The Changes Of Summer Precipitation In Dongting Lake Basin And Its Influencing Factors In The Last 55 Years

Based on summer precipitation data,the corresponding NCEP/NCAR reanalysis data and Hadley center global SST data at 96 weather stations within Dongting Lake basin between 1961 and 2015,the change characteristics of summer precipitation in the Dongting Lake basin in recent 55 years and its influencing factors are analyzed,with main discussion on the relationships between the summer precipitation amount and atmospheric general circulation patterns and global SST anomaly.The results are as follows:(1)In recent 55 years,summer precipitation amount in Dongting Lake basin is relatively small in the central and Northern Plain,and relatively large in the Lishui River basin,the Yuan River basin in the southwest and the hilly region in the southeast,which is greatly influenced by topography.And the stations with heavy precipitation are commonly located in higher altitude or the windward slope.The average summer precipitation amount at stations,characterized by both higher altitude and the windward slope,is higher than that in the entire watershed.Additionally,in some areas where precipitation amount is large,the fluctuation range of precipitation amount is also relatively large,which are vulnerable to drought and flood disasters.(2)In the last 55 years,the summer precipitation amount in the Dongting Lake basin has significant interannual and interdecadal variations.In particular,the summer precipitation amount in the entire basin shows no significant increasing trend,and there is an interdecadal periodical variation with relative less 10-year precipitation amount followed by relative much 10-year precipitation amount.From the 1960s to 1970s,the change in the summer precipitation in the study area is generally stable,however,there is a sudden increase for the middle of 1980s to the early 1990s.And the maximum precipitation amount for 10 years is recorded in the early 1990s to early 20 century over the whole study period.In contrast,from the middle of 2000 to the beginning of the 2010s,the summer precipitation amount tends to decrease with years.(3)For the past 55 years,abnormal summer precipitation in Dongting Lake basin is closely related to the contemporaneous West Pacific subtropical high locations and atmospheric circulation patterns at 500 hPa in mid-high latitude area.During typical summer flood years,when geopotential height at 500 hPa in Eurasia exhibits"positive-negative-positive" in zonal distribution and "north low-south high" in meridional distribution,the cold air mass is too weak to move down southward,the West Pacific subtropical,however,is stronger than normal,combined two effects contribute to a more summer precipitation amount in Dongting Lake basin;in a comparison,during typical summer drought years,when geopotential height at 500 hPa in Eurasia exhibits "negative-positive-negative" in zonal distribution and "north high-south low" in meridional distribution,the cold air mass is stronger than normal,which is favorable for southward invasion of cold air mass.But at that point,the West Pacific subtropical is weaker than normal,which is unfavorable for warm and humid airflow from the Western Pacific to control the Dongting Lake basin,combined two effects contribute to a less summer precipitation amount in Dongting Lake basin.(4)The water vapor transport patterns in the typical year of the anomalous summer precipitation in the Dongting Lake basin show significant characteristics as the follows:During typical summer flood years,four branches of water vapor transport from the southwest,the South China Sea,the Western Pacific and the mid-high latitudes,respectively,confluence in the Dongting Lake basin,causing to a stronger water vapor convergence.During typical summer drought years,the intensity of water vapor transport in the southwest channel is relatively weak than normal affected by the weak subtropical high in the Western Pacific,and the water vapor transport from the West Pacific is greatly reduced.In addition,the water vapor transport from the South China Sea is also relatively reduced,the water vapor path from the mid-high latitudes even disappear.In these cases,there is a strong water vapor divergence in the basin.(5)There is an evident negative correlation between SST over the middle latitudes of North Pacific and the abnormity of Dongting Lake basin precipitation.And a positive relationship between the sea temperature in the equatorial East and Middle Pacific on a period from 12 to 4 month prior to monthly precipitation in the study area and the abnormity of Dongting Lake basin precipitation amount is observed.What's more,a positive correlation between the mean sea surface temperature in the middle latitudes and the low latitudes in China and the abnormity of Dongting Lake basin precipitation amount is also recorded.