The effect of mesoscale ocean eddies on the thermocline and abyss

We investigate the ways in which mesoscale ocean eddies affect both the thermocline and abyssal structure in an idealized, primitive equation model. The model domain is that of an enclosed single basin at low latitudes, with a zonally periodic channel at high latitudes. The model thus contains both a subtropical gyre and a representation of the Antarctic Circumpolar Current. In the non-eddy permitting (but low diffusivity) simulations, the thermocline of the subtropical gyre displays the two-thermocline structure suggested by Samelson and Vallis (1998); that is, an adiabatic ventilated thermocline overlies a diffusive base. In the channel region, the absence of a zonal pressure gradient prohibits meridional transport, the coldest water in the basin is trapped in the latitudes of the channel, and the abyss has a very small vertical temperature gradient. When a mountain barrier that partially blocks the channel is included, meridional transport is supported and abyssal thermal gradients are produced.

The presence of eddies affects the structure of both the thermocline and the channel. In the thermocline, the eddies try to relax the isobars toward the horizontal as they attempt to reduce the available potential energy of the fluid, but these effects are constrained by the wind and buoyancy forcing at the surface which ultimately give rise to the overall structure of the thermocline. In the channel, the eddies allow a net meridional transport even in the absence of a mean zonal pressure gradient, and the role of the eddies in determining the heat balance and water mass formation will be discussed.