Limited surface impacts of the January 2021 sudden stratospheric warming

Subseasonal weather prediction can reduce economic disruption and loss of life, especially during windows of opportunity when noteworthy events in the Earth system are followed by characteristic weather patterns. Sudden stratospheric warmings (SSWs), rare breakdowns of the winter stratospheric polar vortex, are one such event. They often precede warm temperatures in Northeastern Canada and cold, stormy weather throughout Europe and the United States - including the most recent SSW on January 5th, 2021. In this study we assess the drivers of surface weather in the weeks following the SSW through initial condition scrambling experiments using the real-time CESM2(WACCM6) Earth system prediction framework. In these ensemble forecasts the stratosphere, troposphere, and whole atmosphere are separately uninitialized to directly assess the impacts of the SSW, the tropospheric circulation, and ocean/sea ice/land forcing. We find that the SSW itself had a limited impact, and no discernible contribution to the record cold in North America in February. Instead, the tropospheric circulation and its feedbacks with the stratosphere were the dominant contributors to variability.