SGER: Subantarctic Mode Water and Antarctic Intermediate Water: How Well Are They Represented in Climate Models?

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Processes in the Southern Ocean can influence global ocean circulation and density structure on the time scales relevant for global climate change. Evaluation of simulation of the Southern Ocean water masses in U.S. climate models will therefore assist in quantification and reduction of the uncertainty in predicting the response of the global thermohaline circulation to increasing concentration of greenhouse gases in the atmosphere. The goal of this study is to evaluate simulation of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in three U.S. climate Models: the Community Climate System Model (CCSM), the Geophysical Fluid Dynamics Laboratory (GFDL) model, and the Goddard Institute of Space Sciences (GISS) model, using observational data. The research has two main objectives: (i) To evaluate simulation of the properties of SAMW and AAIW by ocean general circulation models (OGCMs) through comparison with the observational data from the World Ocean Atlas 2001. The analysis will focus on the volume, temperature and salinity of SAMW and AAIW and on the stratification along meridional sections at the southern boundary of the Pacific and Atlantic oceans. (ii) To evaluate simulation of circulation and modification of the intermediate water using estimates of mass, heat and salt fluxes by a data-based inverse box model. The analysis will focus on the isopycnal fluxes into the Pacific and Atlantic basins, as well as on the diapycnal fluxes that modify properties of the intermediate water in the Southern Ocean. The results of these analyses will help to identify sources of disagreement between the OGCM-simulated and observed properties of SAMW and AAIW.The broader impacts of the work are that it will aid in assessing the uncertainty of the future climate change projections and will guide future model development. This is important for environmental management and decision-making activities.