COMPENSATION FOR THE NADW OUTFLOW IN A GLOBAL OCEAN GENERAL-CIRCULATION MODEL

Recent observational study on the compensation for the North Atlantic Deep Water (NADW) outflow suggested that the compensation flow loops i nto the south Indian Ocean, whereby the compensating water gains heat and salt before returning to the South Atlantic. A question arises as to whether the heat and salt gain from the south Indian Ocean plays a significant role in determining the thermohaline circulation associate d with the NADW formation. Many low-resolution ocean general circulati on models (OGCMs) for coupled atmosphere-ocean studies fail to produce an Agulhas leakage. The consequence of this missing leakage in these climate models remains unclear. This study examines the role played by the Agulhas leakage in the compensating process for the NADW outflow, and assesses the feasibility of low-resolution ocean climate models. The authors do this in a series of numerical experiments using the Bry an-Cox global OGCM coupled to Schopf's zero heat capacity atmospheric model. The model confirms that in the presence of the Agulhas leakage, the compensating route includes a loop extending into the southwester n Indian Ocean. Part of the compensating water flows to the Indian Oce an through this loop, and returns with Indian Ocean water to the South Atlantic via the Agulhas leakage. All of the compensating water flows with the Benguela Current. A small branch of the Benguela Current the n breaks away from the main stream at about 15 degrees S and heads for the North Atlantic. The Agulhas leakage decreases only slightly when the NADW formation is suppressed. Most of the reduction occurs in the intermediate water. A comparison of model runs suggests that the contr ibution by the Indian Ocean water is less than 35% of the total compen sating water leaving the South Atlantic and that the majority of the I ndian Ocean contribution is intermediate water. Most of the South Atla ntic area gains heat from the atmosphere, and the northward heat trans port in the South Atlantic associated with the compensation can be sus tained by this heat gain. The heat gain accompanies a conversion of in termediate water into surface water, providing the surface water sourc e for the compensating water. The southwestern Indian Ocean loop of th e compensating flow provides a pathway whereby the compensating water may gain heat and salt from the Indian Ocean. In an experiment where t he loop is suppressed, the Atlantic water cools and freshens. However, the cooling and freshening process hardly changes the density field, leading to an almost identical rate of the NADW formation and strength of the NADW outflow, with or without the Agulhas leakage. That the ma jority of the compensating water, whether through the leakage or the D rake Passage, is intermediate water clears the way for the use of low- resolution OGCMs in climatic studies in terms of the compensation for the NADW outflow.