Asymmetry in summertime atmospheric circulation anomalies over the northwest Pacific during decaying phase of El Niño and La Niña

Abstract

The present study has revealed the asymmetric characteristics of summertime atmospheric circulation anomalies over the Indo-Pacific for El Nino and La Nina, as well as the possible mechanism. During the summer when El Nino has dissipated, there are basin-wide warming over the tropical Indian Ocean (TIO) and cooling over the equatorial central and eastern Pacific (CEP). The northwest Pacific (NWP) anticyclone is maintained by a combined effect of the TIO warming and equatorial CEP cooling. The impact of TIO warming is via the Kelvin wave-induced divergence mechanism, and the effect of CEP cooling is by stimulating a Rossby wave response to its northwest. Correspondingly, there are lower (upper) level convergence (divergence) over the TIO and divergence (convergence) over the NWP. The tropical atmospheric waves and large-scale circulation anomalies could couple together and maintain the anticyclone. However, for La Nina, there are easterly wind anomalies over the equatorial western Pacific, and the anomalous NWP cyclone (NWPC) is weak and shifts northwestward, which are largely different from El Nino. The equatorial CEP cooling plays a dominant role in the maintenance of NWPC. The negative rainfall anomalies induced by CEP cooling force the anticyclonic wind anomalies over the WP as a Rossby wave response, which further cause the lower (upper) level convergence (divergence) over the MC and NWP. The resultant convergence leads to the enhanced convection there, inducing the NWPC as a Rossby wave response. Besides, the equatorial easterly wind anomalies trigger the upwelling oceanic Kelvin wave and maintain the CEP cooling as the Bjerknes feedback.