Mechanisms of enhanced ocean surface warming in the Kuroshio region for 1951–2010

Since the early twentieth century, observations have shown that the ocean surface has warmed almost globally, but the rate of sea surface temperature (SST) rise in the Kuroshio region (KR) has been twice that of the global-mean. This study focuses on the mechanisms driving long-term changes in SST in the KR during the period 1951–2010 using observations, reanalysis data, and 10-member ensembles of historical simulations based on a global climate model (GCM) called MIROC 5.2. The observational data indicates that SST in the KR slightly decreased until around 1980, but rapidly increased afterwards. The MIROC5.2 historical experiment reproduced the observed multidecadal changes in the KR SST, and a signal-to-noise analysis showed that one third to half of the changes were attributable to a forced component, represented by the ensemble mean. Comparison of historical runs with another ensemble in which sulfate aerosols were fixed at preindustrial levels, highlighted that multidecadal SST changes in the KR were primarily driven by sulfate aerosols, contributing to the warming for 1981–2010 as much as greenhouse gases. Analyses of the MIROC5.2 ensemble mean revealed that the key factor for enhanced warming after the 1980s in the KR was a high-pressure trend over the North Pacific. This is because it advects warm air to the northwestern Pacific in the lower troposphere and causes a northward shift in the Kuroshio current. The presence of this mechanism was supported by a significant correlation between SST trends in the KR and high-pressure trends over the North Pacific based on historical simulations from 27 GCMs, and a large ensemble of historical simulations using MIROC6. These results demonstrate the importance of an externally forced atmospheric circulation response over the North Pacific as it relates to enhanced ocean surface warming in the KR.