On heat flux boundary conditions for ocean models

Recent modeling studies of thermohaline variability have imposed rapid damping of modeled sea surface temperature (SST) anomalies equivalent to assuming the atmosphere has an infinite heat capacity. Such surface heat Bur parameterizations effectively exclude the possibility of SST playing an active role in the thermohaline circulation. The authors present results of simple thermodynamic modeling of the lower atmosphere that suggest the sensitivity of the surface heat fluxes to variations in SST is much smaller than often assumed. It is found that the flux response is strongly dependent on the scale of the SST anomaly. For the very largest scales the fluxes increase by only a few watts per square meter per kelvin change of SST. For the scales typical of observed anomalies the nonlocality of the response enhances the sensitivity, which may reach up to similar to 15 W m(-2) K-1. This extreme is still less than half of the values typically assumed in ocean models. The small sensitivity arises from the adjustment of the lower atmosphere to the underlying ocean in accord with its relatively much smaller ability to store heat and moisture. The increase in fluxes with SST is dominated by the latent heat Aux but offset significantly by reduced net longwave radiative cooling of the surface.