Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Hide, R.; Boggs, D. H.; Dickey, J. O.; Dong, Danan; Gross, R. S.; Jackson, Andrew

doi:10.1111/j.1365-246x.1996.tb00022.x

Cited by 32 publications

(19 citation statements)

References 41 publications

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“…Electromagnetic core± mantle coupling is too small because the electrical conductivity of the lower mantle is too low (e.g. Recent investigations show that the topographic coupling is probably too small by a factor 10 (Greff- Lefftz & Legros 1995) or 5 (Hide et al 1996) to explain the decadal variations of polar motion. Holme 1998a,b).…”

Section: Introductionmentioning

confidence: 99%

Relative wobble of the Earth's inner core derived from polar motion and associated gravity variations

Greiner-Mai

Barthelmes

2001

Geophysical Journal International

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SUMMARY In this paper an explanation of the observed decadal variations of the polar motion of the Earth is presented. Recent investigations show that the contribution of surface processes is too small to excite the observed magnitudes of the decadal variations of polar motion. After removing the known effects of atmospheric variations from the observed polar motion, we obtain significant residuals that obviously can only be explained by processes in the core. In this paper, we investigate the effect of a relative inner‐core rotation. In particular, we assume that the orientation of the figure axis of the inner core changes with respect to the outer core and mantle. Due to the flattening of the inner core and the density difference between the inner and outer core, these changes contribute to variations of the Earth's polar motion due to internal mass redistributions. The objective of this study is to determine those changes in the orientation of the figure axis of the inner core that produce mass redistributions necessary to excite the decadal variations of the observed polar motion minus the estimated atmospheric influence. Using polar motion data and the atmospheric excitation function, it is possible to determine the excitation function of the internal process superimposed on the free wobble of the Earth by linear approximation of the Liouville equation. On the other hand, provided that only the contribution of the mass redistributions is significant, we can express the time function obtained in terms of the orientation of the figure axis of the inner core. The final expression then contains the corresponding orientation angles as unknowns. Using this expression, we calculate the orientation angles from the numerically determined values of the excitation function. The calculation results in a mean tilt of 1°, and a mean eastward drift of 0.7° yr−1 of the figure axis of the inner core and quasi‐periodic decadal variations of its orientation angles. The associated changes of the mass geometry in the core due to these variations of the figure axis of the inner core are then used to estimate their influence on the Earth's outer gravity field. Finally, we compare the magnitude of the resulting gravity field variations with the accuracy of recent and future gravity field models.

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Section: Introductionmentioning

confidence: 99%

Relative wobble of the Earth's inner core derived from polar motion and associated gravity variations

Greiner-Mai

Barthelmes

2001

Geophysical Journal International

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“…Topographic core-mantle coupling has been considered a possibility, but this means of excitation is unable to reproduce the observed polarization or period, and its magnitude is too small to drive the wobble by a factor of 5 [Hide et al, 1996].…”

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confidence: 99%

Climate‐driven polar motion

Celaya¹,

Wahr²,

Bryan³

1999

J. Geophys. Res.

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Abstract. The output of a coupled climate system model provides a synthetic climate record with temporal and spatial coverage not attainable with observational data, allowing evaluation of climatic excitation of polar motion on timescales of months to decades. Analysis of the geodetically inferred Chandler excitation power shows that it has fluctuated by up to 90% since 1900 and that it has characteristics representative of a stationary Gaussian process. Our model-predicted climate excitation of the Chandler wobble also exhibits variable power comparable to the observed. Ocean currents and bottom pressure shifts acting together can alone drive the 14-month wobble. The same is true of the excitation generated by the combined effects of barometric pressure and winds. The oceanic and atmospheric contributions are this large because of a relatively high degree of constructive interference between seafloor pressure and currents and between atmospheric pressure and winds. In contrast, excitation by the redistribution of water on land appears largely insignificant. Not surprisingly, the full climate effect is even more capable of driving the wobble than the effects of the oceans or atmosphere alone are. Our match to the observed annual excitation is also improved, by about 17%, over previous estimates made with historical climate data. Efforts to explain the 30-year Markowitz wobble meet with less success. Even so, at periods ranging from months to decades, excitation generated by a model of a coupled climate system makes a close approximation to the amphtude of what is geodetically observed.

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“…Interactions between the fluid outer core and mantle also appear to be ineffective in exciting decadal polar motion variations. Electromagnetic coupling between the core and mantle appears to be 2–3 orders of magnitude too weak [ Greff‐Lefftz and Legros , 1995] and topographic coupling appears to be too weak by a factor of three to ten [ Greff‐Lefftz and Legros , 1995; Hide et al , 1996; Hulot et al , 1996]. In addition, the modeled decadal polar motion variations resulting from these studies show little agreement in phase with the observed variations.…”

Section: Discussion and Summarymentioning

confidence: 99%

Atmospheric and oceanic excitation of decadal‐scale Earth orientation variations

2005

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The contribution of atmospheric wind and surface pressure and oceanic current and bottom pressure variations during 1949–2002 to exciting changes in the Earth's orientation on decadal timescales is investigated using an atmospheric angular momentum series computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project and an oceanic angular momentum series computed from a near‐global ocean model that was forced by surface fluxes from the NCEP/NCAR reanalysis project. Not surprisingly, since decadal‐scale variations in the length of day are caused mainly by interactions between the mantle and core, the effect of the atmosphere and oceans is found to be only about 14% of that observed. More surprisingly, it is found that the effect of atmospheric and oceanic processes on decadal‐scale changes in polar motion is also only about 20% (x component) and 38% (y component) of that observed. Therefore redistribution of mass within the atmosphere and oceans does not appear to be the main cause of the Markowitz wobble. It is also found that on timescales between 10 days and 4 years the atmospheric and oceanic angular momentum series used here have very little skill in explaining Earth orientation variations before the mid to late 1970s. This is attributed to errors in both the Earth orientation observations prior to 1976 when measurements from the accurate space‐geodetic techniques became available and to errors in the modeled atmospheric fields prior to 1979 when the satellite era of global weather observing systems began.

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Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Cited by 32 publications

References 41 publications

Relative wobble of the Earth's inner core derived from polar motion and associated gravity variations

Relative wobble of the Earth's inner core derived from polar motion and associated gravity variations

Climate‐driven polar motion

Atmospheric and oceanic excitation of decadal‐scale Earth orientation variations

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