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Spatial and temporal variability of double diffusive structures in Powell Lake, British Columbia Zaloga, Artem

Abstract

The spatial and temporal properties of naturally occurring double diffusive (DD) structures present in the bottom waters of Powell Lake, British Columbia were investigated. Observations were obtained from four annual surveys consisting of vertical cm-resolution conductivity-temperature-depth (CTD) profiles along the 9 km length of the lake, and from a month-long mooring consisting of thirty-eight temperature sensors and two current meters. DD layers were identified by isolating clusters on temperature-salinity (T-S) diagrams, and tracked spatially and temporally throughout each of the CTD surveys. The layers were observed to be persistent over four years, and horizontally coherent over the entire lake length at depths of 336-347 m. In this region the vertical density ratio (a non-dimensional measure of the relative strengths of vertical temperature and salinity gradients), Rρz, and buoyancy frequency, N, were near constant at Rρz = 2.2 ± 0.2 and N = (2.3 ± 0.3) x 10‐³ s‐¹, and the Rayleigh number reached a peak at Ra ≈ 10⁷. Layers just above and below this region were less horizontally-coherent and with larger values of Rρz and N. Spatial variations in layer depth and the background temperature/salinity distribution showed persistent trends throughout the study period. These trends indicated that layer slope and horizontal property gradients are linked and that the horizontal density ratio may be an indicator of the mean layer slope. Linear fits to the layer properties indicated that a horizontal density ratio of Rρx = -0.35 ± 0.17 was accompanied by a mean layer slope of ∆z/ ∆x = 0.05 ± 0.02 m/km. An individual DD step within one of the stable and horizontally coherent DD layers was identified within the moored temperature time series and tracked over the course of a week. The convective regime within the DD step was observed to be composed of intermittent thermal plumes emitted from the bottom diffusive interface. The features appeared as a common peak in the mean DD step temperature and horizontal velocity power spectra. The plumes had a period of ∼22 minutes (coinciding well with the buoyancy period within the diffusive interface), a temperature scale of T' ≈ 0.2 m°C, and horizontal and vertical velocity scales of u' = w' ≈ 0.5 mm/s.

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