Seasonal Variation of the Pacific South Equatorial Current Bifurcation

摘要

The seasonal variation of the South Equatorial Current (SEC) bifurcation off the Australian coast in the South Pacific (SP) is investigated with observations and a nonlinear, reduced-gravity, primitive equation model of the upper ocean. The mean SEC bifurcation latitude (SBL) integrated over the upper thermocline is around 17.5 degrees S, almost 2 degrees south of the position predicted by Sverdrup theory. For its seasonal variation, the SBL reaches its southernmost position in June/July and its northernmost position in November/December. The south-north migration of 2.7 degrees is twice as large as its counterpart in the North Pacific. It is found that the large seasonal amplitude of the SBL results from the combined effect of Low-Lat-SP and Non-Low-Lat-SP processes. The Low-Lat-SP process (referred to as the Rossby wave dynamics forced by the wind stress curl over the low-latitude SP) accounts for almost 2/3 of the SBL seasonal variability, and the Non-Low-Lat-SP processes account for 1/3. Both of these processes are responsible for its south-north migration but in different ways. The Low-Lat-SP wind forcing determines the offshore upper-layer thickness (ULT) via Rossby wave propagation, while the Non-Low-Lat-SP wind forcing determines the alongshore ULT via coastal Kelvin wave propagation. A simple bifurcation model is proposed under the framework of linear Rossby wave dynamics. It is found that the seasonal bifurcation latitude is predominantly determined by the spatial pattern of the wind and baroclinic Rossby wave propagation. This model explains the roles of local/remote wind forcing and baroclinic adjustment in the south-north migration and peak seasons of the bifurcation latitude.