Mechanisms of Subantarctic Mode Water re-emergence in a hybrid-coordinate global GCM

Abstract

This study seeks to investigate the global circulation and re-emergence of Subantarctic Mode Water (SAMW), which is thought to be important in providing the source of nutrients to drive biological production over large proportions of the world ocean. For this purpose, the HYbrid isopycnic-cartesian Coordinate Ocean general circulation Model (HYCOM) is configured to simulate the global ocean circulation for timescales of up to centuries. After development and validation of the model, a control run of 120 years is undertaken in order to reach a near-equilibrium. A tracer is then injected into the SAMW formation regions and used to track the global circulation of SAMW for a further 150 years. The upwelling regions in which SAMW is returned to the upper-ocean mixed layer are identified, and the importance of different mechanisms of SAMW/nutrient re-emergence north of the Southern Ocean is assessed. It is found that for the global ocean north of 30S, entrainment driven by surface buoyancy loss and/or wind stirring is the most important mechanism for SAMW re-emergence in the model (accounting for 33% of the total SAMW upwelling). Substantial regional variation in upwelling mechanisms exists though. Regridding mixing driven by the Ekman pumping effect and shear-induced mixing dominate SAMW upwelling in the equatorial oceans. Induction at isopycnal outcrops is critical to SAMW upwelling in the high-latitude North Atlantic. It is found that SAMW has little influence in the North Pacific when compared with the North Atlantic and the different circulation and upwelling patterns of SAMW in the Northern Hemisphere are investigated in the model. The confined penetration distance of SAMW to the North Pacific is found to be associated with the relatively lighter density levels that the SAMW core resides at in the Pacific Ocean, and the different vertical extensions of the Equatorial Under Current (EUC) and Kuroshio Current (KC) there. The mechanisms of nutrient supply into the North Atlantic Ocean in association with the SAMW has been studied in more detail with a set of perturbation experiments. Intercomparison shows that the relatively high nutrient concentrations and primary production in the upper thermocline isopycnals and surface within the Gulf Stream (GS) are likely to be associated with the imported nutrients through isopycnal transport, and diapycnal mixing need not to be invoked to explain the Gulf Stream's high nutrient concentrations. Based on the tracer experiment, a new nutrient circulation and upwelling model is proposed for the North Atlantic Ocean, which comprises four steps following the seasonal cycle of the thermocline.

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