On the space-time ocean current variability and its effects on the length-of-day

Description

Earth Rotation (polar motion and length-of-day) studies have embarked on a new era with the contributions from space geodesy observation techniques (high accuracy, higher temporal resolution) and the availability of new, global data bases of atmospheric and oceanic observables. Irregular variations in the earth’s rotation rate (length-of-day – LOD – variations) on time scales of 5 years or less are associated with changes in the angular momentum of the solid earth. The problem of transfer of angular momentum between the Earth System components (consisting of the solid earth, the oceans, and the atmosphere) has emerged todays as a problem of great scientific interest, because of the geophysical and environmental implications associated LOD variations. This thesis research chiefly investigates the time-dependent perturbations of LOD on time scales of about two years or less, associated with the space-time fluctuations of the global ocean circulation. Estimates of the space-time variations of ocean currents were derived from both in-situ oceanographic data and two years of satellite altimetry observations from the GEOSAT Exact Repeat Mission. A new technique for extracting ocean current variability from satellite altimetry was developed. The technique is based on the gradient operator and offers advantages over previously existing techniques on the recovery of oceanic variability in that it is conceptually simple and computationally efficient. Through the analysis of ocean current variability, it is established that the oceanic excitation of LOD is at least at the level of 0.1 milliseconds. Variations in the oceanic excitation of ΔLOD have been identified and quantified at periods of 1 year and 0.5 years. At the annual period, the results indicate that the oceanic excitation appears large enough to account for the existing discrepancy between the observed (non-tidal) ΔLOD and the atmospheric contribution. No significant oceanic excitation of ΔLOD is found at the semi-annual period. The results indicate that there exist statically significant higher frequency variations at approximately 121-day, 107-day, and 89-day periods. The analysis of the GEOSAT altimetry data generated many new questions regarding the oceanic behaviour and its dynamical link to LOD variations that require further investigations.