From Internal Waves to Mixing and Transformation Rates: Observations in the Southern Ocean

Amelie Meyer1, Bernadette Sloyan2, Kurt L Polzin3, Helen Elizabeth Phillips4 and N L Bindoff4,5, (1)Norwegian Polar Institute, Ocean and Sea Ice, Tromsø, Norway, (2)CSIRO Marine and Atmospheric Research Hobart, Hobart, TAS, Australia, (3)WHOI, Woods Hole, MA, United States, (4)University of Tasmania, Institute for Marine and Antarctic Studies, IMAS, Hobart, Australia, (5)CSIRO Marine and Atmospheric Research Hobart, Hobart, Australia
Abstract:
Understanding the dynamics that maintain the deep ocean stratification structure is of fundamental importance to understanding large-scale ocean circulation. Dissipating internal waves are the main source of mixing in the stratified ocean. Here we explore the link between internal waves, mixing, and the overturning circulation near the Kerguelen Plateau in the Southern Ocean. Based on observations from profiling floats, we describe the internal wave field properties, the distribution and intensity of mixing, and estimate water mass transformation rates.

The data consist of 914 temperature, salinity, pressure and horizontal velocity profiles from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats deployed northeast of the Kerguelen Plateau in 2008. We first identify and characterize 46 internal waves. We then estimate diapycnal mixing in the upper 1600 m of the water column applying a shear-strain fine-scale parameterization. In the context of the Southern Ocean overturning circulation, we also estimate the water mass transformation rates both locally and scaled to the entire Southern Ocean.

These observational results provide a clear connection between the internal wave field properties, and the distribution and intensity of mixing. The internal wave characteristics are dependent on regional dynamics, suggesting that different generation mechanisms dominate in different dynamical zones. We find evidence that some of the internal wave energy is advected away from the region, contributing to the stratification and driving the overturning circulation downstream. Mixing intensities show strong spatial and temporal variability. Small scale topographic roughness at the seafloor, flow strength, large-scale flow structure and atmospheric forcing are identified as important factors in determining local dynamical regimes for both the internal waves and the mixing rates. In particular, identified fronts of the Antarctic Circumpolar Current are associated with intense internal wave activity, with intense mixing and with substantial water mass transformation rates. We show evidence that local generation of internal waves can set the large-scale stratification of the Southern Ocean.

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