Influence Of Water Vapor Variation In The Upper Troposphere And Lower Stratosphere On South Asia High In Summer

Using the three reanalysis datasets of ERA-Interim,JRA-55 and NCEP-CFSR during 1979-2015,the radiosonde data during 1979-2014 and the Microwave Limb Sounder(MLS)satellite data during 2005-2015 to study variation characteristics of the South Asian High(SAH)and water vapor in the upper troposphere and stratosphere(UTLS).The correlation analysis and causal analysis method were used to reveal the effect of water vapor in UTLS region on SAH in summer,and then influence mechanism of water vapor in UTLS on SAH was verified by CAM5 model.The results are as follows:(1)In the time period 1979-2014,the trends in geopotential height in the control area of the SAH were positive in the JRA-55,NCEP-CFSR and ERA-Interim datasets,but negative in the radiosonde data.The reasons for the differences in the trends of the SAH between the radiosonde data and the three reanalysis datasets in the time period 1979-2014 were updates to the sounding systems around the year 2000.We therefore analyzed the trends in the two time periods of 1979-2000 and 2001-2014 separately,and found that NCEP-CFSR datasets and radiosonde datasets were more consistent,followed by ERA-Interim and JRA-55.During 2005-2015,the spatial distributions of UTLS water vapor in ERA-Interim,JRA-55,NCEP-CFSR and MLS satellites were similar in summer,and were also numerically similar.Compared with the reanalysis datasets of JRA-55 and NCEP-CFSR,the errors between ERA-Interim reanalysis data and MLS satellite data were the lowest,and the water vapor between them were more consistent.(2)In the upper troposphere and stratosphere,the SAH center and water vapor center were inclined slightly to the northwest with altitude.Below 150 hPa,the high values area of water vapor is located on the southeast of SAH,and basically corresponding to the deep convection affected area;above 150 hPa,the center of water vapor was near the center of SAH.The SAH intensity and water vapor showed a significant positive correlation:when SAH was stronger(weaker),the water vapor in the UTLS area of eastern SAH was significantly more(less),while the water vapor anomalies in the northwest of SAH were not significant.The reason that water vapor content is more(less)in the middle of SAH over the UTLS area with stronger(weaker)SAH may be caused by suction and the transmission barrier of SAH to water vapor.The water vapor content is more(less)in the SAH southeast side of the UTLS area when SAH was stronger(weaker),which may be associated with latent heat release generated by transport of deep convection for water vapor.(3)The sensitivity test for water vapor reduction was performed using the CAM5 model.It was found that when the water vapor in the Tibetan Plateau decreased in summer,the SAH intensity in the central and eastern parts of SAH weakened significantly,and the temperature at the corresponding location also decreased,especially at 150 hPa and 200 hPa.Therefore,the positive correlation between water vapor in UTLS and SAH was verified,that is when water vapor changes,the temperature decreased and the SAH intensity also decreased.