On the role of moist processes in tropical intraseasonal variability: Cloud-radiation and moisture-convection feedbacks

Recent observations of the tropical atmosphere reveal large variations of water vapor and clouds at intraseasonal time scales. This study investigates the role of these variations in the large-scale organization of the tropical atmosphere, and in intraseasonal variability in particular. For this purpose, the influence of feedbacks between moisture (water vapor, clouds), radiation, and convection that affect the growth rate and the phase speed of unstable modes of the tropical atmosphere is investigated. Results from a simple linear model suggest that interactions between moisture and tropospheric radiative cooling, referred to as moist-radiative feedbacks, play a significant role in tropical intraseasonal variability. Their primary effect is to reduce the phase speed of large-scale tropical disturbances: by cooling the atmosphere less efficiently during the rising phase of the oscillations (when the atmosphere is moister) than during episodes of large-scale subsidence (when the atmosphere is drier), the atmospheric radiative heating reduces the effective stratification felt by propagating waves and slows down their propagation. In the presence of significant moist-radiative feedbacks, planetary disturbances are characterized by an approximately constant frequency. In addition, moist-radiative feedbacks excite small-scale disturbances advected by the mean flow. The interactions between moisture and convection exert a selective damping effect upon small-scale disturbances, thereby favoring large-scale propagating waves at the expense of small-scale advective disturbances. They also weaken the ability of radiative processes to slow down the propagation of planetary-scale disturbances. This study suggests that a deficient simulation of cloud radiative interactions or of convection-moisture interactions may explain some of the difficulties experienced by general circulation models in simulating tropical intraseasonal oscillations.