T. Marshall Eubanks

An El Nino signal in atmospheric angular momentum and earth rotation

Anomalously high values of atmospheric angular momentum and length of day were observed in late January 1983. This signal in the time series of these two coupled quantities appears to have been a consequence of the equatorial Pacific Ocean warming event of 1982-1983.

Detection of an ENSO signal in seasonal length-of-day variations

Conservation of angular momentum dictates that as the wind-driven axial atmospheric angular momentum changes, so will the length-of-day (LOD). In particular, as the strength of the seasonal zonal winds change, so should the strength of the seasonal LOD signals. Here, observed changes in the strengths of the annual and semiannual LOD signals during 1963–1991 are analyzed and shown to be both significantly correlated (at the 99% significance level) with the Southern Oscillation Index (SOI), and to exhibit trends of comparable magnitude but opposite signs. This reported correlation between the SOI and changes in the amplitude of the seasonal LOD signals demonstrates a linkage between seasonal LOD (and hence seasonal zonal wind) variability and the El Nino/Southern Oscillation (ENSO) phenomenon. Furthermore, this study suggests that observed variations in the amplitudes of the seasonal LOD signals can be used to study changes in the strengths of the seasonal atmospheric zonal winds on interannual to decadal and longer time scales.

Medium-Range Numerical Forecasts of Atmospheric Angular Momentum

Abstract Forecasts of zonal wind fields produced by the medium-range forecast (MRF) model of the National Meteorological Center are used to create predictions of the atmospheres angular momentum at lead times of 1–10 days. Forecasts of this globally integrated quantity are of interest to geodesists and others concerned with monitoring changes in the earths orientation for navigational purposes. Based on momentum forecasts archived for the period December 1985–November 1986, we find that, on average, the MRF exhibits positive skill relative to persistence-based forecasts at all lead times. Over our entire one-year study period, the improvement over persistence exceeds 20% for 2–6-day forecasts and remains as large as 10% even for 10-day forecasts. On the other hand, skill scores for the MRF momentum predictions vary considerably from month to month, and for a sizeable fraction of our study period the MRF is less skillful than persistence. Thus, although our initial impression of the overall quality of th...

Earth Rotation and Polar Motion: Measurements and Implications

Earth rotation and polar motion studies are embarking on a new era with the advent of highly accurate space geodetic techniques and the availability of global atmospheric angular momentum measurements. Space geodesy is opening new doors, answering old questions, and posing new ones. The angular momentum balance and the transfer of angular momentum between the solid Earth, atmosphere, and oceans are emerging as a problem of great scientific interest overlapping many areas, such as atmospheric science, oceanography, geodesy, and geodynamics. Here, the measurements of Earth rotation and polar motion (collectively referred to as Earth orientation) are described; the combination, smoothing, and intercomparison of these results from various techniques are presented. The calculation of atmospheric angular momentum (AAM) excitation functions are outlined; comparisons of the AAM excitation functions with variations in the length of day (LOD) and polar motion results are discussed. The associated geophysical implications (e.g., J2, 50-day oscillations) are stressed; anticipated advances and prospects for the future are high lighted.