Tropical–extratropical interactions conducive to intraseasonal variability in the Northern Hemisphere available potential energy

The goal of this study is to investigate tropical–extratropical interactions occurring on intraseasonal time scales that can significantly influence intraseasonal variability in the Northern Hemisphere (NH) zonal available potential energy (APE) and the “spin-up” of the NH general circulation. The spin-up of the NH general circulation in late 2007 was noteworthy in that the NH zonal APE increased 29% (relative to climatology) 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 850-hPa temperature observed in December at Barrow, AK, between 1948 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 weather events.

The research to be presented will identify tropical–extratropical interactions that occur in conjunction with climatologically significant periods of intraseasonal variability in the NH zonal APE, and will emphasize the contributions of the MJO and the evolution of western North Pacific TCs to this variability. Preliminary results suggest that the MJO can be associated with an intensification of the North Pacific waveguide and concurrent increases in NH zonal APE, whereas the evolution of western North Pacific TCs can contribute to decreases 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 weather events often are observed in conjunction with intraseasonal variability in the NH zonal APE.