Influence of the Indonesian Throughflow on the Circulation of Pacific Intermediate Water

Abstract

A 4½ layer model is used to study intermediate-water circulation in the Pacific Ocean. Solutions are forced by annual-mean winds. They are also driven by a prescribed inflow of water through the southwestern corner of the basin [12 Sv (Sv ≡ 106 m3 s?1)] and a compensating outflow in layers 1, 2, and 3 through the western boundary just north of the equator; this mass exchange simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific, and the same amount of upper water exits via the Indonesian passages. The water in each subsurface layer is formed by specific processes, and hence can be interpreted as corresponding to a distinct water-mass type. The types are thermocline water generated by subtropical subduction (layer 2), upper-intermediate and lower-thermocline water generated by midlatitude subduction in the North and South Pacific (NPIW and SPLTW, respectively; layer 3), and lower-intermediate water that corresponds to Antarctic Intermediate Water (AAIW; layer 4). Solutions are compared for different throughflow vertical structures and without the IOC. For all solutions, the amount of NPIW that moves into the Tropics is almost unchanged (?4 Sv), indicating that it is remotely determined by midlatitude processes. If the layer 3 outflow is sufficiently large (?4 Sv), most of the tropical NPIW exits the basin in the throughflow, and SPLTW fills the northern tropical ocean, consistent with the observed circulation. If it is small, however, most of the tropical NPIW recirculates in the northern Tropics, and no SPLTW enters this region. With the IOC, AAIW crosses into the Northern Hemisphere (3.8 Sv), and more than half eventually moves into subpolar ocean. Without the IOC, the transport of AAIW into the Northern Hemisphere is an order of magnitude less (0.37 Sv), and NPIW occupies most of the tropical ocean in both hemispheres, properties that differ markedly from observations. These results suggest that the Indonesian Throughflow is a possible reason why intermediate waters of Southern Hemisphere origin fill the tropical Pacific and spread to higher northern latitudes.