The project will explore the mechanisms of maintenance of the deep stratification and circulation of the ocean. 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 Merit Understanding 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 Impacts The 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.
The ocean is an integral part of the climate system. Thus, for example, moderates out climate on daily and seasonal timescales -- the difference in the climates of New York and San Francisco, for example, are almost entirely due to the influence of the ocean -- which is much larger in the case of San Francisco. The ocean will, furthermore, moderate the global warming that will occur in the decades ahead. Although the above effects are well understood, we have not hitherto had a good understanding of how the ocean transports heat meridionally. Although we have been able to simulate this transport on big computers we have had no theory of it, and without a theory our large simulations are suspect, for it is always possible to tune a simulation to give the right answers for the wrong reason. In this project we developed a new theory of the deep circulation of the ocean, the part that carries a lot of the heat between equator and pole. The theory is different from a simulation, because it provides a transparent description of the mechanism that is plain for all to see. It proves a new understanding of the phenomenon. We can now predict how the ocean circulation might change if the surface conditions change - as might happen with global warming for example. We can also predict how the ocean might have changed in the far past. The theory also enables us to predict how carbon dioxide is taken up by the ocean, and how this might compare with the heat uptake. This is particularly important because the it is carbon uptake that gave us the cold climate of the last icea age, and the carbon uptake by the ocean will determine the degree to which global warming occurs. Finally, our new theory enables us to better inerpret the way in which the atmosphere and ocean work together, and even compete with each other, in the over heat transport of the climate system. Our new theory doesn't tell us everything. But it is a start in provideing a better understanding of the climate system, an understanding that is vital if we are to better live on this planet, the only one we have.
