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The Summertime Evolution of Temperature and Salinity in an Arctic Lead |
Oregon State University
cpaulson@oce.orst.edu
The focus of this paper is on processes associated with the establishment, evolution, and destruction of a warm, fresh layer at the surface of an Arctic lead. As part of the SHEBA field experiment, measurements were made in a lead from 7 June to 8 August 1998. Measurements were made for several hours nearly every day from a small boat (3 m) and included vertical profiles of temperature (T), salinity (S), and optical properties on vertical sections across the lead. At the beginning of the summer season, T and S in the lead were nearly uniform within an upper mixed layer approximately 30 m in depth with T close to the freezing point. This mixed layer formed during the previous winter under the influence of wind-forced ice motion, surface cooling, and brine rejection associated with ice formation. As solar insolation increased during the summer, the total surface heat flux into leads and ice floes changed from net cooling to net heating, thereby melting ice and snow. The near-surface temperature of the lead was consistently above freezing after 21 June. By 11 July the average temperature of the lead at a depth of 15 cm ranged from 0.9 to 2.2°C with a mean of 1.6°C and the thickness of the warm, fresh layer was approximately 0.5 m with a surface salinity of 4 psu. By 28 July, the fresh surface layer had deepened to 1.3 m and S had decreased to a uniform 2 psu in the upper 0.9 m. On 28 July wind speed increased to 10 m/s and remained near this level for most of the following day. The wind stress caused ice motion and turbulent mixing which deepened the surface fresh layer to 15 m on 1 August. Following the onset of the storm on 28 July the thickness of the fresh layer decreased rapidly to the thickness of the surrounding ice floes, which we ascribe to turbulent mixing associated with the shear stress between the underside of the ice and the ocean. Subsequent mixing within the lead proceeded more slowly under the direct influence of wind stress on the lead surface and on the indirect influence of turbulence generated in the ice/ocean boundary layer. During first part of the summer, the fresh surface layer limited the vertical transfer of heat available to melt bottom ice. Hence the role of fresh surface layers in leads may be to apportion more heat to melting the sides of ice floes than to their bottoms.
