Contrast of Rainfall-SST Relationships in the Western North Pacific between the ENSO-Developing and ENSO-Decaying Summers

While the overall summer rainfall-sea surface temperature (SST) relationship has a negative correlation over the western North Pacific (WNP), this relationship experiences a significant interannual variation. During the ENSO-developing (decaying) summer, the rainfall-SST correlation is significantly positive (negative). The positive correlation is attributed to interplay between the anomalous Walker circulation and the cross-equatorial flows associated with the enhanced WNP summer monsoon. The former leads to negative rainfall anomalies in the western Pacific, whereas the latter leads to a cold SST anomaly resulting from enhanced surface latent heat fluxes. The negative correlation is attributed to the maintenance of an anomalous Philippine Sea anticyclone from the El Nino peak winter to the subsequent summer. The anomalous anticyclone, on one hand, suppresses the local rainfall, and on the other hand induces a warm in situ SST anomaly through both the enhanced solar radiation (resulting from a decrease in clouds) and the reduced surface latent heat flux (resulting from the decrease of the monsoon westerly). The rainfall-SST correlation is insignificant in the remaining summers. Thus, the overall weak negative rainfall-SST correlation is attributed to the significant negative correlation during the ENSO-decaying summers.