A CMIP6 Analysis of Past and Future Arctic Winter Stratospheric Temperature Trends

Reanalysis data reveal a weak warming trend in the midwinter Arctic stratosphere, contrary to the cooling expectation based on the greenhouse gas effect. This trend is also influenced by the occurrence of sudden stratospheric warmings (SSWs). Using Phase 6 of the Coupled Model Intercomparison Project (CMIP6) we investigate temperature trends over a similar timescale as ERA5 and find that CMIP6 models can replicate the positive midwinter temperature trend in the mid-lower stratosphere. However, when considering the multi-model mean, this positive temperature trend is much weaker than ERA5. Extrapolating to the future, we find that the SSW-driven positive temperature trend will likely not continue in the future based on the SSP2-4.5 and SSP5-8.5 climate scenarios. Instead, the models project there will be widespread cooling throughout the Arctic winter stratosphere regardless of the occurrence of SSWs. Using a subsample of CMIP6 models which replicate the seasonality of the Arctic winter stratosphere most similarly to that of ERA5, we also find that the zonal wind strength during SSWs correlates the most with the temperature trends found there. However, trends in the zonal wind strength alone cannot account for the observed temperature trends among the CMIP6 models. During the 1980-2019 period, the temperature in the mid-lower Arctic winter stratosphere has been increasing while the rest of the stratosphere has been cooling. More interestingly, when this period is filtered according to years with and without sudden stratospheric warmings (SSWs), an event associated with a remarkable disruption to the stratospheric circulation and impressive temperature increases, it has been found that the years in which sudden warming events occur are driving the observed temperature trends. To determine if this positive temperature trend will continue in the future, this work uses climate models. We find that a positive temperature trend will not continue in the future. Instead, it is more likely that there will be widespread cooling throughout the winter Arctic stratosphere, regardless of the occurrence of SSWs. We also find that the strength of the zonal wind in the Arctic stratosphere is an important consideration when discussing the temperature trends. However, trends in the zonal wind strength alone cannot account for the observed temperature trends found in the Arctic winter stratosphere. A sudden stratospheric warming (SSW)-induced "cooling hole", or rather a weak warming trend, is observed in the wintertime Arctic stratosphereGCMs replicate this SSW-influenced lack-of-cooling phenomenon in their historical simulationsGCMs project the warming to be less dominant in the future compared to historical data