The Influence of Coupled Sea Surface Temperatures on the Madden–Julian Oscillation: A Model Perturbation Experiment

Abstract

In this study, the authors compare the Madden?Julian oscillation (MJO) variability in the Goddard Laboratory for Atmospheres atmospheric general circulation model for two different sea surface temperature (SST) boundary conditions. In the ?control? simulation, the model employs specified annual cycle SSTs. In the ?coupled? simulation, the model employs the same annual cycle SSTs but in addition is coupled to a slab ocean mixed layer that provides prognostic SST anomalies equatorward of 24°. The results show that the simplified interactive SST facilitates a better simulation with respect to a number of general model shortcomings associated with the MJO that were recently documented by Slingo et al. in an Atmospheric Model Intercomparison Project study. These improvements include 1) increased variability associated with the MJO, 2) a tendency for the timescales of the modeled intraseasonal variability to more closely match and consolidate around the timescales found in the observations, 3) a reduced eastward phase speed in the Eastern Hemisphere, and 4) an increased seasonal signature in the MJO with relatively more events occurring in the December?May period. The above changes are associated with a systematic change in SST, with warming of about 0.10°?0.15°C before the passage of the MJO convection center and cooling of the same magnitude after its passage, both of which appear to be roughly consistent with observations. These changes in SST are primarily due to decreased latent heat flux (?25 W m?2) and slightly enhanced surface shortwave flux (?10 W m?2) to the east of the convection, enhanced latent heat flux and diminished shortwave flux (?25 W m?2) coincident with the convection, and enhanced latent heat flux (?25 W m?2) just west of the convection. The results indicate that the enhanced SST to the east of the convection reinforces the meridional convergence associated with the frictional wave?CISK (conditional instability of the second kind) mechanism that appears to be at work within the model. This enhanced meridional convergence transports more low-level moisture into the region lying just east of convection. The resulting increase in moist static energy helps destabilize the disturbance and/or maintain it against dissipation more effectively relative to the case without SST coupling. The above results are discussed in terms of their relation to current MJO theory, as well as to their implications for medium- to extended-range weather forecasting.