Cold air outbreaks drive near-surface baroclinicity variability over storm track entrance regions in the Northern Hemisphere

Abstract. Cold air outbreaks (CAOs) are key drivers of near-surface baroclinicity in midlatitude oceanic regions, where cold continental air masses interact with warm sea surface temperatures, giving rise to strong surface heat fluxes. Despite their relatively limited spatio-temporal extent, CAOs exert a disproportionate influence on the variability of near-surface baroclinicity, particularly in the entrance regions of the North Atlantic and North Pacific storm tracks. To further clarify this relationship, we use the isentropic slope framework to distinguish between diabatic and adiabatic changes in baroclinicity and quantify the contribution of CAOs to near-surface baroclinicity variability in the Gulf Stream and Kuroshio-Oyashio extension regions. Moderate-intensity CAOs account for up to 40 % of the total near-surface baroclinicity variability in the Gulf Stream Extension, while occupying less than 15 % of the region. In the Kuroshio-Oyashio Extension, CAOs explain a smaller fraction of variability despite their broader spatial extent. We employ phase space analysis to diagnose the typical phasing between adiabatic depletion and diabatic restoration of baroclinicity, with the former leading in time on the latter. Phase portraits and synoptic composites focused on CAO-related variability show that this characteristic phasing is predominantly linked to CAOs, whereas background variability contributes weakly and incoherently. These findings highlight the central role of CAOs in shaping near-surface baroclinicity and suggest that they are essential to the evolution of midlatitude storm tracks.