Why weakening the overturning Walker circulation in the tropical ascent region leads to a reduction in subtropical low clouds

Global Climate Models (GCMs) are essential for predicting the impacts of global and regional 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.  The remote effects of deep convection on subtropical low clouds in the warming climate are poorly understood. Improving our knowledge of how deep convection affects low clouds via the 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 quantify the impact of local and remote changes in the tropical atmospheric circulation on subtropical low clouds.  We conduct a causal intervention analysis by applying a cooling perturbation in the free troposphere above the deep convective western equatorial Pacific Ocean. This method allows us to weaken the large-scale circulation in the ascent region and track the resulting effects on subtropical clouds. We find that when we cool the free troposphere in the tropical west Pacific region, we get a deepening of the subtropical East Pacific boundary layer and a reduction in overall low cloud fraction. This study allows us to determine and present the physical mechanism behind this "tropical ascent → subtropical clouds" interaction and emphasises the benefits of using targeted perturbation methods to conduct causal analyses and disentangle regional and process linkages within models.