Corresponding author: Eric D. Maloney, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Building, Corvallis, OR 97331-5503. maloney@coas.oregonstate.edu.edu
Sensitivity of tropical intraseasonal variability to mixed layer depth is examined in the modified National Center for Atmospheric Research Community Atmosphere Model 2.0.1 (CAM), with relaxed Arakawa-Schubert convection, coupled to a slab ocean model (SOM) whose mixed layer depth is fixed and geographically uniform, but varies from one experiment to the next. Intraseasonal west Pacific precipitation variations during boreal winter are enhanced relative to a fixed-SST (infinite mixed layer depth) simulation for mixed layer depths of 5 to 50 meters, with a maximum at 20 meters, but are strongly diminished in the 2 meter depth simulation. This non-monotonicity of intraseasonal precipitation variance with respect to mixed layer depth was predicted by Sobel and Gildor using a highly idealized model. Further experiments with the same idealized model help to interpret results derived from the modified NCAR CAM.
A sensitivity study shows that the convection-surface flux feedback (wind-induced surface heat exchange, or WISHE) is important to the intraseasonal variability in the CAM. This helps to explain the behavior of the 2 meter SOM simulation and the agreement with the idealized model. Although intraseasonal SST variations are stronger in the 2 meter SOM simulation than in any of the other simulations, these SST variations are phased in such a way as to diminish the amplitude of equatorial latent heat flux variations. Reducing the mixed layer depth is thus nearly equivalent to eliminating WISHE, which in this model reduces intraseasonal variability. The WISHE mechanism in the model is nonlinear, occurring in a region of mean low-level westerlies.
Since a very shallow mixed layer is effectively similar to wet land, it is suggested that the mechanism described here may explain the local minimum in MJO amplitude observed over the Maritime continent region.