David A. Salstein

The Sub-bureau for Atmospheric Angular Momentum of the International Earth Rotation Service - A meteorological data center with geodetic applications

By exchanging angular momentum with the solid portion of the earth, the atmosphere plays a vital role in exciting small but measurable changes in the rotation of our planet. Recognizing this relationship, the International Earth Rotation Service invited the U.S. National Meteorological Center to organize a Sub-bureau for Atmospheric Angular Momentum (SBAAM) for the purpose of collecting, distributing, archiving, and analyzing atmospheric parameters relevant to earth rotation/polar motion. These functions of wind and surface pressure are being computed with data from several of the worlds weather services, and they are being widely applied to the research and operations of the geodetic community. The SBAAM began operating formally in October 1989, and this article highlights its development, operations, and significance.

Sea Level Response to Pressure Forcing in a Barotropic Numerical Model

Abstract A barotropic shallow-water model is used to study the large-scale sea level response to realistic barometric forcing at periods ranging from 1 day to 1 year. Results are presented from coarse resolution “open” ocean experiments (i.e., no shallow continental shelf regions or marginal seas) with coastal geometries and bottom topography representative of the North Atlantic and Pacific basins. The validity of the inverted barometer (IB) approximation is examined in detail, including nonlocal effects which result from taking into account the constant volume of the ocean. These effects are found to be important at low latitudes, where a considerable part of the sea level variability is related to pressure forcing over higher latitudes. Root-mean-square deviations from an IB response in the range of 1–3 cm are typical, with most of the variance occurring at high frequencies. Basin-averaged estimates yield IB deviations of only a few percent at time scales longer than 1 week increasing to 5%–20% over the...

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.

Space geodesy monitors mass transports in global geophysical fluids

Large-scale mass transports in the Earth system produce variations in Earths rotation, gravity field, and geocenter. Although relatively small, these global geodynamic effects have been measured by space geodetic techniques to increasing, unprecedented accuracy, opening up important new avenues of research that will lead to a better understanding of global mass transport processes and the Earths dynamic responses. To take full advantage of these advances, the International Earth Rotation Service (IERS), the organization that monitors the rotational motions of the Earth and related properties, saw the need in 1998 to create an infrastructure to facilitate the link between the space geodetic measurement and the geodynamic “global change” research communities [Dehant et al., 1997]. Hence was born the IERS Global Geophysical Fluids Center (GGFC).

Atmospheric torque on the Earth and comparison with atmospheric angular momentum variations

The purpose of this paper is to compute atmospheric torques on the Earth, including the oceans, with an emphasis on the equatorial components. This dynamic approach is an alternative method to the classical budget-based angular momentum method for viewing atmospheric effects on Earths orientation in space. The expression of the total torque interaction between the atmosphere and the Earth is derived from the angular momentum balance equation. Such a torque is composed of three parts due to pressure, gravitation, and friction. Each of these torque components is evaluated numerically by a semi-analytical approach involving spherical harmonic approximations, and their orders of magnitude are intercompared. For the equatorial components the pressure and gravitational torques have far larger amplitudes than that of the friction torque; these two major torques have the same order of magnitude but opposite signs, and the value of the sum of the torques is shown to be close to the equatorial components of the atmospheric angular momentum time derivative s, as would be expected in a consistent model-based analysis system. The correlation between the two time series is shown to be very good at low frequency and decrease slowly with increasing frequency. The correlation is still significant (≥ 0.7) up to 0.5 cycle per day, but the correlation coefficient reduces to 0.5 at the diurnal frequency band, indicating the difficulty of calculating rapidly changing model-based torques within an atmospheric analysis system.

Considerations concerning the non-rigid Earth nutation theory.

This paper presents the reflections of the Working Group of which the tasks were to examine the non-rigid Earth nutation theory. To this aim, six different levels have been identified: Level 1 concerns the input model (giving profiles of the Earths density and theological properties) for the calculation of the Earths transfer function of Level 2; Level 2 concerns the integration inside the Earth in order to obtain the Earths transfer function for the nutations at different frequencies; Level 3 concerns the rigid Earth nutations; Level 4 examines the convolution (products in the frequency domain) between the Earths nutation transfer function obtained in Level 2, and the rigid Earth nutation (obtained in Level 3). This is for an Earth without ocean and atmosphere; Level 5 concerns the effects of the atmosphere and the oceans on the precession, obliquity rate, and nutations; Level 6 concerns the comparison with the VLBI observations, of the theoretical results obtained in Level 4, corrected for the effects obtained in Level 5.Each level is discussed at the state of the art of the developments.

Regional atmospheric angular momentum contributions to polar motion excitation

We focus on a regional analysis of the equatorial components of the Effective Atmospheric Angular Momentum (EAAM) functions that are responsible for excitation of polar motion. These functions are computed from the NCEP/NCAR 40-Year Reanalysis Project data both globally and in 108 geographic sectors for the period from 1968 to 1997. We investigate regional contributions of these atmospheric angular momentum functions to the short period oscillations of geodetically-determined polar motion directly and to the global EAAM excitation functions themselves. We examine two excitation terms in parallel, both excluding and including the inverted barometer (IB) formulation which adjusts the atmosphere to account for an isostatic equilibrium response from the ocean to overlying pressure; the IB formulation tends to decrease effective atmospheric variability. In the case of pressure terms without IB the largest contributions to the equatorial components of EAAM functions originate in the South Pacific, North Atlantic and North Pacific regions. However, application of the IB correction may result in the dominance of Eurasia and North America instead, with nearly all southern hemisphere contributions disappearing. Oscillations of the polar motion excitation function are mainly coherent with variations of the pressure term of the EAAM excitation functions over northern mid-latitude land areas. Distinct oscillations appear to occur in two frequency bands: 25 – 75 days and 75 – 125 days, in both prograde and retrograde directions which correspond to counterclockwise and clockwise polar motion, respectively. Coherence and cross-spectral analyses are performed to determined degree of agreement and common amplitude, respectively. We examine the atmospheric functions with respect to one important region in Eurasia and note a propagating horizontal influence within the atmosphere.

Earth rotation as a proxy for interannual variability in atmospheric circulation, 1860―present

Abstract Modern atmospheric and geodetic datasets have demonstrated that changes in the axial component of the atmospheres 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 earths 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)....

Regional Sources of Mountain Torque Variability and High-Frequency Fluctuations in Atmospheric Angular Momentum

Abstract The sources of high-frequency (⩽14 day) fluctuations in global atmospheric angular momentum (AAM) are investigated using several years of surface torque and AAM data. The midlatitude mountain torque associated with the Rockies, Himalayas, and Andes is found to be responsible for much of the high-frequency fluctuations in AAM, whereas the mountain torque in the Tropics and polar regions as well as the friction torque play a much lesser role on these timescales. A maximum in the high-frequency mountain torque variance occurs during the cool season of each hemisphere, though the Northern Hemisphere maximum substantially exceeds that of the Southern. This relationship indicates the seasonal role played by each hemisphere in the high-frequency fluctuations of global AAM. A case study reveals that surface pressure changes near the Rockies and Himalayas produced by mobile synoptic-scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high-fre...

Interannual Modes of Variability in Atmospheric Angular Momentum

Abstract The interannual variability of atmospheric angular momentum over a 26-yr period is studied regionally using monthly analyses of zonal winds derived from the global rawinsonde network. Variations in zonal-mean momentum, filtered to emphasize interannual timescales, exhibit a coherent propagating signal emanating from low latitudes, as identified in other studies using shorter records. Applying extended empirical orthogonal function (EEOF) analyses to zonally varying data, the authors isolate a dominant pair of eigenvectors whose principal component time series and spatial patterns are in quadrature with one another, indicating oscillatory behavior. The oscillation described by the two EEOFs has a period of about 36 months and is linked a posteriori to the time evolution of the El Nino-Southern Oscillation phenomenon. Beginning as an anomaly over the Tropics that extends from the Indian Ocean into the Pacific, the signal is observed to progress eastward and poleward into both hemispheres, leading t...