NASA Goddard Space Flight Center Geodesy and Geophysics Laboratory

International Mass Loading Service: Non-tidal Ocean Loading

Introduction

The Earth as a whole responses to external forces as an elastic body. As it was shown by Darwin in 1882, changes of the weight of the column of atmosphere due to variations of pressure result in crustal deformations called atmospheric pressure loading. These variations on average have the rms of 2.6 mm for the vertical component and 0.6 mm for the horizontal component, but peak to peak variations can reach 40 mm for the vertical component and 7 mm for the horizontal one. Pressure loading should be taken into account in a reduction of astronomical and space geodesy observations when the accuracy better than 2 nrad or 1 cm is required.

Variations of the atmospheric pressure changes the sea level. On large time-scales, say more than several months, the change of sea level compensates change of the atmospheric pressure so the bottom pressure remains the same. The ocean responses as an inverted barometer (IB): the higher pressure, the lower sea level. Change of the atmospheric pressure causes ocean water mass redistribution. The ocean obeys the IB model at slow motion, that corresponds to pressure changes at scales a month and longer. At smaller time scales the ocean response strongly deviates from IB model, and the full complexity of ocean dynamics (DR) should be considered. Modeling dynamic ocean (DO) response is feasible with numerical oceanic model that accounts for atmospheric forcing among other factors. Crust deformation caused by the ocean bottom pressure changes is called non-tidal ocean loading. It accounts for atmospheric pressure changes over the ocean and the DO response to that, as well as ocean repines to other forcings, excluding lunar and solar tides. We reserve the use of notion of the atmospheric pressure loading to crust deformation caused by atmospheric pressure changes over the land.

Service for the non-tidal ocean loading

On 2014.04.04 the service of non-tidal atmospheric pressure loading was established by Leonid Petrov and Jean-Paul Boy. The service is based on the spherical harmonics transform of the bottom pressure computed with the OMCT05 model (Dobslaw and Thomas, 2007) developed and maintained by the GFZ. The service provides

Precomputed time series of 3D displacements at a regular 2′ × 2′ global grid. They are available for the time range from 19800101_0000 through 20171130_1800 with time step of 6 hours last modified on [an error occurred while processing this directive] UTC.
3D gridded displacements to download are in NetCDF format. File size: 4–5 Mb per epoch, 4.9Gb/year.
This series has harmonic variations at diurnal and semi-diurnal frequencies removed.

Auxiliary 3D displacements of the center mass of the total Earth with respect to the center mass of the solid Earth without atmosphere caused by atmospheric pressure loading. The sum of this displacement and the displacement wrt of the total mass, is the displacement with respect to the center of solid Earth, i.e. center of figure.
3D gridded displacements to download are in NetCDF format. File size: 4–5 Mb per epoch, 4.8Gb/year.

Precomputed time series of 3D displacements caused by non-tidal ocean loading for 1272 VLBI, GPS, SLR, and DORIS stations. They are available for the time range from 19800101_0000 through 20171130_1800 with time step of 6 hours; last modified on 20171209_1540 UTC. This series has harmonic variations at diurnal and semi-diurnal frequencies removed.
Displacement time series for download are in EPHEDISP format. File size: 227 Kb per epoch, 0.33Gb/year.

Auxiliary 3D displacements of the center mass of the total Earth with respect to the center mass of the solid Earth without atmosphere caused by atmospheric pressure loading. The sum of this displacement and the displacement wrt of the total mass, is the displacement with respect to the center of solid Earth, i.e. center of figure.
3D gridded displacements to download are in EPHEDISP format. File size: 227 Kb per epoch, 0.33Gb/year.

Precomputed coefficients of harmonic variations of 3D displacements 1272 VLBI, GPS, SLR, and DORIS stations at S1 (diurnal) frequency.
The coefficients of S1 harmonic variations to download are in HARPOS format.

Precomputed coefficients of harmonic variations of 3D displacements caused by non-tidal loading for 1272 VLBI, GPS, SLR, and DORIS stations at diurnal and semi-diurnal frequencies. NB:Tidal ocean loading accounts for dynamic oceanic response. It is not recommended to use these variations and ocean loading together, since the same effect will be counted twice.
The coefficients of diurnal and semi-diurnal harmonic variations to download are in HARPOS format.

Auxiliary 3D displacements of the center mass of the total Earth with respect to the center mass of the solid Earth without atmosphere caused by atmospheric pressure loading. The sum of this displacement and the displacement wrt of the total mass, is the displacement with respect to the center of solid Earth, i.e. center of figure.
The coefficients of of diurnal and semi-diurnal harmonic variations to download HARPOS format.

Precomputed coefficients of harmonic variations of 3D displacements of non-tidal loading at diurnal and semi-diurnal frequencies at a regular 2′ × 2′ global grid. The displacements are in NETCDF format.
NB:Tidal ocean loading accounts for dynamic oceanic response. It is not recommended to use these variations and ocean loading together, since the same effect will be counted twice.

Auxiliary 3D displacements of the center mass of the total Earth with respect to the center mass of the solid Earth without atmosphere caused by atmospheric pressure loading. The sum of this displacement and the displacement wrt of the total mass, is the displacement with respect to the center of solid Earth, i.e. center of figure.
The displacements are in NETCDF format.

On-demand online non-tidal ocean loading computation for an arbitrary point at the Earth. You can order computation of the time series for the stations of your interest. You need to prepare a station file in plain ascii that has four columns separated by one or more blanks:

Station_name X-coordinate Y-coordinate Z-coordinate

Station name should have no more than 8 characters. X,Y,Z are Cartesian coordinates of the station of interest in a crust-fixed coordinates system. Units are meters. Here is an example.

Model:

from 19800101_0000 through 20171130_1800

OMCT05

Mode:

Time series

S1 harmonic coefficients

All harmonics coefficients

Frame:

Center of mass

Center of figure

Start date:

Format: YYYY.MM.DD_hh:mm:ss