Coupling of Convection and Circulation over the Tropical Atlantic

Abstract:

Understanding how convection couples with large-scale circulation over the tropical Atlantic is key to reducing persistent model biases in this region. Characteristic biases such as the reversed SST gradient and weak surface easterlies originate from the erroneous Atlantic ITCZ in the atmospheric component of models. While observations show a central Atlantic ITCZ maximum in the mean state, models rain excessively either over the West or over the East Atlantic coast. In this study, factors controlling the Atlantic ITCZ and its accompanying circulation are investigated. Focus is set on two aspects: model resolution and convection scheme. First, we study the effect of horizontal resolution on the Atlantic ITCZ. It is found that atmospheric models which rain excessively at the western basin tend to have low resolution, whereas models which rain more over the east have high resolution. Sensitivity experiments with ECHAM designed to identify the relative contributions of a high resolution orography, atmosphere, and surface are performed. Results show that a high resolution orography induces a stronger westerly monsoon circulation, contributing to more rainfall on the east coast. A high resolution atmosphere triggers convection more easily, leading to more rain over the east and less rain downstream over the west coast. A high resolution surface alone has a minimal effect. In the second part of this study, we look at how spatiotemporal patterns of convection impact the circulation over the Atlantic and examine to what extent the convection scheme controls such patterns. A suite of experiments where convection is artificially strengthened over certain areas and seasons is performed. These experiments are designed such that the convection perturbation is initially decoupled from the circulation. Key areas of convection which strongly influence the large-scale circulation and the Atlantic ITCZ during boreal spring and summer are identified.