700‐HPA geopotential height anomalies from a statistical analysis of the French Hemis data set

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Abstract
The low‐frequency time variations of the Northern Hemisphere 700‐hPa heights have been investigated using the French Hemis grid‐point data set and a statistical model based upon an orthogonal rotated principal component analysis. The data consists of 389 10‐day means of geopotential height (from 1970 to 1980) distributed over an octagonal grid of 1977 points, poleward of 20°. The model summarizes the major modes of 700‐hPa height fluctuations; for each mode, it provides the corresponding spatial pattern and the associated time component.Four seasonal analyses have been performed using 10‐day means. The major modes of the spatial anomaly structure are described in detail and are clearly identified for each season. The stability of these modes through the separate seasonal analyses is examined. The robustness of the resulting patterns is analysed through their consistency with results of other studies.