Another look at the index cycle

Abstract

Relationships between the zonal index, as defined by Rossby, the hemispheric circulation pattern, the amplitude of the zonally asymmetric or “eddy” component of the hemispheric circulation, and the amplitude of the stationary wave pattern during northern hemisphere winter are investigated using 27 years of gridded 5-day mean 500 mb height data from the US National Meteorological Center. In agreement with preliminary results reported in Rossby, the major stationary wave features appear to be displaced to the east of their climatological mean positions during periods of strong zonal flow, and vice versa. The zonal index shows only a weak negative temporal correlation with an index designed to monitor the hemispherically averaged variance of the eddy component of the 500 mb height field. Eddy amplitude is much more strongly correlated with the amplitude of the climatological mean stationary wave pattern. Hence, the notion of an “index cycle”, interpreted in terms of amplitude vacillation, does not appear to have much relevance to the northern hemisphere winter-time circulation. The situation may be different in the southern hemisphere, where the stationary waves are much weaker.