Abstract

Interseasonal variability for a portion of the Northern Hemisphere has been quantified by using grid-point standard deviations of five-day mean 700 mb height for individual seasons, standard deviations of empirical orthogonal function amplitude time series for individual seasons, and a specially derived variability index (VI). The various measures or intraseasonal variability of 700 mb height were used in stratification studies (i.e., using difference maps based on high versus low variability) or correlation analyses so that relationships between intraseasonal variability and seasonal mean circulation or sea-surface temperature anomalies could be determined. Approximately 39 years of data (1948–76) were employed. Principal findings include:

1) Intraseasonal variability, characterized by the VI, generally showed little if any persistence from one season to the next. The exception was the autumn-winter period when the autocorrelation was 0.52. The largest contribution to this persistence came from lower latitudes.

2) The VI time series indicates that intraseasonal variability of 700 mb heights was relatively low in the 1950s to early 1960s, then high until the 1970s when near average conditions occurred, Considerable year-to-year fluctuations occurred, especially in the 1970s.

3) Significant differences occurred overall between winter (also spring) mean 700 mb height in years of highest tercile VI and years of lowest tercile VI. For winter, synoptic interpretation of the 700 mb height difference map showed that high (low) variability winters tend to be associated with expanded (contracted) westerlies. For spring, the relationship was strongest in the North Pacific sector such that high (low) variability springs tended to have weak (strong) Aleutian lows and weak (strong) westerlies in the 30–5O°N band.

4) Empirical orthogonal functions (E0Fs), which depict the important modes of seasonal-mean circulation, were correlated and otherwise related (using contingency tables) to VA. The first three EOFs explain 38% of the VI variance for winter, the most for any season, while only 11% of the VI variance is explained for autumn, the least of any season.

5) Significant differences in eastern tropical Pacific sea-surface temperature in fall-winter were related to intraseasonal fluctuations in the expansion/contraction of the 700 mb circumpolar vortex (i.e., “warm” water implied large intraseasonal fluctuation).

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