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Abstract

We investigate the role of diabatic processes in the evolution of Northern Hemispheric stormtracks using an isentropic slope framework as a measure of baroclinicity. Baroclinicity is depleted through isentropic tilting by eddies while it is restored by diabatic heating. We find, however, that the phasing of depletion and restoration of baroclinicity in the near-surface troposphere is opposite to that in the free troposphere. The opposite phasing of baroclinicity depletion and restoration across the troposphere is observed both in upstream and downstream sectors of the North Atlantic and North Pacific stormtracks, suggesting a common feature of midlatitude stormtracks. Near the surface (900--800hPa), cold air advection, often associated with cold air outbreaks, initially flattens isentropic surfaces, thus reducing near-surface baroclinicity, which is then restored by air-sea heat exchange. Composites of geopotential height at 1000hPa and 500hPa corroborate these stages in the evolution of the near-surface baroclinicity, showing the intensification and decay of cold air advection from the cold continents upstream of the stormtracks or from the Arctic in the downstream regions. In the free troposphere (750–350hPa), on the other hand, generation of baroclinicity by moist diabatic processes precedes its reduction by isentropic tilting associated with evolving cyclones. A correspondence between diabatic generation of baroclinicity, associated baroclinic development, and a local increase in moisture availability emerges from composite analysis for the free-tropospheric lifecycle in the stormtrack. In particular, moisture availability evidently plays a crucial role in the evolution of the stormtrack.