On the representation of the Southern Ocean water masses in an ocean climate model

Abstract

In global ocean general circulation models used for climate studies, the simulated global deep ocean temperature and salinity generally appear to be lower than that observed. This may be due to an insufficient representation of the cryosphere in such models because the deep ocean thermohaline properties are strongly determined by high‐latitude surface conditions. In this study, it is our aim to arrive at an improved simulation of polar water mass characteristics and distribution in the framework of an ocean climate model. As a step toward identifying the critical factors influencing the high‐latitude water masses, in particular those of the Southern Ocean, we have proceeded a series of sensitivity experiments with a global sea ice‐ocean model. Forcing Southern Ocean sea ice with daily instead of monthly fluctuating wind enhances brine release due to new‐ice formation, which leads to a more pronounced core of Lower Circumpolar Deep Water (LCDW) in the Antarctic oceanic regime. Wider Antarctic shelf topography improves the distribution and thickness of sea ice, simultaneously providing a brine reservoir on the shelf, which is a crucial ingredient in the formation process of Antarctic Bottom Water (AABW). As a consequence, the salinity on the Antarctic shelf is increased with depth as observed. Intense southeasterly katabatic winds play a significant role in increasing the salinity, mainly off east Antarctica. It is suggested that modeled Southern Ocean water mass properties are highly sensitive to the large scale and regional forcing of Antarctic sea ice as well as the model representation of the shelf topography, the width of which is crucial to reproducing the first‐order features of near‐boundary convection.