Earth Rotation as a Proxy for Interannual Variability in Atmospheric Circulation, 1860-Present

David A. Salstein Atmospheric and Environmental Research Inc., Cambridge, MA 02139

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Richard D. Rosen Atmospheric and Environmental Research Inc., Cambridge, MA 02139

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Abstract

Modern atmospheric and geodetic datasets have demonstrated that changes in the axial component of the atmosphere's angular momentum and in the rotation rate of the solid earth are closely coupled on time scales of up to several years. We therefore examine the feasibility of using a historical record of the earth's rotation as a proxy for year-to-year changes in the zonal wind held over the globe. The bulk of the earth rotation series acquired for this purpose is based on telescopic observations of the occulation of starts by the moon; semiannual values of changes in the length of day derived from these observations have acceptably small errors from about 1860 onwards. We filter these values to remove decade-scale fluctuations, which are driven primarily by non-atmospheric processes, and we examine the resulting proxy series to see if it contains a signal associated with one of the major modes of interannual variability in the atmosphere, namely that due to the El Nino/Southern Oscillation (ENSO). According to tests of statistical significance, such a signal is present in the historical earth rotation series, in that the day is typically 1onger during the year following an ENSO oceanic warm event than otherwise. We therefore proceed to consider other signals of interannual variability in the proxy series. In particular, we infer that noteworthy trends in atmospheric interannual variability have occurred over the last century, for example, the decade of the 1920s was marked by much year-to-year changes in the zonal circulation over the globe than that of the 1940s. Based on modern atmospheric data, we tentatively suggest that most of these circulation changes have resulted from anomalies in the region between 30°N and 30°S.

Abstract

Modern atmospheric and geodetic datasets have demonstrated that changes in the axial component of the atmosphere's angular momentum and in the rotation rate of the solid earth are closely coupled on time scales of up to several years. We therefore examine the feasibility of using a historical record of the earth's rotation as a proxy for year-to-year changes in the zonal wind held over the globe. The bulk of the earth rotation series acquired for this purpose is based on telescopic observations of the occulation of starts by the moon; semiannual values of changes in the length of day derived from these observations have acceptably small errors from about 1860 onwards. We filter these values to remove decade-scale fluctuations, which are driven primarily by non-atmospheric processes, and we examine the resulting proxy series to see if it contains a signal associated with one of the major modes of interannual variability in the atmosphere, namely that due to the El Nino/Southern Oscillation (ENSO). According to tests of statistical significance, such a signal is present in the historical earth rotation series, in that the day is typically 1onger during the year following an ENSO oceanic warm event than otherwise. We therefore proceed to consider other signals of interannual variability in the proxy series. In particular, we infer that noteworthy trends in atmospheric interannual variability have occurred over the last century, for example, the decade of the 1920s was marked by much year-to-year changes in the zonal circulation over the globe than that of the 1940s. Based on modern atmospheric data, we tentatively suggest that most of these circulation changes have resulted from anomalies in the region between 30°N and 30°S.

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