Large-scale precursors of extreme weather over North America during late October 2007: The role of TC Kajiki

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Benjamin Moore (University at Albany, State University of New York)
Daniel Keyser (University at Albany/SUNY/DAES)
Lance Bosart (University at Albany/SUNY)

This study examines the contribution of western North Pacific tropical cyclone (TC) Kajiki to the development of a high-amplitude Rossby wave train extending into North America along the North Pacific waveguide that was linked to extreme weather events over North America during 22 31 October 2007. These extreme weather events included wildfires in southern California, a cold surge and extreme rainfall in southern Mexico, and widespread extreme rainfall in the central and eastern United States. The processes contributing to the amplification of the large-scale extratropical flow are diagnosed using a potential vorticity (PV) framework combined with trajectory calculations and budgets of 1000 100-hPa vertically integrated eddy kinetic energy (EKE). Results indicate that the evolution of the flow can be viewed in two primary stages: initiation and amplification.

During the initiation stage, two polar PV disturbances perturbed the North Pacific waveguide, resulting in the initial development of two cyclones and two corresponding EKE centers over the western and eastern North Pacific, respectively. During the amplification stage, persistent (~24 h) ridge amplification over the western and central North Pacific, linked to EKE generation via ageostrophic geopotential flux convergence, occurred downstream of the western North Pacific cyclone and TC Kajiki. Trajectory calculations reveal that this ridge amplification was strongly influenced by widespread condensational heating sustained by the poleward transport and deep ascent of moist air originating within the circulation of TC Kajiki. EKE was dispersed downstream from the area of EKE generation associated with TC Kajiki, corresponding to downstream trough amplification and to the intensification of the preexisting EKE center linked to the cyclone over the eastern North Pacific. Downstream of the amplifying trough and associated EKE center, two successive frontal wave disturbances developed along a cold front trailing from the eastern North Pacific cyclone. These disturbances collectively served to reinforce a corridor of strong poleward transport of warm, moist air and contributed to persistent ridge amplification over the eastern North Pacific and western North America. The ridge amplification, in turn, culminated in anticyclonic wave breaking over western North America, establishing a high-amplitude flow pattern favorable for the occurrence of extreme weather over North America.

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