Abstract
A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12–0.40) and too warm in POCM_4C (by >2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.
Highlights
Validating ocean models is an important procedure, for identifying the strengths and weaknesses of particular models, and for their continuing development and improvement.One of the aims of the World Ocean Circulation Experiment (WOCE) was ‘to collect a global dataset for validating and improving ocean circulation models’
POCM_4C does not simulate the eddy observed in the Ocean Circulation and Climate Advanced Modelling Project (OCCAM) flow fields (Fig. 3(b)) and a greater proportion of the flow through Drake Passage is via the southern part of the passage, as noted in the surface flow from the geopotential anomaly data (Fig. 2(e))
The reduced volume of Antarctic Bottom Water in OCCAM, observed in the A23 section comparison, results in slumping of the water masses above and the creation of an anomalous water mass, which together have an adverse effect on the model circulation in the central/southern Scotia Sea forcing much of the flow along the South Scotia Ridge
Summary
Validating ocean models is an important procedure, for identifying the strengths and weaknesses of particular models, and for their continuing development and improvement. The Weddell Front forms the boundary between the Weddell-Scotia Confluence and the waters of the Weddell Sea to the south (Gordon et al, 1977) It is associated with part of the cyclonic flow of the Weddell Gyre. The Scotia Sea is an area of intense water mass modification (Whitworth and Nowlin, 1987; Locarnini et al, 1993; Naveira Garabato et al., 2002) and provides a pathway for the recently ventilated deep waters formed in the Weddell Sea to enter the South Atlantic and join the global circulation
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