Baroclinic Tidal Cusps from Satellite Altimetry

Abstract

Abstract The spatially averaged frequency spectrum of sea level has been computed at 4 cycle-per-year resolution and a Nyquist frequency of 0.5 cycles per hour using dual-satellite crossover data from the Jason and CryoSat-2 satellite altimeter missions. The novelty of the analysis is that it reveals unambiguous peaks due to high-frequency tidal signals, even after removing the predicted barotropic tide, without the usual aliasing caused by altimeter sampling. The tidal continuum, that is, a tidal cusp, is present in the spectrum in the diurnal and semidiurnal tidal bands, and a Lorentzian model spectrum has been fit within each band to identify the properties of the non-phase-locked tidal variability. An interesting feature of the semidiurnal tidal continuum is the unambiguous presence of an inner and an outer band, characterized by different Lorentzian bandwidths of roughly (180 day)−1 and (30 day)−1. Considering different latitude ranges, it is clear that the tidal continuum is most prominent in the range from −30° to 30° latitude. Within this range, it is found that 1.05-cm2 variance is associated with the semidiurnal continuum, and slightly less than half of this variance, 0.41 cm2, is associated with the slower, (180 day)−1 bandwidth, variability. The ratio of non-phase-locked to total baroclinic variability is about 62% in this latitude band, a value that is consistent with previous model-based estimates for this quantity. Quantification of the properties of the tidal continuum poleward of 30° latitude is not possible with the present data, due to the small size of the tidal signal compared to the mesoscale variability and other sources of noise.