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Hydrography of the Labrador Sea during Active Convection

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  • 1 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
  • | 2 Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada
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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.

Corresponding author address: Dr. Robert S. Pickart, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. Email: rpickart@whoi.edu

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.

Corresponding author address: Dr. Robert S. Pickart, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. Email: rpickart@whoi.edu

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