Cold- versus warm-season-forced variability of the Kuroshio and North Pacific subtropical mode water.

The ocean responds to atmospheric variations. Changes in sea surface winds, surface air temperature, and surface air humidity cause upper ocean variability by modulating air-sea momentum and heat exchanges. Upper ocean variability in the mid-latitudes on inter-annual and longer timescales has previously been considered to be attributable to atmospheric variations in the cold season, because atmospheric forcing is stronger in the cold season than in the warm season. However, this idea has not been sufficiently confirmed yet. Although the ocean model is a useful tool to evaluate the impact of the atmospheric forcing in each season, there are no past studies having examined ocean model responses respectively to the cold- and warm-season atmospheric forcing. In this study, we performed numerical experiments with an eddy-resolving ocean general circulation model and investigated oceanic responses to cold- and warm-season atmospheric forcing, focusing on the Kuroshio and North Pacific subtropical mode water (STMW) in the western mid-latitude North Pacific. We found that temporal variations of net Kuroshio transport and STMW distribution/temperature are dominantly controlled by atmospheric forcing in the cold season. These results suggest that cold-season atmospheric variations are key to obtaining insights into large-scale upper ocean variability in the North Pacific subtropical gyre.