Seasonal variability of the North Equatorial Current transport in the western Pacific Ocean

Seasonal variability of the North Equatorial Current (NEC) transport in the western Pacific Ocean is investigated with ECMWF Ocean Analysis/Reanalysis System 3 (ORA-S3). The result shows that NEC transport (NT) across different longitudes in the research area shows a similar double-peak structure, with two maxima (in summer and winter), and two minima (in spring and autumn). This kind of structure can also be found in NEC geostrophic transport (NGT), but in a different magnitude and phase. These differences are attributable to Ekman transport induced by the local meridional wind and transport caused by nonzero velocity at the reference level, which is assumed to be zero in the NGT calculation. In the present work, a linear vorticity equation governing a 1.5-layer reduced gravity model is adopted to examine the dynamics of the seasonal variability of NGT. It is found that the annual cycle of NGT is mainly controlled by Ekman pumping induced by local wind, and westward-propagating Rossby waves induced by remote wind. Further research demonstrates that the maximum in winter and minimum in spring are mostly attributed to wind east of the dateline, whilst the maximum in summer and minimum in autumn are largely attributed to that west of the dateline.