Intraseasonal Variability of the Surface Zonal Currents in the Western Tropical Pacific Ocean: Characteristics and Mechanisms

The surface circulation of the tropical Pacific Ocean is characterized by alternating zonal currents, such as the North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), South Equatorial Current (SEC), and South Equatorial Countercurrent (SECC). In situ measurements of subsurface moorings and satellite observations reveal pronounced intraseasonal variability (ISV; 20-90 days) of these zonal currents in the western tropical Pacific Ocean (WTPO). The amplitude of ISV is the largest within the equatorial band exceeding 20 cm s(-1) and decreases to similar to 10 cm s(-1) in the NECC band and further to 4-8 cm s(-1) in the NEC and SECC. The ISV power generally increases from high frequencies to low frequencies and exhibits a peak at 50-60 days in the NECC, SEC, and SECC. These variations are faithfully reproduced by an ocean general circulation model (OGCM) forced by satellite winds, and parallel model experiments are performed to gain insights into the underlying mechanisms. It is found that large-scale ISV (>500 km) is primarily caused by atmospheric intraseasonal oscillations (ISOs), such as the Madden-Julian oscillation (MJO), through wind stress forcing. These signals are confined within 10 degrees S-8 degrees N, mainly as baroclinic ocean wave responses to ISO winds. For scales shorter than 200 km, ISV is dominated by ocean internal variabilities with mesoscale structures. They arise from the baroclinic and barotropic instabilities associated with the vertical and horizontal shears of the upper-ocean circulation. The ISV exhibits evident seasonal variation, with larger (smaller) amplitude in boreal winter (summer) in the SEC and SECC.