A Coupled Eulerian-Lagrangian Analysis of the Large Scale Tropical Circulation: Implications for Climate Change

Abstract:

The Hadley circulation is a key element of the climate system. It is traditionally defined as the zonally averaged meridional circulation in the tropics, therefore treated as a zonally symmetric phenomenon. However, differences in temperature between land and sea cause zonal asymmetries on Earth, dramatically affecting the circulation. This longitudinal dependence of the meridional circulation evokes questions about where and when the actual large scale tropical circulation occurs. Here, we study the connection between the longitudinally dependent meridional circulation, and the actual large scale transport of air in the tropics using a coupled Eulerian and Lagrangian approach. Decomposing the velocity field into rotational and divergent components, we identify how each component affects the actual circulation. We propose an alternative definition for the circulation, that describes the actual path of air parcels in the tropics, as a tropical atmospheric conveyor belt. We then use an ensemble of CMIP5 models to analyze the response of the tropical conveyor belt to anthropogenic climate change. We find that the changes in the strength and the location of the circulation strongly depend on the longitudinal location. Using the coupled Eulerian-Lagrangian approach, we discuss these localized changes in terms of the distinctive components of the circulation, and in terms of the overall conveyor belt perspective.