Temporal Variability of Diapycnal Mixing in Shag Rocks Passage

Damerell, Gillian M., Heywood, Karen J., Stevens, David P. and Naveira Garabato, Alberto C. (2012) Temporal Variability of Diapycnal Mixing in Shag Rocks Passage. Journal of Physical Oceanography, 42 (3), 370-385. (doi:10.1175/2011JPO4573.1).

Abstract

Diapycnal mixing rates in the oceans have been shown to have a great deal of spatial variability, but the temporal variability has been little studied. Here results are presented from a method developed to calculate diapycnal diffusivity from moored acoustic Doppler current profiler (ADCP) velocity shear profiles. An 18-month time series of diffusivity is presented from data taken by a LongRanger ADCP moored at 2400-m depth, 600 m above the seafloor, in Shag Rocks Passage, a deep passage in the North Scotia Ridge (Southern Ocean). The Polar Front is constrained to pass through this passage, and the strong currents and complex topography are expected to result in enhanced mixing. The spatial distribution of diffusivity in Shag Rocks Passage deduced from lowered ADCP shear is consistent with published values for similar regions, with diffusivity possibly as large as 90 3 1024 m2 s21 near the seafloor, decreasing to the expected background level of ;0.1 3 1024 m2 s21 in areas away from topography. The moored ADCP profiles spanned a depth range of 2400–1800 m; thus, the moored time series was obtained from a region of moderately enhanced diffusivity.

The diffusivity time series has a median of 3.3 3 1024 m2 s21 and a range from 0.5 3 1024 to 57 3 1024 m2 s21. There is no significant signal at annual or semiannual periods, but there is evidence of signals at periods of approximately 14 days (likely due to the spring–neap tidal cycle) and at periods of 3.8 and 2.6 days most likely due to topographically trapped waves propagating around the local seamount. Using the observed stratification and an axisymmetric seamount, of similar dimensions to the one west of the mooring, in a model of baroclinic topographically trapped waves, produces periods of 3.8 and 2.6 days, in agreement with the signals observed. The diffusivity is anticorrelated with the rotary coefficient (indicating that stronger mixing occurs during times of upward energy propagation), which suggests that mixing occurs due to the breaking of internal waves generated at topography.

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Contributors

Author: Alberto C. Naveira Garabato

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