Hemispheric asymmetry of deep water transport modes in the western Atlantic
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Subtropical studies of the Atlantic meridional cold water flow show a hemispheric contrast in the dominant southward transport mode below 2000 m; in the North Atlantic, lower deep water (LDW) (1.8 degrees less than or equal to theta less than or equal to 2.4 degrees C) dominates with small transport of middle deep water (MDW) (2.4 less than or equal to theta less than or equal to 3.2 degrees C), while in the South Atlantic, the opposite is observed. We use numerous observations in the western basins of the tropics to show that the transition occurs rapidly near the equator in the western Atlantic. A meridional section in the central Brazil Basin suggests zonal flows are responsible for the transition. LDW transport from the Guiana Basin (north of the equator) flows eastward in the northern Brazil Basin and is inferred to continue on through the Romanche Fracture Zone into the eastern Atlantic. An opposing flow of MDW from the eastern tropical Atlantic flows toward the western boundary, where it bifurcates to supply MDW to the Deep Western Boundary Current (DWBC) of the Brazil Basin, as well as to feed the northward flow of MDW in the Guiana Basin offshore of the DWBC. The magnitude of each of these oppositely directed flows is roughly 7 Sv. We furthermore speculate that they are connected predominantly by upwelling from LDW to MDW within the low-latitude eastern basin. The overall deep water transport system below 2000 m in the western basins of the mid- and low-latitude Atlantic is thus found to comprise the following three distinct components. (I) A strong DWBC transport of LDW with associated recirculation dominates the Guiana Basin north of the equator. (2) In the northern Brazil Basin (just south of the equator) a narrow eastward flow absorbs the LDW and carries it eastward, while a somewhat broader westward flow imports MDW into the western basin. (3) This MDW flow then bifurcates, with the southward branch causing the MDW dominance in the Brazil Basin, where the MDW dominated DWBC and associated recirculations are the third component of the deepwater transport system.