The Effects on Ocean Models of Relaxation toward Observations at the Surface

Abstract

This paper discusses the errors in surface tracer and flux fields in ocean models induced by using approximate surface boundary conditions involving relaxation toward observed values rather than more physically realistic conditions that involve (often inaccurate) surface fluxes. The authors show theoretically and with a global model example that where there is a net annual surface flux of tracer (balanced by advection), (i) the annual mean surface tracer field is biased compared with the observations and (ii) the annual mean tracer flux is also biased if the surface tracer field has a feedback on the surface tracer advection or diffusion. As previously shown, the amplitude of the annual cycle of tracers is also decreased. The global model indicates that temperature offsets of 1°?2°C (or even greater) and heat flux errors of 30 W m?2 occur in regions of strong advection, such as the equatorial upwelling zone, western boundary currents, and the Antarctic Circumpolar Current. These are all areas crucial for the thermohaline circulation, so that the use of such boundary conditions is likely to yield incorrect estimates for climate simulation models. Zonally integrated meridional heat fluxes may be in error by up to 25%.