Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere

Abstract

study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested 4 , precipitation patterns in the tropics could change, with potential societal consequences. One of the defining features of the Earth’s climate is the fact that tropical rainfall primarily maximizes within the Northern Hemisphere (Fig. 1a,b). This is especially evident in the Atlantic and eastern Pacific oceans, where precipitation near 5 S is much weaker than at the equivalent locations north of the Equator, which are some of the rainiest regions on Earth in the intertropical convergence zone. Throughout the tropics, societies as diverse as subsistence farmers in the Sahel and the technology-based cities of India are reliant, directly or indirectly, on monsoon rains. How tropical rainfall evolves in the future will have important and widespread social consequences. Precipitation in the tropics exists largely within narrow zonal bands owing to the large-scale atmospheric overturning known as theHadleycirculation 5 .Withinthelowerbranchofthiscirculation, the trade winds converge near the Equator, drying the subtropics and bringing moisture into the rain bands. This circulation is thermally direct, and thus transports energy in the directions of its upper branch. The Hadley circulation also plays a primary role in determining the hemispheric asymmetry of tropical precipitation because it transports copious amounts of moisture from the Southern Hemisphere to the Northern Hemisphere. A northward cross-equatorial mass transport in the moist lower branch of the