Zonal index (Z500 35°N − 55°N)

Hemispheric zonal-mean Z500 at 35°N minus 55°N (proportional to the mid-latitude geostrophic westerly). High index = zonal flow, low index = meridional / blocked flow. Classical indices (Rossby 1939, Namias 1950) used 700 hPa over the Western Hemisphere; this is the hemispheric 500-hPa version. — Namias 1950, J. Meteor. 7, 130–139, doi:10.1175/1520-0469(1950)007<0130:TICAIR>2.0.CO;2