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 the changing external radiative forcing (CO₂ and Ozone). In this study, we use a 1-degree GEOS-MITgcm coupled general circulation model to understand the response to different levels of observed external forcing from past decades. Results from an ensemble of multi-year forecasts show the Northern Hemisphere polar stratospheric temperature increased during the period from 1992 to 2000, and decreased during 2000 to 2020. To isolate the influence of external forcing, 30-year long ensemble 'perpetual' experiments were conducted in which the external forcing for a particular year is repeated, 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 does not include any explosive volcanic forcing. This temperature increase from 1992 to 2000 is in contrast to the general expectation that the stratosphere cools as CO₂ increases. The increasing and then decreasing temperature trend is also manifest in several reanalyses, and CMIP6 historical simulations with a well-resolved stratosphere. Results rules out either a response to volcanic emissions or a change in the phase of decadal modes of variability as an explanation for the warming. Analysis of the temperature budget showed that the behavior of the polar stratospheric temperature is dictated by the meridional eddy transport of heat as a result of changes in CO₂ and Ozone in the past decades.