Seasonality and dynamics of the North Equatorial Current and its linkage with Kuroshio: modeling study

In the Western Pacific Ocean, North Equatorial Current (NEC), Kuroshio Current (KC) and Mindanao Current (MC) form the NKM circulation system, which is crucial to the heat and water transport between Pacific and China Seas. Although the NKM circulation system has been investigated in many studies, the linked processes within the system and the underlying dynamics remain largely unknown due to lacking of observational data and geophysical understanding. By utilizing results from the three-dimensional time-dependent China Sea Multi-scale Ocean Modeling System (CMOMS) and momentum/vorticity balance-based dynamic analysis, we investigated the intrinsic physical process and controlling mechanism of the NKM seasonality as well as the associated NEC bifurcation. We found that the seasonality of the NEC stream was governed by the cross-stream pressure gradient force (PGF) due to quasi-geostrophic balance, which was constantly modulated by ageostrophic effect arising from wind forcing in winter, and advection in summer. The origin of the cross-stream PGF is composed of baroclinicity, sloping effect and bottom PGF in the streams, in which barotropic part of the bottom PGF controlled the NEC seasonality and partly offset by baroclinic part. We found that the meridional migration of NEC was controlled by the along-stream PGF, which was formed mainly by beta effect and partly by Joint Effect Baroclinicity and Relief and wind forcing. Combined effect of NEC geostrophically-dominated meridional migration and local Ekman dynamics controlled the variation of the NEC bifurcation.