OceanBound

The currents in the ocean are turbulent, and dominated by "eddy variability" on scales much smaller than the ocean basins. This complex, nonlinear variability makes it impossible to understand the ocean as a whole in all its detail - we can run very expensive computer models at high resolution and get realistic-looking answers, but how do we know whether their long-term predictions are also realistic? That relies on having a good understanding of the processes involved - we have to find a way to sidestep the complications of the eddies and find comprehensible aspects of the system to connect the models, via theory, to the real ocean circulation. Fortunately, when we look at the "sidewall" boundaries of the ocean, we find that the eddy effects are greatly simplified, and we get a picture of the whole ocean which can be connected to theoretical ideas.

The aim of this project is to make the global ocean circulation comprehensible in terms of a small number of clearly defined processes, and hence to improve understanding of its influence on a range of important issues from sea level to heat transport.

Amidst the eddying chaos, there are parts of the ocean circulation which operate on a global scale, carrying water between different ocean basins and carrying heat around the world. These modes are among the most important parts of the global climate system. One mode is the Atlantic Meridional Overturning Circulation, which transports heat to the north throughout the entire Atlantic Ocean and has a large effect on European climate. Others include the Indonesian Throughflow, which carries warm water from the Pacific to the Indian Ocean, and the Antarctic Circumpolar Current, which connects the Atlantic, Indian and Pacific oceans. The currents associated with these modes have an influence on coastal sea levels around the world, causing sea level to be higher along some coasts than others. We call the effect of these modes the "global plumbing" of the ocean.

Although we have computer models which can simulate many aspects of the ocean circulation well, it is hard to model how this plumbing varies over time. Making good predictions of future climate and sea level requires us to understand the causes of variability, and to test our understanding we need good measures of how the plumbing changed in the past.

The ocean's turbulence makes it very hard to measure such large scale modes. To measure the currents themselves would require an enormous number of instruments to be in the ocean at all times. But we now know that pressure on the sidewalls of the ocean encodes information about the large scale modes without the added confusion from turbulence and eddies. Processes occurring at just a few places control the pressures, and therefore the plumbing, over very large distances.

In this project, we will use ocean modelling to learn how those local processes influence ocean boundary pressures, and hence the global ocean plumbing. The new understanding will then be used to determine which future changes we can have confidence in, and to direct improvements of the next generation of climate models.