Linking tropical large-scale circulation and deep convection to subtropical marine low-clouds in the Pacific Ocean

Global Climate Models (GCMs) are essential for predicting the impact of climate change in the coming decades. However, the primary source of uncertainty in these predictions is our limited understanding of cloud feedback and its representation in models. Improving our knowledge of how changes in local heating rates affect low clouds via tropical overturning circulation is crucial to refining climate projections. In this study, we use an AMIP climate assessment configuration (with CMIP6 forcing) of the Met Office Unified Model to test the remote effects on subtropical clouds caused by localised changes in tropical atmospheric circulation.  

We conduct this causal analysis by applying a heating/cooling perturbation in the free troposphere in a typical convecting and in a typical subsiding region in the equatorial Pacific Ocean. This method allows us to perturb large-scale circulation and track the subsequent effects on subtropical clouds. We find that when we apply a heating or cooling in the tropical free troposphere, the subsidence in the subtropics strengthens but we do not find a change in the low-cloud content. However, when we apply a cooling perturbation in the Southeast Pacific subsidence region, which increases subsidence, we get more local low-clouds. This is the opposite of what is suggested by previous studies which use a correlative approach on a global scale. 

We show how changing the intensity of the large-scale circulation in the equatorial Pacific influences subtropical low clouds, while tracking the effects of our perturbations in the transition regions between the tropics and subtropics. Our findings demonstrate a new way to conduct causal studies to better understand and isolate the influence of the free troposphere on large-scale circulation and subtropical clouds in a full GCM setup. Additionally, our findings emphasise how regional influences might differ from global results, highlighting the importance of recognising and quantifying regional contributions which dictate global trends.