Atmosphere Ocean Science Colloquium

Nonlinear Atmospheric Circulation Response to Increased CO2: Role of Atmosphere-Ocean Coupling

Speaker: Dr. Clara Orbe, NASA Goddard Institute for Space Studies

Location: Warren Weaver Hall 1302

Date: Wednesday, November 8, 2023, 3:30 p.m.


It is well-documented that the atmospheric circulation changes in response to increased CO2, although the responses differ among aspects of the circulation, and the causes of these differences are not well understood. More recently, studies have also shown that the atmospheric response can be not only a nonlinear, but also non-monotonic, function of CO2 forcing. We begin by showing that this nonlinearity in the atmospheric circulation response occurs more broadly across the Coupled Model Intercomparison Project (CMIP) Phase 6 archive and occurs in association with a collapse of the Atlantic meridional overturning circulation. To illustrate this last point, we then isolate the climate impacts of a weakened AMOC using a unique ensemble of Shared Socioeconomic Pathway (SSP) 2-4.5 integrations performed using the CMIP6 version of the NASA Goddard Institute for Space Studies ModelE (E2.1). In these runs internal variability alone results in a spontaneous bifurcation of the ocean flow, wherein two out of ten ensemble members exhibit an entire AMOC collapse, while the other eight recover at various stages despite identical forcing of each ensemble member and with no externally prescribed freshwater perturbation. We show that an AMOC collapse results in an abrupt northward shift and strengthening of the Northern Hemisphere (NH) Hadley Cell and intensification of the northern midlatitude eddy-driven jet. Comparisons with a set of coupled atmosphere-ocean abrupt CO2 experiments spanning 1-5xCO2 reveal that this response to an AMOC collapse results in a nonlinear shift in the NH circulation moving from 2xCO2  to 3xCO2. Slab-ocean versions of these experiments, by comparison, do not capture this nonlinear behavior. Finally, provided time, we show that the CO2 forcing at which this nonlinearity occurs can be influenced by stratospheric ozone feedbacks. Overall, our results suggest that changes in ocean heat flux convergences associated with an AMOC collapse—while highly uncertain—can result in profound changes in the NH atmospheric circulation.