FY 1999

Circulation, mixing, and production of Antarctic Bottom Water

Orsi, A.H., G.C. Johnson, and J.L. Bullister

Prog. Oceanogr., 43(1), 55–109, doi: 10.1016/S0079-6611(99)00004-X (1999)

The Antarctic source of bottom water to the abyssal layer of the World Ocean is examined, as well as its large-scale flow pattern and ultimate entrainment rate into the deep water above. We make use of the available high-quality station data in the Southern Ocean to construct bottom maps of neutral density and mean property maps, including Chlorofluorocarbon (CFC), for the abyssal layer underneath a selected neutral density surface. The maximum density at the sill depth of Drake Passage is used to distinguish between the voluminous deep water mass that is a continuous component of the Antarctic Circumpolar Current from the relatively denser bottom water originated along the Antarctic continental margins. Based on water density, Antarctic Bottom Water (AABW) is defined here generically to include all volumes of non-circumpolar water of Antarctic origin. Over the shelf regime multiple localized sources of specific AABW types contribute to the abyssal layer of the adjacent Antarctic basins. Characteristics of these dense bottom waters reflect closely those observed on the parent Shelf Water mass. Spreading paths of newly-formed deep and bottom waters over the slope regime, and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the AABW layer. Interior mixing and interbasin exchanges of AABW are deduced from mean characteristic curves following the southern streamline of the Antarctic Circumpolar Current. Outflow and mixing of AABW from the Weddell Sea to the Argentine Basin is depicted using density and CFC distributions of two zonal hydrographic lines. Recirculation and mixing of deep and bottom waters within the Weddell Gyre are also detailed using a meridional section along the Greenwich Meridian. The strength of all localized sources of AABW combined is estimated by two independent approaches. An estimate of the total production rate of AABW is calculated based on the oceanic CFC budget for the AABW layer offshore of the 2500-m isobath. The sum of all downslope inputs of well-ventilated bottom water types underneath the top isopycnal must account for the measured CFC content in the bottom layer. The resulting total AABW production rate is about 8 Sv, which is a conservative figure that neglects the loss of CFC-bearing waters across the top isopycnal in recent years, whereas about 9.5 Sv is calculated assuming a well-mixed bottom layer. Making use of their transient nature, CFC distributions at the top of the AABW layer indicate that more direct and rapid entrainment of CFC-rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing. A second, more ad-hoc but independent oceanic mass budget of the bottom layer is also constructed. A typical basin-wide rate of deep upwelling of 3 × 10 m s^–1 requires 10 Sv (1 Sv = 10⁶ m³ s^–1) of newly-formed AABW to sink down the slope around Antarctica. We have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the AABW. It is expressed as a combination of wind, topographic, and turbulent components, which in turn are functions of the isopycnal depth, bottom depth, and bottom layer thickness. This non-uniform upwelling field yields about 12 Sv of AABW exported across the top isopycnal. Fortuitously, the overall average of upward speed at the top isopycnal (3.7 × 10^–7 m s^–1) compares well with previous estimates of deep upwelling in the northern basins. A series of likely sites for strong vertical entrainment of AABW are clearly identified in the modeled distribution of deep upwelling, consonant with the observed CFC distributions on the top isopycnal. Altogether, regions with relatively high upwelling rates (>5 × 10^–7 m s^–1) occupy only a quarter of the total areal extent of the top isopycnal, but they account for as much as 45% of the total vertical transport.