Polar boundary layer plumes and bottom water formation: A missing element in ocean general circulation models

Much of the deep water in the world oceans forms as relatively dense water on shelves in polar regions, followed by downslope flow of the dense water as a boundary layer plume. This process is omitted from current global ocean general circulation models (GCMs) which, instead, form bottom water only as a result of the large-scale thermohaline overturning and as a result of deep convection. Here the role of polar boundary layer plumes is investigated by coupling a two-dimensional plume model to a two-dimensional (latitude-depth) dynamical ocean model, which, in turn, is coupled to sea ice and surface climate models. The polar plume mass flux at the shelf brink is parameterized in terms of the rate of sea ice formation, net evaporation, and surface cooling over polar shelves. The presence of plumes has only a modest impact on the high-latitude temperature response to a greenhouse gas increase and the projected sea level rise due to thermal expansion because, in the present model, the overall response is strongly conditioned by changes in the relative strength of northern hemisphere and southern hemisphere bottom waters which occur whether or not plumes are included. These changes, in turn, appear to depend critically on the spin-up flow field and density fields produced by the coupled model. it is concluded that polar plume processes should be incorporated in coupled atmosphere-ocean GCMs but that correct simulation of the ocean density field and surface forcing conditions is a more important priority.