Seasonal Variations of the North Equatorial Current Across the Pacific Ocean

The seasonal variations of North Equatorial Current (NEC) axis ( Y CM ), intensity (INT), and transport across the Pacific basin are investigated based on altimetry data, Argo data, and ocean reanalysis data. In the western Pacific (WP), the NEC shifts southward/northward when it is intensified/weakened in the first/latter half of the year. However, in the interior Pacific (IP), the INT shows antiphase fluctuations with that in the WP with obvious westward propagations of the meridional shift of the Y CM . In the eastern Pacific (EP), the phase relationship between the Y CM and INT is opposite to that in the WP with the strengthening/weakening of the NEC being accompanied by a northward/southward shift of its axis. Different locations of the NEC seasonal thermocline departure maxima/minima in the WP, IP, and EP contribute to its distinct seasonal variations in these three basins. One of the new findings is that the seasonal shallowest thermocline generally corresponds to its largest meridional slope in the IP and EP, hence favoring the intensification of the NEC in these two basins, but not the case in the WP. Model experiments by blocking the baroclinic Rossby wave propagations and Ekman pumpings in different basins suggest that the seasonal variations of the NEC in the WP and EP are mainly controlled by the Ekman pumping, while those in the IP are controlled by the joint effect of the local Ekman pumping and the Rossby waves. The Sverdrup theory tends to overestimate the NEC's mean transport.