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 Antarctica 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.

Freshwater fluxes in the Weddell Gyre: results from δ 18 O

“…The two most southerly-the Southern ACC Front (SACCF) and the Southern Boundary (SB) of the ACC-cross through the northeastern corner of the ANDREX box and mark the furthest poleward extent of Circumpolar Deep Water (CDW). This water mass flows from the ACC into the Weddell Gyre at its eastern edge before recirculating as Warm Deep Water (WDW) and is the source of all water masses south of the SB [42]. A substantial input into the gyre occurs at the Antarctic Slope Front (ASF) at 30 • E, which separates the colder, fresher continental shelf waters from the warmer, saltier waters to the north, and which flows into the gyre transporting relatively recently ventilated varieties of AABW from further east [43][44][45].…”

Potential regime shift in decreased sea ice production after the Mertz Glacier calving

“…In East Antarctica, the local source of AABW to the Australian-Antarctic Basin is the dense shelf water exported from the George V Land coast [3][4][5][6] (Fig. 1).…”

Strong export of Antarctic Bottom Water east of the Kerguelen plateau

“…The high concentration of CFCs near the sea floor 1,13 in the AAB indicates it is the best-ventilated deep basin around Antarctica.…”

“…3b) Several factors make it difficult to assess the relative contribution of the Kerguelen DWBC to the Southern Ocean overturning. These include the presence of recirculation gyres, interaction between the DWBC and other circulation regimes (e.g., the ACC and subpolar gyres) 8 , the fact that mixing and entrainment change the volume and properties of AABW along the export pathway 13 , and the lack of coherent long-term observations in other DWBCs. The only previous coherent current meter measurements of AABW export by a DWBC south of 45°S were obtained north of the Falkland Plateau, where a net transport of 1.9 Sv of Weddell Sea Deep Water (θ < 0.2°C) was found to enter the Argentine Basin (the difference between 8.2 Sv westward and 6.3 Sv recirculating to the east) 8 .…”