It has been suggested that during the suppressed phase of the MJO the diurnal cycle of tropical atmospheric convection is more typical of a continental regime than an oceanic regime during the suppressed phases. This implies that the diurnal cycle of convection may be a direct response to the diurnal SST variability. Furthermore, the feedbacks between the ocean and atmosphere can potentially operate on diurnal time scales. TOGA COARE observations suggest that cumulus congestus clouds are most prevalent during light wind conditions in the presence of a strong diurnal cycle in SST. This type of convection occurs most often during the afternoon or early evening of suppressed and transition phases of the MJO and suggests that the convection is triggered by the diurnal SST cycle. Therefore, coupling with the upper ocean is important on diurnal time scales.
In this talk we present results of a coupled atmosphere-ocean single-column model of the tropical Western Pacific, with a focus on the effects of diurnal sea surface temperature variability on the MJO. We compare results with and without this diurnal forcing and demonstrate that although the diurnal SST warming is greatest during the suppressed phase of the MJO, the diurnal SST warming has the most impact on the atmosphere during the transition phase of the MJO. Increases in low-level and mid-level cloud amounts were seen during the two transition phases observed during the TOGA COARE Intensive Observations Period when using a diurnally-varying SST. Cloud amounts are related to an increase in low-level moistening associated with the diurnal SST variability, as caused by variations in latent and sensible heat fluxes. These results, and comparisons with a cloud-resolving model will be presented, along with implications for forecasting of the MJO.