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Assessing the importance of non-Boussinesq effects in a coarse |
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resolution global ocean model. |
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M. Losch, A. Adcroft, and J.-M. Campin |
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The advent of the GRACE mission presents the opportunity to accurately |
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measure variations in bottom pressure. Such a data source will prove |
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valuable in state estimation and constraining general circulation |
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models (GCMs) in general. However, conventional GCMs make the |
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Boussinesq approximation as a consequence of which mass is not |
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conserved. Thus Boussinesq models have an implicit drift in bottom |
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pressure. By use of the height-pressure coordinate isomorphism |
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implemented in the MIT GCM, we can evaluate the impact of |
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non-Boussinesq effects. We find that although implementing a |
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non-Boussinesq model in pressure coordinates is relatively |
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straight-forward, making a direct comparison between height and |
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pressure coordinate (i.e., Boussinesq and non-Boussinesq) models is |
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not simple. Here we present a careful comparison of the Boussinesq and |
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non-Boussinesq solutions ensuring that only non-Boussinesq effects can |
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be responsible for the observed differences. As a yard-stick, we also |
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compare differences between the Boussinesq hydrostatic and |
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non-hydrostatic models, another approximation commonly made in |
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GCMs. We find that model errors (differences) due to the Boussinesq |
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approximation are apparently smaller than the errors due to the |
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hydrostatic approximation. We also compare these model errors with |
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uncertainties associated with model parameterizations and find that |
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non-Boussinesq and non-hydrostatic effects are much smaller than these |
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uncertainties. We conclude that non-Boussinesq effects are negligible |
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with respect to other model errors. However, since there is no |
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additional cost incurred in using a pressure coordinate model, |
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non-Boussinesq modeling is preferable simply for puristic reasons. |
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