On the total input of Antarctic waters to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements

Abstract

Deep ocean inventories of dissolved chlorofluorocarbon-11 (CFC-11) along representative sections off Antartica provide the first estimate of the overall strength of all dense water sources in the Southern Ocean. Their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the Thermohaline Circulation (THC). The bottom layer is supplied via sinking of Antarctic Bottom Water (AABW) produced at a few continental shelves. The middle layer receives the offshore injection of ventilated Modified Circumpolar Deep Water (MCDW) produced along much of the lengthy Antarctic Slope Front. The top layer is ventilated by northward export of Antarctic Surface Water into the Upper Circumpolar Deep Water of the Antarctic Circumpolar Current. Average Southern Ocean inputs to the upper two layers of the deep ocean for the 1970-1990 period are derived on the basis of the CFC-11 distributions along meridional sections, the mean CFC-11 saturations of all water mass ingredients, and the inferred mixing leading to production of dense source waters. About 5.4 ± 1.7 Sv (1 Sv = 106 m3s-1) of near-freezing Shelf Water ventilate the bottom layer, and 4.7 ± 1.7 Sv and 3.6 ± 1.3 Sv of cold Antarctic Surface Water ventilate the middle and top layers. Therefore the total contribution of ventilated Southern Ocean waters to the lower limb of the global THC is about 14 Sv. This is close to the about 17 Sv estimated for North Atlantic near-surface sources from CFC-11 inventories. Their entrainment of 7.4 ± 2.4 Sv of CFC-poor subsurface Lower Circumpolar Deep Water during the formation and sinking of AABW and MCDW raises the total Southern Ocean input to the deep ocean to about 21 Sv.