Characteristics Of Meridional Circulation Cell On The South Side Of Tibetan Plateau And Its Effects On Precipitation Over The Region

Based on the daily observational precipitation data,snow depth data and ERA-Interim daily reanalysis data from 1979 to 2015,the meridional circulation cell characteristics on the south side of Tibetan Plateau and its effects on precipitation and water vapor transport over the Tibetan Plateau has been investigated.Furthermore,the possible influence factors of the summer meridional circulation are discussed.The main results are shown as follows:(1)The pre-monsoon circulation,monsoon circulation and Hadley circulation constitute seasonal evolution of meridional circulation cell on the south side of the Tibetan Plateau along 80°-900°E.A decreasing trend is observed with a rate of-0.377·s-1/10a in the intensity of pre-monsoon circulation,while an increasing trend is detected with a value of 0.524 m·s-1/10a in the intensity of monsoon circulation.There exists the transition of monsoon circulation and Hadley circulation on the south side of Tibetan Plateau along 90°-105° E.The intensity of monsoon circulation remarkably increases during 1979-2015 with a trend of 0.413 m·s-1/10a.(2)According to the definition of building and ending time of each meridional circulation cell,the setup of pre-monsoon circulation is delayed and the ending time is advanced and hence the maintaining time of pre-monsoon circulation trends to be shortened.The transition of the meridional circulation cell on the south side of the Tibetan Plateau from the winter type to the summer type tends to be faster.Along the 900°-105° E,the maintaining time of monsoon circulation shows an increasing trend while the maintaining time of Hadley circulation presents a decreasing trend.(3)When the pre-monsoon circulation becomes stronger,the water vapor in divergence region trends to divergent and the water vapor in convergence region trends to convergent.Moreover,northeast water vapor transport increases on the southwest Tibetan Plateau and south-west water vapor transport enhances on the northwest Tibetan Plateau.The strong summer monsoon circulation is beneficial to northward water vapor transport in the southern Tibetan Plateau and the Bay of Bengal and south-west water vapor transport from the Indian Ocean to the Tibetan Plateau.The pre-monsoon circulation and summer monsoon circulation have a certain effect on the next period vapor transport and water vapor convergence divergence in the lower reaches of the Tibetan Plateau(the region in the east side of the Tibetan Plateau and the upstream area of Yangtze River).(4)The precipitation decreases in the middle and southwest of the Tibetan Plateau,whereas increases in the southeast and north of the Tibetan Plateau are clear as pre-monsoon circulation enhances.When summer monsoon circulation strengthens,the more/less rainfall occurs in the south/north of the Tibetan Plateau.(5)Based on the theory of thermal adaptation and the principle of thermal wind,the anomalous atmospheric heat sources over the northern plateau,the south side of the plateau and the eastern side of the plateau affect the latitudinal temperature gradient,causing vertical motion anomalies and thus affecting the summer meridional circulation.There are two major modes of Indian Ocean SST anomalies influncing the summer meridional circulation.One mode is that the Indian Ocean SST anomalies are consistent in all area.The other mode is that the Indian Ocean SST anomalies exits east-west inverse difference.The thermal conditions of the Tibetan Plateau and the Indian Ocean have a synergistic effect on the summer monsoon circulation,namely,the effect of sea-land thermal synergy(sea-land temperature difference)on the summer monsoon circulation is more significant than that of the Tibetan Plateau(Indian Ocean)thermal conditions on the summer monsoon circulation.The correlation coefficient between sea-land temperature difference and summer monsoon circulation intensity reaches-0.56.The snow cover on the Tibetan Plateau changes the thermal difference between the plateau and the ocean(the surrounding mainland),resulting in the anomalous vertical movement of the air and then influences the meridional circulation on the south side of the Tibetan Plateau.(6)When the South Asian high(East Asian subtropical westerly jet)moves eastward/southward,the summer monsoon circulation on the south side of the Tibetan Plateau trends to weaken.On the contrary,when the south Asian high(westerly jet)locates further north/west,the summer monsoon circulation becomes stronger.The shift of the south Asian high(westerly jet)influences the convergence and divergence fields in the upper atmosphere,with consequent adjustment of the air mass in the lower atmosphere,resulting in the occurrence of a divergence convergence zone opposite to the upper atmosphere in the lower atmosphere,inducing a vertical airflow anomaly and then affecting the summer meridional circulation.