An analysis of wind-forced, seasonal transport variability at the Florida Straits using linear models of the North Atlantic

Fanning, Augustus F. (1993) An analysis of wind-forced, seasonal transport variability at the Florida Straits using linear models of the North Atlantic. Masters thesis, Memorial University of Newfoundland.

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Abstract

Two linear models of the North Atlantic, a linear barotropic model and a linear two-density layer model, are employed to investigate the effect of using different wind stress climatologies on the model-calculated transport. Particular emphasis is placed on the model-calculated response at the Florida Straits. The model domains extend from 10°S to 65°N and 100°W to 15°E at 1° x 1° resolution. The wind stress climatologies are those of da Silva et al.(1993a; hereafter DS), Hellerman and Rosenstein (1983; hereafter HR), Isemer and Hasse (1987; hereafter IH) and Trenberth et al. (1990; hereafter TR). Comparing the results at the Florida Straits, we find that for each climatology, the barotropic model shows maximum northward transport in the summer and minima in the fall and late winter, in general agreement with transport measurements from cable data (Larsen, 1992). However, the amplitude of the model response differs considerably between the climatologies. In the case of DS the range (maximum transport minus minimum transport) is 2.8 Sv; HR, 3.6 Sv; TR, 5.2 Sv and IH, 5.9 Sv, compared to a range of 4.6 ± 0.4 Sv derived from cable data. When the JEBAR (Joint Effect of Baroclinicity And Relief; Sarkisyan and Ivanov, 1971) forced transport is also considered, using the two-layer model, the amplitude of the model-calculated response changes slightly in each case, with ranges of 3.3Sv, 3.9Sv, 5.8Sv and 6.1Sv for each of DS, HR, TR, and IH respectively. -- We have also conducted experiments using a 1/3° x 1/3° version of the model applied to the region extending from 5°N to 42°N, and 100°W to 70°W. The Bahama/Antilles Island Arc are resolved in this model. Transport through the boundary at 70°W is specified from the 1° x 1° calculations referred to above. The details of the model-calculated response are particularity sensitive to the precise choice of grid point used to represent the offshore boundary of the Florida Straits. If we choose the Grand Bahama Islands, the cases with transport specified on the eastern boundary yield ranges of 1.3Sr, 2.8Sr, 3.0Sr and 3.1Sr for each of DS, HR, TR, and IH respectively. If instead, we choose a region between the Grand Bahama Island, and Andrew Island (the Providence Channel area) the 1/3° x 1/3° model calculated results agree quite well with our 1° x 1° results. This is a consequence of the fact that even at 1/3° x 1/3° resolution, we still do not properly resolve the Florida Straits. In fact, in the model the Straits are much too shallow (roughly half the depth of the true Florida Straits), and hence, does not receive as much information along ƒ/H contours as in the 1° x 1° case, or as the grid-point in the vicinity of the Providence Channel. -- The increased range in the IH case compared to HR in our 1° x 1° case is in general agreement with the finding of Böning el al.(1991b) using the Kiel version of the model that forms the WOCE Community Modelling Effort. However, whereas Böning et al. claim that winds north of 35°N have little influence on the seasonal response of their model at the Florida Straits, we find that winds north of 35°N play an important role in our model. The reason for the behaviour of the Community Model is not clear but may be associated with advection by the western boundary current. In our model, to show the importance of forcing by the meridional component of the wind, although forcing through the zonal component also plays some role in explaining the differences between the cases run under the different climatologies. We also show the importance of forcing associated with the meridional component of the wind along the continental slope region north of the Straits.

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