The contribution of the global thermohaline circulation to the Pacific to Indian Ocean Throughflow via Indonesia


  • Jay F. Shriver,

  • Harley E. Hurlburt


World ocean simulations are used to investigate the pathways feeding the Indonesian throughflow as a function of depth, including the role of the global thermohaline (“conveyor belt”) circulation. The simulations use a horizontal resolution of 1/2° for each variable and the vertical resolution ranges from 1.5-layer reduced gravity to six layers with realistic bottom topography. They are forced by the Hellerman and Rosenstein [1983] monthly wind stress climatology. Contrary to the classical theory of Stommel and Arons [1960], the Naval Research Laboratory model shows the Antarctic Circumpolar Current (ACC) region as the main region of abyssal to upper ocean water upwelling which compensates for the deep water formation in the far North Atlantic, a result corroborated by recent observational evidence [Toggweiler and Samuels, 1993]. We examine the contribution of the global conveyor belt circulation to the throughflow by systematically varying the model dynamics (e.g., by disabling the far North Atlantic ports which parameterize deep water formation in that region). The model simulations show a global conveyor belt circulation contribution of 5.7 Sv to the throughflow, a contribution provided mainly by wind-driven upwelling in the Indo-Pacific ACC region. This is due to a cooperative interaction between the thermohaline and wind-driven circulations. The thermohaline circulation makes the throughflow more surface trapped and less subject to topographic blocking in the Indonesian passageways, while the wind-driven circulation provides the Indonesian throughflow pathway for the thermohaline flow upwelled in the ACC region. Mean layer transport fields, cross-layer mass transfer fields, and Lagrangian tracers are used to identify pathways feeding the Pacific to Indian Ocean throughflow via Indonesia. Starting from the ACC, Sverdrup flow shows a circuitous route that is northward in the eastern South Pacific, then westward in the South Equatorial Current (SEC). The SEC retroflects into the North Equatorial Countercurrent (NECC) followed by cyclonic flow around the Northern Tropical Gyre and into the North Equatorial Current (NEC), then into the Mindanao Current, the Sulawesi Sea, the Makassar Strait, and the Indian Ocean. The depth-integrated pathways from nonlinear simulations show the retroflection from the SEC into the NECC as a secondary route and retroflection into the Equatorial Undercurrent (EUC) as the primary route. The EUC connects with the NECC by westward and then northward flow on the northside of the EUC. The pathways as a function of depth can be presented in three layers: a surface layer, the layer containing the EUC, and layers below the EUC. In the top layer the EUC to NECC connection is via upwelling from the EUC in the central and east-central equatorial Pacific. Some of this upwelled water is returned to the EUC layer via downwelling at midlatitudes where it feeds into the NEC or SEC. Very little water in the South Pacific EUC layer passes into the Indian Ocean without upwelling into the surface layer first. While the pathways in the top two layers are complex and strongly coupled and enter the Indonesian Archipelago from the northern hemisphere, below the EUC layer a very direct Pacific to Indian Ocean route is found: SEC → Sulawesi Sea → Makassar Strait.