Antarctic Bottom Water Formation and Deep-Water Chlorofluorocarbon Distributions in a Global Ocean Climate Model

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Abstract
The ocean distributions of chlorofluorocarbons (CFCs) have been measured extensively in order to determine the mechanisms, rates, and pathways associated with thermohaline deep-water formation. Model temperature, salinity, and CFC-11 fields from the National Center for Atmospheric Research (NCAR) global ocean climate model are compared against observations with emphasis on the patterns of Antarctic Bottom Water (AABW) production, properties, and circulation in the Southern Ocean. The model control simulation forms deep water as observed in both the Weddell and Ross Seas, though not along other sectors of the Antarctic coast. Examination of the deep water CFC-11 distribution, total inventory, and profiles along individual observational sections demonstrates that the decadal-scale deep-water ventilation in the model Southern Ocean is both too weak and too restricted to the Ross and Weddell Sea source regions. A series of sensitivity experiments is conducted to determine the factors contributing to these deficiencies. The incorporation of a simple bottom boundary layer (BBL) scheme leads to only minor reductions in overall model–data error. The limited impact of the BBL may reflect in part other model large-scale circulation problems, for example, the lack of saline Circumpolar Deep Water along the Antarctic slope, and the coarse vertical resolution of the model. The surface boundary conditions in the permanent sea-ice-covered regions are a more major factor, leading to inadequate formation of dense, cold, and relatively saline shelf water, the precursors of AABW. Improved model–data agreement is found by combining the BBL parameterization with reasonably small adjustments in the surface restoring salinities on the Weddell and Ross Shelfs, justified by undersampling of winter conditions in standard climatologies. The modified salinities result in increased AABW production and enhanced signature of shelf water properties in the deep Southern Ocean similar in character to the effect of coupling with an active sea ice model.