Impact of air-sea coupling on the Madden-julian oscillation in a general circulation model

The impact of air-sea coupling on the dynamics of the tropical Madden-Julia n oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, w here climatological sea surface temperature (SST) is specified, realistic s pace-time spectra of near-equatorial zonal wind and precipitation are produ ced, with power concentrated at eastward wavenumbers 1-3 with periods of 35 -90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermo re, during southern summer, when the observed MJO is most dominant across t he Indian and western Pacific Oceans, intraseasonal variance in the uncoupl ed model is overly concentrated to the north and east of Australia with lit tle activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problem s nor did it have any other appreciable impact on the model's MJO. Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equato rial SST anomalies across the warm pool with surface low pressure to the ea st of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not sp atially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radi ation associated with enhanced convection slightly leads enhanced latent he at flux associated with increased surface westerlies. The model does produc e realistic shortwave radiation anomalies, but its latent heat flux anomali es are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of s imulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the M JO.