Angular momentum is a fundamental and conservative property of a system in rotation. In the atmosphere its distribution and fluctuations, which are based on internally changing mass and motion fields, are related to dynamic modes that vary on a range of time scales. The atmosphere also exchanges angular momentum with its surroundings, namely the oceans, hydrosphere, and solid Earth. Two principal means of doing so are by means of torques at the atmosphere's lower interface, either through normal pressure forces against topographic features and through tangential stress forces against the atmosphere's lower interface. Each of these two torques arises from fluctuations of the weather and climate system and they interact with the other geophysical elements with different amplitudes at different temporal scales. The Principal Investigators (PIs) will investigate the details of the angular momentum fluctuation and redistribution, and torque variability, with implications for the angular momentum budget, climate variability, and Earth motions.
The relevance of the angular momentum budget to describe climate variations, especially the dominant tropical El Niño/Southern Oscillation (ENSO), has been well noted in the past, though all the mechanisms linking the two have not been fully explained. Additionally, however, other modes of angular momentum variability can resemble those significant to climate. This research will expand these results by assessing statistically the modes from meteorological analysis fields. Besides the climate modes, angular momentum as an index may be related to other forcing functions, especially the increase in greenhouse gases. The PIs will investigate the relationship of such forced climate fluctuations to the angular momentum cycle.
Besides the atmosphere itself, the Earth's other geophysical fluids including the oceans and the land-based hydrosphere are part of the angular momentum budget. Ocean fluctuations are being considered in the coupled models. The atmospheric analyses and climate models have hydrological parameters as well, and the distribution of the water substance in these reservoirs can have a direct link into the total budget. The hydrological fields themselves are based on a number of models of the soil moisture, precipitation, runoff and storage, and have been summarized in a number of land hydrology models. The PIs seek to explain the role of hydrology further by assessing fields obtained from remote sensing, including the novel gravity measuring missions, like the Gravity Recovery and Climate Experiment.
The intellectual merit of this project will be in further synthesizing a fundamental property of the atmosphere and its role in its surroundings. Climate signals will be sought as part of the spatial patterns of the angular momentum determined by eigenvector analysis. The PIs will see how various climate modes and other changing climate signals may impact the global atmosphere, and impact the motions of the solid Earth. In addition, by using the angular momentum as a diagnostic tool, they will assess the usefulness of atmospheric and hydrologic modeling techniques.
The PIs have been contributing the angular momentum datasets to the broad geo and space science community, including geodesists and astronomers worldwide who are interested in the causes of Earth motions and rotation changes. This activity is organizationally part of the International Earth Rotation and Reference Frame Service. Such information is used in assessing the Earth's orientation changes, its reference frames, and its internal structure. The products and their forecasts are used by national observatories for use in timekeeping operations necessary for exact navigation.
We have researched the area of atmospheric angular momentum (AAM), based fundamentally on the wind and atmospheric mass distributions, and how the atmosphere interacts dynamically with our underlying planet, the Earth. AAM varies on many time scales. The broad merits of the work include a fuller understanding of planet Earth as a system, in this case, dynamically, and the usefulness of atmospheric information for other geophysical and geodetic purposes. Due to the overall dynamics of the Earth’s system, atmospheric changes are accompanied by small motions of the Earth, which are perceptible by space geodetic measurements. We have collected both analyses and forecasts for AAM as well as related data from several of the world’s meteorological centers, and these are used to study the small changes in the Earth’s rotation rate and in the pole position, both of which are important parameters in establishing the Earth’s reference frame and hence, positioning and navigation standards. The regional as well as global changes in AAM are documented. Furthermore, the land-based hydrosphere has changes in mass distribution due to ground and surface water; we note these from models of the land surface water as well as the analysis of the GRACE (Gravity Recovery and Climate Experiment) satellite. The project has studied a number of time scales, from the subdiurnal, to subseasonal, seasonal, interannual and multidecadal. We have analyzed the results of the Coupled Model Intercomparison Project that includes a number of well-recognized atmosphere/ocean models designed to understand the possible scenarios of climate change, and we note that when different amounts of greenhouse gases are injected into the atmosphere, changes in atmospheric angular momentum occur mostly because of the increased winds, according to a number of models and scenarios. Wind changes appear to be particularly noteworthy in the upper troposphere and lower stratosphere, and though the depth of the atmosphere in the southern hemisphere. They increase more with increasing greenhouse gases, thereby increasing the AAM with an expected putative response of slightly altering Earth’s rotation rate. We have examined and streamlined important details of the angular momentum computations. In fact, the standard use of coordinate systems, and the application of the mass at the Earth's surface can be improved by considerations of placing the mass through the depth of the atmosphere, with its inherent increasing radius from the planet. Such a computation is rather cumbersome, so we have investigated the calculations of placing all atmospheric mass in a column at the center of mass in that column. The center of mass approach works quite well for such computations. In examining other phenomena, on shorter time scales, we noted the balances between the wind (motion) terms and the pressure (mass) terms in surface pressure that had not been observed in the angular momentum functions. These relate in a partial cancellation of the effects. Furthermore, the torques, which are the dynamic means by which the changes of angular momentum between certain parts of the system are effected have been considered. Such torques are based on the stress of winds along the Earth surface and the different air pressures hitting the solid Earth on the opposite sides of the topographic features like large mountain ranges. We have compared angular momentum from the three mass changes in the geophysical fluids: atmosphere, land-based hydrology, and oceans. The atmospheric angular momentum has strong variations particularly in the mountainous areas of central Asia, north America and elsewhere; oceanic variations are particularly strong in certain areas including the southern Indian Ocean, and the land-based hydrological functions are strongest in the tropics. We maintain an active data center, the Special Bureau for the Atmosphere of the International Earth Rotation and Reference Systems Service, from which scientists in the United States as well as globally obtain information for atmospheric, geophysical and geodetic purposes.
