We examine the sensitivity of intraseasonal variability in the NCAR Community Atmosphere Model 2.0.1 with relaxed Arakawa-Schubert (RAS) convection to including a simple slab ocean with a range of mixed layer depths. Previous studies have noted that intraseasonal variability in atmospheric general circulation models is improved through coupling to an interactive slab ocean. Watterson showed a monotonic increase in intraseasonal oscillation (ISO) amplitude when progressively decreasing the mixed layer depth to 10 m in a coupled GCM. Sobel and Gildor used a simple zero-dimensional atmosphere model coupled to an ocean mixed layer to show that ISO variability actually decreases with mixed layer depths below 10 m, indicative of the reduced thermal inertia of the ocean. We will test the hypothesis that 1) an interactive ocean can strengthen intraseasonal variability in atmospheric GCMs, and 2) that intraseasonal variability begins to decrease when the mixed layer become sufficiently shallow.

The NCAR CAM2.0.1 with a replacement convection scheme (RAS) will be used as the atmospheric model in the study. The slab ocean model utilized in several of the simulations is applied from 30°N to 30°S and has been modified to have temporally and spatial invariant mixed layer depths. The temperature tendency is determined by the net surface heat flux and an implied oceanic heat transport (Q-flux). This model is similar to that used in Maloney and Kiehl (2002). A control simulation of the CAM with climatological seasonal cycle SSTs was conducted, as well as coupled simulations with mixed layer depths of 50m, 20m, 10m, 5m, 2m, and 0.5 m. Each simulation was 15 years in length, with an additional 5-year spinup conducted for the coupled simulations.

The simulation of intraseasonal variability in the NCAR CAM2.0.1 with RAS convection is sensitive to inclusion of a slab ocean model with variable mixed layer depth. Model intraseasonal variability becomes more realistic as mixed layer depths decrease from 50m to 10m, and then variability degrades with mixed layer depths below 10m. Eastward propagation becomes more robust and ISO amplitudes increase from 50 m to 10m. Intraseasonal precipitation variability is dramatically reduced at slab ocean depths of 2m and 0.5m. The large-scale circulation rapidly imposes temperature anomalies on the ocean at shallow mixed layer depth that force an unrealistic relationship between convection and the large-scale circulation. The reduced thermal inertia associated with decreased mixed layer depths also damps surface latent heat flux anomalies. The simple model of Sobel and Gildor shows that evaporation anomalies are likely reduced at shallow mixed layer depths because SSTs rapidly adjust to an imposed wind speed anomaly to keep evaporation (and precipitation) relatively constant.