A 10-yr Climatology of Diabatic Rossby Waves in the Northern Hemisphere

Abstract

iabatic Rossby waves (DRWs) are low-tropospheric positive potential vorticity (PV) anomalies in moist and sufficiently baroclinic regions. They regenerate continuously by moist-diabatic processes and potentially develop into explosively intensifying cyclones. In this study a specific DRW-tracking algorithm is developed and applied to operational ECMWF analyses to compile a first climatology of DRWs in the Northern Hemisphere for the years 2001?10. DRWs are more frequent over the North Pacific than over the North Atlantic with on average 81 and 43 systems per year, respectively. Less than 15% of these systems intensify explosively, on average 12 per year over the Pacific and 5 over the Atlantic. DRWs are most frequent in summer but most of the explosively intensifying DRWs occur in autumn and winter. DRWs are generated typically between 30°?50°N over the eastern parts of the continents and the western/central parts of the oceans. They propagate fairly zonally along the midlatitude baroclinic zone. The generation of the initial low-tropospheric PV anomalies goes along with precipitation processes in characteristic flow patterns, which correspond to 1) flow around the subtropical high against the midlatitude baroclinic zone, 2) flow induced by an upper-level cutoff or a (tropical) cyclone against the baroclinic zone, 3) upper-level trough-induced ascent at the baroclinic zone, and 4) PV remnants of a tropical cyclone or a mesoscale convective system that are advected into the baroclinic zone where they start propagating as a DRW. In most cases, explosive intensification of DRWs occurs through interaction with a preexisting upper-level trough.