The Impact of a Bottom Boundary Layer Scheme on the North Atlantic Ocean in a Global Coupled Climate Model

Abstract

Abstract Although the overflow and descent of cold, dense water across the Greenland–Iceland–Scotland ridge is the principal means for the maintenance of the thermohaline circulation in the North Atlantic Ocean, this feature is not adequately treated in global ocean numerical models. In this paper, a bottom boundary layer scheme is introduced into the HadCM3 coupled atmosphere–ocean–sea ice general circulation climate model, in order to give an improved representation of cold water formation in the North Atlantic Ocean. The scheme uses a simple terrain-following bottom boundary layer incorporated into the ocean general circulation model; only the tracer tendencies are evaluated in the bottom boundary layer, with the velocities taken from the near-bottom interior values. It is found that with the bottom boundary layer scheme, there are several significant effects on the deep water formation and flow. The overflow of dense water from the Nordic Seas into the North Atlantic Seas is improved with the introduction of the authors’ bottom boundary layer scheme. Further, the thermohaline circulation is reduced in strength, but is also deeper, when compared with simulations without any bottom boundary layer scheme. There is also a stronger flow along the northwestern boundary, a more southerly location of the North Atlantic Current, and a stronger and larger subpolar gyre. Overall, these effects are an improvement when compared with climatology, although some differences remain.