Dynamics of the Pacific Subsurface Countercurrents

Abstract

A hierarchy of models, varying from 2½-layer to 4½-layer systems, is used to explore the dynamics of the Pacific Subsurface Countercurrents, commonly referred to as ?Tsuchiya Jets? (TJs). The TJs are eastward currents located on either side of the equator at depths from 200 to 500 m and at latitudes varying from about 2° to 7° north and south of the equator, and they carry about 14 Sv of lower-thermocline (upper-intermediate) water throughout the tropical Pacific. Solutions are found in idealized and realistic basins and are obtained both analytically and numerically. They are forced by winds and by a prescribed Pacific interocean circulation (IOC) with transport M (usually 10 Sv), representing the outflow of water in the Indonesian passages and a compensating inflow from the Antarctic Circumpolar Current. Analytic solutions to the 2½-layer model suggest that the TJs are geostrophic currents along arrested fronts. Such fronts are generated when Rossby wave characteristics, carrying information about oceanic density structure away from boundaries, converge or intersect in the interior ocean. They indicate that the southern and northern TJs are driven by upwelling along the South American coast and in the ITCZ band, respectively, that the northern TJ is strengthened by a recirculation gyre that extends across the basin, and that TJ pathways are sensitive to stratification parameters. Numerical solutions to the 2½-layer and 4½-layer models confirm the analytic results, demonstrate that the northern TJ is strengthened considerably by unstable waves along the eastward branch of the recirculation gyre, show that the TJs are an important branch of the Pacific IOC, and illustrate the sensitivity of TJ pathways to vertical-mixing parameterizations and the structure of the driving wind. In a solution to the 2½-layer model with M = 0, the southern TJ vanishes but the northern one remains, being maintained by the unstable waves. In contrast, both TJs vanish in the M = 0 solution to the 4½-layer model, apparently because wave energy can radiate into a deeper layer (i.e., layer 4). In the 4½-model, then, the TJs exist because of the Indonesian Throughflow, a remarkable example of remote forcing on a basinwide scale.