Delayed Impact of Indian Ocean Warming on the East Asian Surface Temperature Variation in Boreal Summer

A significant negative relationship is found between the summer mean north Indian Ocean sea surface temperature (SST) and East Asian surface temperature anomalies. However, the relationship is distinctively different for each month and shows a time-lagged relation rather than a simultaneous one. The north Indian Ocean warming in June is responsible for significant cold anomalies over the region of the Korean Peninsula and Japan (KJ region) that peak in July, exhibiting a 1-month leading role. The SST increase is closely associated with enhanced convective activity in the region in June, but the relationship between SST and resultant precipitation is substantially weakened afterward. This dependency of the precipitation sensitivity on SST anomalies is related to the climatological evolution of SST. The relatively low background SST due to the strengthening of southwesterly monsoons in the late summer can weaken the sensitivity of the precipitation to SST anomalies, yet the background SST in June is strong enough to maintain an increased sensitivity of precipitation. Thus, the Indian Ocean warming in June effectively drives atmospheric Kelvin waves that propagate into the equatorial western Pacific. In the western North Pacific (WNP), the resultant Kelvin wave–induced Ekman divergence triggers suppressed convection and anticyclonic anomalies. The WNP suppressed convection and anticyclonic anomalies move slowly northeastward until they are located near 20°N through the local air–sea interaction, and act as a source of the Pacific–Japan teleconnection. This teleconnection pathway brings clod surface anomalies to the KJ region due to the cyclonic circulation that causes the radiative and horizontal advection.