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Modelling the sensitivity of the ocean thermohaline circulation to changing forcing

Ross W. Griffiths1, Graham O. Hughes1, Melissa A. Coman1, Kial D. Stewart1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia


Oceanographers have been examining the question of whether the overturning circulation of the oceans will change in response to global warming, with consequent feedbacks to climate. A particular concern is a potential shut-down of the deep sinking leg of the circulation.

We have modelled the adjustment of a convective circulation to changing surface boundary conditions, such as atmospheric warming or increased freshwater inflow in high latitude oceans. In experiments such a circulation was brought to its equilibrium (balanced) state and then the surface boundary conditions were changed. This disturbs the fine balance within the circulation and causes one of two dramatic responses in the flow. Increased surface cooling rapidly leads to a large increase in the rate of overturning, followed by an exponential decay toward a new finely balanced state similar to the initial state.

The measured exponential timescale is easily predicted from a simple theory. On the other hand, a surface warming can lead to a shutdown of the deep sinking and the circulation quickly becomes confined to a shallow upper ocean layer. The shallow overturning is temporary, and a full-depth overturning circulation comparable to the initial state is eventually restored. A new theoretical solution for the equilibrium state (Hughes et al. 2007) helps us to understand the surface changes that will lead to shut-down. The results are consistent with the effects of increased melt water input, published last year, which indicated that shut-down can occur with a 4% change in the surface buoyancy forcing.

We have also shown this year that the circulation is sensitive to the relative fluxes in northern and southern hemisphere sinking regions. These regions generate waters of different density (the Antarctic Bottom Water and North Atlantic Deep Water) and a few percent change in the density or volume flux in one relative to the other can lead to a substantial modification of both the patterns of ventilation in the deep circulation and cross-equatorial transport in the upper ocean (figure 2). We have undertaken a review of the physics governing horizontal convection (Hughes and Griffiths, 2008) and are currently studying the role of flows between ocean basins, which occur through straits and over sills, in controlling the rate of overturning.

 

Figure 1. A photograph of dye in the convective overturning when a larger polar cooling flux is applied to the right hand quarter of the upper surface, a weaker polar cooling flux is applied to the left hand quarter of the surface, and the equatorial region is heated such that there is no net heat input. The blue dye reveals a weaker sinking and shallow circulation while the red dye shows a strong full-depth overturning. The blue-dyed water is also entrained into the stronger plume and cycled throughout the box.

Figure 2. The stream function from a numerical simulation of the experiment in figure 1.  The black contour lines represent streamlines of the flow; close contour lines mean faster flow velocities.

 


Hughes, G.O., Griffiths, R.W., Mullarney, J.C. and Peterson, W.H. (2007) A theoretical model for horizontal convection at large Rayleigh number. J. Fluid Mech. 581, 251-276.
Hughes, G.O. and Griffiths, R.W. (2008) Horizontal convection. Annu. Rev. Fluid Mech. 40, in press.