In the western North Pacific, a marked increase in wintertime sea surface temperature (SST) has been observed around the subarctic front (SAF), with warming rates reaching approximately +3 °C per century. Remarkable SST warming trends are also observed in the Japan Sea, with zonal band structure (Matsuura and Kida 2024). However, the physical processes responsible for this rapid SST increase over the mid-latitude North Pacific remain unclear.
To investigate the cause of this increase, we focused on the winter season (January-March). Vertical hydrographic sections conducted by the Japan Meteorological Agency along 165°E during 1997–2021 showed that the warming signal extends down to a depth of 300 dbar, leading to enhanced upper-ocean stratification. While a northward shift of the SAF could potentially affect the SST increase, satellite observations indicated that the frontal position has remained nearly stationary since 1982. Atmospheric reanalysis data indicated that the SST increase is primarily driven by changes in atmospheric forcing. In particular, a reduction in cold-air outbreaks originating from the Eurasian continent, associated with large-scale atmospheric circulation changes, plays a dominant role. Consistently, a mixed-layer heat budget analysis using ocean reanalysis data demonstrated that the observed SST increase is mainly attributed to weakened atmospheric cooling, rather than enhanced warm-water transport from the Kuroshio Extension (KE) to the south. We obtained Similar results for the Subpolar front region over the Japan Sea, but a more detailed analysis is needed regarding the influence of the Tsushima Warm Current.
The SST around the SAF strongly influences storm-track activity (Taguchi et al. 2012). Furthermore, the anomalous northward meandering of the KE since 2023 has recently altered the oceanic environment near the SAF (Sugimoto et al. 2025), potentially affecting oceanic advection and the frontal structure. Continuous monitoring will therefore be essential to assess how these recent changes interact with the ongoing multidecadal trend.

Cold-air,Aleutian Low,SST