The Interannual Shift Of The South Asian High And Its Relation With The Asian Summer Monsoon

The South Asian high (SAH) is the most intense and persistent anticyclone system in the upper troposphere and lower stratosphere over the Northern Hemisphere during boreal summer. It is one of the major members of the Asian summer monsoon system at upper levels. The variation of the SAH is closely related to the circulation systems in Asia. By using the monthly ERA-40 reanalysis data and observed rainfall data, we investigated the displacement of the SAH and its role in connecting the Indian summer monsoon (ISM), the East Asian summer monsoon (EASM), the westerly jet, and the western Pacific subtropical high (WPSH). The main conclusions are summarized as follows:On the interannual timescale the zonal and meridional shifts of the SAH are significant. And the eastward (westward) shift of the SAH accompanies with the southward (northward) moving. The correlation coefficient between the zonal and meridional movements is -0.44, exceeding the 0.01 significance level. And the result of the Empirical Orthogonal Function (EOF) analysis reveals that the southeast-northwest (SE-NW) movement is a dominant feature of the SAH.Focusing on the zonal shift of the SAH, it is found that the SAH is closely related to ISM and the summer rainfall over China. When the SAH is located eastward with its center over the Tibetan Plateau (TP), the ISM is weaker than normal, and the Yangtze-Huai River valley is wetter than normal while northern China and southern China are in dry conditions. A possible physical process that the ISM affects the summer rainfall over China via the SAH is proposed. A weaker ISM associated with less rainfall over India corresponds to less condensation heat release and anomalous cooling in the upper troposphere over the northern Indian peninsula. The anomalous cooling stimulates an anomalous cyclone to its northwest and an anomalous anticyclone to its northeast in the upper troposphere, causing an eastward shift of the SAH with its center over the TP. As a result, an anomalous anticyclone is formed over the eastern TP and eastern China in the upper troposphere. The anomalous vertical motions in association with the circulation anomalies are responsible for the rainfall anomalies over China. The SAH plays an important role in the effect of ISM on the East Asian summer monsoon.Focusing on the southeast-northwest (SE-NW) movement of the SAH, it is found that both ISM rainfall and the summer rainfall over China have a closer relation to the SE-NW movement of the SAH than those to the zonal or meridional variation of the SAH, indicating the SE-NW variation of the SAH can better reflect the relationship among the SAH, the ISM and EASM rainfall. The southeastward (northwestward) shift of the SAH is closely related to a less (more) Indian summer monsoon rainfall and more (less) rainfall in the Yangtze River Valley (YRV) over the East Asian summer monsoon region.An anomalous AGCM is utilized to examine the effect of latent heat anomalies associated with the Asian summer monsoon rainfall on the SAH. The negative latent heat anomalies over the northern Indian Peninsular associated with a weak Indian summer monsoon is responsible for the east-west shift of the SAH and more rainfall in the YRV. The positive latent heating release associated with rainfall anomalies in the YRV excites a southward located anticyclone over eastern China, exerting a feedback effect on the SAH and leading to a SE-NW shift of the SAH.By diagnosing the role of the ISM and the SAH in the variation of the upper-level westerly jet and rainfall anomalies over the westerly region, it is found that weak ISM leads to southeastward located SAH and westerly jet, which induces the anomalous convergence over the Central Asia and the anomalous divergence over northern China in the upper level. The circulation anomalies associated with the southeast shift of SAH over the westerly region lead to the positive and negative vorticity advection anomalies over Central Asia and northern China, respectively. As a result, the anomalous ascending motions with more rainfall are induced over Central Asia and the anomalous descending motion with less rainfall appears over northern China.Further studies found that two negative vorticity centers are located between the center of the SAH and the centers of the upper-level westerly jet. The eastern vorticity center is over the TP. Being consistent with the eastern part of the SAH and the eastern center of the westerly jet, the SE-NW variation of the eastern vorticity center is pronounced. It can represent the SE-NW shift of both the SAH and the eastern jet center. When the SAH and westerly jet move southeastward, an anomalous anticyclone is notable over eastern China at 200 hPa. And at 500 hPa, the anomalous anticyclone is located over southern China and the northern South China Sea, which leads the WPSH extend westward. The vorticity equation is diagnosed to investigate the formation mechanisms of the negative vorticity anomalies at 500 hPa. It is found that the anomalous negative relative vorticity is mainly caused by the vertical advection of mean relative vorticity by the anomalous sinking motion. When the SAH extends southeastward, the strong convergence at the southeast flank of the anomalous anticyclone at the upper level induces the descending motion and causes the negative vertical vorticity advection. As a result, the negative vorticity anomaly is formed and makes the WPSH extend westward. The result of the anomalous AGCM also prove that the anomalous anticyclone associated with the southeastward SAH will trigger an anomalous anticyclone at mid-troposphere, and result in the westward extension of the WPSH.