On the interaction of the bottom boundary layer and deep rings

Abstract

The interaction of rings extending down to the ocean bottom and the bottom boundary layer (BBL) is studied numerically with a primitive equation model in which the BBL is resolved using the Mellor Yamada level-2.5 closure scheme. The lower portions, below the thermocline, of rings with radii of ≈50, 100 and 200 km, are considered. For rings of comparable strength, the larger ones are found to have the stronger velocity “footprint” on the bottom. The BBL is found to spin down this portion of rings by ≈60% in times 0(6 months), with most of the energy transfer lost irretrievably to bottom friction for the larger rings and to potential energy for the smaller rings. The classical constant eddy viscosity BBL formulation is found to yield similar spindown results as the model, provided the eddy viscosity is chosen to give realistic bottom stress rather than BBL thickness. The numerical model indicates that Ekman pumping out of the BBL affects the bottom mixed layer thickness more so than the Ekman pumping into the BBL, and that maximum bottom mixed layer thicknesses are found not in regions of maximum flows but in upwelling regions of relatively strong flows. Because of the presence of the Cold Filament, the HEBBLE site is found to be unsuitable for testing the model conclusions about bottom layers.