Research On The Factors Regulating Summer Precipitation Over The Tianshan Mountains On Interdecadal And Interannual Time Scales

The Tianshan mountains are usually known as the “wet island” of the arid region of Central Asia.Its abundant atmospheric precipitation,especially in summer,is of great importance in supplying and regulating the flows of rivers and the volume of water in lakes in Xinjiang and therefore has a profound impact on local agriculture,animal husbandry and ecosystems.Based on the observation data and NCEP/NCAR reanalysis data and NOAA SST data,the impact factors and mechanisms of the summer precipitation over the Tianshan mountains are explored,respectively on interdecadal and interannual scales.The major conclusions are as follows:(1)A clear interdecadal increase of summer precipitation over the Tianshan mountains has occurred since the 1990 s during 1961 – 2016.The sea surface temperature anomalies(SSTAs)in the high-latitude northwest Atlantic and the Mediterranean Sea are partly responsible for this change.The warmer SSTAs in these two regions can excite the high pressure anomaly over the northwest Atlantic and to the north of the Mediterranean Sea,respectively,further synergistically modulating the downstream atmospheric circulation near Lake Baikal through the eastward dispersion of Rossby wave energy.As a result,a high pressure anomaly with the corresponding anomalous anticyclone circulation has occurred near Lake Baikal since the 1990 s,inducing an anomalous easterly flow over the Tianshan mountains.This anomalous easterly flow caused an interdecadal decrease in the eastward export of water vapor on the eastern boundary and thus resulted in an interdecadal increase in the total water vapor budget over the mountains.This has facilitated the interdecadal increase in summer precipitation over the Tianshan mountains since the 1990 s.(2)The variability in summer precipitation over the Tianshan mountains was further explored on interannual timescales.On the interannual timescale of ?5 years,the variability of summer precipitation over Tianshan Mountains is in accord with that over Tarim Basin,which seems to be affected by the Bimodality of the South Asia High.On the interannual timescale of 5 – 6 years,the summer precipitation over the Tianshan mountains and Central Asia shows an in-phase variability and therefore can be regarded as a whole.This larger-range precipitation may be jointly regulated by sea surface temperature anomalies in the Mediterranean Ocean and surface air temperature anomalies over its adjacent northern land.Through exciting the dispersion of Rossby wave energy along the Norwegian Sea,the West Siberian and the Central Asia,the warmer underlying surfaces over these two regions can synergistically result in a high pressure anomaly over the Mediterranean Sea – Europe region,a high pressure anomaly over the West Siberian and a low pressure anomaly over Central Asia.Such circulation anomalies guide the cold air flow southward and trigger active upward motion over Central Asia,leading to an increase in precipitation.(3)The key forcing factors mentioned above are responsible for the variabilities of summer precipitation over the Tianshan Mountains on these different timescales,and thus they have a good ability in fitting the precipitation on the selected timescales.In general,the time series regressed upon these sea and land temperature anomalies can successfully capture the trend and important turning points of observational Tianshan summer precipitation and explain a large part of the total variance of precipitation.In addition,the high-frequency variability below 5 years also accounts for a considerable part of the entire variability in summer precipitation over the Tianshan Mountains,which should be further investigated in the future.