The influences of East Asian Monsoon on summer precipitation in Northeast China

A unique dataset of 53-year (1961–2013) rainfall measurements from 104 stations uniformly distributed in the Northeast China, combined with the observation-based NCEP/NCAR atmospheric reanalysis, is used to analyze the precipitation anomalies in Northeast China during late boreal summer (July–August) and their relationship with the anomalous moisture transport associated with the fluctuations of the East Asian Summer Monsoon (EASM) circulation. Based on this analysis, a new EASM influence index (IEASM) is proposed to quantify the EASM effects on the Northeast China summer precipitation. The relationship between the IEASM variations and patterns of the anomalous regional atmospheric circulation is demonstrated. The characteristics of several precursors that lead to the major fluctuations of the IEASM index are also explored. The results show that the EASM influence index is closely linked to the anomalous rainfall in Northeast China and can be used as a major factor to measure the physical processes that affect the regional dry and wet conditions. The IEASM index responds to the large-scale anomalies of the atmospheric circulation sensitively. Specifically, the high IEASM values are associated with the intensified Mongolia cyclone, blocking developing near the Ural Mountains and a northwestward shift of subtropical high over the western Pacific. The low IEASM values are associated with a reversed pattern of these features. The IEASM anomalous fluctuation has some precursors. A major high (low) index during the summer is likely preceded with the pattern of the sea surface temperature anomalies of an El Niño (La Niña) event in the Pacific from the previous early fall to early winter.