Zonal-Wind Oscillations over the Western Hemisphere during Winter: Further Evidence of a Zonal-Eddy Relationship

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) 500-mb height analyses and 7-day forecasts are examined during ten winters (1981?90) for oscillations of geostrophic zonal wind over the western hemisphere. A well-defined zonal-wind oscillation is isolated in the first two eigenvectors. This zonal-wind oscillation accounts for about 55% of the total zonal-wind variance over the western hemisphere during this ten-winter period. The oscillation is characterized by zonal-wind anomalies that are in phase between 30° and 70°N and out of phase with zonal-wind anomalies along 50°N. The oscillation clearly displays southward propagation from 85° through 30°N, with standing components along 30°, 50°, and 70°N. The dominant temporal period associated with the oscillation is found to be in the range of 15?35 days with large interannual variability. Composites of 500-mb heights through 25 cycles of zonal-wind oscillations over ten winters were performed for unfiltered and 15?39-day filtered data. In both filtered and unaltered composites, the zonal-wind oscillation is associated with southward propagation of positive and negative height anomalies from 85°N southward past 30°N. Composite 500-mb height anomalies change from zonal to meridional over North America as positive and negative zonal-wind anomalies pass through the 50°?40°N latitude belt. Characteristics of meridional height anomalies associated with the zonal-wind oscillation are similar to numerically modeled height anomalies produced by orographic forcing from the Rocky Mountains. Preliminary indications show that there does not appear to be a significant association between the zonal-wind oscillation isolated here (15?35 days) and oscillations in tropical convection (40?50 days). The geostrophic zonal-wind oscillation is also found in 1?7-day 500-mb height forecasts made by the ECMWF global spectral model; however, the temporal character of this oscillation is significantly different in 7-day forecasts as compared to those observed. As a consequence, it is shown that the zonal-wind oscillation is associated with a significant source of 500-mb height forecast errors in the 7-day ECMWF model, since the pattern of 7-day forecast errors is similar to the height-anomaly pattern associated with the observed zonal-wind oscillation. This study is an extension of preliminary results by Lyons (1989) and gives further evidence for a zonal-eddy relationship over the western hemisphere during winter.