Intensified anticyclonic anomaly over the western North Pacific during El Niño decaying summer under a weakened Atlantic thermohaline circulation

It has been well documented that there is an anticyclonic anomaly over the western North Pacific (WNPAC, hereafter) during El Niño decaying summer. This El Niño-WNPAC relationship is greatly useful for the seasonal prediction of summer climate in the WNP and East Asia. In this study, we investigate the modification of the El Niño-WNPAC relationship induced by a weakened Atlantic thermohaline circulation (THC) in a water-hosing experiment. The results suggest that the WNPAC during the El Niño decaying summer, as well as the associated precipitation anomaly over the WNP, is intensified under the weakened THC. On the one hand, this intensification is in response to the increased amplitude and frequency of El Niño events in the water-hosing experiment. On the other hand, this intensification is also because of greater climatological humidity over the western to central North Pacific under the weakened THC. We suggest that the increase of climatological humidity over the western to central North Pacific during summer under the weakened THC is favorable for enhanced interannual variability of precipitation, and therefore favorable for the intensification of the WNPAC during El Niño decaying summer. This study suggests a possible modulation of the El Niño–Southern Oscillation-WNP summer monsoon relationship by the low-frequency fluctuation of Atlantic sea surface temperature. The results offer an explanation for the observed modification of the multidecadal fluctuation of El Niño-WNPAC relationship by the Atlantic multidecadal oscillation.