Poleward shift of the Pacific North Equatorial Current Bifurcation

The classic Sverdrup theory predicts the Pacific North Equatorial Current bifurcation latitude (NBL) should lie at about 14.6°N, where the basin-scale zonally integrated wind stress curl is generally zero. However, the NBL is not vertically distributed, and it shifts with increasing depth from 13.5°N at the surface to farther north below the thermocline. It is also the case that equatorial currents bifurcations in other basins share the same vertical structure, i.e., shifting poleward with increasing depth. Here we use a 5.5-layer reduced-gravity model to investigate this feature and found that the poleward shift of the bifurcation is associated with the asymmetric intensity of the wind stress curl (WSC) input to the tropical and subtropical gyres. Stronger WSC over the subtropical gyre leads to a relatively stronger circulation, which requires more lateral dissipations at the western boundary to overcome the vorticity induced by wind forcing. In this sense, the NBL in the upper layer should shift to the weaker side rather than locating at the zero line of zonally integrated WSC. Therefore, the NBL exhibits a northward shift feature, with its position being more southward than the zero line at the upper layer and more northward at the lower layer. In other oceans, the WSC input to the subtropical gyre is generally much stronger than that to the tropical gyre, resulting in the bifurcation with a poleward shift with increasing depth.