Variations in atmospheric angular momentum on global and regional scales and the length of day

Abstract

Twice‐daily global analyses of the zonal winds prepared by the U.S. National Meteorological Center for the period from January 1, 1976, through December 31, 1981, have been used to construct a time series of the atmosphere's angular momentum (M). This series is spectrally analyzed and compared with independent observations of changes in the solid earth's rate of rotation. The most prominent feature in the M time series is an annual signal, and this is mirrored by similar behavior in changes in the length of the day (l.o.d.). Other noteworthy fluctuations in M and l.o.d. on time scales of less than a year also occur, however, and these too agree well with each other. In this regard, a near 50‐day fluctuation in both time series, also noted in other studies, is especially visible. We conclude on the basis of these comparisons that indeed most of the variability in our time series for M is real and that, as others have suggested, the atmosphere plays a dominant role in forcing changes in l.o.d. on time scales of about a year and less. We divide the globe into a number of belts to investigate the contributions made by different parts of the atmosphere to the changes found in the global statistic M. Our approach is to calculate empirical orthogonal functions that define the major spatial modes of variation among the regional time series of angular momentum. As expected, we find the annual cycle in M is associated with seasonal changes in the major jet streams of the two hemispheres, with the larger of these changes occurring in the northern hemisphere. Nonseasonal changes in M emerge from the imbalances among regional variations in momentum, with the largest contributions on time scales less than a year coming from the regions 10°–25°S and 20°–35°N.