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Robert L. Molinari, Elizabeth Johns, and John F. Festa

Abstract

Total meridional heat flux through a zonal oceanic section at 26.5°N in the Atlantic Ocean is computed from hydrographic, direct current and surface wind observations. The oceanic current and temperature fields are decomposed into depth-averaged and depth-dependent (including Ekman and geostrophic) components to perform the calculation. The mean annual boat flux is estimated to be 1.21 ± 0.34 PW. Mean monthly values of net heat flux are also computed from the data. The annual cycle of net heat flux determined from these values ranges from a minimum of 0.69 PW in February to a maximum of 1.86 PW in July. Thus, in contrast to an earlier estimate of the annual cycle of oceanic heat flux derived indirectly from surface energy fluxes and upper-layer heat content changes, there is no net southward heat flux during the Call. Results from a simulation of the circulation of the North Atlantic give an annual cycle of heat flux similar to our calculations with a summer maximum and winter minimum. However, the simulated mean value and range of the annual cycle are less than observed.

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Rodert L. Molinari, John F. Festa, and David W. Behringer

Abstract

Monthly mean dynamic height topographies for the upper 500 m of the Gulf of Mexico, seasonal mean topographies for the upper 1000 m and annual topographies for the deep flow are presented. The dynamic height values on a 1° × 1° grid were determined from observed temperature values and salinities derived from mean T-S relations. The seasonal intrusion of the Loop Current is observed and found to vary directly with the geostrophic transport through the Yucatan Straits. At the Straits, the transport in the upper 500 m is a maximum in June. The transports in the upper 500 m of an anticyclone in the western Gulf are a maximum in winter and summer, and a minimum in spring and fall. There is a permanent westerly flow on the Texas Shelf. After turning cyclonically, this flow joins the eastward transport of the northern limb of the anticyclone in the western Gulf of Mexico. Most of this eastward flow recirculates in the anticyclone; however, a portion flows cast across the central Gulf to become entrained in the Loop Current. The deep circulation between 1500 and 3000 m is dominated by an anticyclonic gyre which fills the entire deep basin.

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Elizabeth Johns, Rana A. Fine, and Robert L. Molinari

Abstract

In June–July 1990, hydrographic, chlorofluorocarbon (CFC), and velocity observations were taken along the western boundary of the North Atlantic south of the Blake Bahama Outer Ridge from 30° to 24°N between the northern Bahamas and 71°W. The deep flow in the region, associated with the deep western boundary current, forms a pattern of strong, narrow currents and cyclonic gyres close to the continental slope with broad, slower southward flow offshore. The CFCs reveal that the most recently “ventilated” water (i.e., having the highest CFC concentrations due to more recent contact with the atmosphere in the northern North Atlantic) is found along the western boundary in two distinct cores between potential temperatures 4°–6°C and 1.9°–2.4°C. Geostrophic transport streamlines were constructed for the deep flow, referenced using direct velocity observations at 26.5°N and assuming mass conservation between closed areas bounded by the hydrographic sections. The tracers and transports are used together to describe the deep circulation in the region, to determine the origins and pathways of the various flow components, to define the spatial scales and strengths of the deep currents and recirculation gyres, and to examine their relationship to bottom topography and their possible role in ventilating the interior. The close correspondence of the tracer distributions with the regional bottom topography implies that the major topographic features in this region strongly influence the deep circulation. The geostrophic transport for the narrow branch of current having the highest CFC concentration, which transits the region and continues equatorward adjacent to the western boundary, is 31 Sv (Sv ≡ 106 m3 s−1) below 6°C. A cyclonic gyre with one or more embedded gyres extends offshore of the narrow boundary current out to about 74°W, transporting 12 Sv of water with intermediate CFC concentrations. Farther offshore, a broad band of southward flow contributes an additional 16 Sv of water with considerably lower CFC concentrations. Thus there is a total deep (<6°C) equatorward transport through the study area along the western boundary of 47 Sv at 24°N. The layer containing the shallower CFC core (4°–6°C) appears to be less constrained by the bottom topography. Within this temperature layer, one current branch with high CFC and low salinity flows southward along the Blake Escarpment. However, there is another branch of flow within this layer that forms an extended zonal high CFC and high salinity distribution from the eastern to the western bounds of the study region. This second branch apparently originates in the Gulf Stream recirculation and carries the higher salinity influence of the Mediterranean Water.

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John C. Swallow, Robert L. Molinari, John G. Bruce, Otis B. Brown, and Robert H. Evans

Abstract

Near-surface observations of temperature, salinity and current are used to describe the seasonal reversal of the Somali Current during 1979, in response to the onset of the southwest monsoon winds. During April, prior to the reversal of the winds north of the equator, the northward flowing East African Coastal Current (EACC) and the southward flowing Somali Current (SC) converged near the equator. The EACC was characterized by surface waters with salinities less than 35.1%, and the SC by salinities greater than 35.3%. The winds reversed north of the equator during the first week of May, and the boundary current intruded in the form of an anticyclonic gyre to 2.5°N. Most of the low-salinity water was recirculated back south of the equator by the offshore limb of the gyre. It did not flow continuously at the surface into the eastward equatorial jet, which was present farther offshore during May and June. That current was fed by high-salinity water from the region to the north of the low-latitude gyre. Surface winds increased dramatically in early June; and subsequently, the gyre intruded farther north and east; recirculation southward across the equator was still observed. A second gyre spun up north of the southern feature, apparently in response to the increase in winds. During July and early August the southern gyre intruded farther north, the northern gyre intensified and the equatorial jet disappeared. The data are inadequate to resolve the rapid changes which occurred in late August. The net result was the replacement of the offshore flow between the equator and 5°N by onshore flow along the equator and advection of low-salinity water from south of the equator to 12°N. The observations are discussed in the context of model results and implications for the redistribution and modification of local water masses.

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