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The Geoid and Earth Eotation
Published in Petr Vaníček, Nikolaos T. Christou, GEOID and Its GEOPHYSICAL INTERPRETATIONS, 2020
On longer temporal scales, the most important phenomenon is probably the post-glacial rebound,29 the uplift of the mantle following the rapid melting of the ice sheets that covered much of the North America and Northern Europe during the last ice age ending some 8000 years ago. The rebound, which occurs strongest in high latitudes, acts to reduce J2. It is also believed to cause a secular drift in the polar motion as the response of the Earth’s rotation axis to the mass void left behind by the melting.30 This polar drift should continue until the post-glacial rebound of the mantle material gradually “refills” the void. The estimates for the post-glacial rebound given in Table 1 are consistent with reasonable Theological models for the mantle.31,32
Trends of sea level in the Bay of Bengal using altimetry and other complementary techniques
Published in Journal of Spatial Science, 2018
S. Ghosh, S. Hazra, S. Nandy, P. P. Mondal, T. Watham, S. P. S. Kushwaha
Changes in gravity over time can reveal important details about polar ice sheets, sea level, ocean currents, Earth’s water cycle and the interior structure of the Earth. The GRACE mission has been measuring temporal gravity fields that reveal the mass variations both on and in the Earth (Tapley et al. 2004, Feng and Zhong 2015). The raw GRACE-based ocean mass time series is dominated by an annual cycle caused by the annual exchange of water between land and oceans (Cazenave et al.2009). In this study, monthly interval 1° × 1° gridded data of the GRACE-based equivalent water height rendered to 500-km Gaussian smoothing were used for the period 2004 to 2010. Swenson and Wahr (2006) found that if the Gaussian smoothing is combined with filtering in the spectral domain that effectively removes the stripe pattern, an averaging radius as small as 500 km may be sufficient. Therefore, in this study, we adopted the approach of Swenson and Wahr (2006) as followed by Munekane (2007), Chen et al. (2006), Belda et al. (2015) and many others. This has become the standard approach because it is conceptually straightforward, simple to implement, and at a suitable averaging radius produces normally distributed errors (Wahr et al. 2006, Davis et al. 2008). For consistency with the altimeter data, a 60-day running mean smoother was applied. This time series represents the changes in MSL related to the exchange of fresh water between the land and the oceans. Errors in the estimated monthly mass component of the global mean sea level are 2 mm for each month (Willis et al. 2008). A correction for glacial isostatic adjustment (GIA) was also applied to justify the displacements of the Earth’s crust of the last ice age. In situ global navigation satellite system (GNSS) measurements for determining the vertical land movements in the Bay of Bengal region are often not available. So, GIA corrections of vertical land movement are recommended for consistency of the sea level rise trends for the north Indian Ocean with global values (Unnikrishnan and Shankar 2007). The ICE-5G VM4 model was used for GIA corrections (Peltier 2004). Vertical land movements are associated mainly with two processes, local tectonic activity and GIA, the latter caused by post-glacial rebound of land. Since GPS estimates were not available, vertical land movement due to local tectonic activities cannot be accounted as GPS measurements were not available. Hence, we only applied GIA correction. The sea level signal from the glacial cycle exhibits significant spatial variability from its globally averaged value because of the combined deformation and gravitational response of the Earth and ocean to the changing ice-water load. During ice-sheet decay, the crust rebounds beneath the ice sheets and subsides beneath the melt-water-loaded ocean basins; the gravitational potential and ocean surface are modified by the deformation and changing surface load, and the planet’s inertia tensor and rotation changes, further modifying equipotential surfaces. Together, this response of the land-ocean system to glacial cycles is referred to as the GIA (Lambeck et al. 2014).