Implied Ocean Heat Transports in the Standard and Superparameterized Community Atmospheric Models

Implied ocean heat transport (T-o) based on net surface energy budgets is computed for two versions of the Community Atmospheric Model (CAM, version 3.0) general circulation model (GCM). The first version is the standard CAM with parameterized convection. The second is the multiscale modeling framework (MM in which parameterized convection is replaced with a two-dimensional cloud-resolving model in each GCM grid column. Although global-mean net surface energy totals are similar for both models, differences in the geographic distributions of the component errors lead to distinctly different T-o for each model, with CAM's T-o generally agreeing with observationally based T-o estimates, and the MMF's T-o producing northward transport at all latitudes north of similar to 50 degrees S. Analysis of component error sources in the T-o calculation identifies needed improvements in the MMF. Net surface shortwave radiation and latent heat fluxes over the oceans are the primary causes of T-o errors in the MMF. Surface shortwave radiation biases in the MMF are associated with liquid and/or ice water content biases in tropical and extratropical convection and a deficit of marine stratocumulus clouds. It is expected that tropical ice water contents in the MMF can be made more realistic via improvements to the cloud microphysics parameterization. MMF marine stratocumulus clouds are overly sensitive to low-level relative humidity and form only with nearly saturated conditions and a shallow boundary layer. Latent heat flux errors in the MMF are amplifications of those found in the CAM and are concentrated in the trade wind regime and the Asian monsoon region and the adjacent western Pacific Ocean. Potential improvements to T-o are estimated by replacing either simulated net surface shortwave or latent heat fluxes with those from observations and recomputing T-o. When observed shortwave fluxes are used, both CAM and MMF produce greatly improved T-o curves for both hemispheres. When T-o is computed using observed latent heat fluxes, CAM T-o degrades slightly and MMF T-o improves, especially in the sign of Southern Hemisphere transport.