Impact of sudden stratospheric warming on Eurasian cold anomalies on subseasonal time scales using stratospheric nudging experiments

Using stratospheric nudging experiments in a global climate model, we examined the role of sudden stratospheric warming (SSW) in driving Eurasian cooling on subseasonal timescales. Nudging was applied to extratropical stratospheric conditions derived from observations, imposing the full observed stratospheric state including zonal asymmetries. Two nudging experiments were performed with different lower boundaries within the stratosphere (~88 hPa vs. ~200 hPa). Both experiments successfully reproduced Eurasian cooling following SSW events, linked to cold air advection into Eurasia driven by changes in upper tropospheric circulation resulting from stratosphere–troposphere coupling. The results were generally insensitive to the vertical extent of nudging, especially in cases with strong coupling. However, the nudging experiments also exhibited adverse effects. Most notably, a pronounced cold bias developed over Alaska, attributed to the model’s tendency to produce overly strong coupling between the stratosphere and upper troposphere in this region compared to observations. This bias was mitigated when nudging was extended deeper into the lower stratosphere. These findings underscore the importance of accurately representing stratosphere–troposphere coupling strength to reliably capture the surface impacts of stratospheric variability.