The global Meridional Overturning Circulation's response to variable buoyancy forcing

Abstract

The meridional overturning circulation (MOC) is a large-scale circulation throughout the global ocean and plays a significant role in the complex global climate system. However, our traditional understanding of the processes driving the MOC has been questioned in recent years. In particular, it has been suggested that surface buoyancy forcing plays little energetic role in driving the MOC. Furthermore, doubt has also been cast over the relationship between meridional overturning and meridional density gradients. In this thesis, we revisit these two ideas using a series of equilibrium and transient numerical ocean simulations.

We begin by diagnosing the global ocean's gravitational potential energy (GPE) budget at steady state in the coupled atmosphere-ocean model HadCM3, utilising the available gravitational potential energy (AGPE) framework to distinguish between the effects of adiabatic and diabatic processes on background GPE and the portion available for adiabatic conversion to kinetic energy (AGPE). We show that surface buoyancy forcing plays an active energetic role in diabatically sustaining large-scale ocean circulation, making available background GPE generated by diapycnal mixing. We then carry out a series of transient experiments using a global configuration of the NEMO ocean modelling framework, periodically varying North Atlantic surface buoyancy forcing over a broad range of timescales. It is shown that the ocean exhibits a highly nonlinear, frequency-dependent response to forcing of the same amplitude, which is well predicted by a scaled overturning streamfunction based on twice-integrated density differences at multi-decadal timescales and longer. Finally, we analyse the AGPE generated by surface buoyancy forcing during each forcing cycle, finding a strong relationship between the AGPE power input by surface buoyancy fluxes and the magnitude of the MOC response. We conclude by discussing the remote effect of North Atlantic buoyancy forcing on lower cell overturning in the Southern Ocean, considering regional changes in AGPE generation.

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