Mechanisms by Which Surface Drying Perturbs Tropical Precipitation Fields

Abstract

The observed precipitation climatology in austral summer has a pronounced longitudinal gradient across Africa and South America. A low-resolution general circulation model (GCM) with a simple continent centered on the equator is used to understand how the presence of the land surface generates this gradient, and the role of surface wetness in determining its magnitude. In the model, precipitation is enhanced on the east coast of the continent in the summer hemisphere tropics with magnitude and location independent of surface wetness. Precipitation rates are lower in the continental interior and in the west as the surface becomes drier, resulting in a longitudinal precipitation gradient that is similar to observations. Modeled low-level moisture convergence and wind convergence anomalies mimic the precipitation anomalies over all but the driest land surfaces. A linearized primitive equation model is used to identify the physical mechanisms responsible for the GCM's dynamical response. Dry convection and condensational heating force most of the anomalous convergence over the land surface in the GCM, with sensible heating and transient eddies playing more minor roles. At the latitude of the intertropical convergence zone (ITCZ), dry convection drives anomalous convergence at low levels, and this convergence is larger over drier (warmer) surfaces. Anomalous divergence develops in response to decreased condensational heating below 680 mb. The dependence on surface wetness arises because the relative strength of these opposing responses depends on the degree of warming over the land surface. Low-level convergence over the eastern portion of the land surface in the model is forced by condensational heating in the middle and upper troposphere. Here, diabatic heating is balanced by adiabatic cooling, and the positive vertical velocities induce convergence below 830 mb by continuity. The magnitude of the response is largely independent of land surface drying and warming. The longitudinal precipitation gradient develops when even moderate surface drying affects precipitation in the continental interior and west, but not in the east.