South Asian High Research Articles

The Research on Abnormal Features and Formation Causes of Tropical Cyclones Landing Southeastern Coastal Zone in 2005

AbstractUsing 1949~2005 data from the Tropical Cyclone (Typhoon) Year‐Book of CMA, this paper investigates the features of the TC (tropical cyclone) activities in the western Pacific as well as TCs landing on the coastal areas of southeastern China, indicating that the numbers have dropped slightly, while the intensity of the landing TCs increased significantly. In 2005, the number of TCs of the western Pacific origin was fewer than normal but the landing typhoons were greatly stronger than the mean. Analysis of the abnormal phenomena resulting from the large‐scale circulation background shows that prior to the generation, the south‐Asian high (SAH) and the western Pacific subtropical high (sub‐high) are stronger than the mean, together with the weaker sub‐high and East Asian summer monsoon, vertical shear of zonal wind etc. in the TC season all working together prevent TCs from formation. On the other hand, the onland TCs are stronger than normal, which may be the result of abnormal vapor transport in the west part of the western Pacific, weak vertical wind shear, SSTA and the interaction between weather systems at mid‐ and low‐ latitudes.

Analysis of the simulated climatic characters of the South Asia High with a flexible coupled ocean-atmosphere GCM

The ability of a climate model to reproduce the climatic characters of the South Asia High (SAH) is assessed by analyzing the 110-yr output of a Flexible Coupled GCM, version 0 (FGCM-0). Comparing the results of FGCM-0 with the NCEP/NCAR reanalysis data, the major findings show that FGCM-0 has better results in simulation of the geopotential height field at 100 hPa, and reproduces fairly the main atmospheric circulation centers. However, there are still some differences in the simulated results compared with the reanalysis data. The coupled model also successfully reproduces the mean seasonal variation of the SAH, that is, it moves from the Pacific Ocean to the Asian continent, remaining over the Tibetan Plateau from winter to summer, and then withdraws from the Tibetan Plateau to the Pacific Ocean from summer to winter. However, such observed relationships between the SAH positions and the summer precipitation patterns cannot be fairly reproduced in the FGCM-0.

Relationship between the boreal spring Hadley circulation and the summer precipitation in the Yangtze River valley

The connection between the boreal spring Hadley circulation (HC) and variability of the following summer east Asian atmospheric circulations and precipitation in the Yangtze River valley is investigated through an analysis of the observed data in this study. It is found that there is a significantly positive correlation between HC and the summer rainfall in the Yangtze River valley. This relationship is well supported by the changes of atmospheric general circulation backgrounds and water vapor conditions related to the variation of the preceding boreal spring HC. The summer situations of strengthened western Pacific subtropical high (WPSH), intensified South Asian high (SAH), southward located east Asian jet (EAJ) and enhanced water vapor corresponding to strong spring HC provide favorable conditions for increasing the precipitation in the Yangtze River valley and vice versa. The possible mechanism how the boreal spring HC affects summer atmospheric circulations is identified preliminarily in the study. Results show that sea surface temperature (SST) anomalies in the Indian Ocean and South China Sea may play an important role in linking the spring HC and summer atmospheric circulations. Spring HC may evoke SST anomalies in the Indian Ocean and South China Sea, which can persist from spring to summer and in turn give rise to anomalous east Asian summer monsoon. As a result, summer rainfall in the Yangtze River valley may be influenced.

Large-scale atmospheric singularities and summer long-cycle droughts-floods abrupt alternation in the middle and lower reaches of the Yangtze River

The daily precipitation data at 720 sta- tions over China for the 1957―2003 period during summer (May―August) are used to investigate the summer subseasonal long-cycle droughts-floods abrupt alternation (LDFA) phenomenon and a long-cycle droughts-floods abrupt alternation index (LDFAI) in the middle and lower reaches of the Yangtze River (MLYRV) is defined to quantify this phenomenon. The large-scale atmospheric circula- tion features in the anomalous LDFA years are ex- amined statistically. Results demonstrate that the summer droughts-to-floods (DTF) in the MLYRV usually accompany with the more southward western Pacific subtropical high (WPSH), negative vorticity, strong divergence, descending movements develop- ing and the weak moisture transport in the low level, the more southward position of the South Asia high (SAH) and the westerly jets in the high level during May―June, but during July―August it is in the other way, northward shift of the WPSH, positive vorticity, strong convergence, ascending movements and strong moisture transport in the low level, and the northward shift of the SAH and the westerly jets in the high level. While for the summer floods-to-droughts (FTD) in the MLYRV it often goes with the active coldair mass from the high latitude, positive vorticity, strong convergence, ascending movement develop- ing and the strong moisture transport in the low level, and the SAH over the Tibetan Plateau in the high level, but during July―August it is often connected with the negative vorticity, strong divergence, de- scending movements developing and the weak moisture transport in the low level, the remarkable northward shift of the WPSH, the SAH extending northeastward to North China and the easterly jets prevailing in the high level over the MLYRV. In addi- tion, the summer LDFA in the MLYRV is of significant relationship with the Southern Hemisphere annual mode and the Northern Hemisphere annual mode in the preceding February, which offers some predictive signals for the summer LDFA forecasting in the MLYRV.

Simulations of the 100-hPa South Asian High and precipitation over East Asia with IPCC coupled GCMs

The South Asian High (SAH) and precipitation over East Asia simulated by 11 coupled GCMs associated with the forthcoming Intergovernmental Panel on Climate Change’s (IPCC) 4th Assessment Report are evaluated. The seasonal behavior of the SAH is presented for each model. Analyses of the results show that all models are able to reproduce the seasonal cycle of the SAH. Locations of the SAH center are also basically reproduced by these models. All models underestimate the intensity and the extension of coverage in summer. The anomalous SAH can be divided into east and west modes according to its longitudinal position in summer on the interannual timescale, and the composite anomalies of the observed precipitation for these two modes tend to have opposite signs over East Asia. However, only several coupled GCMs can simulate the relationship between rainfall and SAH similar to the observed one, which may be associated with the bias in simulation of the subtropical anticyclone over the West Pacific (SAWP) at 500 hPa. In fact, it is found that any coupled GCM, that can reproduce the reasonable summer mean state of SAWP and the southward (northward) withdrawal (extension) for the east (west) mode of SAH as compared to the observed, will also simulate similar rainfall anomaly patterns for the east and west SAH modes over East Asia. Further analysis indicates that the observed variations in the SAH, SAWP and rainfall are closely related to the sea surface temperature (SST) over the equatorial tropical Pacific. Particularly, some models cannot simulate the SAWP extending northward in the west mode and withdrawing southward in the east mode, which may be related to weak major El Nino or La Nina events. The abilities of the coupled GCMs to simulate the SAWP and ENSO events are associated partly with their ability to reproduce the observed relationship between SAH and the rainfall anomaly over East Asia.

South Asian high and Asian-Pacific-American climate teleconnection

Growing evidence indicates that the Asian monsoon plays an important role in affecting the weather and climate outside of Asia. However, this active role of the monsoon has not been demonstrated as thoroughly as has the variability of the monsoon caused by various impacting factors such as sea surface temperature and land surface. This study investigates the relationship between the Asian monsoon and the climate anomalies in the Asian-Pacific-American (APA) sector. A hypothesis is tested that the variability of the upper-tropospheric South Asian high (SAH), which is closely associated with the overall heating of the large-scale Asian monsoon, is linked to changes in the subtropical western Pacific high (SWPH), the mid-Pacific trough, and the Mexican high. The changes in these circulation systems cause variability in surface temperature and precipitation in the APA region. A stronger SAH is accompanied by a stronger and more extensive SWPH. The enlargement of the SWPH weakens the mid-Pacific trough. As a result, the southern portion of the Mexican high becomes stronger. These changes are associated with changes in atmospheric teleconnections, precipitation, and surface temperature throughout the APA region. When the SAH is stronger, precipitation increases in southern Asia, decreases over the Pacific Ocean, and increases over the Central America. Precipitation also increases over Australia and central Africa and decreases in the Mediterranean region. While the signals in surface temperature are weak over the tropical land portion, they are apparent in the mid latitudes and over the eastern Pacific Ocean.

Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS’s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June–July–August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

The effects of the thermal anomalies over the Tibetan Plateau and its vicinities on climate variability in China

The evident effects of the thermal anomalies over the Tibetan Plateau (TP) and its vicinities are summarized and discussed in this paper. By the singular value decomposition (SVD) technique and numerical simulations of the effect of the snow depth anomaly over the TP, it is shown that the snow depth anomaly, especially in winter, is one of the factors influencing precipitation in China, and the winter snow anomaly is more important than the spring one. The relations between the sensible heat anomaly over the TP and the intensity of the South China Sea summer monsoon (SCSSM) are studied, too, and two key areas of the sensible heat anomaly over the TP are found. The relationships between the South Asia High (SAH) and the precipitation in the years with typical droughts or floods in the mid to lower valleys of the Yangtze River (MLVYR) and North China are investigated in some detail. It is found that not only the intensity of the SAH over the TP, but also the 100-hPa height in a large area influences the precipitation in the above two regions. The effects of the SAH on the onsets of the tropical Asian summer monsoon (TASM) including the SCSSM and the tropical Indian summer monsoon (TISM) are studied as well. It is found that the onset times of both the SCSSM and the TISM are highly dependent upon the latitudinal position of the SAH center.

Seasonal variation and heat preference of the south asia high

By use of the NCEP/ NCAR reanalysis data, the seasonal variation of the South Asia high (SAH) is analyzed. The influences of temporal and spatial variations of the middle and upper level atmospheric temperatures, the visible heat sources, and the diabatic heating rates in the whole atmospheric column on the seasonal variation of the SAH are discussed. Results show that the SAH has two seasonal balancing modes, one of which is the land high in summer and the other the ocean high in winter. The land high itself can be divided into two patterns as well, that is the Tibetan high and the Iranian high. Heating fields have important impacts on the seasonal variation of the SAH. The SAH is a warm high and its center has the property of heat preference, usually locating over or moving to an area with relatively larger heating rates. The annual cycle of the SAH is mainly controlled by the seasonal process of the latent and sensible heating in South Asia. Strong shortwave radiative heating in the north at high latitudes and over the Tibetan Plateau also has an effects on the northward movement and maintenance of the SAH. The cooling effect of infrared radiation is an important cause in weakening the SAH.

The Bimodality of the 100 hPa South Asia High and its Relationship to the Climate Anomaly over East Asia in Summer.

The NCEP/NCAR pentad mean reanalysis data from 1980 to 1994 are employed to examine the activities of the 100 hPa South Asia High (SAH) during the boreal summer. The results show that there exists bimodality in the longitude location of the SAH. According to the two preferred regions for the SAH, the SAH is classified into the Tibetan Mode (TM) and the Iranian Mode (IM), respectively. The studies on the maintenance mechanism, both from circulation structure and thermal structure, manifest that the SAH has the feature of warm preference. The diagnosis based on the thermodynamic equation further reveals that the TM is closely related to the diabatic heating of the Tibetan Plateau, whereas the IM is more associated with the adiabatic heating in the free atmosphere, as well as the diabatic heating near the surface. The statistical composites, and case studies, corresponding to the two modes show that the SAH bimodality is strongly related to the climate anomalies over East Asia. In the case of TM, the southerly airflow over the subtropics and the northerly airflow over the middle-high latitude at 850 hPa are enhanced, forming a convergence zone along 30°N and resulting in increased rainfall extending from the south Japan, Korea Peninsula, and Yangtze-Yellow river valley of China to the Tibetan Plateau. In the case of IM, at 850 hPa the mid-latitude East Asia is dominated by a huge anomalous anticyclone, thus results in the decreased rainfall over the area.

Conversion characteristics between barotropic and baroclinic circulations of the sah in its seasonal evolution

In the context of 1958–1997 NCEP/ NCAR re-analyses, the South Asia high (SAH) was divided into two components, barotropic and baroclinic, the former based on mass weighed vertical integration and the latter on the difference between the measured circulation and the barotropic component counterpart, whereupon the barotropic and baroclinic circulation conversion features were addressed of the research SAH during its seasonal variation. Evidence suggests that i) in summer (winter), the SAH is a thermal (dynamical) system, with dominant baroclinicity (barotropicity), either of the components accounting for approximately 70% of the total contribution; ii) as time progresses from winter to summer, accompanied by the barotropic SAH evolving into its baroclinic analog, the SAH is moving under the “ thermal guidance” of its baroclinic component circulation, suggesting that the component circulation precedes the system itself in variation; iii) the reversal happens when it goes from summer to winter, with the SAH displacement under the “ dynamic steering” of its barotropic component circulation.