Abstract The Pacific South Equatorial Countercurrent (SECC) is a crucial but poorly characterized component of the equatorial current system. Here, we investigate its seasonal and interannual variability using Argo‐derived absolute geostrophic currents. During boreal spring, the SECC attains its widest meridional (5°S–13°S) but narrowest zonal (150°E−170°W) extent, with deepest penetration of over 300 m and a maximum transport of 12.8 Sv. In contrast, the SECC in summer weakens substantially as it shoals to depths less than 150 m, resulting in a transport of no more than 6.5 Sv. By autumn, the SECC extends eastward, reaching a peak zonal range (150°E−140°W). Subsequently, its western branch strengthens and deepens, while the eastern part retreats. These seasonal shifts are closely linked to the first‐mode baroclinic Rossby waves forced by remote wind stress curl anomalies, particularly over 180°–140°W. Contrasting responses are also observed during El Niño and La Niña events. The SECC expands horizontally but contracts vertically during El Niño, with positive velocity anomalies progressing from north to south; and vice versa during La Niña. Correspondingly, the SECC transport shows little difference between El Niño and La Niña during the developing phases, but by summer, during its decaying phase, the El Niño transport reaches nearly twice that of La Niña. Sensitivity experiments show that the wind stress curl anomalies east of 140°W primarily control the SECC on El Niño‐Southern Oscillation (ENSO) timescales between 180° and 140°W, while anomalies over 180°–140°W govern the SECC west of 180°.
Wu et al. (Thu,) studied this question.