Spectrum and Cross Spectrum Analysis of Hemispheric Westerly Index

The following tentative conclusions are drawn from the results of cross spectrum analysis applied to the hemispheric 500-mb zonal index at latitudes of 25° N, 40° N, and 60° N.

1. Most of the variations of the indices at 40° N and 60° N are produced by oscillations with periods of the order of 25 days. This result is obtained from two independent samples of data, each at least 2 1/2 years in length. At latitude 25° N, longer periods seem to be predominant. High-frequency fluctuations with periods of the order of five days may also be important, but have been filtered out by the analysis.

2. The indices at latitudes 60° N and 40° N are correlated negatively. Most of this correlation is again contributed by oscillations with a period of the order of 25 days. The correlation between latitudes 25° N and 40° N is also negative, but so small as to be hardly significant. Again, both sets of observations agree fairly well in furnishing these results.

3. There is an indication of a slight lag of the long-period variation of index at latitude 40° N behind that at 60° N.