Content area
Full text
ABSTRACT
The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (128N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of theWBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of theWBTcan be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.
(ProQuest: ... denotes formulae omitted.)
1. Introduction
Thewestward-flowingNorth Equatorial Current (NEC) is an important component of the tropical Pacific circulation system. East of the Philippine coast, the NEC bifurcates to form the northward-flowing Kuroshio and the southward-flowing Mindanao Current. The bifurcation of the NEC plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres (e.g., Qu et al. 1999; Toole et al. 1990).
The Sverdrup balance (Sverdrup 1947) predicts that the mean bifurcation latitude of the NEC should occur where the zonally integrated wind stress curl is zero, around 128-148N (e.g., Qu and Lukas 2003). The NEC bifurcation latitude also experiences seasonal variations (Qu and Lukas 2003;...