Dynamical controls on tropical circulation and precipitation–evaporation responses to cloud radiative changes

While a range of processes have been linked to uncertainty in tropical precipitation minus evaporation (P–E) and circulation changes, growing evidence links cloud-radiative changes to inter-model spread. Radiation-locking studies further demonstrate strong sensitivities of circulation and P–E to cloud-radiative changes in aquaplanet models; however, the physical mechanisms linking CO₂-driven cloud-radiative changes to tropical circulation and P–E responses remain poorly understood. Here, we use the radiation-locking technique to elucidate these mechanisms in a climate model configured with realistic continents, sea ice, and a seasonal cycle, with the ocean represented by a slab ocean model with prescribed climatological q-fluxes. We introduce a novel analytical framework in which the P–E response is analysed as a function of climatological P–E, enabling direct comparison with thermodynamic scaling arguments.

Despite inducing weak surface warming, CO₂-driven cloud-radiative changes substantially modify the tropical hydrological response, driving a robust wet-gets-drier, dry-gets-wetter P–E pattern that opposes the canonical wet-gets-wetter, dry-gets-drier signal associated with climate warming. Moisture and moist static energy budget analyses show that this response is driven by a weakening of the tropical overturning circulation associated with enhanced upper-tropospheric cloud-radiative heating. Sea surface temperature pattern changes induce additional P–E responses, including a poleward shift of precipitation maxima over the Indian and western Pacific Oceans. Our results demonstrate that circulation changes strongly shape tropical P–E responses to cloud-radiative changes, and that the balance between dynamic and thermodynamic responses may be a key control on inter-model spread. We further highlight the coupling between cloud-radiative heating and latent heat release as critical for the resulting circulation response.