Why a Workshop on Tropical Rainfall? Climate models are our primary tool for understanding what climatic shifts we will experience, as pollution continue to alter the composition of the atmosphere. This is especially true when it comes to changes in precipitation, which cannot be expected to be positive everywhere—contrary to the case, by and large, of temperature. Yet, it is difficult to trust projections of precipitation changes because Coupled General Circulation Models (CGCMs) fail to accurately reproduce the position and strength of the Tropical precipitation maxima (monsoons and the Inter-tropical Convergence Zone, or ITCZ) both in the current climate and in the past. For example—despite great efforts that span decades—most models overestimate rainfall south of the Equator in their climatology, both in the Pacific and, to a lesser extent, in the Atlantic. Moreover, we cannot assume that the models reproduce the correct changes about the climatology when the external forcings are perturbed; for example, there are still doubts about whether  the latest inter-comparison of Mid-Holocene simulations produce enough rainfall over Africa to be consistent with some of the paleo record of a Green Sahara. These modeling stumbles derive from (and exacerbate) our limited understanding of what controls tropical precipitation.

Yet, there is great hope for progress. Recently, the World Climate Research Program Grand Challenge Workshop on Clouds, Circulation and Climate Sensitivity identified the question of  “what controls the strength and position of tropical precipitation maxima?” as one for which concerted effort could lead to scientific breakthroughs and more reliable projections of future changes in climate patterns. This optimism stems from parallel advances in modeling ability, the availability of observations, and theoretical understanding.

To foster progress on this stubborn problem, we are organizing a workshop, to be held at Columbia University in late summer 2015. The focus will be on modeling and theoretical understanding of the annual cycle of oceanic ITCZs and land monsoons  in the present climate, the past, and the future.