Thermal contrast between the middle-latitude Asian continent and adjacent ocean and its connection to the East Asian summer precipitation

To analyze the middle-to-lower-troposphere atmospheric thermal contrast between the middle latitude over the Asian continent and over its eastern adjacent ocean near Japan, an empirical orthogonal function (EOF) analysis of the 40-yr ECMWF Re-Analysis (ERA-40) data of the June-August (JJA) 500-hPa geopotential height over the Asia-Pacific area (10 degrees-80 degrees N, 60 degrees-180 degrees E) during 1958-2000 was done. It shows that the dominating pattern of the thermal contrast may well be represented by a "seesaw" of 500-hPa geopotential height anomalies between a land area (40 degrees-55 degrees N. 75 degrees-90 degrees E) and an oceanic area (3 degrees'-42.5 degrees N. 140 degrees-150 degrees E). An index showing the difference between the two areas is defined as the middle-latitude land-sea thermal contrast index (LSI). The LSI has significant interannual and interdecadal variability. Its interannual variation is mainly attributed to the atmospheric thermal condition over the ocean, which has a remarkably regional unique feature. while the interdecadal variability is greatly attributed to that over the land. The LSI has a close connection to the East Asian summer precipitation. The results show that large (small) LSI is related to high (low) Summer precipitation in the middle to lower reaches of the Yangtze River. Korea, Japan, and its eastern adjacent ocean at the same latitude, and low (high) precipitation in file South China Sea and tropical western Pacific, as well as low (high) precipitation in north China and high-latitude northeast Asia. The pattern of correlation between LSI and precipitation resembles the spatial distribution of the principle EOF mode of year-to-year precipitation variations. Furthermore. the variation of I-SI is highly correlated to the time series of the first EOF made of summer precipitation anomalies. This suggests that the middle-latitude land-sea thermal contrast is one of important factors to influence on the summer precipitation variations over the area from the whole East Asia to the western Pacific. The possible physical mechanisms of file land-sea thermal contrast impacting the East Asian summer monsoon precipitation are also investigated.