Separating and Quantifying the Distinct Impacts of El Niño and Stratospheric Sudden Warmings on North Atlantic and Eurasian Wintertime Climate

Stratospheric sudden warmings (SSWs) have been shown to have a major influence on Eurasian wintertime climate. By increasing the frequency of SSWs, the El Niño phase of the El Niño-Southern Oscillation (ENSO) also affects the region through the stratosphere. However, the majority of SSWs are unrelated to El Niño, and their relative importance compared to the tropospheric pathway of El Niño remains to be carefully quantified. Here, we separate and contrast these distinct sources of variability by using two 200-member ensembles of one-year integrations from the Whole Atmosphere Community Climate Model, one ensemble using sea surface temperatures (SSTs) for El Niño conditions and one ensemble imposing neutral-ENSO SSTs. We form composites of surface climate anomalies with and without SSWs in each ensemble and contrast them to a basic state represented by winters without SSWs in neutral-ENSO. We find that El Niño and SSWs have impacts of comparable magnitude on the North Atlantic Oscillation and European precipitation, with SSWs causing larger cooling over Eurasia. Results from this study have implications for predictability of wintertime Eurasian climate.