The bifurcation of the North Equatorial Current in the Pacific

A new climatology using historical temperature and salinity data in the western Pacific is constructed to examine the bifurcation of the North Equatorial Current (NEC). Integrating dynamically calculated circulation from the sea surface to 1000 m and combining it with surface Ekman transport, it is shown that the bifurcation of the NEC occurs at the southernmost position (14.8 degrees N) in July and the northernmost position (about 17.2 degrees N) in December. This annual signal lags behind the seasonal meridional migration of the zero zonally integrated wind stress curl line by 4-5 months but corresponds pretty well with the local Ekman pumping associated with the Asian monsoon winds. The bifurcation latitude of the NEC is depth dependent. On the annual average, it shifts from about 13.3 degrees N near the surface to north of 20 degrees N at depths around 1000 m. There is a time lag of 1-2 months from the sea surface to the subsurface (300-700 m) for the annual cycle. Below 700 m, the bifurcation of the NEC approaches as far north as 22 degrees N during the northeast monsoon (November-January), and as a result an anomalous transport of subtropical water is shown to flow equatorward along the western boundary. The bifurcation of the NEC below 700 m becomes unrecognizable when the prevailing wind is from the southwest (June-August).