EGM2008: The Earth Gravitational Model 2008 is described in Pavlis et al. (2008) and distributed by the International Centre for Global Gravity Field Models (http://icgem.gfz-potsdam.de/ICGEM/shms/egm2008.gfc). It is available up to spherical harmonic degree and order 2159, and contains additional coefficients extending to degree 2190 and order 2159. For consistency with the EGM96 and GRACE05, the geoid heights have been calculated using up to the spherical harmonic degree and order 360, with a spatial resolution of 0.5°. Like EGM96, EGM2008 is the outcome of fusing different data from several sources, including altimetry data.

GOCE’s level 2 time-wise product: The time-wise product is obtained using only direct measurements of the gravity field, without any altimetry contribution. Details about the GOCE mission can be found on the ESA web page (http://earth.esa.int/GOCE) and in the ESA (1999) report; data may be accessed on the ESA web page. Three different products are available, namely the direct solution, the space-wise solution and the time-wise solution; the last of these was used in this study as it does not rely on auxiliary information (EGG-C 2012). We used the third release, the last one available at the time of the study, based on 16 months of data collected over 18 months (from November 2009 to April 2011). The spatial resolution of the GOCE level 2 products is 0.5°, with degree and order 250.