Characteristics and Mechanism of Subseasonal Eastward Extension of South Asian High

The South Asian High is a planetary-scale anticyclonic circulation system in the upper troposphere, with its major body located over the Tibetan Plateau (TP) and its neighboring areas during boreal summer. This study investigates the features of the eastward extension of the SAH on subseasonal timescale, and its connection with diabatic heating/rainfall. The causes of SAH's eastward extension are examined by performing potential vorticity (PV) diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wavetrain. Based on a definition of SAH's eastward extension index (EI), a total of 40 eastward extension events are identified for the summers of 1979-2012. The lagged composites of 12520-line at 200-hPa for the eastward events demonstrate an eastward-then-westward migration of the SAH. The SAH eastward extension is featured by eastward propagation of wavetrain across the Eurasian continent in the regression fields of geopotential height and PV at 200-hPa on EI. When the SAH extends to its easternmost position, a strong negative PV (positive geopotential height) center prevails to the east of the Tibetan Plateau at 200-hPa. Three zonally elongated regions with anomalous diabatic heating/rainfall over East Asia is connected with the SAH's eastward extension. The anomalous heating/above-normal rainfall occurs over the South China Sea (SCS)-subtropical western Pacific twelve-day before SAH's easternmost stretch, followed by anomalous cooling/below-normal rainfall over south China and anomalous heating/above-normal rainfall over north China-east Mongolia a week later. The heating over the SCS-western Pacific induces an anomalous cyclone at 850 hPa over the East Asian coastal region, favoring the development of below-normal rainfall/ cooling over south China. The PV diagnosis indicates that the anomalous heating and ascending motion generate negative PV anomalies at 200-hPa directly over north China-east Mongolia. While anomalous cooling and descending motion produce positive PV anomalies over south China. Those south/north dipolar structure of PV generation indicates large value of meridional gradient of PV anomalies. As a consequence, the negative PV anomalies over the north lobe are transported southwardly by the advection of climatological northerly located to the east and southeast of the TP. Above processes contributes to eastward extension of the SAH.