The asymmetric influence of the two types of El Niño and La Niña on summer rainfall over Southeast China
Abstract In this study, we demonstrate that the two types of the El Niño Southern Oscillations (ENSO) have asymmetric features with respect to the impact of their positive and negative phases on boreal summer rainfall over the Yangtze River Valley (YRV) and South China (SC). The relationship between rainfall over the YRV and the warm pool (WP) La Niña is positive and significant, whereas the relationship with the WP El Niño is not. In the case of the cold tongue (CT) ENSO, its positive phase has a positive influence, while there is no significant relationship with the negative phase. In contrast, rainfall over SC has a significant positive relationship with the WP El Niño, but a non-significant relationship with the WP La Niña. The positive phase of the CT ENSO has a significant negative influence on SC rainfall, while the negative phase has a non-significant impact. An asymmetric atmospheric response to the asymmetric sea surface temperature anomalies (SSTA) was also observed in the lower troposphere. The location of the center of the anomalous circulations over the study region differs during the opposite phases of the two types of ENSO. This asymmetric response is likely to be linked to the different spatial patterns of the two types of El Niño and La Niña. Atmospheric general circulation models confirm our analysis of the observed data. Numerical simulations show that the asymmetric response of the lower atmosphere is driven mainly by differing SSTA patterns in the equatorial Pacific Ocean.