Wintertime Weakening of Low-Cloud Impacts on the Subtropical High in the South Indian Ocean

Abstract

To elucidate the unique seasonality in the coupled system of the subtropical Mascarene high and low-level clouds, the present study compares wintertime cloud radiative impacts on the high with their summertime counterpart through coupled and atmospheric general circulation model simulations. A comparison of a fully coupled control simulation with another simulation in which the radiative effects of low-level clouds are artificially switched off demonstrates that the low-cloud effect on the formation of the Mascarene high is much weaker in winter. Background climatology plays an important role in this seasonality of the Mascarene high reinforcement. Relative to summer, the suppression of deep convection due to low-level clouds that acts to reinforce the high is much weaker in winter. This arises from 1) seasonally lower sea-surface temperature in concert with the smaller sea-surface temperature reduction due to the deeper ocean mixed layer and the weaker cloud radiative effect under weaker insolation, and 2) seasonally stronger subtropical subsidence associated with the Hadley circulation in winter. As verified through atmospheric dynamical model experiments, enhanced cloud-top radiative cooling by low-level clouds acts to reinforce the wintertime Mascarene high in comparable magnitude as in summer. The present study reveals that the self-sustaining feedback with low-level clouds alone is insufficient for replenishing the full strength of the wintertime Mascarene high. This implies that another internal feedback pathway and/or external driver must be operative in maintaining the wintertime high.

Radiative impacts of Californian marine low clouds on North Pacific climate

“…In -h). Thus, the precipitation response is key to the seasonality in the subtropical anticyclonic response, as found for the Mascarene high over the South Indian Ocean (Miyamoto et al 2021(Miyamoto et al , 2022a.…”

“…Following Miyamoto et al (2021Miyamoto et al ( , 2022a, radiative impacts of low clouds are evaluated by setting maritime cloud fraction to zero over a given geographical domain for radiation calculations in a fully coupled AOGCM, CM2.1 (Delworth et al 2006). Its atmospheric component has To isolate the SST influence simulated in CM2.…”

“…Recently, Miyamoto et al (2021Miyamoto et al ( , 2022a applied techniques of the Clouds On-Off Klimate Model Intercomparison Experiment (COOKIE; Stevens 2012;Voigt et al 2021) to a fully coupled AOGCM. Specifically, low-cloud radiative effect (CRE) was artificially removed regionally to evaluate specific low-cloud impacts.…”

“…The resultant anomalous diabatic cooling reinforces the surface Mascarene high and promotes the WES feedback. By contrast, the low-cloud feedback is modest in winter, when the suppression of deep-convective precipitation by low clouds is less effective due to climatologically low SST (Miyamoto et al 2022a).…”

“…This study applies the same methodology as in Miyamoto et al (2021Miyamoto et al ( , 2022a to the North Pacific. Figure 1…”