Northern Hemisphere stratospheric temperature response to external forcing in decadal climate simulations

Abstract. To predict the future state of the Earth system on multiyear timescales, it is crucial to understand the response to changing external radiative forcing (CO2 and Ozone). Analyzing the Northern Hemisphere (NH) winter stratospheric polar vortex temperature, we found a general temperature decrease in the reanalysis data (1982–2020), the expected trend with increasing CO2, except for a sharp warming during the period 1992–2000. Results from 1° GEOS-MITgcm coupled general circulation model simulations of past decades show a similar increase in the NH polar stratospheric temperature during 1992–2000 and a decrease during 2000–2020. To isolate the influence of external forcing, we conducted a series of 30-year-long “perpetual” time-slice experiments in which the external forcing for a particular year is held fixed at its values for 1992, 2000, and 2020. Each simulated year of these perpetual experiments is forced with the CO2, Ozone, anthropogenic aerosol emissions, and trace gases of that year, but none of the simulations include any explosive volcanic forcing. The increasing and then decreasing temperature trend is also manifest in the CMIP6 historical simulations performed with models that include a well-resolved stratosphere. The configuration of the perpetual experiments rules out a direct response to volcanic emissions or a change in the phase of decadal modes of variability as explanations for the warming rather than the expected cooling behavior. Analysis of the temperature budget showed (only significant terms are discussed) that the polar stratospheric temperature behavior is dictated by meridional eddy transport of heat resulting from changes in CO2 and Ozone over the past decades.