Oceanic Fronts Shape Hemispheric Contrasts in Polar Stratospheric Extremes

Extreme stratospheric events such as Sudden Stratospheric Warmings (SSWs) and Polar Stratospheric Clouds (PSCs) exhibit striking inter-hemispheric asymmetries: SSWs are frequent in the Arctic but rare in the Antarctic, while PSCs are more persistent in the Antarctic. Although land-sea thermal contrast and orography (LSCO) have been traditionally invoked to explain these asymmetries, we show that LSCO alone cannot fully account for the observed differences. Using targeted model experiments, we reveal that midlatitude oceanic Sea Surface Temperature (SST) fronts represent a crucial but overlooked additional driver of hemispheric stratospheric differences. Like LSCO, SST fronts enhance stratospheric convergence of planetary wave activity, strengthening the Brewer-Dobson Circulation and inducing high-latitude adiabatic warming. This warming significantly enhances Arctic SSW frequency and strongly reduces PSC formation. The oceanic impact is dominated by Pacific SST fronts. Our results highlight the indispensable role of SST fronts in shaping Arctic-Antarctic asymmetries in stratospheric dynamics and associated extremes .