Tidal ocean loading

Abstract

This page provides a summary of activities in modeling ocean loading in Astrogeo Center

Introduction

The Earth as a whole responses to external forces as an elastic body. Tides in the ocean redistribute water mass with respect to land. Therefore, the weight of the column of water above the given point periodically changes. These changes may reach 40 mm vertical and 5 mm horizontal. Tidal ocean loading should be taken into account in reduction of astronomical and space geodesy observations when precision better than 5 nrad or 4 cm is required.

Results

Ocean tide can be characterized be a sum of harmonic constituents. For computing site displacements caused by tidal ocean loading a global ocean tidal maps for major harmonic constituents should be used. A tidal map represents the amplitude of the sea level due to harmonic constituent and the phase with respect of the astronomical tide. The global map is convolved with Green's functions which describes Earth's crust response on a loading of the point mass. I used software SPOTL developed by D. Agnew for computation of the displacements due to ocean loading. The following ocean tidal maps were used:

Major diurnal and semidiurnal tides: K₂ S₂ M₂ N₂ K₁ P₁ O₁ Q₁ -- GOT00.2 model (R. Ray).

Zonal tides: Mtm, Mf, Msf, Mm, Msm, Ssa, Sa -- NAO99.b model (K. Matsumoto)

The Chandler and annual components of polar tide, the tide with period of 18.6 years -- equilibrium model.

Minor tides with frequencies closed to the frequencies of major tides, so-called companion tides: K₂-a S₂-a M₂-a K₁-a K₁-b P₁-a O₁-a Q₁-a Mtm-a Mf-a -- the ocean tide map obtained from the ocean tide map by a multiplying by a complex scaling admittance factor. It is assumed that the ratio of the ocean tide amplitudes for the companion tide to the the major tide is the same as the ratio of the amplitudes of the tide-generating potential.

In total, ocean loading for 26 frequencies is modeled.

Validation of the model

The model was extensively tested against the whole set of VLBI observations. More details can be found here.

Files for downloading

3-D displacements of VLBI stations caused by ocean loading.