Relationship between the astrometric z-term, the Earth rotation and the southern oscillation index.

The z-term series derived from astronomical observations is related to plumb-line deflection. Its analysis over the interval 1962-1982 reveals interannual variations with amplitudes of about 0.01". These variations appear to be significantly correlated with both the length-of-day variations and the Southern Oscillation Index characterizing El Nino events, and this with a time lag of about 12 months. This z-term series appears to be of great interest in the study of relationships between Earth rotation and geophysical process like in particular oceano-atmospheric phenomena.