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Abstract:

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.

Abstract:

Midlatitude storms transport warm and moist air poleward and upward, releasing latent heat. Latent heating is thus organized by thecirculation but then modifies temperature gradients and winds, constituting a nonlinear feedback. We define the latent heating feedbackas the effects that arise from latent heating being coupled with the circulation. Because of its nonlinearity, the climatic effects of thisfeedback are difficult to isolate and remain poorly understood.By decoupling latent heating from the circulation in an atmospheric general circulation model, we show that the latent heating feedbackenhances storm track eddy diffusivity, modifying eddy heat fluxes beyond changes in mean baroclinicity. Simultaneously, tracked stormsoccur at lower latitudes, intensify more, and propagate further poleward, while the subtropical jet strengthens as coupled latent heatingpreserves lower latitude baroclinicity. The feedback response 91̽��s the idea that diabatic effects cause the “too zonal, tooequatorward” storm track biases in climate models.Finally, we extend the analysis to climate change experiments where we isolate the contribution from the latent heating feedback onstorm intensity and eddy kinetic energy as the world warms. The feedback is most important in summer where it accounts for most of thechanges in eddy kinetic energy. In winter, the feedback is constrained. Isolating the latent heatingfeedback helps to quantify how storminess changes as the atmosphere warms, which climate models currently struggle with.

Abstract:

The Quasi-Biennial Oscillation (QBO) is the dominant mode of tropical stratospheric variability that modulates global circulation and climate. Although a long-term weakening of QBO amplitude has been observed under global warming, the relative roles of different tropical oceans remain unclear. We perform sensitivity experiments forced by sea surface temperature perturbations over the tropical Pacific, Atlantic, and Indian Oceans, as well as their combined warming, to separate individual and joint effects. Pacific warming produces the strongest weakening and slowest descent of the QBO, whereas Atlantic warming slightly strengthens the amplitude and extends the vertical structure. Indian Ocean warming slightly weakens the amplitude and accelerates the descent. When all three oceans warm simultaneously, the QBO exhibits a weaker amplitude and faster descent, consistent in sign with the combined single-basin responses but with a reduced magnitude owing to diminished zonal and inter-basin SST gradients. Momentum budget analyses further show that basin-dependent competition between equatorial wave forcing and tropical upwelling underlies these contrasting responses.