What controls the rate of equatorial warm water mass formation?
Abstract:Equatorial warm water formation is one important factor in the mass balance of Tropical Surface Water (TSW) in the Atlantic Ocean; another is drainage by the North Brazilian Coastal Current (NBCC). Both are affected by the depth of the TSW layer at the western boundary: deepening of this layer increases the transport of the NBCC and reduces warm water mass formation by shortening the eastern upwelling zone. Strengthening westward wind-stress steepens the thermocline in the western sector of the equatorial band, enhances upwelling in the eastern sector, affecting mass balance on both counts. Recirculation of TSW via a loop containing the North Equatorial Counter Current (NECC), and inflow from the south at the eastern boundary are also important elements of this mass balance: they depend on the wind-stress field over a wider region.
The various elements of this mass balance are parameterized in terms of western boundary layer depth and wind-stress in the western sector. The resulting first order equation describes the response of the system to the annual cycle of wind-stress. With quantitative inputs typical of the equatorial Atlantic, output variables are simulated realistically: upwelling varies from 3 to 18 × 106 m3/s, NBCC transport from 13 to 26 × 106 m3/s, both in accord with observation, as is simulated storage, and TSW depth. An interesting finding is that the east-west length of the upwelling region varies relatively little while everything except storage varies in phase with the wind-stress (the storage lags by 2 months). The lesser variation of upwelling sector length comes about because the steepening of the thermocline in response to increasing wind-stress is accompanied by deepening in the western end, necessary to allow the escape of excess fluid.
The results show the interplay of wind-stress, upwelling and western boundary current transport in the control of the oceanic heat gain in the equatorial band.
Document Type: Research Article
Publication date: 1987-08-01
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