Relative importance of tropical SST anomalies in maintaining the Western North Pacific anomalous anticyclone during El Niño to La Niña transition years

Abstract

This study investigates the relative importance of tropical Indian Ocean warming (IOW) and equatorial central to eastern Pacific cooling (EPC) in sustaining an anomalous Western North Pacific anticyclone (WNPAC) during the transition from an El Nino in the preceding winter to a La Nina in the subsequent summer through a suite of numerical experiments. The numerical results indicate that the WNPAC is maintained by a combined effect of IOW and EPC during the La Nina developing years. The contribution of IOW in maintaining the WNPAC sustains from spring to early summer, but appears to weaken after that as IOW decays. The role of IOW is via an eastward-propagating Kelvin wave induced Ekman divergence mechanism. The decay of IOW is because of reduction in downward solar radiation associated with above normal precipitation in situ. As the cooling develops over central to eastern Pacific from spring to summer, EPC starts to contribute to the maintenance of the WNPAC during summer through stimulating a Rossby wave response to its northwest. In this study, we have identified that the cooling over the central to eastern Pacific plays an important role in sustaining the WNPAC during La Nina developing summers. This finding may help improve the prediction of the East Asian summer monsoon, which is closely associated with the WNPAC.