Multi-scale Variation Of Summer Precipitation And Its Moisture Source Diagnosis In Three-rivers Headwater Region,China

Studies have investigated a wetting trend over the Three-Rivers Headwater Region(TRHR)in recent decades,although there are still uncertainties in the possible mechanisms of this change.Based on daily precipitation observations,this paper first analyzed the multiple time scale characteristics of summer precipitation in TRHR,highresolution simulations were then carried out for several years(1980-2017)based on the Lagrangian Particle Diffusion Model FLEXPART driven by ERA-Interim reanalysis data.Through the backward tracking of all the air parcels entering the TRHR,the moisture source diagnosis technology is adopted to quantitively reveal the spatialtemporal distribution characteristics of the summer water vapor source in TRHR.Combining statistical methods such as EOF analysis and correlation analysis,the causes and possible mechanisms of multi-scale changes of precipitation in the TRHR were analyzed from the perspective of water vapor source changes.The main conclusions are summarized as follows:(1)There are obvious interannual and interdecadal oscillations of summer precipitation in TRHR,especially since the middle and late 1990s(1995),there has been a decadal transition from a decrease to a sudden increase in precipitation.The summer precipitation in TRHR presents a long-term increasing trend,showing spatial inhomogeneity,and the increase is even more significant in the northwest region.The long-term increase trend of summer precipitation in TRHR is mainly caused by the change of heavy precipitation events,which is reflected in the increase of heavy precipitation days and the enhancement of precipitation intensity.(2)The results of moisture source analysis based on high-resolution Lagrange simulation show that the moisture of summer precipitation in TRHR can be roughly divided into three parts: Water vapor transported from large-scale monsoon circulation(Arabian Sea-Indian Ocean),Water vapor transport by westerlies(Eurasia)and Local Water Vapor Recycling(Water Vapor around the TRHR and the Qinghai-Tibet Plateau).There is a close relationship between summer precipitation and water vapor transport in the TRHR,especially the change of precipitation and the moisture contribution are consistent,indicating that it is feasible to reveal the causes of precipitation change from the perspective of water vapor source changes.(3)On the interannual scale,the moisture contribution of the Arabian Sea-Indian Ocean and the local recirculation process over the plateau are important factors affecting the interannual variation of regional precipitation.Composite analysis in drought and flood years shows that the moisture contribution of the area around the plateau and distant ocean in the southwest increases during flood years,the control of strong South Asian monsoon leads to the convergence of water vapor,which in turn increases precipitation.The negative SST anomaly mainly presented over the equatorial Middle East Pacific during the same period,which strengthened the Walker Circulation and transported the warm and wet moisture from the Pacific to the land area.What's more,the water vapor evaporation caused by the positive anomaly of the Indian Ocean sea temperature is more conducive to the South Asian monsoon water vapor transport.On the contrary,the precipitation is mainly dependent on the water vapor transported by westerlies from Central Asia and Eurasia in the dry years.At this time,water vapor divergence over the Plateau under the control of northerly airflow,and the positive SST anomaly in the same period is not conducive to the formation of precipitation.(4)The summer precipitation in the TRHR experienced an interdecadal transition from decrease to increase in the mid to late 1990 s,influenced by the variation of local recirculation and long-distance land-ocean moisture sources.Specifically shown as the enhanced water vapor sources with short transport time are mainly located in the northeast of the Tibetan Plateau,while the enhanced water vapor sources with longer transport time scale are located in the north of the Indian Peninsula and the vast area of Arabian Sea-Indian Ocean under the control of South Asian summer monsoon.These results indicate that the interdecadal increase in summer precipitation over TRHR is due to the combined effects of enhanced local water recirculation,increased monsoon water vapor transport,and the changes in the relative strength of South Asian monsoon and mid-latitude westerlies.The analysis of the characteristics of atmospheric circulation field and sea surface temperature further confirms the above results.The abnormal anticyclone system in the northern plateau triggered by the mid-high latitude wave trains provides better condensation conditions for the precipitation in the TRHR.With the weakening of the driving force of the Indian summer monsoon,the positive sea surface temperature anomaly in the Arabian Sea-Indian Ocean still increases the specific humidity of the atmosphere,which is conducive to the increase of summer precipitation.(5)The water vapor that caused the long-term increase in summer precipitation in the Three Rivers Source area originated from the eastern area of the entire Three Rivers Source area,the northern Bay of Bengal and the local area in the northwest side of the TRHR.In particular,the steady increase of water vapor contribution from the land area on the northwest side of TRHR is the main moisture source that causes the long-term increase in summer precipitation in the TRHR.Further analysis shows that this is mainly related to the enhanced contribution of water vapor sources around the ThreeRiver Headwaters(especially the local precipitation recirculation process in the northwest)associated with heavy precipitation events.