On the formation of anomalous summer precipitation in the years of 2010 and 1998: A comparison of the El Nino’s impact between Modoki and typical El Nino cases

In the summers of 1998 and 2010, severe floods occurred in the middle and lower reaches of the Yangtze River. Although both the summers follow an El NiNo event, difference between the events is significant. The 1997/1998 El NiNo is a conventional one with warming in the central eastern Pacific whereas the 2009/2010 event is an El NiNo Modoki with warming in the central Pacific. In this study, summer rainfall anomalies in China in the two years are first compared by using the gauged rainfall dataset in 160 stations within mainland China with each other. The results show a significant difference. To understand the underlying mechanism for the difference, the atmospheric monsoonal circulation systems, particularly the western North Pacific anticyclone (WNPAC) and the western Pacific subtropical high (WPSH) as well as the lower level air flow, are compared by using the NCEP/NCAR reanalysis data. The results display that the WNPAC in 2010 is stronger, in comparison with 1998, 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 in 2010 shifted northward. Difference in the WNPAC is caused by the anomalous ascending motion associated with the different warming location in the two El NiNo events from one another. Furthermore, the role of tropical SSTs in modulating these differences is investigated by conducting sensitive experiments using the atmospheric model, GFDL AM2.1. Two kinds of experiments are performed, one with the observed monthly SSTs and the other with the persistent June SSTs in summer for the two years. The results suggest that the primary difference in the monsoonal circulation system 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 of 1998 and 2010. At last, the case of 2010 is compared with the composite of the historical El NiNo Modoki events, and difference is significant. This indicates that the impact of El NiNo Modoki events is more complicated than that revealed previously.