Energetics of monsoons and deserts: role of surface albedo vs water vapor feedback

Abstract. Despite receiving similar solar energy, the top-of-the-atmosphere (TOA) radiation budget is negative over deserts and positive over monsoon domains. This contrast has been attributed to differences in the surface albedo between the two regions. Here, we show that this difference in TOA radiation budget is primarily driven by the absorption of longwave radiation by water vapor, while the surface albedo only plays a secondary role. As a greenhouse gas, water vapor absorbs the surface longwave radiation and enhances the local radiative heating of the atmosphere. Due to the aridity of the deserts and prevalent clear sky conditions, long wave energy is efficiently radiated to space. We demonstrate that this is the dominant cause of the net radiative cooling of the atmosphere. Our hypothesis is confirmed by a climate model experiment in which the Earth’s rotation is reversed. This flips the zonal asymmetries producing a monsoon over the Sahara (in spite of high albedo) and a desert over South East Asia (where low albedo persists). We find that radiative feedback of water vapor on the large-scale circulation drives these changes initially, with further amplification by cloud feedbacks. Our results suggest that this radiation-circulation feedback due to water vapor enhances monsoon-desert contrast.