School of Meteorology

The Occurrence of Extreme Weather Events over North America Arising from Interactions of the North Pacific Jet Stream with Tropical and Polar Disturbances

Dr. Lance F. Bosart
Department of Atmospheric & Environmental Sciences
University at Albany/SUNY

31 March 2015, 3:30 PM

National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Four sequentially linked extreme weather events (EWEs) that occurred over North America during 22–31 October 2007 and their antecedent atmospheric circulations illustrate important aspects of the intraseasonal variability prediction problem. These EWEs featured devastating wildfires in southern California, prominent early-season cold surges in northern and eastern Mexico, flood-producing heavy rain in the eastern United Sates, and crippling heavy rains in southern Mexico. A dynamically driven amplification of the upper-level flow across the North Pacific and North America preceded EWE formation. This flow amplification was associated with the formation of a high-amplitude Rossby wave train (RWT) that propagated eastward in association with downstream baroclinic development. The formation and maintenance of the RWT involved multiple tropical and polar disturbances that interacted with the North Pacific jet stream from 22–31 October 2007. These disturbances included two upper-level polar disturbances, a diabatic Rossby vortex, a western North Pacific tropical cyclone (TC) (Kajiki), and migratory extratropical cyclones (ECs).

Deep subtropical and tropical moisture plumes resembling “atmospheric rivers” extended poleward from western Pacific TC Kajiki and from the subtropical eastern North Pacific into the warm sectors of polar disturbance-generated ECs over the western and eastern North Pacific, respectively. The moisture plumes that were drawn poleward along warm conveyor belts into the warm sectors of these ECs played a critical role in amplifying upper-level ridges located within the RWT. The poleward-displaced moisture plumes bolstered latent heat release, negative potential vorticity (PV) advection by the irrotational wind into downstream ridges that contributed to upper-level flow amplification and ridge building within the RWT. The EWEs occurred subsequent to ridge amplification and anticyclonic wave breaking over western North America and the concomitant downstream formation of a meridionally elongated PV streamer over the central United States. The resulting high-amplitude flow pattern over North America favored the aforementioned EWEs by promoting an extensive meridional exchange of air masses from high and low latitudes. The behavior of the sequentially linked EWEs will be illustrated by Lagrangian trajectories and Eulerian PV analyses.

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