The Maintenance of Deep Circulation and Stratification in the Ocean

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The project will explore the mechanisms of maintenance of the deep stratification and circulation of theocean. A central aspect will be to further develop and to test a theory for the mid-depth and abyssal circulation and stratification. There is no body of accepted theory for the stratification of the mid-depth and abyssal ocean. Traditionally, it has been supposed that the deep stratification is maintained by the presence of multiple water masses with different temperature and salinity properties and/or by diapycnal mixing which, in association with a meridional overturning circulation, can maintain a stratified abyss. However, recent numerical investigations have suggested that, in the presence of a re-entrant Antarctic Circumpolar Channel, deep stratification can be maintained even in the limit of vanishing diapycnal diffusivity. If true, this would be an important result both at a fundamental level and because of the implications it would have for ocean mixing and the need for large values of diapycnal diffusivity, for the behavior of the ocean in past climate regimes, and for the carbon cycle. The investigators have developed a conceptual model for the stratification of the ocean below the thermocline, which will be further developed and extended into a quantitative and testable mechanistic theory. The theory will be tested with numerical simulations, and used to better understand whether the results apply to the real ocean and what the consequences are for ocean circulation in past climates.Intellectual MeritUnderstanding the stratification of the ocean is a challenging fundamental problem in oceanography. We are proposing to further develop a novel theory and to perform numerical simulations, and to compare with alternative hypotheses, to determine the cause of the deep stratification and associated overturning circulation.Broader ImpactsThe deep circulation of the ocean transports heat meridionally and so is of direct importance to the climate system. Understanding the ocean stratification and circulation is a necessary aspect of improving numerical models that predict that transport. The deep circulation is also likely tied to the uptake and release of carbon dioxide, and is an important factor determining the levels of carbon dioxide in the atmosphere. Understanding the deep circulation will therefore help us to understand the nature of climate variability on very long timescales. The proposal will support a junior scientist at the threshold of a promising career. Finally, the lead investigator will be revising a textbook (Vallis 2006) that has been adopted by a number of universities for the teaching of atmospheric and oceanic dynamics, and he is beginning to write a second, undergraduate level, book on the ocean's impact on climate.