On the interbasin-scale thermohaline circulation


  • William J. Schmitz Jr.


The global-scale circulation has long been one of oceanography's most challenging and exciting research topics. A few features of the abyssal (near bottom) and deep circulation of the Atlantic Ocean have been known for over 50 years, and in the past decade or so there has been a developing focus on the world oceans' thermohaline circulation. The term thermohaline circulation as used here applies not only to a direct response to atmospheric buoyancy fluxes but also in the general sense of water mass modification or conversion, where mechanisms may be associated with internal mixing processes and even wind forcing (i.e., wind-induced upwelling or wind-driven mixing). The thermohaline circulation components reviewed and summarized in the following are associated with water mass conversion processes that are involved with interbasin exchange. Updated summary maps of the volume transports (in sverdrups; 1Sv = 106 m³ s−1) for the interbasin-scale pathways of the abyssal and deep thermohaline circulation and associated upper level compensating flows are developed for two to four vertical layers or potential density intervals, based primarily on a synthesis of published observational results. The cell(s) involving the largest worldwide exchange transport-wise (53 Sv) are associated with an interaction between various deep and bottom water components via Circumpolar Deep Water (CDW). The first major conversion step in the replacement path for the renewal (14 Sv) of North Atlantic Deep Water (NADW) is taken to be primarily to CDW. Bottom water in the Indian Ocean originates as lower CDW which recirculates while also moving equatorward in deep western boundary currents with eventual conversion to both deep and intermediate layer flows. Some of the intermediate water so formed in the Indian Ocean moves through the Agulhas Current system (ACS) and may “leak” into the Benguela Current regime (BCR), although probably primarily flowing through the ACS into the Subantarctic Frontal Zone (SFZ). It is modified throughout its transit in the SFZ south of the Indian Ocean, south of Australia, and across the South Pacific. Up to 10 Sv of the least dense brand of intermediate water flows through the northern sector of Drake Passage, becomes involved in a Malvinas Current-Brazil Current-Subtropical Gyre interaction, and then joins the BCR after perhaps also interacting with the ACS again. This compensating flow is warmed and becomes more saline in the South Atlantic and is later further modified and upwelled in the equatorial Atlantic, crossing the equator and moving through the Gulf Stream system to replace NADW. There is also an NADW replacement path of secondary importance westward around the tip of Africa (∼4 out of 14 Sv) associated with an interbasin circulation pattern throughout the southern hemisphere oceans involving an O(10 Sv) Indonesian Throughflow.