M2 Internal Tides and Their Observed Wavenumber Spectra from Satellite Altimetry

Abstract

A near-global chart of surface elevations associated with the stationary M2 internal tide is empirically constructed from multimission satellite altimeter data. An advantage of a strictly empirical mapping approach is that results are independent of assumptions about ocean wave dynamics and, in fact, can be used to test such assumptions. A disadvantage is that present-day altimeter coverage is only marginally adequate to support mapping such short-wavelength features. Moreover, predominantly north–south ground-track orientations and contamination from nontidal oceanographic variability can lead to deficiencies in mapped tides. Independent data from Cryosphere Satellite-2 (CryoSat-2) and other altimeters are used to test the solutions and show positive reduction in variance except in regions of large mesoscale variability. The tidal fields are subjected to two-dimensional wavenumber spectral analysis, which allows for the construction of an empirical map of modal wavelengths. Mode-1 wavelengths show good agreement with theoretical wavelengths calculated from the ocean’s mean stratification, with a few localized exceptions (e.g., Tasman Sea). Mode-2 waves are detectable in much of the ocean, with wavelengths in reasonable agreement with theoretical expectations, but their spectral signatures grow too weak to map in some regions.

DUACS DT2014: the new multi-mission altimeter data set reprocessed over 20 years

“…The increase in the variance of the differences between altimetry and drifter measurement seems to underscore a noise-like signal in the SLA gridded products. This could correspond to the signature of the internal tidal signal, which is more prominent in the DT2014 gridded products, as shown by Ray et al (2015). This is certainly reinforced by reduced filtering and the smaller temporal/spatial correlation scales used in this version (Sect.…”

“…It is characteristic of the residual M2 internal tide, visible in both along-track (Dufau et al, 2016) and gridded products (Ray and Zaron, 2015). The presence of this signal leads to local degradation of DT2014 quality in specific areas.…”

Using CryoSat‐2 altimeter data to evaluate M2 internal tides observed from multisatellite altimetry

“…CryoSat4yr is noisier because of its short observational time. Our work, integrating with Zaron [] and Ray and Zaron [], demonstrates that CryoSat‐2 altimeter data are useful for observing and calibrating global internal tide models.…”

“…[] construct a global map of mode‐1 M 2 internal tides (labeled as MultiSat20yr herein) by a two‐dimensional plane wave fit method. Using the same data set, global internal tides have also been extracted by frequency‐wavenumber analysis [ Dushaw , ] and harmonic analysis (with prior mesoscale correction using AVISO‐gridded SSH products) [ Ray and Zaron , ], respectively. In addition, Egbert and Erofeeva [] have employed a reduced gravity model to map low‐mode coherent internal tides by assimilating the harmonically fitted altimetric internal tide signals.…”

Mapping the nonstationary internal tide with satellite altimetry

“…The peak at , the wavenumber of the mode‐1 baroclinic M 2 tide, is evident. The second version of the spectrum, , is computed with SSH data from which an estimate of the mesoscale sea level anomaly (SLA) has been removed [ Ray and Byrne , ; Ray and Zaron , ]. This spectrum is much flatter and makes the mode‐1 tidal peak even more prominent relative to the background spectrum.…”