Compensation for the NADW Outflow in a Global Ocean General Circulation Model

Abstract

Recent observational study on the compensation for the North Atlantic Deep Water (NADW) outflow suggested that the compensation flow loops into 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 associated with the NADW formation. Many low-resolution ocean general circulation 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 Bryan–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 southwestern Indian Ocean. Part of the compensating water flows to the Indian Ocean 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 then breaks away from the main stream at about 15°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 contribution by the Indian Ocean water is less than 35% of the total compensating water leaving the South Atlantic and that the majority of the Indian Ocean contribution is intermediate water. Most of the South Atlantic area gains heat from the atmosphere, and the northward heat transport in the South Atlantic associated with the compensation can be sustained by this heat gain. The heat gain accompanies a conversion of intermediate water into surface water, providing the surface water source for the compensating water. The southwestern Indian Ocean loop of the compensating flow provides a pathway whereby the compensating water may gain heat and salt from the Indian Ocean. In an experiment where the 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 majority of the compensating water, whether through the leakage or the Drake 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.