Sensitivity of modelled Antarctic climate to surface and boundary-layer flux parametrizations



The Hadley Centre climate model version HadAM2 is used to study the sensitivity of modelled Antarctic climate to the parametrization of surface and boundary-layer heat fluxes under stable conditions. Specifically, the impact of changing the dependence of surface exchange coefficients and eddy diffusivities on the Richardson number is investigated. Three alternative parametrizations are implemented; in all of these the exchange coefficients decrease more rapidly with increasing stability than they do in the standard parametrization used in the model. When only the surface flux scheme is replaced by one of these alternatives, cooling is largely restricted to the surface, with some compensating warming occurring at the lowest atmospheric levels, and little change is seen in the low-level wind field over Antarctica. If alternative schemes are implemented both at the surface and in the boundary layer, widespread cooling occurs at the surface and at the lowest one or two atmospheric levels. The increased negative buoyancy thus generated causes significant increases in the speed of katabatic winds blowing down the coastal slopes of Antarctica. Colder and stronger offshore winds lead to increased cooling of the Antarctic coastal waters. In a coupled model, this could impact on the production of sea ice and ocean-bottom water. The modelled temperature changes appear to show both a direct response to changed boundary-layer heat-flux divergence and an indirect response as a result of the consequent changes to the low-level circulation.