The Meridional Variation And The Zonal Variation Of The South Asian High And The Relationship With The Summer Precipitation Over China

The South Asian High (SAH) is the most intense and persistent circulation at200hPaover the Northern Hemisphere during summer. It is a part of the subtropical high system andalso a part of the Asian Summer Monsoon system. The variation of SAH has close relationshipwith the Indian summer monsoon, and the summer rainfall in China.An Empirical Orthogonal Function analysis (EOF) has been performed on the summergeopotential height anomalies at200hPa. The first three EOF modes indicate that the intensityvariation, the meridional variation and the zonal variation are the three most important modesof SAH. The long-term linear trend, interdecadal change and the interannual variation of SAHcould be explained by the first three EOF modes respectively. The positions of centers of SAHhave been counted. The results show that, at200hPa, the latitudes of centers lie between25o–30oN, and the longitudes of center lie in80o–90oE and57.5o–62.5oE. Both the meridionaland zonal variation of SAH are significant.An index of SAH was defined to measure its meridional variation in summer and toanalyze its relationship with the summer precipitation over China.The results show asignificant correlation between the meridional position of SAH and the summer rainfall overChina in both the interannual timescale and the long-term linear tendency. When the SAHlocated further northward (southward), the rainfall is stronger in North China and South China,while it is weaker in the middle and lower reaches of the Yangtze River valley. An anomalousanticyclone controls eastern China with its center tilted southward from200hPa to850hPa,which forms a divergence zone over North China at the upper atmosphere and makes the flowsdiverge over the middle and lower reaches of the Yangtze River valley and converge overNorth China at the lower atmosphere. Besides, the northward movement of SAH would causethe upper-level westerly jet and the Western Pacific Subtropical High move northward, and asa result, the rainbelt locates in North China.The meridional variation of SAH is closely related to the sea surface temperature (SST)of the tropical Indian Ocean (TIO), the central and eastern equatorial Pacific, and the northernPacific, and might be modulated by the SST of TIO more directly. High SST in TIO mightlead to a southward expansion of SAH. Due to having a strong relationship with the summerrainfall over China and being modulated by the TIO SST, the meridional variation index ofSAH could be considered as an important indicator used to predict the regional climateanomaly.The zonal variation of SAH and its role in connecting the Indian summer monsoon (ISM)and the East Asian summer monsoon (EASM) are analyzed. It is found that the east-west shift of the SAH is significant in the interannual timescale. On one hand, it is influenced by the ISM.When the ISM is stronger (weaker) than normal, the SAH shifts westward (eastward) to theIranian Plateau (Tibetan Plateau). On the other hand, the east-west movement of the SAH hasa close relationship with the summer precipitation over China. When the SAH moves farthereast (west) than normal, the Yangtze-Huaihe River valley is wetter (drier) than normal whileNorth China and South China are in dry (wet) conditions. A possible mechanism that the ISMaffects the summer rainfall over China via the SAH is proposed. The strong ISM with heavyrainfall in India leads to a strong condensation heating source above the northern Indianpeninsula, which causes a positive height anomaly to its northwest while a negative heightanomaly to its northeast at the upper atmosphere. As a result, the SAH moves westward withits center over the Iranian Plateau. An anomalous anticyclone extends from the Iranian Plateauto the western Tibetan Plateau and an anomalous cyclone extends from the eastern TibetanPlateau to eastern China. The anomalous cyclone might lead to a convergence at the upperatmosphere over eastern China and cause a strong ascending motion in the Yangtze Rivervalley. Therefore, the precipitation in the Yangtze River valley is strong. Results of the presentwork give a new route that the ISM affects the EASM. The SAH plays an important role inconnecting the ISM and the EASM.