On the Formation Dynamics of the North Equatorial Undercurrent

Abstract

AbstractBased on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.