The Deep Water Regime in the Equatorial Atlantic
The importance of the Deep Western Boundary Current (DWBC) in the Atlantic for the interhemispheric exchange of water masses and of heat is well known, but data to estimate transports and to follow its pathways are sparse, especially in the equatorial Atlantic. New insight into the distribution of water masses in the DWBC, their transports and their variability off Brazil were gained in a contribution to the WOCE (World Ocean Circulation Experiment) program. In these studies, moored current meter measurements were combined with shipboard data from three cruises in the years 1990, 1991 and 1992. Besides tracer (Chlorofluoromethanes CFMs, components F11 and F12) and hydrographic data, direct velocity measurements were carried out using a lowered ADCP attached to the CTD, and the Pegasus profiling system.
The estimated transports of deep water in the equatorial Atlantic, net eastward transport of 19–22 Sv at 44°W, and 26.8 ± 7.0 Sv at 35°W, net southward transport of 19.5 ± 5.3 Sv at 5°S, are in the range of previously published estimates farther west and south. The data show significant spatial and temporal variability of the flow field and of the estimated transports as well as variability in the tracer distributions. This can lead to large uncertainties in the interpretation of single cruise observations.
The transient tracer distributions (CFMs and tritium) along the DWBC from the northern North Atlantic to 10°S have been used within a box model to estimate the mean spreading velocities of the tracer bearing water masses of the DWBC. Together with assumptions about the mean spatial extent of the DWBC and the vertical velocity structure a mean transport of the DWBC of 9–12 Sv is obtained. These numbers represent the spatially and temporally averaged net DWBC transport, i.e. the deep part of the thermohaline circulation. Thus, the tracer derived estimates are comparable to the estimates from inverse calculations. The different results between the transports obtained from direct observations on one side and from inverse calculations and tracer distributions on the other side are thought to be caused by various recirculation cells along the path of the DWBC. Indications for various recirculation paths of part of the NADW have also been found off Brazil.
KeywordsConvection Tritium Verse Rhein Chlorofluoromethane
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