Predictions of crustal deformation and of geoid and sea-level variability caused by oceanic and atmospheric loading

Abstract

SUMMARY In this paper we consider several small effects related to oceanic loading of the Earth and atmospheric loading of the oceans. Non-tidal ocean loading induces changes in oceanic bottom pressure, which in turn deform the geoid and the nearby crust. Changes in bottom pressure are derived from estimates of sea-surface height variability and density changes output from an oceanic general circulation model. Sea-surface heights measured by the TOPEX/POSEIDON altimeter are also used to infer changes in bottom pressure. We estimate that non-tidal ocean loading can typically cause % 5 mm of peak-to-peak (2 mm root-mean-square, rms) vertical motion at sites near the shore, with displacements of up to 10 mm possible. Amplitudes of horizontal displacements are about one-third those of vertical ones. Deformation-associated gravity changes are usually of the order of 2-3 pGal; however, peak-to-peak changes of 5 pGal are also predicted. Loading-induced geoid perturbations in mid-ocean regions are typically 5-10 mm peak-to-peak (3 mm rms) and can extend over a region of 20. In the case of atmospheric loading of the oceans, we estimate the difference between the inverted barometer correction, usually used to describe the ocean's response to atmospheric pressure, and the complete equilibrium response. There are two sources of this difference. One is that the inverted barometer correction does not conserve oceanic mass. We use National Meteorological Center (NMC) global pressure data to estimate the time-dependent, average change in pressure over the oceans. A time-series of the net pressure over the oceans contains an annual signal with a peak-to-peak amplitude of 1.2 mbar superimposed on a linear trend of 0.09 mbar yr-'. The secular signal is caused by a redistribution of atmospheric mass from over continental land masses to the oceans. We conclude that this effect introduces additional variability into the ocean's response to pressure at annual, secular, and multi-year timescales, with a total peak-to-peak amplitude of a few tens of millimetres. The second difference between the inverted barometer response and the complete equilibrium response is due to gravitational forcing by the atmosphere and by the displaced mass in the solid Earth and oceans caused by the atmospheric loading. This effect is largest within about 1000 km of the coast and at high latitudes. We use NMC pressure data to estimate the effect, and find that it causes differences from the inverted barometer solution in those regions of up to 10-30mm peak-to-peak, with values as large as 50-60 mm possible at certain locations. The time dependence of this effect is negatively correlated with the inverted barometer response. The effect causes an apparent reduction of the inverted barometer response by a few per cent within a few hundred kilometres of the coast, with reductions of more than 6-7 per cent at a few places.