Middle East Jet Stream Research Articles

The Correlation between the Middle East Jet Stream and the East Asian Winter Monsoon

The Correlation between the Middle East Jet Stream and the East Asian Winter Monsoon

Dynamical linkages between Arctic Oscillations and winter precipitation patterns over the Indian Region

Winter precipitation over the Indian region is mainly beneficial for the maintenance of the glaciers in the mountainous region and associated atmospheric conditions benefit various sectors, including agriculture, water resources, power generation, and tourism, among others. Hence, understanding the variability and teleconnections of winter precipitation is indispensable. The present study focuses on the understanding of dynamical linkages between the Arctic Oscillations (AO) and winter precipitation variability over the Indian region using reanalysis and observational datasets. The analysis reveals the dipole pattern in the precipitation between the western Himalayas and the central Indian region, which is mainly linked with the positive (negative) phase of the AO. The positive (negative) phase of the AO is mainly associated to the Rossby wave with wavenumber 2 and dominated by the quasi-barotropic Euro-Atlantic and Siberian High (Ural-Siberian high and high over Northwest Pacific) which is accompanied by the wave train pattern across Europe through subtropical north Africa and west Asia. The wave train pattern abets the strengthening (weakening and southward movement) of the middle east jet stream and induces anticyclonic (cyclonic) circulation over the Arabian Sea in a positive (negative) regime of the AO. The precipitation dipole between the western Himalayan region and central India is primarily due to the moisture transport from an outflow of anticyclonic (cyclonic) circulation positioned over the north Arabian Sea which is allied with the positive (negative) phase of the AO and associated atmospheric dynamics.

Dynamical Analysis of the Winter Middle East Jet Stream and Comparison with the East Asian and North American Jet Streams

Abstract The wintertime Middle East jet stream (MEJS) is an important upstream signal for the East Asian winter monsoon and the subsequent Asian summer monsoon. Thus, the maintenance and interannual variations of the MEJS as well as its similarities and differences with the East Asian jet stream (EAJS) and the North American jet stream (NAJS) are studied dynamically using the geopotential tendency equation and empirical orthogonal function analysis. Analysis reveals that the MEJS is mainly maintained by tropical diabatic heating and the low-frequency transient eddy (TE) vorticity forcing. It is different from the EAJS, which is maintained by both tropical diabatic heating and high-frequency TE vorticity forcing, and the NAJS, which is mainly sustained by high-frequency TE vorticity forcing. Furthermore, while temperature advection plays a considerable role in the maintenance of EAJS and NAJS, it is less important for the MEJS. On interannual time scales, the meridional shift of the MEJS is related to low-frequency TE heating, while the variation of the jet’s intensity is associated with temperature advection. For both EAJS and NAJS, the interannual variations are mainly contributed by high-frequency TE vorticity forcing, although temperature advection also promotes their meridional shifts. These results suggest that whether or not the maintenance of the jet streams is related to tropical diabatic heating, their interannual variations are not directly induced by this forcing. Significance Statement The wintertime Middle East jet stream (MEJS) is a narrow and strong westerly wind belt over the Middle East whose variations in intensity and location can affect the Asian monsoon significantly. However, little effort has been devoted to investigating the MEJS. Thus, dynamical diagnosis and statistical analysis are applied in this study to understand the MEJS and its variability comprehensively. Analysis reveals that low-frequency transient eddies, which are the mobile atmospheric systems with a lifespan longer than 10 days, are important for both the maintenance and the interannual variability of the MEJS. This phenomenon is apparently different from the East Asian and North American jet streams, in which synoptic transient eddies (lifetime shorter than 10 days) play an essential role.

Zonal Extension of the Middle East Jet Stream and Its Influence on the Asian Monsoon

Abstract Investigation into the interannual variation of the Middle East jet stream (MEJS) and its influence on the Asian monsoon indicates that the eastward extension of MEJS is closely related with a wetter and colder winter in southern China and a later onset of the subsequent Asian summer monsoon, compared with normal conditions. When the MEJS extends eastward, a significant barotropic anomalous anticyclone is located over the Arabian Sea (AS), associated with the southeastward-propagating wave train from Europe. Intense divergence in the southwest of the AS anomalous anticyclone favors more convection over the western tropical Indian Ocean, which excites an anomalous upper-level anticyclone to the north as a Rossby wave response, further intensifying the AS anticyclonic anomaly. This positive feedback loop maintains the AS anomalous anticyclone and results in the eastward extension of the MEJS. Accordingly, intense northeasterly anomalies over the Mediterranean Sea and the subtropical westerly anomalies bring abundant cold air from the mid- to higher latitudes to subtropical regions, resulting in a widespread cooling in subtropical Eurasia including southern China. Barotropic anomalous westerlies occur around the Tibetan Plateau in the south and deepen the India–Burma (now Myanmar) trough, favoring more water vapor transport from the Bay of Bengal to southern China. These wetter and colder conditions in subtropical Eurasia can persist from winter to spring, leading to the much later onset of the Asian summer monsoon. Therefore, the winter MEJS variability can be considered an important indicator for the Asian monsoon. Significance Statement Diabatic heating over the western tropical Indian Ocean exerts significant influence on the wintertime Middle East jet stream (MEJS), whose eastward extension leads to a wetter and colder winter in southern China and a later onset of the Asian summer monsoon. When the MEJS extends eastward, an anomalous anticyclone appears over the Arabian Sea and strengthened northeasterlies occur over the Mediterranean Sea. The westerly anomalies bring cold air from the high latitudes, resulting in cooling in subtropical Eurasia. Anomalous westerlies also occur around the Tibetan Plateau and deepen the India–Burma trough, favoring water vapor transport from the Bay of Bengal. These wetter and colder conditions persist from winter to spring, leading to later onset of the Asian summer monsoon.

Surface air temperature variability over the Arabian Peninsula and its links to circulation patterns

This study investigates the long‐term variability of surface air temperature (SAT) over the Arabian Peninsula (AP), using data from the Climate Research Unit (TS 3.22) for the 1960–2010 period. The long‐term climatology suggests that the warmest AP mean temperatures occur during summer, with the highest temperatures over the northern AP (NAP), due to the monsoon–desert mechanism. During winter, the NAP exhibits low SATs under the influence of western disturbances originating from the Mediterranean. The southwestern AP exhibits the lowest temperatures because of its proximity to the Arabian Sea cold waters, and also because of the orographic effects. The inter‐annual variability of the SAT is stronger during winters. A linear trend analysis reveals a significant increase in the SAT anomaly (0.10 °C/decade) across the AP, consistently with the global temperature anomalies. Besides the local convective heating, summer SAT variability is associated with the weakening of the Asian jet stream and a Rossby wave train from the Indian Ocean. This variability is also influenced by the anomalous low pressure over the North Atlantic and the Sahara, a high‐pressure system over Siberia and the northwest Pacific. Both in spring and autumn, sea surface temperature (SST) variations over the Indo‐western Pacific are highly influenced the AP SATs, whereas winter SATs are modulated by the subtropical jet stream and the Middle East jet stream. In all seasons, the AP SAT is strongly influenced by the SST variations over the tropical oceans. The temperature variability is closely associated with the El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). The warm phase of ENSO (i.e., El Niño) is one possible reason behind the inter‐annual increase in SAT over the southern AP. The negative phases of NAO and AO also play a role in increasing AP SAT.

Contrasting Impacts of the Arctic Oscillation on Surface Air Temperature Anomalies in Southern China between Early and Middle-to-Late Winter

AbstractIn the boreal winter, the Arctic Oscillation (AO) evidently acts to influence surface air temperature (SAT) anomalies in China. This study reveals a large intraseasonal variation in the relationship between the winter AO and southern China SAT anomalies. Specifically, a weak in-phase relationship occurs in December, but a significant out-of-phase relationship occurs in January and February. The authors show that the linkage between the AO and southern China SAT anomalies strongly depends on the AO-associated changes in the Middle East jet stream (MEJS) and that such an AO–MEJS relationship is characterized by a significant difference between early and middle-to-late winter. In middle-to-late winter, the Azores center of high pressure anomalies in the positive AO phase usually extends eastward and yields a significantly anomalous upper-level convergence over the Mediterranean Sea, which can excite a Rossby wave train spanning the Arabian Sea and intensify the MEJS. In early winter, however, the Azores center of the AO is apparently shifted westward and is mainly confined to the Atlantic Ocean; in this case, the associated change in the MEJS is relatively weak. Both observational diagnoses and experiments based on a linearized barotropic model suggest that the MEJS is closely linked to the AO only when the latter generates considerable upper-level convergence anomalies over the Mediterranean Sea. Therefore, the different impacts of the AO on the MEJS and the southern China SAT anomalies between early and middle-to-late winter are primarily attributed to the large intraseasonal zonal migrations of the Azores center of the AO.

Multidecadal Variations in the Relationship between the NAO and Winter Precipitation in the Hindu Kush–Karakoram

Abstract Winter precipitation over the Hindu Kush–Karakoram (HKK) range in the western Himalayas is generated by westerly perturbations whose dynamics is affected by the North Atlantic Oscillation (NAO). Larger precipitation is typically recorded during the positive NAO phase. In this work, the relationship between the NAO and winter precipitation in the HKK is explored further, using an ensemble of precipitation datasets and the 40-yr ECMWF Re-Analysis (ERA-40) and Twentieth Century Reanalysis (20CR) data. The mechanisms underlying this relationship are discussed, with a focus on the secular variations that occurred in the last century. The NAO exerts its control on HKK precipitation by altering the intensity of westerly winds in the region of the Middle East jet stream (MEJS). Results indicate that evaporation from the Persian Gulf, the northern Arabian Sea, and the Red Sea plays an important role. During positive NAO phases, westerlies are strengthened and enhanced evaporation occurs from these basins owing to higher surface wind speed. The extra moisture combines with stronger westerlies and results in enhanced moisture transport toward the HKK. Precipitation datasets covering the twentieth century show an alternation of periods of strong and weaker influence of the NAO on precipitation in the HKK. It is found that these variations are associated with changes in the spatial pattern of the NAO: the relative position of the two centers of action of the NAO determines to what extent it can modulate the MEJS, affecting precipitation in the HKK.

Dominant Modes of Wintertime Upper-Tropospheric Temperature Variations over Asia and Links to Surface Climate

AbstractIn this study, the authors investigate the variations and predictability of wintertime upper-tropospheric temperature (UTT) over Asia, which are often linked to severe climate anomalies, and the associated features of large-scale circulation and surface climate. The ECMWF Interim Re-Analysis (ERA-Interim) and hindcast of the NCEP Climate Forecast System, version 2 (CFSv2), are mainly analyzed.The first empirical orthogonal function mode of UTT shows a dipole structure, with a strong positive center over southern China and a weak negative center over Mongolia. The second mode is featured by a monopole variation, with a positive center appearing from the northwestern Tibetan Plateau (TP) to Japan. The third mode exhibits a tripole pattern, with two positive centers over Pakistan and the Sea of Japan and a negative center over central Asia. The first mode is linked to El Niño–Southern Oscillation, accompanied by surface warming over the southeastern TP and deficient precipitation over southern China, the Korean Peninsula, and from equatorial East Africa to the east of the TP. The second mode is associated with circulation anomalies similar to those associated with the Arctic Oscillation, with significant warming over East Asia. The third mode features two wave trains and is linked to the Middle East jet stream, and is associated with excessive precipitation from the eastern TP to southern Japan. The CFSv2 can predict the first mode skillfully by several months in advance, but it shows little skill in predicting the second and third modes.

Simulation of the westerly jet axis in boreal winter by the climate system model FGOALS-g2

The major features of the westerly jets in boreal winter, consisting of the Middle East jet stream (MEJS), East Asian jet stream (EAJS) and North Atlantic jet stream (NAJS), simulated by a newly developed climate system model, were evaluated with an emphasis on the meridional location of the westerly jet axis (WJA). The model was found to exhibit fairly good performance in simulating the EAJS and NAJS, whereas the MEJS was much weaker and indistinguishable in the model. Compared with the intensity bias, the southward shift of the WJA seems to be a more remarkable deficiency. From the perspective of Ertel potential vorticity, the profiles along different westerly jet cores in the model were similar with those in the reanalysis but all shifted southward, indicating an equatorward displacement of the dynamic tropopause and associated climatology. Diagnosis of the thermodynamic equation revealed that the model produced an overall stronger heating source and the streamfunction quantifying the convection and overturning Hadley circulation shifted southward significantly in the middle and upper troposphere. The two maximum centers of eddy kinetic energy, corresponding to the EAJS and NAJS, were reproduced, whereas they all shifted southwards with a much reduced intensity. A lack of transient eddy activity will reduce the efficiency of poleward heat transport, which may partially contribute to the meridionally non-uniform cooling in the middle and upper troposphere. As the WJA is closely related to the location of the Hadley cell, tropopause and transient eddy activity, the accurate simulation of westerly jets will greatly improve the atmospheric general circulation and associated climatology in the model.

Interannual linkage between Arctic/North Atlantic Oscillation and tropical Indian Ocean precipitation during boreal winter

In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Nino/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of −0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.

Linkage between the Arctic Oscillation and winter extreme precipitation over central-southern China

In this study, we analyzed the relationship between the Arctic Oscillation (AO) and extreme precipitation events over China during boreal winters from 1954 to 2009. The extreme precipitation events are defined as those with daily precipitation above the 80th (or 90th) per- centiles. The AO shows a significantly positive correlation with the frequency of extreme precipi- tation events over China during January to February. Of all 287 stations in China, 238 stations have positive correlation coefficients and 82 stations are positively significant above the 95% con- fidence level. These stations that have significantly positive correlation are mainly located over central-southern China. The correlation between the AO and the average frequency of extreme precipitation events over central-southern China is 0.49, which is significant at the 99% confi- dence level. This correlation is still significant even when the El Nino/Southern Oscillation signal is excluded from the time series. In association with the AO-precipitation extreme linkage, the Middle East jet stream (MEJS) and the southern branch trough (SBT) over the Bay of Bengal covary consistently. A positive AO phase is accompanied by a stronger than normal MEJS and a deepened SBT. The deepened SBT enhances synoptic scale disturbances in vertical motions in the low to middle troposphere over central-southern China. More moisture transport by the deepened SBT and active synoptic scale disturbances in vertical motion over central-southern China would provoke more extreme precipitation events there.

Extreme drought event of 2009/2010 over southwestern China

The extreme drought of 2009/2010 over southwestern China is the driest event with the lowest percentage rainfall anomaly and the longest non-rain days during winter season (October–February) in the past 50 years, and also the severest one with the lowest percentage rainfall anomaly at the same period since 1880. The drought domain is characterized with anomalous warming and drying in the mid-lower troposphere as well as an evident anomalous subsidence. The favorable circulation anomalies for this drought are associated with the following two factors. One is the strongest negative-phase Arctic Oscillation during 2009/2010 winter that accompanies with a weakened Middle East Jet Stream (MEJS), the cyclonic anomaly over Arabian Sea (AS), the anticyclonic anomaly over Tibet and the cyclonic anomaly over Lake Baikal. The weakened MEJS, the AS cyclonic anomaly and the Tibet anticyclonic anomaly weaken the Southern Branch Trough (SBT) that directly decreases the moisture transport toward the southwestern China; the cyclonic anomaly over the Lake Baikal causes a deepened and westward shifted East Asian Major Trough (EAT) so that dry cold air behind the EAT easily invades down to southwestern China. The AS cyclonic anomaly favors the westward extension of Western Pacific Subtropical High (WPSH). The westward extension of WPSH is also associated with the second factor that is the El Nino Modoki event during 2009/2010 autumn–winter. The intensification and westward extension of WPSH enhance the local subsidence, weaken the SBT and exacerbate this drought.

Is extreme Arctic sea ice anomaly in 2007 a key contributor to severe January 2008 snowstorm in China?

AbstractWe conduct numerical experiments with an atmospheric general circulation model to answer the question whether the extreme Arctic sea ice anomaly in 2007 contributed to the severe January 2008 snowstorm in China. Results show that the record low Arctic sea ice in September 2007, and follow‐up slow recovery of the Arctic sea ice in October and November 2007 could excite two stationary wave structures. One wave train propagates southeastward from the Barents/Kara Seas to central Asia, strengthening the Middle East Jet Stream (MEJS), and the other wave train propagates southward from central Arctic Ocean/eastern Siberia Sea to the mid‐latitude and subtropics of the north Pacific, weakening the East Asian Jet Stream (EAJS). The anomalous westerlies associated with the strengthening of the MEJS and the anomalous easterlies associated with the weakening of the EAJS form a strong convergence zone over central and southern China, providing favorable conditions for the severe January 2008 snowstorm. Copyright © 2011 Royal Meteorological Society

Climatic background of cold and wet winter in southern China: part I observational analysis

This study explores the climate background of anomalous wet and cold winter in southern China, focusing on results in January when most of its disastrous snowstorms and freezing rainfall events were observed. Based on the ERA-40 reanalysis and Climate Research Unit (CRU) observed precipitation and surface temperature monthly data for the period of 1959–2001, the difference between normalised monthly precipitation and temperature is used to define a simple index which reflects the intensity of the wet and cold condition in the region. It offers a good agreement with an index defined by daily weather station data observed in the region. Then, through simple correlation analyses we focus on exploring the dominant physical and dynamical processes leading to such climatic anomalies. While we acknowledge the contribution of the cold/dry air penetrated from the north, the importance of maintaining a warm and moist airflow from the south is highlighted, including an enhanced Middle East Jet Stream (MEJS) and southwesterly flow over Indochina Peninsula and South China Sea region. Strong vertical share of meridional wind, with enhanced northerly flow near the surface and southerly flow in the low to middle troposphere, leads to significant temperature and moisture inversions. These are consistent with results from synoptic analyses of the severe January 2008 event which was not included in the correlation calculations and thus suggest the 2008 event was not an unusual event although it was very intense. In the third part, we use a partial least-square statistical method to uncover dominant SST patterns corresponding to such climatic conditions. By comparing results for the periods of 1949–1978 and 1978–2007, we demonstrate the shift of dominant SST patterns responsible for the wet and cold anomalies. Shifting from “conventional” ENSO SST patterns to ENSO Modoki-like conditions in recent decades partially explains the unstable relationship between ENSO and Asian winter monsoon. Meanwhile, the importance of SST conditions in extra-tropic Pacific and Indian oceans is acknowledged. Finally, we developed a forecasting model which uses SST condition in October to predict the occurrence of the anomalous wet and cold January in the region and reasonable forecasting skill is obtained.

Roles of Anomalous Tibetan Plateau Warming on the Severe 2008 Winter Storm in Central-Southern China

Abstract Anomalous warming occurred over the Tibetan Plateau (TP) before and during the disastrous freezing rain and heavy snow hitting central and southern China in January 2008. The relationship between the TP warming and this extreme event is investigated with an atmospheric general circulation model. Two perpetual runs were performed. One is forced by the climatological mean sea surface temperatures in January as a control run; and the other has the same model setting as the control run except with an anomalous warming over the TP that mimics the observed temperature anomaly. The numerical results demonstrate that the TP warming induces favorable circulation conditions for the occurrence of this extreme event, which include the deepened lower-level South Asian trough, the enhanced lower-level southwesterly moisture transport in central-southern China, the lower-level cyclonic shear in the southerly flow over southeastern China, and the intensified Middle East jet stream in the middle and upper troposphere. Moreover, the anomalous TP warming results in a remarkable cold anomaly near the surface and a warm anomaly aloft over central China, forming a stable stratified inversion layer that favors the formation of the persistent freezing rain. The possible physical linkages between the TP warming and the relevant resultant circulation anomalies are proposed. The potential reason of the anomalous TP warming during the 2007–08 winter is also discussed.

Anomalous winter temperature and precipitation events in southern China

This paper analyzed the anomalous low-temperature events and the anomalous rain-abundant events in January since 1951 and winter since 1880 for southern China. The anomalous events are defined using ±1σ thresholds. Twelve cold Januaries are identified where temperature anomaly below −1σ, and ten wet Januaries are identified where precipitation anomaly above +1σ. Among these events there are three patterns of cold-wet Januaries, namely 1969, 1993 and 2008. The NCEP/NCAR reanalysis data are used to check the atmospheric circulation changes in association with the anomalous temperature and precipitation events. The results show that the strong Siberian High (SBH), East Asian trough (EAT) and East Asian jet stream (EAJS) are favorable conditions for low-temperature in southern China. While the anomalous southerly flow at 850 hPa, the weak EAT at 500 hPa, the strong Middle East jet stream (MEJS) and the weaker EAJS are found to accompany a wetter southern China. The cold-wet winters in southern China, such as January of 2008, are mainly related to a stronger SBH, and the circulation in the middle to upper troposphere is precipitation-favorable. In wet winters, the water vapor below 500 hPa is mainly transported by the anomalous southwesterly flow and the anomalous southern flow over the Indo-China Peninsula and the South China Sea area. The correlation coefficients of MEJS, EAMW (East Asian meridional wind) and EU (Eurasian pattern) to southern China precipitation in January are +0.65, −0.59 and −0.48 respectively, and the correlations for high-pass filtered data are +0.63, −0.55 and −0.44 respectively, the significant level is all at 99%. MEJS, EAMW and EU together can explain 49.4% variance in January precipitation. Explained variance for January and winter temperature by SBH, EU, WP (west Pacific pattern) and AO (Arctic Oscillation) are 47.2% and 51.5%, respectively. There is more precipitation in southern China during El Nino winters, and less precipitation during La Nina winters. And there is no clear evidence that the occurrence of anomalous temperature events in winter over southern China is closely linked to ENSO events.

An Analysis of the Large-Scale Climate Anomalies Associated with the Snowstorms Affecting China in January 2008

Abstract Extraordinarily frequent and long-lasting snowstorms affected China in January 2008, causing above-normal precipitation, below-normal temperature, and severe icing conditions over central–southern China. These snowstorms were closely linked to the change in the Middle East jet stream (MEJS), which intensified and shifted southeastward. The change in MEJS was accompanied by southeastward shifts of the ridge and the trough over Europe and western Asia. The intensified MEJS also strengthened the trough embedded in the southern branch of the subtropical westerlies over the southern Tibetan Plateau, enhancing the water vapor transport from western Asia and the Bay of Bengal to China. In the meantime, the subtropical western Pacific high (SWPH) was stronger and its ridgeline was farther north than normal. The anomalous high slowed down the eastward propagation of weather systems to the Pacific and favored convergence of water vapor over central–southern China. The MEJS is usually strong when the Arctic Oscillation (AO) is positive and the SWPH is farther north than normal in La Niña winters. Compared to the SWPH and the Niño-3.4 sea surface temperature (SST), the MEJS and the AO exert stronger influences on the temperature and the precipitation over central–southern China, despite the fact that these possible impacting factors are not completely independent from each other. Although the La Niña event might contribute to the climate anomalies through its relation with the SWPH in January 2008, an analysis of historical events indicates that La Niña conditions alone can hardly cause severe and persistent snow conditions over central–southern China. In addition, compared to the Niño-3.4 SST and the SWPH, the conditions of December MEJS and AO exhibit stronger precursory signals of the variability of January temperature over central–southern China.

Upstream Subtropical Signals Preceding the Asian Summer Monsoon Circulation

AbstractIn this study, the authors address several issues with respect to the antecedent signals of the large-scale Asian summer monsoon that were earlier identified by Webster and Yang. In particular, they revisit the changes in the subtropical upper-tropospheric westerlies preceding the monsoon, depict the detailed structure of the monsoon's antecedent signals, and investigate the physical processes from the signals to the monsoon. They also explore the teleconnection of these signals to various large-scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the Tibetan Plateau and the monsoon.Before a strong (weak) Asian summer monsoon, the 200-mb westerlies over subtropical Asia are weak (strong) during the previous winter and spring. A significant feature of these signals is represented by the variability of the Middle East jet stream whose changes are linked to the Arctic Oscillation, North Atlantic Oscillation, El Niño–Southern Oscillation, and other climate phenomena. When this jet stream intensifies and shifts southeastward, cold air intrudes frequently from eastern Europe into the Middle East and southwestern Asia. As a result, in subtropical Asia, snow and precipitation increase, the ground wetness increases, and surface temperature decreases. A strengthening Middle East jet stream is also accompanied by increases in both stationary wave activity flux and higher-frequency eddy activities. The Tibetan Plateau acts to block these westerly activities propagating eastward and increase the persistence of the low-temperature anomalies, which in turn prolongs the atmospheric signals from winter to spring.A strong link is found between the persistent low-temperature anomalies and the decrease in geopotential height over southern Asia, including the Tibetan Plateau, in spring. The latter indicates a late establishment of the South Asian high, and implies a delay in the atmospheric transition from winter to summer conditions and in the development of the summer monsoon. The preceding scenario for a strong Middle East jet stream and a weaker Asian monsoon can be applied accordingly for the discussion of the physical processes from a weak jet stream to a strong monsoon.The current results of the relationship between the extratropical process and Asian monsoon resemble several features of the tropical–extratropical interaction mechanism for the tropospheric biennial oscillation (TBO). While the role of tropical heating is emphasized in the TBO mechanism, compared to the variability of the sea surface temperature related to El Niño–Southern Oscillation, the extratropical process examined in this study is more strongly linked to the Asian summer monsoon.