Thermal Contrast Between Middle-Latitude Asian Continent And Adjacent Ocean And Its Connection To The East Asian Summer Circulation And Precipitation

The land-ocean thermal contrast is the primary driver for the Asian monsoon. Much work has gone into the connection of East Asian Summer Monsoon circulation and the large scale surface thermal contrast between Tibetan Plateau and Indian Ocean and Pacific; while the middle-latitude land-sea thermal contrast between Asian continent and adjacent ocean was paid to less attention. In this paper, the main feature of the middle-latitude troposphere atmospheric thermal contrast between the land and the sea in the boreal summer was studied, the impact mechanism of the land-sea thermal contrast on the East Asian summer monsoon circulation and precipitation were examined, and then explored the impact factors on the LSI.The main conclusions are as follows:1. The middle-latitude Land-Sea thermal contrast Index (LSI) was defined and studied its variation.An empirical orthogonal function (EOF) analysis to the ERA-40 reanalysis data of June-July-August (JJA) 500-hPa geopotential height over Asia to Pacific area (60o-180oE, 10°-80°N) during 1958-2000 was done. The leading mode reveals the main feature of opposite year-to-year variation in phase of every JJA atmospheric internal thermal conditions over the middle-latitude East Asian continent and over the adjacent marginal ocean. The difference of the 500 hPa geopotential height anomalies between the land area A (75°-90°E, 40°-55°N) and the oceanic area B (140°-150°E, 35-42.5°N) is regarded as a representative of the middle-to-lower troposphere atmospheric temperature or thermal contrast between the continent and the ocean. The difference is defined as a middle-latitude Land-Sea thermal contrast Index (LSI).The LSI has significant interannual and interdecadal variability. There exists an abrupt climate change near 1978. The climatological mean of LSI before 1978 is obviously lower than that after 1978. The surging periods of LSI are 3years and 10 years. Its interannual variation is mainly attributed to the atmospheric thermal condition over the ocean,which has 3-5 year period; while the interdecadal variability is mainly attributed to the atmospheric thermal condition over the land, which has 10 years period.2. The LSI has close connection to the East Asian summer precipitation. The results show that large/small LSI is related to rich/poor summer precipitation in the middle-to-lower reaches of the Yangtze River,Korea,Japan and its eastern adjacent ocean at the same latitude and poor/rich precipitation in the South China Sea and tropical western Pacific,as well as poor/rich precipitation in North China and high-latitude northeast Asia,respectively. The pattern of correlation between LSI and precipitation resembles to the spatial distribution of the principle EOF mode of year-to-year precipitation variations. Furthermore,the variation of LSI is highly correlated to the time series of the first EOF mode 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 physical mechanism of LSI impacting on the East Asian summer precipitation is also investigated. In high LSI summers,strong land-sea thermal contrast i.e. warm air over land and cold air over ocean causes anticyclonic anomalies over Asian middle-latitude inland centered in the eastern part of Kazakhstan and the northwestern part of China and a cyclonic anomalies over East Asian middle-latitude ocean near Japan and around ocean. At the same time,an anomalous anticyclone over the tropical region from South China Sea to the western Pacific also appears to match with the anomalous cyclone near Japan. This spatial pattern causes more-than-normal summer monsoon precipitation from the middle-to-lower reaches of the Yangtze River valley to Japan and west Pacific,and less-than-normal precipitation to its north and south sides i.e. the south China and the north China. The anticyclonic anomalies over Asian inner continent likely play an important role in strengthening the anomalous north wind in north China and also positively act on the maintenances of the anomalous cyclone over Japan. In addition,the anomalous cyclone over Japan adjacent Ocean possibly acts as a bridge connecting the high-latitude systems like blocking highs and the lower latitude systems like Asian summer monsoon.3. The impact of the surface heat flux and atmospheric diabatic heating over the land and the ocean on the LSI.The results show that the surface heat flux over Eurasian inner land has little influence to the land thermal index, while the variation of the surface latent heat flux and long-wave radiation over the Pacific adjacent to Japan has highly correlation with the ocean thermal index. The changes with height of the atmospheric diabatic heating rates over the Eurasian inner land and the Pacific adjacent to Japan have different features.The variations of the middle troposphere atmospheric long-wave and short-wave radiation heating have significantly influences on land thermal index, and that of the low troposphere atmospheric long-wave radiation, short-wave radiation and deep convective heating also have impact on the yearly variation of the land thermal index. For the ocean thermal index, the variations of the surface layer atmospheric vertical diffuse heating, large-scale latent heating and long-wave radiation heating are more important, low and middle troposphere atmospheric large-scale latent heating and shallow convective heating also have impact on the yearly variation of the ocean thermal index. And then the ocean thermal index has closely connection with the low troposphere atmospheric temperature, while the land thermal index has closely connection with the middle troposphere atmospheric temperature.4. Effect of the preceding global SST anomalies on the LSI.The relations of land thermal index and ocean thermal index and the global SST anomalies in the preceding autumn, winter, spring and same summer are observed, the results show that the preceding SST anomalies in North Indian Ocean (50o-130oE,10oS-20oN) has closely relationship with the land thermal index, and the preceding SST anomalies in Northwest Pacific (140oE-180o,10oN-20oN) are highly positively correlated with ocean thermal index. Utilizing EOF analysis, we have discussed the spatial distribution of SSTA in preceding winter over North Pacific, and it is found that the variation of SSTA in the area located in Northwest Pacific could reflect the variation of SSTA in North Pacific. The spacial distribution of preceding SSTA in the high and low LSI year confirmed the important effect of these two areas.The relationship of the preceding winter and spring SSTA in the area located in Northwest Pacific and the ocean thermal index, and that of the preceding winter and spring SSTA in the area located in North Indian Ocean and the land thermal index, reveal that the preceding winter and spring SSTA in these two areas could be used as forecasting factors of the LSI. The effect of the preceding winter SSTA in these two areas on the summer circulation in East Asian was also observed. The results show that, when the preceding winter SSTA in Northwest Pacific is warmer (colder), the anticyclonic (cyclonic) anomalies appears in summer over Japan and adjacent ocean; when the preceding winter SSTA in North Indian Ocean is warmer (colder), the anticyclonic (cyclonic) anomalies appears in summer over Kazakhstan area in Eurasian inner land.5. The conceptual model of the preceding winter SST impacting on the summer circulation and precipitation in East Asian.When the preceding winter SSTA in Northwest Pacific is warmer, the anticyclonic in summer over Japan and adjacent ocean, and the weak cyclonic anomalies appears over Kazakhstan area in Eurasian inner land; and at the same time the preceding winter SSTA in North Indian Ocean is always colder, then the cyclonic anomalies appears in summer over Kazakhstan area in Eurasian inner land. The anticyclonic in Japan and adjacent ocean and the cyclonic in Eurasian inner land make the difference of geopotential height between these two areas be significant, and the LSI is higher, and result in the rich summer precipitation in the middle-to-lower reaches of the Yangtze River,Korea,Japan and its eastern adjacent ocean at the same latitude and poor summer precipitation in the South China Sea and tropical western Pacific,as well as poor precipitation in North China and high-latitude northeast Asia.