Seasonal variations in global mean sea level and consequences on the excitation of length-of-day changes

Global mass redistribution between the Earth subsystems oceans, atmosphere and continental hydrosphere causes a predominantly seasonal signal in Earth rotation excitation that superimposes the effects of each individual Earth subsystem. Especially for annual length-of-day variations a consistent consideration of the global mass balance among atmosphere, ocean and continental water is necessary to compare the simulated effective angular momentum functions for Earth rotation from geophysical models with geodetic observations. In addition to atmospheric, oceanic and hydrological contributions, we estimate the contributions due to the global mass balance effect using the new ESMGFZ SLAM product as well as estimates of the barystatic ocean bottom pressure anomalies from the GRACE Level 3 GravIS products. For the annual cycle the global mass balance effect overcompensates the contributions to length-of-day variations from terrestrial hydrology. Moreover, most of the atmospheric surface pressure contribution is also compensated. The global mass balance effect has to be calculated for each combination of geophysical Earth system models individually. Considering the global mass balance, model based mass induced excitation on seasonal length-of-day variations coincide well with estimates from satellite gravimetry. Moreover, the mass terms can be determined accurate enough to attribute the remaining gap in the length-of-day excitation budget between models and observation clearly to an underestimation of atmospheric wind speeds in the global European weather forecast model. Magnifying its wind speeds by +7 per cent the sum of all ESMGFZ angular momentum functions can almost perfectly explain the total length-of-day excitation.