the upper level near the equator, which keeps the SAH in the lower latitudes (promotes the SAH to the North), and delays (advances) the mutual promotion between the SAH and ASM rainfall. As a result, the entire ASM onset process is later (earlier) than normal.

Revisiting the Circulation of Hudson Bay: Evidence for a Seasonal Pattern

AbstractThe Hudson Bay Complex is the outlet for many Canadian rivers, receiving roughly 900 km3/year of river runoff. Historically, studies found a consistent cyclonic flow year‐round in Hudson Bay, due to the geostrophic boundary current induced by river discharge and cyclonic wind forcing that was supported by available observations at that time. Using a high‐resolution ocean general circulation model, we show that in summer, the mean circulation is not cyclonic but consists of multiple small cyclonic and anticyclonic features, with the mean flow directed through the center of the bay. Absolute Dynamic Topography and velocity observations also show this seasonal flow pattern. We find that this summer circulation is driven by geostrophic currents, generated by steric height gradients, which are induced by increased river discharge during the spring freshet, and reinforced by anticyclonic seasonal wind patterns.

Impacts of recent decadal changes in Asian aerosols on the East Asian summer monsoon: roles of aerosol\u2013radiation and aerosol\u2013cloud interactions

Anthropogenic aerosols (AA) can affect cloud and precipitation through aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI). Over the past few decades, anthropogenic aerosol emissions have exhibited remarkable changes in the magnitude and in spatial pattern. The most significant changes are the increased emissions over both South Asia and East Asia. In this study, the atmospheric component of a state-of-the-art climate model that includes eight species of tropospheric aerosols, coupled to a multi-level mixed-layer ocean model, has been used to investigate the impacts of Asian anthropogenic aerosol precursor emission changes from 1970s to 2000s on large scale circulation and precipitation in boreal summer over East Asia. Results reveal significant changes in circulation and clouds over East Asia and over the tropical and western North Pacific (WNP). Increased Asian AA emissions lead to anomalous cyclonic circulation over the Maritime continent (MC) and anomalous anticyclonic circulation over the WNP, resulting in anomalous moisture transport convergence over the MC and therefore increased precipitation. They also lead to anomalous moisture flux divergence over both the WNP and large land areas of East Asia, especially over northern China, and therefore decreased precipitation there. These large scale circulation anomalies over the adjacent oceans are related to aerosol change induced ocean feedbacks, predominantly through ACI. It is the slow responses over the adjacent oceans (e.g., SST changes) through coupled atmosphere–ocean interaction in pre-monsoon seasons and summer that shape the changes of the East Asian summer monsoon and local precipitation. The results in this study suggest that increased Asian AA emissions from 1970s to 2000s may have played an important role for the observed southward shift of the Pacific intertropical convergence zone and precipitation belt, weakening of East Asian summer monsoon and reduced precipitation over northern China in East Asia during the latter half of the twentieth century.

Contourite distribution and bottom currents in the NW Mediterranean Sea: Coupling seafloor geomorphology and hydrodynamic modelling

Contourites are common morphological features along continental margins where currents encounter the seafloor. They can provide long-term archives of palaeoceanography, may be prone to sediment instability, and can have a great potential for hydrocarbon exploration. Despite their importance and increasingly recognised ubiquitous occurrence worldwide, the link between oceanographic processes and contourite features is poorly constrained. In particular, it is unclear under which specific conditions sediments are mobilised, modified and deposited by bottom currents. Here, we aim to determine key bottom current characteristics (velocity and bottom shear stress) affecting contourite deposition, by assuming that recent oceanographic regimes may be extended back in time over the past glacial-interglacial cycles, with strong winter circulation assumed similar to glacial conditions and weak summer circulation to interglacials. We present an integrated study from the NW Mediterranean Sea that couples results of the MARS3D hydrodynamic model with high-resolution sedimentological and geophysical data (piston cores, multibeam bathymetry and high resolution seismic data). Near bottom circulation was modelled during winter and summer 2013 as representative of past periods of high and low current intensity, respectively. Model results match well with the extent of contourite depositional systems and their different localised morphologic elements. We deduce that higher intensity events control the formation of erosional features such as moats and abraded surfaces. The heterogeneous distribution of bottom-current intensity on slopes explains the development of different types of contourite drifts. Plastered drifts form in zones of low bottom-current velocities constrained upslope and downslope by higher current velocities. Separated elongated mounded drifts develop where fast bottom-currents decelerate at foot of the slope. In contrast, no mounded contourite morphologies develop when the current velocity is homogeneous across the slope, especially in margins prone to downslope sediment transport processes. In confined basins, gyres may transport sediment in suspension from a margin with a high sediment supply to an adjacent starved margin, favouring the development of fine-grained contourites in the latter. Our results provide new insights into how detailed bottom-circulation modelling and seafloor geomorphological analyses can improve the understanding of palaeoflow-regimes, at least over time spans when the overall paleogeography and the distribution of contourite drifts is comparable to present-day conditions. The approach of coupled hydrodynamic models and geomorphological interpretations proposed here for depositional, erosional and mixed contourite features may be used to understand other areas affected by bottom currents, and for a better conceptual understanding of bottom-current processes and their interactions with the seafloor.

Climatological influence of Eurasian winter surface conditions on the Asian and Indo‐Pacific summer circulation in the NCEP CFSv2 seasonal reforecasts

This study evaluates the possible influence of the winter surface conditions in Eurasia on the summer circulation over the Asian continent and Indo‐Pacific region. We have analysed multi‐seasonal ensemble reforecasts for 30 years (1979–2008) using the National Centers for Environmental Prediction Climate Forecast System version 2 initialized at the beginning of each month from January to May. It is found that the reforecasts initialized in winter (e.g., February) overestimate the snow cover fraction, depth and water equivalent, as well as surface albedo in the excessively snow‐covered portion of Eurasia from March to June. These biases are generated and perpetuated by a snow‐albedo feedback, leading to excessive upwards shortwave radiation reflected from the overly snow‐covered surface and an intense cold bias from the surface to mid‐troposphere. Originating over land, the cold bias is extended eastwards over the northwestern North Pacific by the advection of prevailing westerly winds. The cold air temperature in the broad mid‐latitude Asian‐Pacific region causes significantly lower geopotential heights at pressure levels in the middle and upper troposphere and thus increases the upper‐level westerly winds on its southern flank over the Asian continent and Indo‐Pacific. A slower than observed snow melting rate helps the winter cold bias persists well into the summer season in these runs. As a result, compared with the reforecasts initialized in spring (e.g., May), winter‐initialized reforecasts feature lower geopotential heights in the upper troposphere over Eurasia and a stronger subtropical jet over the Asian continent and the North Pacific from May to September, especially in early summer. The CFSv2 reforecasts in both sets of cases have too little total cloud fraction over Eurasia during June–August, leading to enhanced downwards shortwave radiation.

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).

Improving Seasonal Prediction of East Asian Summer Rainfall Using NESM3.0: Preliminary Results

It has been an outstanding challenge for global climate models to simulate and predict East Asia summer monsoon (EASM) rainfall. This study evaluated the dynamical hindcast skills with the newly developed Nanjing University of Information Science and Technology Earth System Model version 3.0 (NESM3.0). To improve the poor prediction of an earlier version of NESM3.0, we modified convective parameterization schemes to suppress excessive deep convection and enhance insufficient shallow and stratiform clouds. The new version of NESM3.0 with modified parameterizations (MOD hereafter) yields improved rainfall prediction in the northern and southern China but not over the Yangtze River Valley. The improved prediction is primarily attributed to the improvements in the predicted climatological summer mean rainfall and circulations, Nino 3.4 SST anomaly, and the rainfall anomalies associated with the development and decay of El Nino events. However, the MOD still has biases in the predicted leading mode of interannual variability of precipitation. The leading mode captures the dry (wet) anomalies over the South China Sea (northern East Asia) but misplaces precipitation anomalies over the Yangtze River Valley. The model can capture the interannual variation of the circulation indices very well. The results here suggest that, over East Asia land regions, the skillful rainfall prediction relies on not only model’s capability in predicting better summer mean and ENSO teleconnection with EASM, but also accurate prediction of the leading modes of interannual variability.

Mid-late Holocene rainfall variation in Taiwan: A high-resolution multi-proxy record unravels the dual influence of the Asian monsoon and ENSO

Taiwan is particularly sensitive to changes in monsoonal precipitation and to typhoon-induced heavy precipitation events, however, rainfall variability in Taiwan on centennial and millennial time scales during the Holocene has not been well understood. This study describes mid-Holocene rainfall features of Taiwan based on pollen, total organic carbon (TOC), total nitrogen (TN), and C/N ratio records of core MD05-2908. The step-wise increase in sedimentation rate, fern spore percentage and concentration, TOC content, and C/N ratio suggests an increasing terrestrial material supply due to the intensified rainfall in Taiwan since 6800 cal. yr BP. This rainfall pattern shows an inverse pattern to the decreasing East Asian summer monsoon (EASM) strength represented by the multi-proxy records from North China. Variation of the East Asian summer circulation and associated moisture transport may account for the long-term rainfall changes in Taiwan. Superimposed on this trend, we interpreted three prominent rainfall changes, which focus on the periods of 6800–6600, 1090–880 and 490–190 cal. yr BP. These centennial time scale rainfall variations in our records are linked to the intensity of El-Niño Southern Oscillations.

Long term variability of the northerly winds over the Eastern Mediterranean as seen from historical wind observations

Abstract The summer circulation over the eastern Mediterranean is characterized by a persistent northerly regime whose interannual variability is modulated both by mid and tropical latitudes. In this paper, we use historical wind observations taken aboard ships to assemble the first purely instrumental index quantifying this wind system since 1880. This has allowed evaluating the multidecadal and interannual variability of the northerlies in the eastern Mediterranean over the longest time period so far. Our results indicate that the first half of the 20th century was characterized by more frequent and persistent northerly winds in the eastern Mediterranean than the second half. We have also found that the well-known teleconnection between the eastern Mediterranean summertime winds and the Indian Summer Monsoon, with enhanced northerlies concurrent with a stronger monsoon, has not been steady along the 20th century. It is shown that the Indian summer monsoon modulation of convection over the western Indian Ocean plays a crucial role in the strength of this connection.

Coupled Summer Circulation and Dynamics Between a Bay and the Adjacent Shelf Around Hong Kong: Observational and Modeling Studies

AbstractObservational and numerical modeling studies are conducted to investigate the coupled circulation between Mirs Bay to the east of Hong Kong and the adjacent shelf sea during an upwelling season. Long‐ and short‐term observations are synthesized to characterize the circulations in the bay‐shelf region. A three‐dimensional‐coupled bay‐shelf‐estuary circulation model was developed with realistic topography and forced with time‐dependent wind, tides, and lateral fluxes to investigate the processes and physics in the circulation of the coupled bay‐shelf regime. Based on the validated model, it was found that a strong northeastward coastal upwelling jet persisted over the shelf with highly variable topography outside the bay, and a strong upslope current occurred where the topography was sharply convex. This upslope current intruded into the bay in the lower layer (>10 m) as a cold‐water stream. A horizontal anticyclonic circulation formed inside the bay with a seaward outflow in the upper layer (<10 m). Momentum and vorticity analyses showed that a southwestward along‐isobath pressure gradient force over the convex isobaths off the bay intensified that bay‐ward intrusion. Negative relative vorticity advection from the jet was responsible for this pressure gradient force. The horizontal anticyclonic circulation and elevation fluctuation inside the bay were determined by the interaction between the intruding shelf current and the topographic trough inside the bay, and they were also baroclinically modulated by the intrusion of denser shelf waters. Winds over Mirs Bay intensified exchange flow across its entrance but suppressed the anticyclonic circulation inside the bay.

Spatial Distribution and Seasonal Variation of Explosive Cyclones over the North Atlantic

Spatial distribution and seasonal variation of explosive cyclones (ECs) over the North Atlantic from October 2000 to September 2016 are investigated using the reanalysis data of Final Analysis (FNL), Mean Sea Level Pressure (MSLP) and Optimum Interpolation (OI) Sea Surface Temperature (SST) provided by the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Oceanic and Atmospheric Administration (NOAA), respectively. Considering the meridional distribution of ECs and 10-m height wind field associated with the ECs, the definition of EC given by Yoshida and Asuma (2004) is modified. It is found that the ECs occurred mainly in four regions during winter season, namely, North America continent (NAC), the Northwest Atlantic (NWA), the North-central Atlantic (NCA), and the Northeast Atlantic (NEA), depending on the spatial distribution of EC’s maximum deepening rate of central sea level pressure (SLP). According to the magnitude of maximum deepening rate, the trend of EC numbers basically decrease with the increase of EC’s maximum deepening rate over the North Atlantic during the whole time period. Over the North Atlantic basin, for monthly statistics, the NEA, NCA, and NWA cyclones occur mainly in December, from December to March, and from January to February, respectively. NWA, NCA and NEA cyclones in winter are associated with low-level baroclinicity, both low-level baroclinicity and upper-level forcing and upper-level forcing, respectively. According to monthly variation, the averaged maximum deepening rate of central SLP firstly increases and then decreases from July to June. Overall, the distribution of ECs’ tracks is basically in the southwest-northeast direction. During winter circulation stage (from October to May), the averaged maximum deepening rate of central SLP and the averaged minimum central SLP of ECs decrease, and the averaged explosive-deepening duration of ECs shortens from west to east over the North Atlantic basin. During summer circulation stage (from June to September), the number of ECs is far less than that of winter circulation. NCA cyclones are the lowest in the average minimum central SLP of ECs, and the longest in the average explosive- deepening duration of ECs. NEA cyclones are the strongest in the average maximum deepening rate of central SLP.

Probabilistic dependence between streamflow and hydroclimatic variables and the possible linkages to large-scale atmospheric circulation: A case study in Baden-Württemberg, Southwest Germany

This study presents an investigation of probabilistic relationships between streamflow and hydroclimatic variables (including precipitation, temperature and soil moisture) and the potential links to large-scale atmospheric circulation over Baden-Württemberg, Southwest Germany. First, the joint dependence between seasonal streamflow and hydroclimatic variables was established by using copulas. On the basis of the joint dependence structure, we estimated the probabilities of hydrological droughts and prolonged high-streamflow events conditioned upon two different scenarios (high and low) of hydroclimatic variables for various seasons over the study area. The results indicate that both precipitation and soil moisture are positively related to the streamflow variation in Baden-Württemberg for each season and strongly impact the likelihoods of hydrological droughts and high-streamflow events. Temperature tends to have less impact on the streamflow variation, and an inverse connection between streamflow and temperature is found in spring and summer. Then, the connections between streamflow variability and large-scale atmospheric circulation in summer and winter were explored by using composite analysis. Although the atmospheric circulation patterns vary in each season, it can be found that in summer high streamflow anomalies over Baden-Württemberg are related to a cyclonic activity over central Europe while low streamflow anomalies are linked to anticyclonic patterns over western Ireland. The high streamflow anomalies in winter are strongly impacted by the westerlies that play an important role in favoring warm and moist airstreams from the Atlantic Ocean towards the study area, while an opposite atmospheric pattern is found for the years with low streamflow anomalies. The presented probabilistic methodology could also be applied in other regions worldwide.

The role of the Indian Summer Monsoon variability on Arabian Peninsula summer climate

This study investigates the influence of the Indian Summer Monsoon (ISM) on the atmospheric circulation over the Arabian Peninsula (AP) using the European Centre for Medium Range Weather Forecasts’ twentieth century reanalysis (ERA-20C) for the period 1901–2010. After describing the summer climate of the AP using various dynamic and thermodynamic parameters, we investigate the link between extreme ISMs and atmospheric circulation over the AP on inter-annual time scale. Analysis of composites of different parameters during extreme monsoon (strong and weak) years reveals that the ISM plays an important role in the summer circulation over the AP and adjoining regions. The major noticeable changes in modulating circulation during extreme monsoons are: (1) a strengthening of lower tropospheric northerly winds, westerly winds passing through the Tokar Gap, Shamal winds, and the upper tropospheric easterly jet stream during strong ISM; (2) a northward (southward) shift of the subtropical westerly jet stream during strong (weak) monsoon years; (3) the development of strong upper level ridge above the surface thermal low during strong ISM years, which result in a baroclinic structure over the AP and adjoining regions; (4) an increase in adiabatic warming, and hence aridity, over the AP during strong monsoon years, caused by intense subsidence of the middle to upper troposphere due to zonal overturning circulation; and (5) convective instability during strong monsoon years caused by an intensification of the upward motion over the southern AP. Furthermore, during strong monsoons, the availability of excess moisture leads to atmospheric instability, which in turn triggers the formation of clouds that lead to more rainfall over the southwestern AP. Finally, the westward propagation of a Gill-type Rossby waves induced by the ISM play an important role in the variations of the AP summer climate by enhancing the warm core structure over the AP and through their interaction with the midlatitude westerlies during strong monsoons.

SEASONAL VARIABILITY OF THE SURFACE WATER CIRCULATION IN THE NORTHERN JAPAN SEA BY DATA OF DRIFTING BUOYS

Data of observations on drifters in 1988–2015 are used to study seasonal variability of the surface water circulation in the Japan Sea. The well-known circulation patterns are observed, as the Tsushima Current, the East-Korean Current, the Primorye Current, and the northern cyclonic gyre. In the fall season (October), the main features of the mean flow are a very strong inflow through the Korean/Tsushima Strait and the intensive Tsushima Current and East-Korean Current. In winter (January), the cyclonic gyre over the norther basin and the Primorye Current along Primorye coast are strengthened. These seasonal changes of the flow field are presumably caused by monsoon winds variation. In spring (April) and summer (July), the water circulation is generally weakened; one of important patterns of the summer circulation in the northern Japan Sea is the westward branch of the Tsushima Current toward Primorye coast.

A Statistical Investigation of the Impact of the Indian Monsoon on the Eastern Mediterranean Circulation

The Indian summer monsoon (ISM) is a prominent feature of the summer circulation in the Northern Hemisphere (NH) and has been found to modulate the weather and climate conditions in many remote regions. This study investigates the most recurrent patterns of summertime midlatitude circulation, over the eastern Mediterranean (EM) and also globally, that are most associated with the ISM. Monthly data of 44 summers from the ERA40 dataset are used and two multidimensional statistical methods, the Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA), are implemented. The ISM is found to be related to subsidence anomalies in the middle and more extendedly in the upper troposphere over the central and eastern Mediterranean and with an Etesian-like pattern regarding the field of the lower troposphere winds. An equatorial Rossby wave pattern, extending westward from an ISM heat source up to EM and N. Africa, was identified to be associated with the variability of ISM. The observed relationship between the ISM and the EM circulation features can be attributed to this equatorial Rossby wave response to the monsoon forcing. CCA implementation revealed the interconnection of the aforementioned PCA results with an ISM action center over the northern Arabian Sea and the monsoon trough region.

Influence of Western Tibetan Plateau Summer Snow Cover on East Asian Summer Rainfall

AbstractThe influence of boreal winter‐spring eastern Tibetan Plateau snow anomalies on the East Asian summer rainfall variability has been the focus of previous studies. The present study documents the impacts of boreal summer western and southern Tibetan Plateau snow cover anomalies on summer rainfall over East Asia. Analysis shows that more snow cover in the western and southern Tibetan Plateau induces anomalous cooling in the overlying atmospheric column. The induced atmospheric circulation changes are different corresponding to more snow cover in the western and southern Tibetan Plateau. The atmospheric circulation changes accompanying the western Plateau snow cover anomalies are more obvious over the midlatitude Asia, whereas those corresponding to the southern Plateau snow cover anomalies are more prominent over the tropics. As such, the western and southern Tibetan Plateau snow cover anomalies influence the East Asian summer circulation and precipitation through different pathways. Nevertheless, the East Asian summer circulation and precipitation anomalies induced by the western and southern Plateau snow cover anomalies tend to display similar distribution so that they are more pronounced when the western and southern Plateau snow cover anomalies work in coherence. Analysis indicates that the summer snow cover anomalies over the Tibetan Plateau may be related to late spring snow anomalies due to the persistence. The late spring snow anomalies are related to an obvious wave train originating from the western North Atlantic that may be partly associated with sea surface temperature anomalies in the North Atlantic Ocean.

Activity of the Southern Annular Mode during 2015–2016 El Niño event and its impact on Southern Hemisphere climate anomalies

ABSTRACTPrevious studies documented that El Niño (EN) events are in general associated with negative phases of the Southern Annular Mode (SAM). EN 2015–2016 (EN15–16) was one of the three strongest events ever recorded. However, it was associated with a SAM positive phase of extreme intensity. Furthermore, while the negative linear relationship between ENSO and SAM during the most recent period (1986–2014) was significant and associated with a narrow uncertainty band, the combined condition of both climate patterns in the EN15–16 event was an outlier. The EN15–16 influence on the austral summer circulation anomalies at the extratropical and polar regions of the Southern Hemisphere was considerably altered by the strong SAM positive phase, which was evident not only at the troposphere but also at the stratosphere. Such circulation changes resulted in unusual regional impacts, such as negative anomalies of surface air temperature in western Antarctic Peninsula and negative precipitation anomalies in southeastern South America, ever recorded for previous strong EN events. Further research is needed to better understand the mechanisms explaining the SAM behaviour during 2015–2016 and its implication for climate predictability on seasonal timescales.

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.

Spatiotemporal variation of upper-air and surface wind speed and its influencing factors in northwestern China during 1980–2012

Scientists generally believe that human activities and atmospheric circulations have important effects on wind speed changes; however, the main driving factors of wind speed at different times and in different areas are far from clear. Here, based on monthly wind speed data in the northwestern China during 1980–2012, we analyzed the spatiotemporal variations in wind speed and their relationship with atmospheric circulations. The results showed that (1) annual surface wind speed (SWS) displayed a decreasing trend (− 0.032 m/s/10a) during 1980–2012; SWS increased only in winter. In contrast, annual upper-air wind speed (UWS) exhibited an increasing trend; UWS presented a declining trend only in summer. (2) Human activities were likely the major cause of decrease in SWS, whereas the enhancement of the Asian Meridional Circulation (AMC, R = 0.50, P < 0.01) was an important factor for the increase in SWS in winter. (3) Annual UWS exhibited an increasing trend (0.176 m/s/10a) in the troposphere. This trend was closely related to the weakening of the Eurasian Zonal Circulation (EZC, R = − 0.52, P < 0.01) and the enhancement of the Siberian High (SH, R = 0.48, P < 0.01) during the winter, whereas the enhancement of the Eurasian Zonal Circulation (EZC, R = − 0.60, P < 0.001) in summer was the prime reason for the decrease in UWS in summer. (4) UWS increased significantly (0.38 m/s/10a) in the lower stratosphere, and this trend was mainly associated with the weakening of the Pacific Decadal Oscillation (PDO, R = − 0.40, P < 0.05).

The Summer Circulation in the Gulf of Suez and Its Influence in the Red Sea Thermohaline Circulation

AbstractThe Gulf of Suez is accepted as an important location for Red Sea Deep Water formation, but the circulation and exchange with the Red Sea around the year remains elusive. A summer cruise in the area gives the opportunity to investigate features of the summertime hydrological structure and exchange with the Red Sea. An inverse estuarine circulation and exchange with the Red Sea is evident. The topographic patterns of the gulf play an important role in the circulation. Two sills, one in midbasin and a second at the mouth of the gulf, inhibit the bottom flow, topographically trapping waters that were formed in the cold season. Although the water mass characteristics of the outflowing waters during the other seasons are not directly related to the deep waters, they can influence the water column structure of the northern Red Sea. A simple box model shows that their contribution can have a significant influence in the formation of the intermediate layer. A hypersaline (40.6 psu) but relatively warm (23°C) water mass, originating in the Gulf of Suez, is detected at intermediate depths (100–150 m), with a strong signal in the western part of the Red Sea.