Seasonal Variability of the Pacific South Equatorial Current during the Argo Era

The tropical Pacific currents from January 2004 to December 2018 are computed based on the gridded Argo temperatures and salinities using the P-vector method on an f plane and the geostrophic approximation on a beta plane. Three branches of the South Equatorial Current (SEC) are identified, i.e., SEC(N) (2 degrees S-5 degrees N), SEC(M) (7 degrees-3 degrees S), and SEC(S) (20 degrees-8 degrees S), with the maximum zonal velocity of -55 cm s(-1) and total volume transport of -49.8 Sv (1 Sv equivalent to 10(6) m(3) s(-1)) occurring in the central-east Pacific. The seasonal variability of each branch shows a distinct and different westward propagation of zonal current anomalies, which are well mirrored by the SLA differences between 2 degrees S and 5 degrees N, between 3 degrees S and 6 degrees S, and between 8 degrees S and 15 degrees S, respectively. Most of the seasonal variations are successfully simulated by a simple analytical Rossby wave model, highlighting the significance of the first-mode baroclinic, linear Rossby waves, particularly those driven by the wind stress curl in the central-east Pacific. However, the linear theory fails to explain the SEC(M) variations in certain months in the central-east Pacific, where the first baroclinic mode contributes only around 50% of the explained variance to the equatorial surface currents. A nonlinear model involving higher baroclinic modes is suggested for a further diagnosis. Considering the crucial role played by the tropical Pacific in the natural climate variability via the El Nino-Southern Ocean dynamics and the ocean response to anthropogenic forcing via the ocean heat uptake in the eastern tropical Pacific, advancing the process understanding of the SEC from observations is critical.