La Niña obscures the North Atlantic response to sudden stratospheric warmings in C3S seasonal forecasts

Abstract

Sudden stratospheric warmings (SSWs) can alter the North Atlantic circulation on subseasonal-to-seasonal timescales, typically leading to an equatorward shift of the eddy-driven jet. However, this response is highly variable and can be modulated by other sources of climate variability. Using seasonal forecasts, this study investigates the modulation of the North Atlantic response to SSWs by El Niño–Southern Oscillation (ENSO). Forecast systems reproduce the tropospheric variability following SSWs found in reanalysis, including an increase of events followed by a poleward-positioned jet during La Niña winters. Composite analysis reveals that SSWs increase the likelihood of an equatorward-shifted jet and negative North Atlantic Oscillation compared to non-SSW members during both ENSO phases, but La Niña masks this response due to their opposing influence on the North Atlantic mean state. This study highlights the importance of accounting for ENSO when predicting SSW impacts on the circulation and surface extremes on subseasonal timescales.