Effects on gravity from non-tidal sea level variations in the Baltic Sea

Artikel i vetenskaplig tidskrift, 2009

The main purpose of this paper is to investigate numerically the effects of non-tidal sea level variations in the Baltic Sea on gravity with special emphasis on the Swedish stations in the Nordic Absolute Gravity Project. To calculate the ocean loading effect on gravity the method described by Farrell (1972) is widely used. This method is based on convolution of a Green's function for gravity with the ocean load, but does not include the direct attraction depending on the height of the observation point. It is described how this effect is included in the Green's functions and how numerical integration is performed over a dense grid bounded by a very high resolution coastline. The importance of this high resolution is shown. The major part of the direct attraction for stations close to the coast comes from relatively small water masses close to the station. The total effect from the Baltic Sea, crustal loading and direct attraction, is calculated for 12 Swedish and one Finnish absolute gravity stations. The distance from the coast for these stations varies from 10 m to 150 km. It is shown that the total non-tidal gravity effect is significant, easily reaching values of 2–3 μgal for stations with high elevation close to the coast. In modelling the Glacial Isostatic Adjustment (GIA), the relation between the change of gravity and the absolute land uplift ((dg/dt) / (dh/dt)) contains information about the viscoelastic properties of the upper mantle. The Baltic Sea is located in the Fennoscandian postglacial land uplift area and experiences therefore a long-term sea level decrease. It is also shown that the magnitude of this long-term effect is not negligible for determination of the unknown part of (dg/dt) / (dh/dt).