The influence of the 1997-99 El Nino Southern Oscillation on extratropical baroclinic life cycles over the eastern North Pacific

The El Ni (n) over tildeo 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 1997-99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and predictability.