The influence of El Nino-Southern Oscillation events on the Northern Hemisphere 500 hPa circumpolar vortex

The relationship between El Nino, the Southern Oscillation, and their joint effect (ENSO), and Northern Hemisphere 500 hPa midlatitude circulation for the 1946-1994 observational record is determined. Seasonally standardized vortex positions at a spatial resolution of 5 degrees longitude are correlated with an equatorial Pacific sea surface temperature: anomaly index and the Southern Oscillation Index (SOI). The 500 hPa circumpolar vortex is consistently expanded over the central Pacific during and for the entire year following warm ENSO periods. The vortex is contracted over North: America concurrently with warm ENSO and to a lesser degree during the following season. Expansion/contraction variability is also evident over east Asia, although the linkages to warm or cold ENSO are weak. Sea surface temperatures have a stronger association with the vortex than does the SOI.