University of Cambridge > > BPI Seminar Series > The response of the Meridional Overturning Circulation and Antarctic Circumpolar Current to changes in surface forcing

The response of the Meridional Overturning Circulation and Antarctic Circumpolar Current to changes in surface forcing

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The ocean stratification and circulation is largely controlled by surface fluxes of heat, water and momentum. Understanding the response of the ocean to changes in the distribution and magnitudes of these forcing fields is vital to predictions of future climatic changes, given the role of the Meridional Overturning Circulation (MOC) in the poleward transport of heat and the importance of the deep ocean as a sink of anthropogenic carbon dioxide.

I will discuss the stratification and circulation of an idealised pole-to-pole ocean basin with a circumpolar channel , forced by surface wind stresses and buoyancy fluxes. Firstly, the phenomena relevant to the maintenance of the MOC in such a basin, namely boundary currents, baroclinic eddies and their parameterisation, Ekman transport and vertical diffusion, will be reviewed and combined into a simple conceptual model of the ocean circulation. This new model resolves some of the problems associated with previous models such as Gnanadesikan (1999). The model predicts simple scaling laws for the response of the ocean stratification and overturning circulation to changes in the air-sea fluxes of buoyancy and momentum. The key result is that both buoyancy and wind forcing are equally important in determining the circulation and stratification of the ocean basin. The effect of the surface buoyancy fluxes in modifying the ocean stratification, and thus via thermodynamic balance modifying the ocean circulation, has not been previously quantified.

Secondly, I present results from a high resolution numerical simulation of the same idealised ocean basin. These results show good agreement with the predictions of the simple conceptual model. Furthermore, an analysis of the energy budget of the numerical model demonstrates a positive feedback between the wind and buoyancy forcings. Changes in the rate of generation of available potential energy (APE) by buoyancy fluxes at the ocean surface are positively correlated with changes in the rate of generation of kinetic energy (KE) by wind stress, and vice versa. This result emphasises that the overturning circulation is the result of a complex and subtle balance of surface forcing mechanisms.

This talk is part of the BPI Seminar Series series.

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