Observational Evidence That Enhanced Subsidence Reduces Subtropical Marine Boundary Layer Cloudiness

Conventional wisdom suggests that subsidence favors the presence of marine stratus and stratocumulus because regions of enhanced boundary layer cloudiness are observed to climatologically co-occur with regions of enhanced subsidence. Here it is argued that the climatological positive correlation between subsidence and cloudiness is not the result of a direct physical mechanism connecting the two. Instead, it arises because enhanced subsidence is typically associated with stronger temperature inversions capping the marine boundary layer, and stronger temperature inversions favor greater cloudiness. Through statistical analysis of satellite cloud data and meteorological reanalyses for the subsidence regime over tropical (30 degrees S-30 degrees N) oceans, it is shown that enhanced subsidence promotes reduced cloudiness for the same value of inversion strength and that a stronger inversion favors greater cloudiness for the same value of subsidence. Using a simple conceptual model, it is argued that enhanced subsidence leads to reduced cloud thickness, liquid water path, and cloud fraction by pushing down the top of the marine boundary layer. Moreover, a stronger inversion reduces entrainment drying and warming, thus leading to a more humid boundary layer and greater cloud thickness, liquid water path, and cloud fraction. These two mechanisms typically oppose each other for geographical and seasonal cloud variability because enhanced subsidence is usually associated with stronger inversions. If global warming results in stronger inversions but weaker subsidence, the two mechanisms could both favor increased subtropical low-level cloudiness.