Changes in Internal Wave Driven Diapycnal Mixing

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The oceans are the largest sink of anthropogenic warming, and a large portion of this heat is accumulating in the deep ocean. Understanding what is driving this deep warming, and how the climate system will respond to it, is vital to accurately forecasting how the ocean will take up anthropogenic heat in the future. One important process is ocean mixing between different densities (diapycnal mixing), which both depends on and directly affects heat distribution within the ocean. Yet to our knowledge there have been no studies using observations to explore how ocean mixing may be changing in response to climate change, which would directly affect how heat is taken up and stored within the ocean in the future. The ultimate goal of this study is to provide the first such observational estimate of mixing changes in the ocean over the last few decades. One undergraduate summer student will be supported on this grant, in addition to the support of an early career observationalist. Much of the mixing in the ocean interior is due to internal gravity waves whose generation, propagation, and breaking all depend on the background stratification. This background stratification evolves from changes in the temperature and salinity of the water, which vary by location in the ocean. The mixing due to internal waves is also expected to change. However, it is not known whether that will make a substantial contribution to the energy or heat budgets of the ocean. This project aims to place observational bounds on the change in mixing, addressing the questions: Where, and to what extent, is diapycnal mixing in the ocean changing on climate scales? Is this change large enough to influence abyssal heat budgets? Is the planned observational infrastructure sufficient for measuring the changes in mixing? These questions will be addressed through analysis of data collected during global repeat hydrographic surveys. Using these hydrographic sections, the investigator will explore the stratification change with time on a global scale, and link these observations to known water property changes. The change in diapycnal mixing and associated uncertainties will then be calculated over the hydrographic sections using estimates of mixing from temperature, salinity, and pressure data. Finally, the importance of these changes will be considered by applying the calculated changes in mixing to heat budgets of enclosed abyssal basins.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.