UNDER-ICE CIRCULATION AND THE RESIDENCE TIME OF A SHALLOW BAY

Vertical temperature profiles measured along the axis of Wupaw Bay in March of three consecutive years showed that the water column was inversely stratified under ice. The depth of the (inverse) thermocline varied monotonically along the length of the bay, with the most rapid change occurring in the channel connecting the bay to Southern Indian Lake. Using prototype instrumentation, the associated two-layer current in the connecting channel was measured. The current near bottom often exceeded 1 cm/s and was directed out of the bay. The upper layer current generally flowed into the bay but was more variable than the bottom current and at times reversed direction. Although the circulation is relatively weak, the water exchange through the channel makes an important contribution to the residence time of water in Wupaw Bay during March. A short (2 h) time series showed that the temperature structure was highly variable below the surface mixed layer. High-frequency random fluctuations in temperature were observed together with active overturning events indicative of significant levels of turbulence under ice. Nonlinear effects also dominated regions with the largest temperature gradients where temperature oscillations appeared wavelike but were higher in frequency than the measured Brunt-Vaisala frequency.