Collaborative Research: Non-Linearity and Feedbacks in the Atmospheric Circulation Response to Increased Carbon Dioxide (CO2)

Efforts to address climate change begin with the simple fact that adding carbon dioxide (CO2) to the atmosphere warms the globe. But the consequences of greenhouse warming that matter for people are regional rather than global and involve other factors besides temperature. One such factor is warming-induced change in atmospheric circulation, which can, for example, cause some regions to dry out while others become more flood prone. The importance of circulation change for regional climate challenges efforts to address climate impacts since the dynamical mechanisms through which warming induces circulation change are not well understood, and model simulations used to inform decision making do not show strong agreement as to how much circulation change will occur. A further consideration is that the amount of circulation change is not necessarily proportional to the amount of global temperature increase. Previous work by the Principal Investigators (PIs) of this project found several examples of nonlinear behavior, for instance the strength of the Northern Hemisphere Hadley Cell (the overturning cell with rising motion near the equator and subsidence over the Northern subtropics) decreases for a doubling of CO2 but increases if CO2 is further increased to a tripling.Work under this award seeks to understand the circulation response to CO2 increase, focusing specifically on the reasons for differences in circulation change from one climate model to another and the mechanisms responsible for nonlinearity in circulation change. One mechanism for nonlinearity is the rapid weakening of the Atlantic Meridional Overturning Circulation (AMOC), which occurs at a particular level of CO2 increase and causes circulation change by creating a patch of colder sea surface temperatures in the North Atlantic. The rapid weakening of the AMOC happens at different CO2 levels in different climate models and is thus a source of inter-model spread in circulation change. Another mechanism is CO2-induced change in stratospheric ozone, as CO2 increase affects the amount and latitudinal distribution of stratospheric ozone, which in turn can influence the jet stream because ozone causes radiative heating which produces upper-level temperature contrasts. The CO2-ozone feedback, in which CO2 influences temperature which in turn affects ozone causing further temperature change, has only recently been recognized as a contributing factor to climate and circulation change. The research is conducted through analysis of model simulations available through several Model Intercomparison Projects, along with new simulations generated using the Community Earth System Model (CESM) and the climate model of the Goddard Institute for Space Studies (GISS Model E2.2).The work is of societal as well as scientific interest given the influence of circulation change on regional climate change, as noted above. A related consideration is that assessments of regional climate change often assume that regional climate changes will be proportional to global temperature increase, thus research on the nonlinearity of circulation change has direct bearing on regional climate change assessments. The PIs are well positioned to disseminate their results to the climate impacts community as both have been authors on previous assessments including reports of the Intergovernmental Panel on Climate Change and the World Meteorological Organization. In addition, the PIs participate in outreach programs within their communities including the Baltimore Ingenuity Project (IP), and the work provides support and training to two graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.