Reduction in the Fractional Anvil Cloudiness Due to an Indirect Weakening of the Hadley Circulation

Although the net radiative effect of tropical anvil clouds is negligible, their shortwave and longwave cloud radiative effects are individually large, such that even small changes in the fractional cloudiness of tropical anvils can significantly impact the Earth’s radiation budget. Most general circulation models and cloud resolving models depict a decrease in the tropical anvil cloud cover with surface warming. According to the “stability-iris” hypothesis, this reduction is thermodynamically controlled by the changes in the upper-tropospheric static stability, which is linked to the altitude of peak net detrainment in turn governed by the radiatively-driven clear-sky convergence. However, the influence of the changes in the atmospheric dynamics independent of the local SST changes remains relatively less explored due to the difficulty in segregating the dynamical influence from the local thermodynamic influence on the tropical anvil cloud cover.

Using idealized general circulation model simulations using the Met Office Unified Model, our study aims to understand the dynamical impact on the fractional cloudiness of tropical high clouds with global warming. To achieve this, we propose a novel method to separate the dynamical effects from the local thermodynamical effects by warming the extratropics and keeping the tropical sea surface temperatures unchanged. We thereby focus on the mechanisms underpinning the changes in the tropical high clouds resulting from changes in the atmospheric dynamics induced by extratropical warming. We find that the net depositional growth of ice cloud condensates has relatively greater significance than the net convective detrainment of condensates in controlling the reduction of the fractional cloudiness over a considerable altitude range of the upper troposphere in the deep tropics.