Linked Recurving Western Pacific Tropical Cyclones, Transient Polar Disturbances, Downstream Baroclinic Development, and Sequential Extreme Weather Events over North America

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, widespread heavy rain in the eastern United Sates that produced local flooding but relieved long-term drought conditions, 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 TC (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 along warm conveyor belts into the warm sectors of eastward-propagating ECs. These moisture plumes played a critical role in amplifying upper-level ridges located within the aforementioned RWT. The associated latent heat release and diabatically driven upper-level outflow increased the magnitude of the negative potential vorticity (PV) advection by the irrotational wind into downstream ridges which further contributed to upper-level flow amplification and ridge building within the RWT over the North Pacific. The downstream 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 dynamical processes associated with the sequentially linked EWEs will be illustrated by Lagrangian trajectories and Eulerian PV analyses.