Deep-Ocean Tides in the South-West Indian Ocean: Comparing Deep-Sea Pressure to Satellite Data

Abstract

Deep ocean pressure measurements in two regions of the South-West Indian Ocean (West and East of Madagascar), covering one to two years of data, are analysed for tidal motions. The pressure data are taken both from Bottom Pressure Recorders as well as from mid-water column instruments. Coherent tides are characterised by fixed amplitudes and phases. Those inferred from bottom measurements compare well to tides obtained from satellite altimetry, and cover up to 99 % of the pressure variance in the frequency band having periods shorter than 29 h. Long-period tides, in the low-frequency band, are regularly overshadowed by (unwanted) eddy-induced mooring motion (‘blow-down’), which events have therefore been eliminated. In the Mozambique Channel, semidiurnal surface tides are stronger than East of Madagascar, and all appear to be near resonance with a basin mode. Away from the bottom, coherent internal tides were determined. Evidence of the presence of incoherent internal tides has been obtained by applying Harmonic Analyses over a moving time window of 1 year duration. East of Madagascar internal tides appear to be very strong, although its source remains unclear.