Subthermocline and Intermediate Zonal Currents in the Tropical Pacific Ocean: Paths and Vertical Structure

The mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0-2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3 degrees. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 108: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10 degrees. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 1558W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms.