Intraseasonal variation of the East Asian summer monsoon in La Niña years

Abstract

Based on the composite result of six major La Niña events during 1979–2012, the authors reveal the intraseasonal variation of the East Asian summer monsoon (EASM) and summer rainfall in East Asia in La Niña years. Due to a higher SST over the western Pacific warm pool in the proceeding winter and spring, warm pool convection in summer is enhanced, leading to a cyclonic anomaly in the subtropical western Pacific. As a result, the western Pacific subtropical high is located more northeastward, and the seasonal march in East Asia is thus accelerated. This anomalous pattern tends to change with the seasonal march, with a maximum anomaly in July. Besides, there is less Mei-yu rainfall in the Yangtze River basin, with an earlier start and termination. The rainfall distribution in East Asia during La Niña years is characterized by a zonal pattern of less rainfall in eastern China and more rainfall over the oceanic region of the western Pacific. By comparison, a meridional pattern is found during El Niño years, with less rainfall in the tropics and more rainfall in the subtropics and midlatitudes. Therefore, the influence of La Niña on the EASM cannot be simply attributed to an anti-symmetric influence of El Niño.