Response to Limited Surface Impacts of the January 2021 Sudden Stratospheric Warming

AER Principal scientist Judah Cohen led a study that received wide attention on how the polar vortex contributed to the historical cold air outbreak and snowstorm in the Central United States (US) in February 2021.  Texas was particularly hard hit with a catastrophic breakdown in the energy infrastructure leading to billions of dollars in insured losses and hundreds of deaths across the state.  A subsequent modeling study from NCAR argued that the polar vortex played no role in the cold air outbreak of February 2021.  Cohen led a response to the NCAR study raising concerns regarding the methodology in the NCAR experiments as well as the ability of the model to reproduce the observed event over the US.  Furthermore, Cohen’s team questions the conclusion that polar vortex variability can be excluded as an important factor in the February 2021 US cold wave.  Cohen’s team concludes with suggested additional modeling experiments and observational analysis necessary to determine the causal role of the polar vortex in the February 2021 US cold wave. 


Figure 1. NCAR Model forecasts simulate North American cold due to internal variability but miss the forced cold.  Surface temperature anomalies from (a) MERRA2 (observations), and (b) ensemble-mean surface temperature anomalies in the period February 8-12, 2021 as shown in NCAR study.  We included a blue box to delineate North America and a red box to delineate the United States. The predicted North American cold wave is limited to Canada in contrast to what was observed where the largest cold anomalies occurred in the US.

Citation:  Response to Limited Surface Impacts of the January 2021 Sudden Stratospheric Warming

J. Cohen, L. Agel, M. Barlow, C. I. Garfinkel, I. White

Nature Communications, accepted.

Principal Scientist and Director of Seasonal Forecasting