Abstract
AbstractThis article explores how atmospheric radiative heating, due to the presence of clouds, influences the Madden‐Julian Oscillation (MJO) as simulated by four comprehensive atmosphere general circulation models. Simulations in which clouds are transparent to electromagnetic radiation (“clouds‐off”) are compared with control simulations in which clouds are allowed to interact with radiation (“clouds‐on”). Making clouds transparent to radiation leads to robust changes of the mean state: the westerly winds in the equatorial Indo‐Pacific area weaken and the precipitation reveals a shift from single to double Intertropical Convergence Zones. These changes are accompanied by weaker MJOs. Also, the moisture sensitivity of precipitation changes, however not consistently within our group of models. Further analyses show that within the active phase of intraseasonal variability, cloud‐radiative effects amplify the heating profiles compared to clouds‐off. Heating from nonradiative processes is dominated by the parameterized convection, but large‐scale heating associated with cloud microphysical processes acting on the grid‐scale modifies the shape of the heating profile, leading to a top‐heaviness when cloud‐radiative effects are accounted for. The radiative heating due to clouds slows down the phase speed of the MJO. Averaged over the entire MJO life cycle, the column‐integrated radiative heating due to clouds lags the vertically integrated moist static energy by 40°–60° of longitude (equivalently 7–10 days assuming a period of 60 days). All four models studied reveal more pronounced Kelvin waves when clouds are transparent to radiation, suggesting that cloud‐radiative effects on large‐scale heating profiles damp smaller scale, or faster, Kelvin waves and amplify MJO‐like disturbances.
Highlights
The Madden-Julian Oscillation (MJO) represents the dominant mode of intraseasonal variability in the tropics
Past work has demonstrated that both the zonal extent of the mean surface westerly winds across the Indo-Pacific region and the mean precipitation are relevant for a realistic MJO simulation [Slingo et al, 1996; Sperber et al, 2005; Figure 1
The contribution of cloud-radiative heating on the mean tropical climatology and its intraseasonal variability is investigated using four comprehensive atmosphere general circulation models
Summary
The Madden-Julian Oscillation (MJO) represents the dominant mode of intraseasonal variability in the tropics. Among the various processes associated with the MJO, the interaction between clouds and radiation has repeatedly been identified to be an important one This interaction has been investigated theoretically and in GCM studies. Lin et al [2007] and Lee et al [2001] found that cloud-radiative effects influence Kelvin waves and their propagation speed. Their control experiment as well as their experiment excluding cloud-radiative effects did not reveal MJO-like variability, and so they could not study the radiative effects of clouds on the MJO. In contrast to the aforementioned studies, Lau et al [2005] found that the MJO simulated in their GCM strengthens and is more eastward propagating when cloud-radiative effects are reduced
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