This presentation is motivated by Professor Krishnamurti's pioneering subtropical jet (STJ) research 50 years ago and his use of multiscale analyses to determine the structure and evolution of the Asiatic monsoon on intraseasonal time scales. Observations suggest that the latitudinal location, longitudinal extent, and overall strength of the North Pacific STJ can vary on intraseasonal and synoptic time scales in response to tropical heating anomalies, recurving tropical cyclones (TCs), and higher latitude transient disturbances. In this presentation we will show how extreme weather events over the Americas can be related to changes in the structure of the North Pacific STJ that result in downstream baroclinic development and associated amplified Rossby wave trains. These changes in the structure of the STJ are investigated during periods when recurving TCs and transient disturbances of higher latitude origin interact with the jet. The linkage between changes in the structure of the STJ and the subsequent occurrence of high-impact weather events downstream over the Americas are illustrated by two cases studies.

The first case, from late October and very early November 2007, featured four extreme weather events including destructive wildfires in California, a severe cold outbreak over central Mexico, catastrophic flooding in southern Mexico, and devastating flooding in parts of the Caribbean. Six recurving western North Pacific TCs earlier in October collectively strengthened and amplified the STJ, resulting in subsequent downstream baroclinic development and anticyclonic wave breaking that helped to trigger these extreme weather events. The second case, from late October 2010, featured a record-breaking cyclone (SLP < 960 hPa) over the Upper Midwest. This cyclone originated from a lee trough east of the Rockies and rapidly developed in conjunction with a very strong (150+ kt) zonally elongated North Pacific STJ that moved inland and crossed the Rockies. The North Pacific STJ was characterized by the formation of a zonally elongated and strong meridional thermal gradient in the upper troposphere that was produced by the juxtaposition of an eastward-moving plume of very warm air on the equatorward flank of the STJ that originated from a nonrecurving western Pacific TC and an eastward-moving potential vorticity disturbance of polar origin that interacted with the STJ on its poleward flank.