Speaker: Xiyue Zhang (Johns Hopkins Univ)
Topic: Response of Tropospheric Transport to Abrupt CO2 Increase: Dependence on the Atlantic Meridional Overturning Circulation

Understanding how the transport of gases and aerosols responds to climate change is necessary for policy making and emission controls. There is considerable spread in model projections of transport in climate change simulations, largely because of the substantial uncertainty in projected changes in large-scale atmospheric circulation. A potential contributor to the uncertainty is the response of the Atlantic Meridional Overturning Circulation (AMOC). Here we examine the connections between the atmospheric circulation and tropospheric transport response to increased CO2, and their dependence on changes in the AMOC using a passive tracer in abrupt 2xCO2 and 4xCO2 experiments. We show that AMOC weakening leads to a poleward shift of the northern midlatitude jet and enhanced eddy mixing along isentropes on the poleward flank of the jet. As a result, for a tracer of northern midlatitude surface origin, its tropospheric concentration decreases in the midlatitudes and increases in the Arctic. This mechanism is robust in individual abrupt 2xCO2 or 4xCO2 simulations, the nonlinearity to CO2 forcing, and the differences between two versions of a climate model (GISS E2.2-G). Preliminary analysis of realistic chemical tracers suggest that the same mechanism can be used to provide insights into the climate change response of anthropogenic pollutants.