Characteristics And Causes Of Interannual And Interdecadal Variation Of Summer Precipitation In Greater Mekong Subregion

The Greater Mekong Subregion(GMS)is a geographical area composed of six countries and regions,mainly including Yunnan Province of China,Myanmar,Thailand,Laos,Cambodia and Vietnam.In this paper,the spatial distribution of summer precipitation over the GMS,and its temporal variations on the interannual and interdecadal time scales have been thoroughly investigated based on the high-resolution daily Asian land precipitation data of 1958 to 2015 from the APHRODITE(Asian Precipitation-High Resolved Observational Data Towards Evaluation of Water Resources),JRA55 monthly mean reanalysis data and the SST data from the Hadley Centre of the Met Office.This study has employed the following research methods: the empirical orthogonal function(EOF)decomposition,Lanczos filter,regression and composite analysis and the significance test.The main conclusions are as follows:(1)The spatial distribution of mean annual and summer precipitation and relative variability of precipitation in the GMS are similar during 1958-2015,and the summer precipitation accounts for 75% of the annual precipitation.The relative variability of annual and summer precipitation in central Myanmar and Southeast Vietnam is significantly higher than that in the rest of the GMS.In this area,the precipitation fluctuates greatly,and drought and flood disasters are more likely to occur.The spatial distribution of precipitation and its relative variability in other seasons are quite different from those in summer.(2)The variance contribution rate of the first EOF mode of summer precipitation in the GMS is 27.8%,and it shows significant interannual and interdecadal variation characteristics of quasi-3 years and quasi-19 years,respectively.(3)The first EOF mode of the interannual variation of summer precipitation in the GMS exhibits the “?-Pattern” with the explained variance of 27.4%,which is mainly attributed to ENSO.When the La Ni?a event occurs in previous winter and persists until the next spring,the summer precipitation in the GMS is in the positive phase of "?-Pattern";On the contrary,if the El Ni?o event occurs in the last winter and persists until the next spring,the summer precipitation in the GMS is in the negative phase of "?-Pattern".(4)The second EOF mode of the interannual variation of summer precipitation in the GMS shows a “South-North Reversal Pattern” that accounts for 13.5% of the total variance,which is mainly affected by the sea surface temperature anomalies(SSTAs)in the Western Pacific and the South Pacific.When the SSTAs in the Western Pacific Warm Pool and the South Pacific are positive during spring and summer,and the tropical East Pacific SSTAs are not significant,the summer precipitation in the GMS is in the positive phase of "South-North Reversal Pattern";On the contrary,if the SSTAs in the Western Pacific Warm Pool and the South Pacific are negative,and the tropical East Pacific SSTAs are still insignificant,the summer precipitation in the GMS is in the negative phase of the "North-South Reversal Pattern".(5)The first EOF mode of the interdecadal variations of summer precipitation in the GMS roughly shows a "Uniform Consistency Pattern" with explained variance of 53.2%,and its interdecadal turning point occurred around 2000.The Atlantic SSTA is a key factor for the interdecadal variation of summer precipitation in the GMS.When the Atlantic SSTA is anomalously cold,the summer precipitation on the interdecadal time scale in the GMS is in the positive phase of the " Uniform Consistency Pattern ";Conversely,when the Atlantic SSTA is anomalously warm,the summer precipitation on the interdecadal time scale in the GMS is in the negative phase of the " Uniform Consistency Pattern ".