Anthony MEMIN

Geophysics and Geodesy

250 rue Albert Einstein
Les Lucioles 1, Sophia Antipolis
06560 Valbonne, France
+33 (0)4 83 61 85 70
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New perspective on geocentre motions estimate from ground gravity observations

The geocentre motion is the motion of the centre of mass of the entire Earth, considered an isolated system, in a terrestrial system of reference. We first derive a formula relating the harmonic degree-1 Lagrangian variation of the gravity at a station to both the harmonic degree-1 vertical displacement of the station and the displacement of the whole Earth's centre of mass. The relationship is independent of the nature of the Earth deformation and is valid for any source of deformation. Next, we consider the geocentre motion caused by surface loading. In a system of reference whose origin is the centre of mass of the solid Earth, we obtain a specific relationship between the gravity variation at the surface and the geocentre displacement which demands the Earth's structure and rheological behaviour be known. We then select 6 well distributed stations of the Global Geodynamics Project, which is a world network of superconducting gravimeters, to invert actual gravity data for the degree-1 variations and determine the geocentre displacement between the end of 2004 and the beginning of 2012, assuming it to be due to surface loading. We find annual and semi-annual displacements with amplitude 0.5 to 2.3 mm.

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Geodetic velocity errors induced by surface loading

Geodetic vertical velocities derived from data as short as 3 years are often assumed to be representative of linear deformation over past decades to millennia. We use two decades of surface loading deformation predictions due to variations of atmospheric, oceanic and continental water mass to assess the effect on secular velocities estimated from short time series. The interannual deformation is time-correlated at most locations over the globe, with the level of correlation depending mostly on the chosen continental water model. Using 5-year-long time series, we found median vertical velocity errors of 0.5 mm/yr over the continents (0.3 mm/yr globally), exceeding 1 mm/yr in regions around the southern Tropic. Horizontal velocity errors were 7 times smaller. Unless an accurate loading model is available, a decade of continuous data is required in these regions to mitigate the impact of the interannual loading deformation on secular velocities.

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Map of vertical velocity errors (mm/yr) for 5 years of continuous data.

Possible detection of the Antarctic Circumpolar Wave in satellite gravimetry and altimetry over Antarctica

Assessment of the long term mass balance of the Antarctic Ice Sheet, and thus the determination of its contribution to sea level rise, requires an understanding of interannual variability and associated causal mechanisms. We jointly analyze surface-mass and elevation changes from satellite gravimetry (GRACE) and altimetry (Envisat) data. We find a 4.7-yr quasi-periodic wave propagating eastward over Antarctica between 08/2002 and 10/2010. This wave induces snow changes reaching maximum amplitude along the Antarctic coast. It contributes up to 35% to the annual rate of snow accumulation in Antarctica. Extending the analysis to 09/2014 using data from the GRACE mission, we have found anomalies with a periodicity of about 4–6 yrs that circle the AIS in about 9–10 yrs. These properties connect the observed anomalies to the Antarctic Circumpolar Wave (ACW) which is known to affect several key climate variables, including precipitation. It suggests that variability in the surface-mass balance of the Antarctic Ice Sheet may also be modulated by the ACW.

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Time longitude diagram of surface-mass changes obtained from GRACE solutions (a). Map of amplitude of a 4.7-yr periodic signal that best fits the surface-mass time series (b). The slope of the arrow is 9.4 yrs/360 degrees.

Snow and ice-mass change in Antarctica

Determining the variations in snow and ice mass in Antarctica is fundamental to understand the response of the ice sheet to climate changes. We have recently combined satellite gravimetry and altimetry data from GRACE and Envisat missions to examine snow and ice-mass changes at the surface of the Antarctic Ice Sheet. Our estimates of snow accumulation rates agree well with predicted surface-mass balance rates obtained from a regional climate model opening new insight to understand Antarctic processes.

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Regional rates of change of snow (left) and ice (right) height in Antarctica estimated using GRACE solutions and Envisat data.

Non-linear motions of Australian geodetic stations induced by non-tidal ocean loading and the passage of tropical cyclones

Taking into account the deformation of the Earth due to a range of well-known geophysical processes is required to accurately define our position on the Earth' surface. Models describing the dominant tidal signals (from phenomena such as solid Earth tides, pole tide and ocean tide loading) are routinely employed following the International Earth Rotation and Reference Systems Service recommendations. However to achieve greater accuracy at the millimetre level, non-tidal deformations must also be considered. ​

We investigated these small but significant signals around Australia and found that deformation due to non-tidal ocean loading can be large along the northern Australian coast and the Spencer Gulf. Indeed, the storm surge caused by the tropical cyclone Yasi between January 29 and February 3, 2011​, may have induced a surface motion of up to 3 centimetres in Cardwell over just a few hours. High-resolution dynamic ocean models are required to take into account such sub-daily deformations and make small improvements to the variability of GPS time series.

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Predicted changes in the vertical coordinates of the TOW2 GPS station induced by non-tidal oceanic and atmospheric loading (D-NTOAL) during the passage of tropical cyclone Yasi using the global ocean model TUGO-m (green) and a regional model (magenta).

Changes in Arctic deformation and gravity opens up the Earth and its history

The Arctic coastline and the fjords of the Svalbard archipelago were ice free when the last ice age ended there 10,000 years ago. Since then, the glaciers have fluctuated to some extent, with widespread glacier thinning occurring over recent decades. This contrasts to the significant, but poorly observed, thickening of the glaciers that occurred between about 1700 and 1930, during the “Little Ice Age”, and then followed by glacier thinning. In a paper published in Geophysical Journal International, we seek to understand more about the Little Ice Age thickening and also the mechanical behaviour of Earth by taking advantage of the fact that Earth’s shape and gravity field change as surface masses (e.g., water, ice) are redistributed.

We have shown that the Earth’s response to these past and recent ice-mass redistributions explains the vertical ground velocity and gravity variation rate observed at the Ny-Alesund geodetic observatory, Svalbard, provided that the Earth's asthenosphere viscously deforms with viscosities ranging between 1.0 and 5.5 × 10^18 Pa s. We have also shown that in that case, the thickness of the lithosphere has to range between 50 and 100 km with the asthenosphere’s thickness varying between 120 and 170 km. The existence of a low-viscosity asthenosphere differs from the traditional mantle structure made of two layers, with viscosities of the order of 10^21-10^22 Pa s, and revealed by observations of the last major deglaciation. These results emphasise that the transient mechanical behaviour of the Earth remains poorly understood, but Earth’s response to quite recent ice loading changes offers an exciting new opportunity to do this.

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Absolute (AG) and relative (SG) gravity measurements at Ny-Alesund.