Effects of self-attraction and loading on annual variations of ocean bottom pressure.

Abstract

The impact of self-attraction and loading (SAL) on ocean bottom pressure ξ, an effect not previously considered, is analyzed in terms of the mean annual cycle based on decade-long estimates of changes in land hydrology, atmospheric pressure, and oceanic circulation. The SAL-related changes in ξ occur as a result of deformation of the crust due to loading and self-gravitation of the variable fluid loads. In the absence of SAL, net freshwater input and changes in mean atmospheric pressure over the ocean give rise to a spatially constant ξ annual cycle with an amplitude ∼1–2 cm in equivalent water thickness. Consideration of SAL physics introduces spatial variations that can be significant, particularly around continental boundaries, where the amplitude of deviations can exceed 1 cm. For the spatial variability induced by SAL effects, changes in both land hydrology and atmospheric pressure are important. Effects related to the changing ocean circulation are relatively weaker, apart from a few shallow coastal regions. Comparisons with a few in situ, deep ocean observations indicate that for the most accurate ξ estimates, one needs to consider spatially varying SAL-related signals, along with the effects of mean atmospheric pressure and net freshwater input into the oceans. Nevertheless, the most complete estimates, including also effects of ocean circulation, are able to account for only ∼1/3 of the observed annual variances. Sources of the remaining contribution remain unclear.