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Rainer J. Zantopp and Kevin D. Leaman

Abstract

The existence of a tight T-S relationship in the southwestern North Atlantic is used to convert temperature measurements from moored sensors to dynamic heights. Seven hydrographic cruises with intensive CTD coverage during 1980–81 allow us to establish a close correlation between temperature and specific volume anomaly, which then is integrated vertically as a function only of temperature to derive dynamic heights. The systematic errors arising from the method are smaller than the natural variability of temperature from the mesoscale field.

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Donald B. Olson, Friedrich A. Schott, Rainer J. Zantopp, and Kevin D. Leaman

Abstract

Observational evidence from a two-ship synoptic survey, from one-year-long current measurements at four sites, and from two independent long-term expendable-baththermograph (XBT) data sets is combined to describe the circulation east of the Bahamas. The results indicate that a weak Antilles Current actually exists at mid-depths, about 300–100 m, with a northward transport along this boundary of about 4 × 106 m3 s−1. However, both the dynamic height fields and water mass distributions suggest that this current does not represent return flow from the gyre-wire Sverdrup recirculation but instead is part of the northwestern Gulf Stream recirculation system, west of about 50°W. The near-surface flow east of the Bahamas is more complicated and consists of two isolated anticyclonic circulation cells along the boundary. The most intense of these is centered just northeast of Abaco Island in the northern Bahamas; the southern, weaker cell is centered at about 24°N, 71°W. In the area just north of Puerto Rico a narrow band of water with characteristics originating in the southern and southeastern subtropical gyre is found. Dynamic height maps and water mass distributions imply that this flow passes through the island passages into the Caribbean. No evidence for significant seasonally reversing barotropic or baroclinic currents could be found in Antilles Current area.

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Thomas N. Lee, William E. Johns, Rainer J. Zantopp, and Eve R. Fillenbaum

Abstract

A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation.

Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer, and minimums occur in fall and spring due to a predominantly barotropic response to remote and local seasonal wind forcing. Transport variability on timescales less than semiannual is dominated by mesoscale eddies that propagate westward into the Bahamas boundary in the thermocline at periods of 70–100 days, wave speeds of about 4 cm s−1, and wavelengths of about 335 km. These events are frequently correlated with offshore shifts of the DWBC core.

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Eve R. Fillenbaum, Thomas N. Lee, William E. Johns, and Rainer J. Zantopp

Abstract

Data from almost five years of current meter moorings located across the Bahamas Escarpment at 26.5°N are used to investigate meridional heat transport variability in the section and its impact on transatlantic heat flux. Estimates of heat transport derived from the moored arrays are compared to results from the Community Modeling Effort (CME) Atlantic basin model and to historical hydrographic section data. A large fraction of the entire transatlantic heat flux is observed in this western boundary region, due to the opposing warm and cold water flows associated with the Antilles Current in the thermocline and the deep western boundary current at depth. Local heat transport time series derived from the moored arrays exhibit large variability over a range of ± 2 PW relative to 0°C, on timescales of roughly 100 days. An annual cycle of local heat transport with a range of 1.4 PW is observed with a summer maximum and fall minimum, qualitatively similar to CME model results. Breakdown of the total heat transport into conventional “barotropic” (depth averaged) and “baroclinic” (transport independent) components indicates an approximately equal contribution from both components. The annual mean value of the baroclinic heat transport in the western boundary layer is 0.53 ± 0.08 PW northward, of opposite direction and more than half the magnitude of the total southward baroclinic heat transport between Africa and the Bahamas (about −0.8 PW) derived from transatlantic sections. Combination of the results from the moored arrays with Levitus climatology in the interior and historical Florida Current data yields an estimate of 1.44 ± 0.33 PW for the annual mean transatlantic heat flux at 26.5°N, approximately 0.2 PW greater than the previously accepted value of 1.2–1.3 PW at this latitude.

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