Formation of the Anomalous Summer Precipitation in East China in 2010 and 1998: A Comparison of the Impacts of Two Kinds of El Niño

In the summers of 1998 and 2010, severe floods occurred in the middle and lower reaches of the YangtzeRiver. Although an El Niño event took place preceding each of the summer floods, significant differencesbetween the two summer floods and the two El Niño events were identified. The 1997/98 El Niño is aconventional one with strongest warming in the central-eastern Pacific, whereas the 2009/10 event is an El Niño Modoki with strongest warming in the central Pacific. In this study, summer rainfall anomalies (SRA)in the two years were first compared based on the rainfall data at 160 stations in mainland China, and asignificant difference in SRA was found. To understand the underlying mechanism for the difference, theatmospheric circulation systems, particularly the western North Pacific anticyclone (WNPAC), the westernPacific subtropical high (WPSH), and the low-level air flows, were compared in the two years by using theNCEP/NCAR reanalysis data. The results display that the WNPAC was stronger in 2010 than in 1998, alongwith a northwestward shift, causing weakened southwesterly from the Bay of Bengal to the South China Seabut intensified southerly in eastern China. This resulted in less water vapor transport from the tropical IndianOcean and the South China Sea but more from the subtropical western Pacific to East Asia. Subsequently,the rainband in 2010 shifted northward. The difference in the WNPAC was caused by the anomalousascending motion associated with the warming location in the two El Niño events. Furthermore, the roleof tropical sea surface temperature (SST) in modulating these differences was investigated by conductingsensitivity experiments using GFDL AM2.1 (Geophysical Fluid Dynamics Laboratory Atmospheric Model).Two experiments were performed, one with the observed monthly SST and the other with June SST persistingthrough the whole summer. The results suggest that the model well reproduced the primary differences inthe atmospheric circulation systems in the two years. It is found that the difference in El Niño events hasshaped the rainfall patterns in the two years of 1998 and 2010. At last, the case of 2010 was comparedwith the composite of historical El Niño Modoki events, and the results indicate that the impact of El Niño Modoki varies from case to case and is more complicated than previously revealed.