Zonal shift of the South Asian High on the subseasonal time-scale and its relation to the summer rainfall anomaly in China

摘要

By using daily National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis and observed rainfall data, we investigated the east-west shift of the South Asian High (SAH) on the subseasonal time-scale and its relationship with the rainfall anomaly in China. It is found that the zonal shift of the SAH at 200 hPa displays a dominant periodicity of 10-50 days, with two preferred locations, one over the northeast Tibetan Plateau (eastern location) and the other over the northeast Iranian Plateau (western location). The zonal shift of the SAH is closely related to the southward movement of intraseasonal perturbations originating in middle latitudes. A vorticity budget analysis reveals that the advection of mean vorticity by the perturbation of meridional wind primarily contributes to the vorticity tendency over both the eastern and western locations. The zonal shift of the SAH closely connects to the summer rainfall anomalies over China. An eastern (a western) location of the SAH corresponds to generally positive (negative) rainfall anomalies in northwestern China and negative (positive) anomalies in southern China. A further diagnosis reveals that the strengthening of the intraseasonal perturbation during the southward journey in the eastern location is attributed to a positive convection-circulation feedback. On the one hand, a positive geopotential height anomaly in the upper troposphere and associated upper-level divergence induce anomalous ascending motion and condensational heating in the middle troposphere. On the other hand, the positive heating anomaly increases mid-tropospheric temperature and strengthens upper-level geopotential height. The southward propagation leads to the zonal shift of the SAH.