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The relationship between the tropical cloud radiative effect (CRE) and tropical surface temperature variability on El Nino-Southern Oscillation (ENSO) time scales is investigated in preindustrial control simulations from the fifth Climate Model Intercomparison Project (CMIP5) archive. The tropical CRE is binned according to midtropospheric vertical velocities and then regressed in frequency space versus tropical mean surface temperatures. Low clouds play a leading role in the relationship between clouds and surface temperature variability, amplifying ENSO-induced surface temperature anomalies through thermodynamically driven changes in the shortwave CRE. Changes in CRE driven by changes in the large-scale dynamics have a minor influence on surface temperature variability. It is shown that the regression coefficients at ENSO frequencies between the CRE in regions of moderate subsidence and of weak ascent, and tropical mean surface temperatures are well correlated with models' climate sensitivities, constituting a potential emergent constraint on climate sensitivity.

Plain Language Summary The relationship between clouds and surface temperature anomalies generated by El Nino-Southern Oscillation (ENSO) events is still poorly understood. This is concerning, both because we would like to better understand the dynamics of ENSO and because we would like to use this relationship to constrain the long-term response of clouds to increased CO2 concentrations. Here a new approach is taken to investigate the behavior of different cloud types during ENSO events in ENSO events in state-of-the-art climate models. It is found that in the models, ENSO-induced surface temperature anomalies are mainly affected by changes in the amount of low clouds, with variations in high clouds and in the relative fraction of low versus high clouds playing minor roles. Furthermore, the changes in low clouds during ENSO events are strongly correlated with the models' climate sensitivities and their long-term warming in response to doubling CO2 concentrations. This shows that the changes of tropical low clouds during ENSO events can be used to better constrain the warming that Earth will experience if CO2 concentrations continue increasing.