Seasonality of the intraseasonal variability in the upper equatorial western Pacific Ocean currents

Quantifying spatiotemporal characteristics of intraseasonal variability (ISV) of upper ocean (100 m) zonal currents (U) in the equatorial western Pacific remains difficult due to a lack of direct observations. Here we investigate the characteristics by analyzing five subsurface mooring data at 140°-142°E, 1.7°S-4.7°N, from January 2014 to April 2021. Our analysis revealed that U ISV has an amplitude exceeding 40 cm s-1, comparable to their long-term mean, and pronounced seasonality, with a peaking period in boreal winter-spring (October-April) and a weakening period in summer-autumn (May-September). U ISV intensity in the south of the equator is 50% stronger than that in the north. Analysis of satellite/reanalysis data and model experiments suggests that atmospheric intraseasonal oscillation (ISO) accounts for 80% of the U ISV seasonality through wind forcing, while the oceanic internal process contributes 20% through nonlinear baroclinic instability. The consistent variation in mixed layer depth indicates the potential roles of oceanic ISVs in seasonal phase locking of El Niño–Southern Oscillation. Our results highlight the significance of direct observations for better understanding and estimating ISVs of ocean circulation.