Internal Variability Reshapes Externally Forced Trends in Marine Cold‐Air Outbreak Frequency Over the Northern Hemisphere

Abstract

Marine cold‐air outbreaks (MCAOs) are key conduits of polar‐to‐midlatitude air‐mass exchange, strongly modulating wintertime air–sea heat fluxes, storm‐track activity, and the development of extreme cold conditions over adjacent continents. The drivers of recent trends of MCAOs in the Northern Hemisphere remain unclear. Using the CESM1 Large Ensemble, we decompose wintertime MCAO frequency trends during 1979–2025 into externally forced and internally generated components. In the Arctic and sub‐Arctic, MCAOs increase through the combined effects of forcing and internal variability, consistent with sea‐ice loss enhancing lower‐boundary thermal contrast and open‐water area. Over the North Pacific and North Atlantic, external forcing imposes a basin‐scale decline, but internal variability determines its regional manifestation by reorganizing that decline into dipole‐like structures and offsetting it over parts of the North Atlantic. Recent MCAO trends therefore do not provide a simple fingerprint of forcing: forcing sets the basin‐scale tendency, whereas internal variability governs the regional expression.