Uma S. Bhatt, Dept. of Atmospheric and Oceanic Sciences, Univ. of Wisconsin, Madison, WI 53706
Michael A. Alexander, CIRES, Univ. of Colorado, Boulder, CO, 80309
We present a new model for the upper North Atlantic Ocean and use it to hindcast the sea surface temperature (SST) from 1950-88. The model consists of a matrix of one-dimensional (independent) columns in which a variable depth, bulk mixed layer overlies a diffusive convective thermocline. The climatological annual cycle of heat flux convergence by the oceanic circulation is implicitly icluded in the formulation of the forcing.
The 38 year control integration of the model includes as surface forcing the shortwave and net longwave radiation from a control integration of the Community Climate Model. Sensible and latent heat fluxes are determined from instantaneous values of surface temperature, humidity, and wind speed from the atmospheric model, and the SST simulated by the ocean model using the bulk formulae. The hindcast is performed by repeating the control integration adding the monthly mean surface anomalies in surface temperature, humidity and wind speed for the period 1950-88. Thus, the simulated SST anomalies are generated explicitly by the anomalies in the latent and sensible heat flux. A separate hindcast integration is presented, using as forcing the "observed" sensible plus latent heat flux anomalies rather than the surface atmospheric field anomalies to demonstrate the major results are not pre-determined by the formulation of the coupling.
The ability of the model to hindcast the wintertime interannual variations in SST is demonstrated by simple correlations with observed anomalies, and by comparing the composite of warm and cold events observed with those simulated by the model. There is a good quantitative agreement between simulated and observed SST anomalies throughout most of the North Atlantic Ocean. Since the model formulation explictly excludes any effects due to anomalies in the ocean advection, our results confirm the hypothesis that wintertime interannual to sub-decadal variability in SST is mainly due to local anomalies in the air-sea flux of sensible and latent heat and not to anomalies in ocean advection. Significant disagreement between the hindcast and simulated SST anomalies is limited to a small region extending along the coast from Cape Hatteras to Nova Scotia. Here, the observed surface flux anomalies are anti-correlated with the SST anomalies, implicating important changes in ocean advection to the interannual wintertime SST anomalies.
Both the sensible and latent heat flux anomalies are shown to contribute substantially to the wintertime anomalies in SST in the subpolar Atlantic, while the heat fluxes anomalies are predominantly determined by the latent heat fluxes in the subtropics. Entrainment anomalies contribute to a lesser extent to the mixed layer temperature anomalies throughout the basin. Sensitivity studies are performed to highlight the atmospheric processes and variability that account for the surface heat flux anomalies.