East-west Oscillation Of The South Asia High And Its Related Theromdynamic Forces

The South Asia high(SAH)is a large-scale anticyclone eddy in the upper troposphere and lower stratosphere which is located at mid-latitude in boreal summer.The SAH oscillates zonally around its mean position on a quasi-biweekly interval.The subtropical westerly jet(SWJ)locates on the northern side of SAH at 200 hPa,the tropical easterly jet(TEJ)locates on the southern side of SAH at 100 hPa,and the west Pacific subtropical high(WPSH)locates on the eastern side of SAH at middle troposphere.This study investigated relationships and interactions between SAH,SWJ,TEJ and WPSH through describing the heat and momentum flux around the SAH and Rossby waves which propagates on the Eurasia on intraseasonal timescales in summer and dynamic diagnosing.The major contents and results as follows:The SAH is a large-scale anticyclone between easterly and westerly jet in the climate mean in summer,which forces the heat and momentum flux exchange between south and north.The heat fluxes on the eastern and western sides of the SAH have different directions due to the opposite directions of meridional wind.The heat flux is also reversed in the stratosphere(100 hPa)and troposphere(200hPa)due to the thermal structure change.The maximum momentum flux is located on the four maximum curvature corners of SAH,the directions on the northwest,northeast,southwest and southeast is different from each other,and present as opposite dipole structure.But the momentum fluxes in the stratosphere and troposphere have similar directions.The net heat and momentum always transport from the easterly to westerly(poleward)in the climate mean in summer.When the SAH first occurs in the Tibetan Plateau(TP)(eastern Pattern-EP)due to the increasing diabatic heating from spring to summer,the monsoon cell on the TP replaces the Hadley cell and the Tibetan high is primary controlled by the ascent motion.The SAH in TP presents a warm high in the upper troposphere and a cold high in the stratosphere duce to the strong diabatic heating.The net heat and momentum flux around the SAH is poleward for EP.Once the strength of SWJ and TEJ increase,and a perturbation occurs in the upstream of westerly jet,the net heat flux on the western side of SAH and the poleward westerly momentum flux on the northwestern side of SAH will be enhanced and lead to the westward movement of the SAH.When the SAH reaches the Iranian Plateau(western Pattern-WP)which is mainly controlled by the Hadley cell,the thermal structure of SAH has no change,i.e.,the SAH in IP presents a warm high in the upper troposphere and a cold high in the stratosphere.However,the causing reason of this thermal structure has been changed,which is mainly due to the adiabatic heating induced by the descend motion of Hadley cell.The net heat and momentum flux of the SAH begins to turn toward the equator.After about 4 days,the strength of westerly and easterly jet begins to decrease and the SAH moves eastward and back to the TP due to the increasing diabatic heating in the TP,thereby completing a west-east oscillation.In short,increasing and perturbation of the zonal flow induces heat and momentum flux exchange in westerly and easterly jets imbalance on either sides of SAH,which induces SAH moving in a west direction.The west-east oscillation of SAH makes the local geopotential gradient change,and further causes the strength of local mean flow change in the upper level.Namely,for WP,the strength of SWJ and TEJ increases in West and Center Asia,and the SWJ in East Asia moves southward thus makes the increasing precipitation in the middle and lower reaches of Yangtze River.And the SWJ stretches eastward into the Pacific ocean for WP.However,for EP,the strength of SWJ and TEJ increases in East Asia,and the SWJ is East Asia moves northward thus makes the increasing precipitation in the Northern China.The entirety strength of SWJ decreases and breaks in the west Pacific.Otherwise,the SWJ mainly presents baroclinicity in summer,and changes with the SAH center moving.The TEJ on the Indian Ocean also presents baroclinicity.This implies the diabiatic heat is the main reason of the formation of SWJ and TEJ on the Indian Ocean.The TEJ on the Pacific Ocean and North Africa mainly presents barotropy which implies the larger-scale circulation is the mainly formation reason in these two places.The temperature gradient on the northern and sourthern sides of SAH enchances the mean flow strength at 200 hPa and decreases the mean flow strength at 100 hPa through the thermal wind effects.This induces the SWJ at 200 hPa and TEJ at 100 hPa.The strength and location change of upper-level jet streams and the zonal moving of the SAH thus lead to summer precipitation anomalies in eastern China.Otherwise,in the WP and EP events,at higher latitude,a Rossby wave train locates in the Polar Regions at 200 hPa in Eurasia(north branch).Another Rossby wave train propagates along the SWJ in the mid-latitude which starts from the Iranian Plateau and stretched eastward to west Pacific Ocean(south branch).The north branch of Rossby wave has the different wave phases for WP and EP and occurs earlier than the south branch of Rossby wave,and the south branch of Rossby wave becomes strongest when the WP/EP onset which is mainly motivated by the disturbance in the entrance of SWJ and the east-west oscillation of SAH.The Rossby wave which propagates along the SWJ transports Rossby wave energy to Iranian high for WP and to Tibetan high for EP.I.e.,in the WP events,there occurs a divergence of Rossby wave activity flux in SWJ due to the Rossby wave propagation on the north side of Iranian plateau(IP),the Rossby wave energy is emitted from the SWJ and absorbed by the Iranian high,thus the strength of SWJ enhances there and the geopotential height(GPH)in Iranian high increases then forms the West Pattern(WP)of SAH.In the EP events,the divergence of Rossby wave activity flux occurs in the SWJ due to the Rossby wave propagation on the north side of Tibetan Plateau,the wave energy transports from the SWJ to Tibetan high,thus the SWJ strengthens on the north side of Tibetan high and the GPH in the Tibetan high increases then forms the East Pattern(EP)of SAH.The propagation of Rossby wave along the SWJ contributes to the west-east oscillation of SAH on the dynamic aspect.The west-east oscillation of SAH further motivates another eastward Rossby wave train in the extratropics which starts from the IP(TP)and stretched northeastward to Pacific Ocean.The Rossby wave which propagate in the mid-latitude forms a positive wave phase on the Korean Peninsula and South Japan for WP,and a negative wave phase there for EP.The Rossby wave activity flux is convergence at 200 hPa on the Korean Peninsula and South Japan,and propagates downward to 500 hPa,then diverges in this area,and forms a westward Rossby wave train at 500 hPa which has the same phase pattern with it at 200 hPa.The convergence at 200 hPa and the divergence at 500 hPa on the Korean Peninsula and South Japan are stronger in WP than in EP events,which effects on the eastward retreat of WPSH at WP and westward stretch of WPSH at EP.The Rossby wave train motivated by the west-east oscillation of SAH contributes to the east-west oscillation of WPSH,and further influence the summer precipitation in east China.