Major Driver Leading to Winter SST Variability in the Kuroshio Recirculation Gyre Region and Its Decadal Changes: Refreshening Versus Spring‐Initiated Reemergence Processes

The analyses using reanalysis data and one-dimensional ocean model during the past decades (1963-2012) revealed that the epoch-dependent refreshening and reemergence mechanisms alternatively influenced the winter sea surface temperature (SST) in the Kuroshio recirculation gyre region. During 1971-1985, the refreshening mechanism due to the strong East Asian winter monsoon significantly contributed to the interannual winter SST variability, consistent with previous works. In contrast, the spring-initiated reemergence process better explains the winter SST variability for 1991-2005, a period corresponding to a declined winter monsoon. The classical winter-to-winter reemergence did not occur during either studied period; thus, the observed spring-to-winter reemergence is an original feature in the Kuroshio recirculation gyre region. The winter monsoon-related strong atmospheric forcing and prominent year-to-year variability of spring SST are important prerequisites for the regional refreshening and reemergence of the winter SST anomaly, respectively. Plain Language Summary Based on the reanalysis data and one-dimensional ocean model, we showed that the interannual variability of the winter sea surface temperature (SST) in the gyre region south of the Kuroshio are controlled by two different mechanisms depending on the epoch before and after the 1990 regime shift. Before 1990, the strong East Asian winter monsoon mostly determined the interannual winter SST variability. In contrast, the spring-initiated reemergence process, which is a recurrence of previous spring SST anomaly, appears to be the main factor affecting the winter SST variability after 1990. Normally, the reemergence mechanism initiates from the winter; however, we have discovered here an original reemergence mechanism initiating from spring in the study region.