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R. Allyn Clarke and A. R. Coote

Abstract

Oxygen, nutrient, and tritium concentrations observed in the western Labrador Sea in March 1976 during deep convective renewal of Labrador Sea water are analyzed to show how a newly formed water mass obtains its characteristics. Common to other winter observations of deep mixed layers, the oxygen concentrations are some 6% undersaturated, even in the upper 20 m. A gas transfer model coupled to a simple mixed layer model illustrates the difficulty of transferring sufficient oxygen across the air-sea boundary to fully oxygenate the mixed layer when the mixed layer depth exceeds a few hundred meters. The nutrient concentrations of the mixed layers are fairly well mixed as is consistent with the fairly narrow range of nutrient concentrations of the source waters. Only the tritium concentrations exhibit any structure within the mixed layer, and it is argued that this is due to the much larger range of tritium concentrations in the source waters that make up the mixed layer.

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R. Allyn Clarke and Jean-Claude Gascard

Abstract

Data obtained in the western Labrador Sea during March 1976 by Hudson are analysed to show that new Labrador Sea Water was being formed at this time. On the basis of hydrographic and moored current-meter data, it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western Labrador Sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to Labrador Sea Water to take place. Using a model, it is demonstrated that water columns found along the western boundary of the Labrador Sea can be modified by cooling, evaporation and mixing to form deep mixed layers with the properties of Labrador Sea Water.

Approximately 105 km3 of new Labrador Sea Water was formed in 1976, an estimate that is consistent with earlier estimates of mean annual production rates. This water, 2.9°C, 34.84‰, is some 0.6°C cooler and 0.06‰ fresher than that defined by Lazier (1973) from his data collected in 1966. The variation of Labrador Sea Water and its rate of production over the last 50 years is discussed.

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Jean-Claude Gascard and R. Allyn Clarke

Abstract

In a previous paper, Clarke and Gascard argued that the formation of Labrador Sea Water was taking place in a cyclonic gyre set up each winter in the western Labrador Sea.

Within the gyre and at its boundaries, a number of different scales of organization are believed to be important in the formation processes. The longest of these scales is the mesoscale (50 km), which appears to be related to topographic Rossby waves generated in the Labrador Current and propagating offshore. The next smaller scale is an eddy scale (20 km) believed to arise because the mesoscale is baroclinically unstable, as shown by applying a two-layer model of Tang. This instability is believed to promote mixing by generating frontal structures and vertical motions along them, thus bringing subsurface T-S maxima nearer the surface. Then within the mesoscale and eddy-scale structures, intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions. These events are short-lived and occur in response to particularly intense air-sea exchanges.

Most of these processes have already been recognized in the Mediterranean Sea (MEDOC): that is, baroclinic instability of mesoscale features generating mixing at an eddy scale which is quite small because the scale is related to the internal radius of deformation (5–10 km). What is new is the link between the unstable mesoscale structures and the large-scale general circulation through the generation of topographic Rossby waves.

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Robert S. Pickart, Daniel J. Torres, and R. Allyn Clarke

Abstract

The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February–March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (∼200 W m−2). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself—something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current “throughput” and offshore “recirculation.” Finally, the overall trends in stratification of the observed mixed layers are described.

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R. Allyn Clarke, Harry W. Hill, Robert F. Reiniger, and Bruce A. Warren

Abstract

During April–June 1972 three ships conducted a survey of the region between the Grand Banks and the Mid-Atlantic Ridge, including a grid of hydrographic stations, and two long lines of near-bottom current-meter moorings across the Gulf Stream and North Atlantic Current, respectively. The purpose was to map the property distributions and current field where the Gulf Stream branches, in greater detail and with less ambiguity than hitherto; that material is described here. Worthington's hypothesis that the primary current system there is not a branching Gulf Stream but portions of two separate (and nongeostrophic) gyres is criticized at length in terms of the observed property distributions; it is shown that, given a moderate degree of lateral mixing, they are consistent with the branching, geostrophic flow field, and that there is no need to abandon established physics in order to rationalize them. The deep motions recorded by the current meters on the North Atlantic Current line were roughly suggestive of the prevailing flow field inferred at shallower levels. No evidence of the Gulf Stream was found on the other line, however: rather, there was a burst of low-frequency eddy flow, which masked any prevailing extension of the Stream into the near-bottom water.

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