Effects of Bottom Boundary Layer Parameterization on Reproducing Deep and Bottom Waters in a World Ocean Model

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Abstract
A simple bottom boundary layer (BBL) model is developed to be incorporated into a z-coordinate medium resolution ocean general circulation model. In the BBL model, velocity is calculated from the pressure gradient, which is also calculated within the BBL. Preliminary experiments using an idealized basin model clearly document that, for reproducing realistic overflow/downslope flow, it is essential to adopt the horizontally distributed Rayleigh drag coefficient in the BBL model and also that, for avoiding warming of the abyssal ocean owing to unphysical strong flows created by the pressure gradient error along the steep slope, it is necessary to limit the area of the BBL. This BBL model is successfully incorporated into a World Ocean model with 1° × 1° resolution, producing the overflow/downslope flow in the northern North Atlantic and around Antarctica. The dense overflow/downslope flow water provides the nucleus of the abyssal water in the World Ocean, leading to the better representation of the abyssal water. Thus, the incorporation of the BBL model can reduce the warming bias for the abyssal water in a coarse resolution model without modifying surface boundary conditions. In addition, it can also alleviate the model bias of too shallow extension of North Atlantic Deep Water. Without the BBL model, the dense water from the Nordic seas does not flow southward and remains south of Iceland, forming a strong front there. The effect of Gent and McWilliams parameterization for mesoscale eddies to flatten isopycnal surfaces leads to unrealistic cooling south of Iceland. Unrealistic cooling also takes place around Antarctica. The incorporation of the BBL model drastically reduces this cooling by preventing the formation of the artificial front.