Rotation of the atmospheres of the Earth and planets

The most striking features of the general circulation of the Earth’s atmosphere are its average ‘super-rotation’ relative to the solid Earth at about 10 m s -1 , and the concentration of much of the motion in jet streams with speeds of about 30 m s -1 . Changes in the pattern of winds, and variations in the distribution of mass within the atmosphere produce fluctuations in the angular momentum associated with this ‘super-rotation’, namely, the axial component H 3 of the total angular momentum of the atmosphere H i ,i = 1,2,3, and also in the equatorial components H 1 and H 2 . Fluctuations in H 3 during the Special Observing Periods of the recent ‘First GARP Global Experiment’ (FGGE, where GARP is the Global Atmospheric Research Programme) later have been investigated. They are well correlated with short-term changes of up to about 10 -3 s in magnitude in the length of the day (allowing for lunar and solar tidal effects on the moment of inertia of the solid Earth), exhibiting pronounced contributions on timescales of about seven weeks and one year and they are consistent with the sum of H 3 and the axial component of the angular momentum of the Earth’s crust and mantle being conserved on short timescales, without requiring significant angular momentum transfer between the Earth’s liquid core and overlying solid mantle on such timescales. Fluctuations in H 1 and H 2 on timescales much less than the Chandlerian period (14 months) but rather more than a few days make a major contribution to the observed wobble of the instantaneous pole of the Earth’s rotation with respect to the Earth’s crust, which has a variable amplitude of several metres. A theoretical basis has now been established for future routine determinations of fluctuations in H i for the purposes of meteorological and geophysical research, including the assessment of the extent to which movements within the solid Earth associated with major earthquakes (magnitude much greater than 7.9) and motions in the liquid metallic core might occasionally contribute to the. excitation of the Chandlerian wobble. Venus’s atmosphere ‘super-rotates’ at an average speed of about ten times that of the underlying planet, which, owing to angular momentum exchange with the atmosphere, should (like the solid Earth) undergo detectable changes in rotation period. Each of the giant planets Jupiter and Saturn exhibits a sharply-bounded equatorial atmospheric jet stream moving at 100 m s -1 (Jupiter) or 400 m s -1 (Saturn) in a positive (i.e. eastward) direction relative to higher latitude parts of the atmosphere, the average rotation of which is close to that of the hydrogen-metallic fluid interior (as determined from radio-astronomical observations). The theoretical interpretation of these observations presents challenging problems in the dynamics of rotating fluids.

Download Full-text

Atmospheric angular momentum fluctuations, length-of-day changes and polar motion

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1983.0050 ◽

1983 ◽

Vol 387 (1792) ◽

pp. 31-73 ◽

Cited By ~ 354

Keyword(s):

Angular Momentum ◽

Solid Earth ◽

Polar Motion ◽

Meteorological Data ◽

Liquid Core ◽

Atmospheric Angular Momentum ◽

Axial Component ◽

Length Of Day ◽

Momentum Exchange ◽

Geophysical Research

Variations in the distribution of mass within the atmosphere and changes in the pattern of winds, particularly the strength and location of the major mid-latitude jet-streams, produce fluctuations in all three components of the angular momentum of the atmosphere on timescales upwards of a few days. In a previous study (Hide et al . 1980) it has been shown that variations in the axial component of atmospheric angular momentum during the Special Observing Periods in 1979 of the First GARP Global Experiment (FGGE, where GARP is the Global Atmospheric Research Program) are well correlated with changes in length-of-day. This would be expected if the total angular momentum of the atmosphere and ‘solid’ Earth were conserved on short timescales (allowing for lunar and solar effects) but not if angular momentum transfer between the Earth’s liquid core and solid mantle, which is accepted to be substantial and even dominant on timescales upwards of several years, were significant on timescales of weeks or months. Fluctuations in the equatorial components of atmospheric angular momentum should contribute to the observed wobble of the instantaneous pole of the Earth’s rotation with respect to the Earth’s crust, but this has not been shown conclusively by previous studies. In this paper we re-examine some aspects of the underlying theory of non-rigid body rotational dynamics and angular momentum exchange between the atmosphere and solid Earth. Since only viscous or topographic coupling between the atmosphere and solid Earth can transfer angular momentum, no atmospheric flow that everywhere satisfied inviscid equations (including, but not solely, geostrophic flow) could affect the rotation of a spherical solid Earth. Currently available meteorological data are not adequate for evaluating the usual wobble excitation functions accurately, but we show that partial integration leads to an expression involving simpler functions ─ here called ‘equatorial angular momentum functions’ ─ which can be reliably evaluated from available meteorological data. The length-of-day problem is treated in terms of a similar ‘axial angular momentum function’ ; and ‘effective angular momentum functions’ are defined in order to allow for rotational and surface loading deformation of the Earth. Daily values of these atmospheric angular momentum functions have been calculated from the ‘initialized analysis global database’ of the European Centre for Medium-Range Weather Forecasts (ECMWF). They are presented for the period 1 January 1981─30 April 1982, along with the corresponding astronomically observed changes in length-of-day and polar motion, published by the Bureau International de l’Heure (BIH). Changes in length-of-day during this period can be accounted for almost entirely by angular momentum exchange between the atmosphere and solid Earth, and the existence of a persistent fluctuation in this exchange, with a timescale of about 7 weeks, is confirmed. We also demonstrate that meteorological phenomena provide an important contribution to the excitation of polar motion. Our work offers a theoretical basis for future routine determinations of atmospheric angular momentum fluctuations for the purposes of meteorological and geophysical research, including the assessment of the extent to which movements in the solid Earth associated with very large earthquakes contribute to the excitation of the Chandlerian wobble.

Download Full-text

Towards a theory of irregular variations in the length of the day and core-mantle coupling

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1977.0030 ◽

1977 ◽

Vol 284 (1326) ◽

pp. 547-554 ◽

Cited By ~ 62

Keyword(s):

Angular Momentum ◽

Liquid Core ◽

Theoretical Research ◽

Artificial Satellites ◽

Momentum Exchange ◽

Planetary Scale ◽

The Core ◽

Long Baseline ◽

The Earth ◽

Length Of The Day

Determinations of fluctuations in the length of the day reveal changes due to the transfer of angular momentum between the Earth’s ‘solid’ mantle and the overlying atmosphere on time scales upwards of a few weeks, as well as the slower but more pronounced ‘decade variations’ due largely (according to current ideas) to angular momentum transfer between the mantle and the Earth’s liquid core. Improvements in techniques for monitoring the Earth’s rotation, such as those afforded by recent advances in methods of ranging to artificial satellites and the Moon and of very long baseline interferometry, should therefore lead to results of interest to meteorologists concerned with planetary-scale motions in the atmosphere and to geophysicists concerned with the magnetohydrodynamics of the core and the origin of the main geomagnetic field. The consideration of the stresses at the Earth’s surface and at the coremantle interface that bring about angular momentum exchange between the solid and fluid parts of the Earth raises a number of basic hydrodynamical questions requiring further experimental and theoretical research. In the case of the core, quantitative difficulties encountered by the suggestion that the stresses are electromagnetic in origin led to the idea of topographic coupling associated with hypothetical undulations of the core-mantle interface.

Download Full-text

Forecasting short-term changes in the Earth's rotation

Symposium - International Astronomical Union ◽

10.1017/s007418090011962x ◽

1988 ◽

Vol 128 ◽

pp. 287-288 ◽

Cited By ~ 1

Author(s):

Raymond Hide

Keyword(s):

Angular Momentum ◽

Solid Earth ◽

Weather Forecasting ◽

Meteorological Data ◽

Earth’S Rotation ◽

Momentum Exchange ◽

Short Term ◽

Earth's Rotation ◽

Tidal Changes ◽

Geophysical Processes

Summary of PosterIt has long been appreciated that atmospheric motions must contribute to the excitation of fluctuations in the Earth's rotation (Munk and MacDonald 1960, Lambeck 1980, Rochester 1984) but the exploitation of modern meteorological data, collected largely to meet the demands of daily global weather forecasting, in the routine evaluation of angular momentum exchange between the atmosphere and the solid Earth was not initiated until comparatively recently (Hide et al. 1980). This procedure constitutes a necessary step towards the accurate separation of these features of the observed non-tidal changes in the length of day and polar motion and that are of meteorological origin from those that must be attributed to other geophysical processes, such as angular momentum transfer between the solid Earth and other fluid regions of the Earth (liquid metallic core, oceans, etc.), and to changes in the inertia tensor of the solid Earth associated with earthquakes, melting of ice, etc.

Download Full-text