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Temperature and Salinity Staircases in the Northwestern Weddell Sea

R. D. MuenchScience Applications International Corporation, Bellevue, Washington

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H. J. S. FernandoDept. of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona

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G. R. StegenScience Applications International Corporation, Bellevue, Washington

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Abstract

Temperature and salinity data obtained from the northwestern Weddell Sea during March 1986 reveal numerous thermohaline staircases in the thermocline separating warm deep water from the overlying colder, lower salinity winter water. Staircases in the upper, steeper portion of the thermocline were characterized by layers having vertical extents of 1–5 m. Layer thicknesses in the deeper, weaker portion of the thermocline were far greater, sometimes exceeding 100 m. The former staircases are referred to as Type A, and the latter as Type B. Vertical gradients in temperature and salinity decreased abruptly across the boundary between Type A and Type B staircase regions. Mean density ratios Rρ were 1.52 and 1.36 over the depth intervals containing Type A and Type B staircases, respectively. Type A staircases were present at all sites sampled, whereas Type B staircases were present over approximately the central 50% of the area sampled.

Laboratory-derived results show that the observed time and vertical space scales for the Type B staircases are consistent with the notion that they are maintained by double diffusive processes. These results, combined with temperature-salinity analyses, lead us to suggest that the Type B staircase regime may have originated as a vertically convective feature within which staircases have formed and evolved continually through double diffusion. Laboratory-derived flux laws are used to estimate upward buoyancy flux due to heat flux through the Type B staircase regime of order 2 × 10−1 m2 s−1, consistent with values derived previously using oceanographic, atmospheric and sea ice data and an order of magnitude greater than computed double diffusive heat fluxes through the Type A staircase regime. The broad areas coverage of Type B staircases, coupled with previous observation of these features at scattered sites throughout much of the Weddell Sea, suggests that they are widespread there and may play a significant role in regional vertical heal transfer.

Abstract

Temperature and salinity data obtained from the northwestern Weddell Sea during March 1986 reveal numerous thermohaline staircases in the thermocline separating warm deep water from the overlying colder, lower salinity winter water. Staircases in the upper, steeper portion of the thermocline were characterized by layers having vertical extents of 1–5 m. Layer thicknesses in the deeper, weaker portion of the thermocline were far greater, sometimes exceeding 100 m. The former staircases are referred to as Type A, and the latter as Type B. Vertical gradients in temperature and salinity decreased abruptly across the boundary between Type A and Type B staircase regions. Mean density ratios Rρ were 1.52 and 1.36 over the depth intervals containing Type A and Type B staircases, respectively. Type A staircases were present at all sites sampled, whereas Type B staircases were present over approximately the central 50% of the area sampled.

Laboratory-derived results show that the observed time and vertical space scales for the Type B staircases are consistent with the notion that they are maintained by double diffusive processes. These results, combined with temperature-salinity analyses, lead us to suggest that the Type B staircase regime may have originated as a vertically convective feature within which staircases have formed and evolved continually through double diffusion. Laboratory-derived flux laws are used to estimate upward buoyancy flux due to heat flux through the Type B staircase regime of order 2 × 10−1 m2 s−1, consistent with values derived previously using oceanographic, atmospheric and sea ice data and an order of magnitude greater than computed double diffusive heat fluxes through the Type A staircase regime. The broad areas coverage of Type B staircases, coupled with previous observation of these features at scattered sites throughout much of the Weddell Sea, suggests that they are widespread there and may play a significant role in regional vertical heal transfer.

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