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Influence of non-stationary ocean loading on the M2 tidal parameters investigated with time-stepping, non-linear ocean models

Influence of non-stationary ocean loading on the M2 tidal parameters investigated with time-stepping, non-linear ocean models
sws:

23.07.2019

Referent:

Eva Schroth

Abstract

An annual variation of the tidal parameters of wave group M2 is observed
globally and is in the order of 1.e-4 for the gravimetric factor (GF) and
1.e-2° for the phase at almost all European superconducting
gravimeters (SG).
With sea surface heights (SSH) from five non-stationary, non-linear ocean
models which potentially describe the annual variation of the M2 ocean tide
the gravity loading can be calculated. We add the loading signal to synthetic
Earth tides and analyse the resulting signal with a moving window tidal
analysis (MWA). The amplitude of the annual variation of the tidal parameters
obtained that way is of the same order of magnitude, as the tidal parameters
obtained from measurements.
The annual changes of the M2-amplitude are largest in shelf areas. As they are
close to the SG stations on the continents they probably have a large
influence on the gravity recordings. On the other hand, although the
amplitudes in the pelagic areas are small, they cover large areas and could
add up to a large loading signal. In this study we investigate which areas of
the global ocean contribute most to the observed variation at a SG station.
The amplitudes of the annual variation of M2 obtained with harmonic analysis
of the SSH of the global ocean model 'Stormtide' are used for calculating the
contribution that the annual variation of the M2 ocean tide at every single
grid point causes at the SG station. We use this to define thresholds for the
analyses of synthetic data, taking into account only those grid points whose
contributions are above the threshold. The variations of the tidal parameters
obtained with these reduced data sets are compared to the tidal parameters we
obtain for the total ocean model data set and the measured data.
At three stations, BFO, Moxa and Sutherland, about 3% of all grid points have
a maximal contribution that exceeds a 1.e-6 nm/(s*s)-threshold which
is sufficient to explain 80% to 90% of the result for the global ocean.
These points are distributed over large areas in the North Atlantic as well as
smaller shelf areas in even more than 140° distance. For the other
stations all grid points whose contribution exceed 1.e-7 nm/(s*s) are
necessary to reach the same level of similarity with the result from the total
model. These are more than 40% of all grid points; they cover almost the
entire globe.
We conclude that not only the very close coastal area but distant ocean
regions have a significant contribution to the annual variation of the M2
tidal parameters. Accordingly, the influence of the annual variation of the M2
amplitude on SG stations has to be studied on a global scale. At the same time
a comparison of MWA for synthetic loading with MWA of recorded data from
several globally distributed SGs can be used to evaluate global time-varying
tidal ocean models.