On the Temporal Variability of Tidal Atmospheric Signals

The reduction of geophysically induced high-frequency harmonic variations inherent in space geodetic measurements is carried out on the basis of physically-driven empirical models that may vary spatially, under the implicit assumption that they do not vary in time. Motivated by the fact that the parameters driving processes such as the atmospheric tides (largely induced by the absorbtion of ultraviolet and infrared radiation by Ozone and water vapor) that in turn partly excite the oceanic tides are affected by climate change, in this contribution we put the time-invariance modelling assumption to the test with a focus on harmonic deformation and atmospheric delay. To study temporal variations in atmospheric tides, we have analyzed hourly series from ECMWF’s latest reanalysis, the ERA5, including its back-extension. To validate the harmonic estimates and the temporal evolution thereof, we have resorted to two largely independent reanalyses: the MERRA2 and the JRA55. Recovering the evolution of harmonic coefficients has been carried out employing a square-root information filter (SRIF) and a Dyer-McReynolds smoother. Subsequently, the pressure harmonic fields are convolved with load Green’s kernels to simulate crustal displacements and harmonic pressure, temperature, and humidity fields are used to evaluate the refractivity integrals via ray-tracing. We found that the most important high-frequency waves, that is, the solar diurnal and semi-diurnal exhibit marked secular and seasonal variations. We find that harmonic S2 estimates for sites in the tropics from an hourly five-year long time series segment vary by up to 25% depending on the temporal boundaries thereof and the annual amplitudes of the S2 amplitude series from the SRIF can exceed 10 Pa.