On convection and the formation of Subantarctic Mode Water in the Fine Resolution Antarctic Model (FRAM)

Abstract

We investigate the formation of Subantarctic Mode Water (SAMW) in the Fine Resolution Antarctic Model (FRAM). FRAM velocity fields are applied to advect an ideal tracer in an off-line diffusion and advection model of the Southern Ocean, and the results from two computational experiments are reported. In the first of our experiments, the tracer was released to the south of the Antarctic Polar Front (APF), in the second experiment to the north of the front. In this manner, we obtain insight into the SAMW formation process and the relative importance of convection, downwelling, vertical mixing and subduction in FRAM. The modelled tracer distribution was not in a steady state; however, good overall agreement was found between the modelled and the observed climatological distribution of ventilated water. The formation of SAMW in the model is found to be only weak for the southeast Pacific Ocean. In contrast, Indian Ocean SAMW is ventilating the Indian Ocean thermocline and is adverted eastward into the Pacific Ocean. We give a quantitative estimate for the amount of Antarctic and Subantarctic surface water found north of the APF and find that its contribution to ventilated water in the upper thermocline is significant.