Climatic signals observed by VLBI

VLBI observations at various stations all over the globe have been carried out since the beginning of the eighties. As VLBI allows to determine tropospheric parameters such as hydrostatic and wet zenith path delays stable and long time series of these parameters exist that can be used for climatological studies. From the wet zenith delays information about trends and periodic variations of the amount of water vapour in the troposphere can be obtained. For example a trend of ~+0.7 mm/year in the wet zenith delay will correspond to a trend of ~ +0.1 mm/year in precipitable water. For assessing the reliability of the linear trends and the amplitudes of annual and semiannual components, data from VLBI, GPS, and the ECMWF (European Centre for Medium-Range Weather Forecasts) were compared. Good agreement was found for the seasonal variations, though the linear trends showed some disagreement. Possible reasons for this disagreement are discussed. Finally, the effect of El Niño events on tropospheric mapping functions and on length-of-day time series is shown