Characteristics and Mechanisms of the Subseasonal Eastward Extension of the South Asian High

Abstract

his study investigates the features of eastward extension of the South Asian high (SAH) and its connection with diabatic heating and rainfall over eastern Asia on subseasonal time scales. The causes of SAH?s eastward extension are examined by potential vorticity (PV) diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wave train. The SAH?s eastward extension features eastward propagation of a wave train across Eurasia. Among the wave train, the migration of weak high from the western flank of the Tibetan Plateau (TP) to the east of TP contributes to the SAH?s eastward extension at the early stage. When the SAH approaches its easternmost position, a strong negative PV (positive geopotential height) center prevails to the east of the TP at 200 hPa. The associated anomalies in diabatic heating and rainfall include the anomalous heating and above-normal rainfall over the South China Sea (SCS) and subtropical western Pacific occurring 12 days before the SAH?s easternmost stretch, and then anomalous cooling and below-normal rainfall over the southern foot of the TP and southern China and heating and above-normal rainfall over the northern TP and northern China a week later. The anomalous heating and ascending motion over the northern TP and northern China act to increase negative PV locally at 200 hPa. The cooling and descending induce positive PV over southern China. The north?south dipolar structure of PV anomaly with the climatological northerly flow is favorable to southward advection of a negative PV anomaly at 200 hPa. The anomalous heating over the SCS?western Pacific helps to develop a below-normal rainfall condition over southern China via inducing a lower-level anomalous cyclone over coastal region. These processes are conducive to the SAH?s eastward extension at its later stage.