The Influence of the 1997 - 1999 ENSO on Extratropical Baroclinic Life Cycles Over the Eastern North Pacific

Melvyn A. Shapiro

NOAA/Environmental Technology Laboratory, Boulder, Colorado


The El Nino-Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time-mean upper-level jet associated with ENSO modulates the time-mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper-level potential vorticity (PV) waves. The present study suggests that differences in the time-mean flow associated with the 1997-1999 ENSO cycle had a similar impact on tropopause PV structure and meridional eddy fluxes of momentum and temperature, and predictability. The seminar concludes with a discussion of high-spatial resolution (~40 km) dropwindsondes deployed in collaboration with the NCEP/Winter storm Reconnaissance Program. These observations raise questions regarding the spatial/temporal representativeness of synoptically spaced (~500 km) radiosonde/dropwindsonde soundings and the current difficulties in height assignment water-vapor drift winds in the vicinity of upper-level jet streams.