Relationship between Two Types of East–West Oscillations of the South Asia High in Summer and Their Influences on Weather

The South Asia high (SAH) center is characterized by bimodal distribution i.e., the Tibetan Plateau (TP) mode and Iranian Plateau (IP) mode, showing an east–west oscillation pattern. In addition, the eastern edge of the SAH also regularly extends eastward to East Asia or retreats westward to the TP, manifesting another type of east–west oscillation. Using NCEP1 daily reanalysis data, APHRODITE daily precipitation data, and daily India precipitation data, this paper investigates the relationship between the two types of east–west oscillations of the SAH and the differences in their impacts on the circulation and weather in Asia. The results show that bimodal east–west oscillation of the SAH center can significantly affect the occurrence and amplitude of the eastward extension/westward retreat of the eastern edge of the SAH. Although the eastern edge of the SAH can extend eastward when the SAH center is in the TP or IP mode, the frequency of the eastward extension of the eastern edge of the SAH in the TP mode is significantly higher than that in the IP mode. In the IP mode, the eastern edge of the SAH is more inclined to retreat westward, and the magnitude of the eastward extension of the eastern edge of the SAH in the TP mode is larger than that in the IP mode. Further analysis reveals that the bimodal east–west oscillation of the SAH center is closely related to the rainfall anomaly pattern in North India and the TP region and is coupled with the variation of thermodynamic effect related to rainfall anomaly. The eastward extension/westward retreat of the eastern edge of the SAH is related to the dipole rainfall anomaly pattern in East Asia (i.e., rainfall anomalies in the central and eastern TP and the middle and lower reaches of the Yangtze Riverand Yellow River are opposite to those in the southern region of the Yangtze River), resulting in the westward extension/eastward retreat of the western Pacific subtropical high. Furthermore, when the SAH is in the TP mode and its eastern edge extends eastward and when the SAH is in the IP mode and its eastern edge retreats westward, rainfall anomaly in the western TP is always opposite to that in the central and eastern TP.