Sea levels at Aldabra Atoll, Mombasa and Mahé, western equatorial Indian Ocean, related to tides, meteorology and ocean circulation

Abstract

Sea levels at Aldabra Atoll, Mombasa and Mahé, for the period June 1975 to June 1976, have been analysed for tides and for local and regional meteorological effects. At Aldabra mean sea level has been related to a permanent bench mark for future comparisons; tides in the ocean at Aldabra are significantly different from those previously measured at the entrance to the lagoon. Radiational tides at S 1 are probably generated along the African coast by strong diurnal winds, from whence they propagate to Aldabra and Mahé. The S 2 radiational tide is close to that expected from the atmospheric pressure forcing, assuming a similar ocean response to that for the gravitational S 2 tide. Strong seiching occurs throughout the year in the Seychelles. Seiches at 10-min period are related to the local wind, but 50-min oscillations are not. The 50-min oscillations may also be observed at Aldabra, where there is no evidence for island-trapped waves. Cross spectral analysis shows that there are also oscillations with periods of 60 and 130 h. 30-day mean sea level variations at Mombasa appear to lag behind those at Aldabra by about a month and bear little relation to those at Mahé. A simple model of lagoon tides is developed to estimate the rate of dissipation of tidal energy. This rate is so low that the total dissipation by all such atolls is unlikely to exceed 0.3% of that dissipated globally.