Effect of El Nino on the response ratio of Hadley circulation to different SST meridional structures

The role of El Nino in influencing the response of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures is investigated over the period 1979-2016. The explained variance of the equatorially asymmetric principal mode of HC variability is enhanced during El Nino events that are characterized by equatorially symmetric El Nino SST anomalies. It is shown that the response of the HC to different SST meridional structures is greatly enhanced in El Nino events; this is because the SST over the Indo-Pacific warm pool (IPWP) shows inhomogeneous warming in El Nino events, with greater values in the southern IPWP. The asymmetric warming over the IPWP is opposite to the climatological meridional structure of the SST over the IPWP, which intensifies the equatorially symmetric variation of SST, and contributes to the enhanced response of the HC to SST. This point is further verified by the suppression of this enhanced response in El Nino events of the HC to SST when the effects of SST over the IPWP are removed, implying that the different warming amplitude within the IPWP during the El Nino events contributes to the enhanced response. The response in La Nina events is also explored, indicating a difference between El Nino and La Nina events. These results may help explain the corresponding variations of the HC during the El Nino events, and highlight the different influences of the El Nino and La Nina events on the response of the HC to SST.