Impact of Atmosphere-Ocean-Land Interactions on Short-Term Variations of Earth Rotation Parameters

The Earth's variable rotation is precisely observed by modern space geodetic techniques down to subdiurnal timescales. For the interpretation of observed rotational parameters a reliable and consistent representation of mass redistributions within and mass exchanges among the Earth's subsystems is crucial. Here, causative physical processes of observed rotational variations are investigated by means of a numerical model approach allowing mass and impulse fluxes among the subsystems atmosphere, ocean, and continental hydrology. Operational analysis data from ECMWF are used to consistently force a hydrological discharge model and a global model for the ocean's baroclinic circulation and ephemeral tides. Effects from sea ice, pressure tides as well as gravitational tides, loading, self-attraction and nonlinear interactions between the various dynamic components are considered. The unconstrained hydrology and ocean models are coupled via continental discharge in order to close the hydrological cycle. Thus, resulting estimates of excitations of rotational variations implicitely take into account effects of continental discharge on near-shore ocean dynamics as well as the impact of variations in total ocean mass due to time-varying atmospheric and continental freshwater fluxes. Focussing on the period 2001 - 2005, individual angular momentum contributions of the atmosphere, the oceans, and continental hydrology are separated and typical variation patterns of underlying physical processes are identified. Agreement of simulated and observed excitations are significantly improved when water mass fluxes from the continents into the oceans are applied as additional boundary condition in the ocean model, since accompanying modifications of oceanic angular momentum partly compensate hydrological angular momentum contributions. As atmospheric analyses from ECMWF are routinely available on a near real-time basis, numerical estimates of Earth rotation excitations as presented here are principally suitable for operational purposes.