A spectral analysis of the Earth's angular momentum budget

Abstract

Changes in the circulation of the earth's atmosphere cause fluctuations in the length of day (LOD) by exchanging angular momentum with the surface of the earth. In this paper we examine the earth's polar angular momentum budget using earth rotation data from optical astrometry and lunar laser ranging together with simultaneous estimates of the atmospheric angular momentum Matm derived from meteorological data. The atmospheric excitation can explain most of the observed LOD fluctuations at periods between 700 and 40 days. The observed Matm spectral power is proportional to the inverse frequency squared for periods between 1 year and 3 days and flattens out at higher frequencies. The LOD and Matm spectra are very similar except at periods of <40 days, where the LOD measurement noise dominates. The Matm ‐ LOD coherence is highly significant between periods of 700 and 40 days. There is no evidence for phase shifts between the Matm and LOD data at periods between 600 and 40 days. A broad peak is clearly evident at periods of 40–60 days in the spectra of all three time series. The LOD data have a long‐term drift, not matched by the Matm data, that is probably the result of a core‐mantle angular momentum exchange. There are significant imbalances in the angular momentum budget. Most of the seasonal imbalance can be attributed to the effects of systematic errors in the meteorological data, but some of this discrepancy may be caused by exchanges of angular momentum with the oceans. The semiannual, monthly, and fortnightly tidal variations in the earth rotation were detected, and the observed amplitudes of these variations are consistent with theoretical estimates.

Phase Space Structure, Attractor Dimension, Lyapunov Exponent and Nonlinear Prediction from Earth's Atmospheric Angular Momentum Time Series

“…However, recognition of these limited numbers of well defined parameters is quite difficult and requires a deep understanding of the physical mechanism of underlying dynamics. It is known that short-period AAM changes in the higher frequency range are due to thermally forced seasonal and coupled air-sea level oscillations (ROSEN and SALSTEIN, 1983;EUBANKS et al, 1985;MORGAN, et al, 1985). On short time scales underlying dynamics also exhibits a near equilibrium state (NICOLIS and NICOLIS, 1984) and coupling with self-regulating atmospheric and ocean processes.…”

Atmosphere and Earth's rotation

“…Since the moment of inertia of the system mantle-crust is only slightly influenced by atmospheric pressure loading, this requires mainly a change of the angular velocity of the solid Earth, i.e., a change of the length of the day (LOD). Presently, LOD can be measured to a high accuracy with integration times of only a few hours (e.g., Robertson, 1991), and general circulation models of the atmosphere allow a determination of changes in the AAM (e.g., Barnes et al, 1983;Eubanks et al, 1985). A comparison between changes in AAM and LOD shows that both are indeed highly correlated (see Figure 5).…”

Observations of stratospheric sudden warmings in Earth rotation variations

“…Variations of ΔLOD on this time scale are almost entirely explained by variations in the wind AEF, which is why the mass term (8), which is about an order of magnitude smaller [Eubanks et al, 1985], is omitted here.…”

“…In all three parameters, we have removed the daily climatology (in order to remove the seasonal cycle) as well as the 151 day average around the central date (in order to remove interannual variability due to, e.g., the quasi-biennial oscillation or El Niño-Southern Oscillation (ENSO)). This leaves the subseasonal fluctuations, which are typically on the order of tens of milliarcseconds for the polar motion angles, and microseconds for the length-of-day anomalies [Salstein and Rosen, 1989;Eubanks et al, 1985;Rosen et al, 1991]. Polar motion angle 2 in particular shows a negative anomaly of 30 mas about 3 weeks before the central date, while the length-of-day anomaly shows a steady decline as the central date is approached and passed.…”

“…All Rights Reserved. subseasonal time scales the atmosphere is the dominant source of axial angular momentum [Rosen and Salstein, 1983;Eubanks et al, 1985] and a major source, along with the ocean, of equatorial angular momentum [Dobslaw et al, 2010].…”