This grant will fund research on the Hadley cell, an overturning atmospheric circulation with rising motion in the rainy regions near the equator and subsiding motions over the subtropics on either side of the equator. The Hadley cell is quite important for the climate of the tropics and subtropics, as regions under the subsiding branches of the circulation are characterized by arid or semi-arid climates. The research addresses the behavior of the Hadley cell, including its width, strength, and the central latitude of the rising motions, under a variety of simulated planetary climate states. The project has three main objectives: 1) To determine how the Hadley circulation changes over a wide range of climates, in order to place possible past, present, and future changes in a larger context. The research will determine, for example, whether the Hadley circulation generally widens as the climate warms, even in climates very different from the present. 2) To determine how the Hadley circulation responds to climate changes in climate simulations of increasing complexity, including simulations in which the world's surface is entirely ocean with no landmasses, ocean-only simulations including sea ice, and configurations with idealized continents. These simulations are intended to identify mechanisms through which the Hadley cell is influenced by external factors such the insolation changes associated with the ice age cycles. 3) To develop scientific theories that account for observed and simulated changes of the Hadley circulation.
The issue of Hadley cell change in a warming climate has societal impacts, as observations and simulations suggest that the Northern and Southern Hemisphere Hadley cells may be widening due to global warming, which could result in aridification in some regions. In addition, the work will promote the education and training of the next generation of scientists by funding a postdoctoral scholar, two graduate students, and two summer undergraduate research fellows.
The Hadley circulation is the tropical overturning circulation that has its ascending branch at the intertropical convergence zone (ITCZ), has descending branches in the subtropics, and has north-south flows near the surface and in the upper troposphere connecting the ascending and descending branches. Rainfall on Earth is most intense in the ITCZ, where warm and moist air masses ascend, cool, and dry. By contrast, the descending branches of the Hadley circulation bring dry air from the upper troposphere toward the surface, and the regions underneath are dry and contain Earth's largest deserts. Therefore, the strength and extent of the Hadley circulation are of fundamental importance for Earth's climate and the distribution of climatic zones. Yet the dynamics controlling the Hadley circulation are unsufficiently understood. This project aimed at elucidating the dynamics of the Hadley circulation and closely related monsoon circulations using analyses of observational data and experiments with climate models. We have developed new theories that can explain, for example, the widening of the Hadley circulation that occurs in climate models in response to global warming, as well as the changes in the strength of the Hadley circulation under different climate change scenarios--from those that may occur in the future to those that may have occured in the distant past. We have developed a theoretical framework that relates the position of the ITCZ to the energy balance of the atmosphere and can help pinpoint how the well known biases climate models exhibit in simulating the ITCZ can arise. Thus, results of this award contribute to reducing uncertainties in climate simulations. We have also developed a new theory of how rainfall in Earth's tropics can vary on geological timescales, in response to changes in Earth's orbit. This theory helps in the interpretation of geological and geochemical records, which clearly show large tropical rainfall changes on long timescales. The results of this research have been disseminated through publications in the peer-reviewed scientific literature (10 publications so far), including one review paper, through conference contributions, and through numerous invited talks, for example, at workshops and summer schools. Three graduate students have been trained under this award and will soon be beginning or have begun their own independent research careers.
