Extending Abyssal Mixing Observations and Parameterizations

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Intellectual Merit: Observational studies suggest the existence of only weak diapycnal mixing in the ocean interior, with orders of magnitude more mixing near the bottom in regions of rough topography. In contrast, numerical circulation and climate models generally employ spatially uniform parameterizations of abyssal diapycnal mixing. However, several recent numerical studies demonstrate the sensitivity of the large scale abyssal ocean circulation to the spatial distribution of abyssal diapycnal mixing, leading modelers to recognize that mixing parameterizations mimicking the observed spatial variability are required to improve simulations of the ocean circulation.As is typical in oceanography, a major problem hampering both the development and validation of improved abyssal mixing parameterizations is the scarcity of observations. Microstructure observations, the most direct and accurate tool with which to infer diapycnal mixing, have only been accomplished in a few select regions of the global ocean and will unfortunately not become a routine measurement in the near future due to their cost and the necessity for very high levels of quality control. Consequently, much work has focused on ways to infer diapycnal mixing indirectly from standard hydrographic data, potentially yielding a much more densely sampled global map of diapycnal mixing. A now common approach is the application of fine-scale parameterizations, relating turbulent dissipation to shear and/or strain variance on scales of order 10 meters, based on nonlinear internal wave-wave interaction theory. However, it is becoming increasingly clear that fine-scale parameterizations are reliable predictors only for a very limited range of conditions. The method is known to break down in special environments (e.g., the coastal slope, canyons) and to be a poor predictor of diapycnal mixing in weakly stratified water. The stratification limitation is severe since both the global abyssal ocean (depth 1000 m) and the Southern Ocean fall in that category. The abyssal and Southern Ocean are important branches of the Meridional Overturning Circulation and thus the credibility of climate modeling studies depends on accurate representation of diapycnal mixing in these ocean volumes. The World Ocean Circulation Experiment (WOCE) hydrographic data will be re-analyze using Thorpe scale analysis. Thorpe scale analysis has been shown to be a more robust predictor of dissipation and diapycnal mixing than fine-scale parameterizations, in particular in weak stratification and regions where the characteristics of the internal wave field deviate from the canonical Garrett-Munk model, yet has not been applied extensively to the WOCE hydrographic data set. The expected results from this work are an improved observational knowledge of the spatial distribution of mixing in the abyssal ocean. We also propose a comprehensive comparison and validation of existing abyssal mixing parameterizations intended for use in general circulation and climate models. Broader Impacts: The funds requested in this proposal are intended for the support of a postdoctoral investigator. The results from this project will be disseminated in the refereed literature, made available online and presented at appropriate scientific meetings. The spatial variability of mixing significantly affects the abyssal circulation and stratification, the strength and depth of the Antarctic Circumpolar Current as well as various aspects of the meridional overturning circulation (MOC). In turn, these relate to the ability of the ocean to store and transport heat and greenhouse gasses, and thus the response of the climate system to anthropogenic and natural forcing. This project is expected to result in a better observational knowledge of the spatial distribution of mixing than achieved to date, and to contribute to the improvement of diapycnal mixing parameterizations intended for ocean general circulation models and climate models.