| In the summers of2010and1998, severe floods occurred in the Yangtze River Valley and Northeast China. Although both the summers follow an El Nino event, difference is significant between the events. The1997/98El Nino is a conventional one with warming in the central-eastern Pacific whereas the2009/2010event is an El Nino Modoki with warming in the central Pacific. Based on the160China stations rainfall dataset, the NCEP/NCAR atmospheric data, the extended reconstruct sea surface temperature (ERSST) and outgoing long wave radiation dataset, regression and composite analyses as well as atmospheric general circulation model experiments have been used to compare the influences of anomalous SST in the tropic during two types of El Nino events on the following summer rainfall between the two years. The characteristics of rainfall anomalies in the Yangtze River Valley and Northeast China and the associated atmospheric circulations are compared. The impacts of tropical SSTA on the large-scale atmospheric circulation in two cases and their possible mechanisms are investigated. Additionally, the relationships between different summer rainfall anomaly patterns in Northeast China and two types of El Nino events are revealed. Main conclusions are as follows:(1) Summer rainfall anomalies in China show a significant difference. In the summer of2010, there are positive rainfall anomalies in the middle and lower reaches of Yangtze River and the region from Huaihe River to Yellow River, negative in the upper reaches of Yangtze River, and in Northeast China the rainfall anomalies appeared to be dipole pattern. By contrast, more rainfall occurred in the whole region of Yangtze River Valley, and almost the whole region of Northeast China in1998.(2) The difference of western North Pacific anticyclone (WNPAC) between2010and1998is the main reason. In2010, the WNPAC is stronger in addition to a shift to the northwest, causing weakened southwesterly in the region from the Bay of Bengal to the South China Sea but intensified southerly in eastern China. This results in less water vapor transport from the tropical Indian Ocean and the South China Sea but more from the western subtropical Pacific to East Asia. Subsequently, the rainfall band in2010shifted northward.(3) It is found that there is a statistic relationship between different rainfall anomaly patterns in Northeast China and two types of El Nino events. Influenced by the tropical SSTA,"PJ" like pattern appears along the coast of the East Asia in the following summer of a typical El Nino event. Then, summer monsoon become more strength, and more water vapor can be transported to Northeast China, and at the same time, anomalous cyclonic circulation with great convergence appears in Northeast China, blocking circulation strongly persists in the north, causing more rainfall in whole region; by contrast, the anomalous large-scale circulations in the following summer of an El Nino Modoki event entirely shift northward, which presents that the East Asian subtropical westerly jet and western subtropical high both move northward. Correspondingly, the anomalous cyclone in Northeast China in typical El Nino cases moves to the north in El Nino Modoki cases, causing the "dipole pattern" in which positive rainfall centers in the south part of Northeast China, while negative rainfall centers in the north.(4) By comparing the two chosen cases and composites of two flavors of El Nino, the influences of tropical SSTA in1997/98and2009/10on summer rainfall and large-scale atmospheric circulations are conformed in further. But when2010is compared with the composite of historical El Nino Modoki events, and difference is seen. This indicates that the impact of El Nino Modoki is more complicated than revealed previously, and there perhaps exists difference from case to case.(5) Difference of the anomalous ascending motion associated with the warming location in the two El Nino events is the key mechanism. Furthermore, the role of tropical SST in modulating these differences is investigated by conducting sensitive experiments in atmospheric general circulation model, GFDL AM2.1. The results suggest that the primary difference in monsoonal circulation systems in the two years is well simulated. It is concluded that the difference in El Nino events shapes the rainfall patterns in the two years of1998and2010. |
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