Diagnostic and prognostic calculations of the North Atlantic circulation and sea level using a sigma coordinate ocean model

The North Atlantic circulation and sea surface height, determined from hydrographic and wind stress data are calculated with a free surface, primitive equation ocean model. The model grid, a vertical sigma - coordinate and horizontal curvilinear orthogonal system, makes it possible to resolve coastal regions with complicated topography that were unresolved in previous calculations. When running in a diagnostic mode, in which the temperature and the salinity fields are fixed and equal to the Levitus annual mean fields, the vertically integrated North Atlantic circulation is very similar to though more detailed than that obtained from previous calculations using simpler diagnostic models. On the other hand, the meridional, zonally averaged flows and the poleward heat transport from the purely diagnostic calculations are noisy and unrealistic. However, short prognostic calculations of only 30 days following the diagnostic run allow intensification of the western boundary current and removal of noise due to inconsistencies between the hydrographic data and bottom topography and produce a more realistic meridional circulation and poleward heat transport, with a maximum value of 1.2×1015 W which is, comparable to estimates based on observations. The calculated sea level along the North America continent agrees with previous estimates but provides more spatial detail. Analysis of the dynamic adjustment process shows that this process is dominated by the effect of bottom topography through the action of the bottom pressure torque. This study is a first step in applying a realistic numerical model, previously used primarily for estuaries and coastal regions, to basin-scale ocean problems.

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