A fine-resolution barotropic model of the North Atlantic driven by wind and atmospheric pressure forcing

Lu, Youyu (1994) A fine-resolution barotropic model of the North Atlantic driven by wind and atmospheric pressure forcing. Masters thesis, Memorial University of Newfoundland.

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Abstract

We investigate the response of the North Atlantic to wind and atmospheric pressure forcing with a two-dimensional, fine-resolution barotropic model. The model domain extends from the equator to 65ﾟN and from 100ﾟ W to 14ﾟE with a resolution of 1/3ﾟ in latitude and 2/5ﾟ in longitude. The forcing field to drive the model is the twice-daily wind and atmospheric pressure data from the European Center for Medium Range Weather Forecasts (ECMWF). The model results are compared with sea level observed at coastal tide gauges, volume transport derived from voltage measurements made using a submarine cable across the Florida Straits, and bottom pressure data collected on the Labrador and Newfoundland Shelf. The time scales being studied range from several days to seasonal. -- The primary model experiment is a two-year (1985-1986) run driven by both wind and atmospheric pressure forcing. Three one-year runs are used to determine the contributions from the individual forcing, and the influence of Hudson Bay on the Labrador Shelf. Model results show the best agreement with observed sea level data at locations with broad shelves where the stratification is weak. Significant coherence between observed and modeled adjusted sea level is obtained at periods beyond ~3 days, at four representative stations along the western boundary. Contributions from atmospheric pressure forcing are not important for periods beyond ~2-3 days. The primary model experiment explains the observed volume transport variation through the Florida Straits at synoptic time scales, which is mainly due to wind. The model captures the variation at longer time scales in 1985, but not in 1986. The exclusion of advection by the mean flow may be the reason for the drop of coherence at ~10 days in the case of the Florida Straits volume transport and at ~12 days in the case of sea level at Fernandina Beach, Florida. -- Model output is also coherent with the observed bottom pressure data from the Labrador and Newfoundland Shelf. The significant non-iostatic response in the observational data at synoptic time scales in reproduced by model experiments with the Hudson Bay-Hudson Strait system included, with energy peaks at ~2-6 days. The contribution from atmospheric pressure forcing is only important in generating the energy peak at ~2-6 days when the Bay/Strait system is included, verifying the Helmholtz-like resonance mechanism proposed by former researchers. Wind forcing dominates over atmospheric pressure forcing at synoptic time scales. In particular, the wind over Hudson Bay-Hudson Strait is shown to be important.

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