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Abstract:
We examine the representation of Southern Ocean water mass properties, circulation and transformation in an ensemble of CMIP6 models, under historical climate forcing conditions and future climate scenarios. By using a dynamically defined water mass classification scheme based on physical characteristics we are able to compare water masses across a range of models, well as within single models where water mass properties change under climate forcing. We find that under strong climate forcing scenarios the heat content of SubAntarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW) and Circumpolar Deep Water (CDW) all increase consistently across models, while the volume of Antarctic Bottom Water (AABW) declines significantly. Importantly this change is strongly modulated by using dynamic definitions. These analyses are complimented by Walin budgets, identifying the factors driving change in water mass volumes, transformation and formation rates.
There is a consistent ‘fingerprint’ of temperature change in density space across all models, with CDW experiencing surface intensified warming and SAMW/AAIW demonstrating cooling and freshening in their subducted layers. We show that the upper cell of the residual overturning circulation consistently increases across all models evaluated, by 10-50%, while the lower cell is dramatically decreased in strength, declining by up to 70%.
These water mass changes are examined in conjunction with surface fluxes of buoyancy and momentum, stratification and mixed layer depth. It is found that in the historical scenarios there is a trend towards shallower and more stratified mixed layers, in contrast to observations, and that this is increased under strong forcing scenarios.
