Tropical-extratropical interactions conducive to intraseasonal variability of Northern Hemisphere available potential energy

A noteworthy aspect of the “spin-up” of the Northern Hemisphere (NH) general circulation in late 2007 was a 29% increase (relative to climatology) in the NH zonal available potential energy (APE) between 27 October and 11 November. This period of increasing NH zonal APE coincided with the progression of anomalous tropical convection associated with the Madden–Julian Oscillation (MJO) from the Maritime Continent into the western Pacific Ocean. Subsequently, the NH zonal APE decreased 32% between 15 November and 3 December in conjunction with the tropical and extratropical phases in the life cycles of western North Pacific tropical cyclones (TCs) Mitag, Hagibis, and 26W. This period of decreasing NH zonal APE coincided with the warmest observed December upper-tropospheric temperatures poleward of the Arctic Circle between 1979 and 2008. The evolution of the tropical and extratropical North Pacific flow during late 2007 represents an extraordinary example of tropical–extratropical interaction on intraseasonal time scales (e.g., one-to-two weeks) that is conducive to intraseasonal variability in the NH zonal APE and to the occurrence of downstream high-impact extreme weather events.

The research to be presented will identify tropical–extratropical interactions that occur in conjunction with climatologically significant periods of NH zonal APE variability, and will emphasize the contributions of the MJO and the evolution of western North Pacific TCs to intraseasonal variability in the NH zonal APE. Preliminary results suggest that the MJO can be associated with an intensification of the North Pacific waveguide and a concurrent increase in NH zonal APE, whereas the evolution of western North Pacific TCs can contribute toward a decrease in NH zonal APE during periods of cyclogenesis, downstream development, and large-scale flow amplification over the eastern North Pacific and North America. As a result, high-impact extreme weather events often are observed in conjunction with intraseasonal variability in the NH zonal APE.