Dynamics of the Coastal Oyashio and Its Seasonal Variation in a High-Resolution Western North Pacific Ocean Model

Abstract

The Coastal Oyashio (CO) carries the cold, fresh, and relatively light water mass called the Coastal Oyashio Water (COW) westward along the southeastern coast of Hokkaido in winter and spring. To investigate dynamics of the CO and its seasonal variation, model experiments are executed using a western North Pacific general circulation model with horizontal resolutions of approximately 2 and 6 km. The 2-km resolution model reproduces the properties of COW with temperature of 0°?2°C and salinity of 32.2?32.6 and reproduces its distribution. COW is less dense than offshore water by 0.2 kg m?3, and it forms a surface-to-bottom density front with a width of 10 km near the shelf break. The CO appears as a baroclinic jet current along the front with a maximum velocity of approximately 40 cm s?1. The velocity and density structures and the front location relative to bathymetry indicate that the CO can be understood in terms of a simplified dynamical model developed for the shelfbreak front in the Middle Atlantic Bight. In contrast to the 2-km resolution model, the 6-km model cannot realistically reproduce the COW distribution. This is because only the 2-km model can represent the sharp density structure of the shelfbreak front and the accompanying CO. The CO exists during the limited period from January to April. This is directly connected with seasonal variation of the COW inflow from the Okhotsk Sea to the North Pacific Ocean through the Nemuro and Kunashiri Straits, indicating that the seasonal variation of the CO is ultimately controlled by the variation of the circulation in the Okhotsk Sea induced by the monsoon.