Diurnal sea surface temperature variability and the transition phase of the MJO

Intraseasonal variability in atmospheric convection and circulation in the tropical Eastern Hemisphere is dominated by the large-amplitude Madden-Julian Oscillation (MJO). The MJO is of particular interest because of its presence in the tropical oceans where it has been found to substantially impact the diurnal variability of the western Pacific warm pool region. The interaction of the MJO and the near-surface ocean has become an increasingly important consideration with regard to our understanding of both climate and weather and our ability to simulate them in models. This is due to the MJO's extensive interaction with other components of the climate/weather system, in conjunction with evidence that air-sea coupling may play an important role in defining the characteristics of the MJO. This implies that the MJO is a coupled ocean-atmosphere phenomena, or at least strongly influenced by ocean-atmosphere interaction. The warm pool SST intraseasonal variability associated with the passage of the MJO is well documented, yet little attention has been given to the diurnal SST variability. TOGA COARE IOP observations show that SST diurnal variations during the suppressed phases of the MJO are on the order of 1°C and often reach over 2°C. This diurnal variability may impact longer time scales and more specifically might be an important factor in the mean hydrological cycle and energy balance of the region.

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.