Mapping the nonstationary internal tide with satellite altimetry

Zaron, Edward D.

doi:10.1002/2016jc012487

Cited by 74 publications

(108 citation statements)

References 53 publications

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“…Figures and , respectively, show global maps of the diurnal and semidiurnal tidal band variance after the stationary part of the tide has been removed. Low‐latitude and equatorial regions tend to display the largest signals in the nonstationary diurnal and semidiurnal steric (internal tide) maps [ Zaron , ], and the high variance regions are correlated with the total internal tidal signals (Figures a and a). The global HYCOM25 maps of nonstationary steric SSH (internal tides) have a spatially averaged global variance of

0.05 {cm}^{2}

in the diurnal band and

0.43 {cm}^{2}

in the semidiurnal band (Table ), the latter being comparable to the

\sim 0.33 {cm}^{2}

estimated from Zaron [].…”

Section: Resultsmentioning

confidence: 99%

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

et al. 2017

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High horizontal‐resolution ( 1/12.5° and 1/25°) 41‐layer global simulations of the HYbrid Coordinate Ocean Model (HYCOM), forced by both atmospheric fields and the astronomical tidal potential, are used to construct global maps of sea surface height (SSH) variability. The HYCOM output is separated into steric and nonsteric and into subtidal, diurnal, semidiurnal, and supertidal frequency bands. The model SSH output is compared to two data sets that offer some geographical coverage and that also cover a wide range of frequencies—a set of 351 tide gauges that measure full SSH and a set of 14 in situ vertical profilers from which steric SSH can be calculated. Three of the global maps are of interest in planning for the upcoming Surface Water and Ocean Topography (SWOT) two‐dimensional swath altimeter mission: (1) maps of the total and (2) nonstationary internal tidal signal (the latter calculated after removing the stationary internal tidal signal via harmonic analysis), with an average variance of 1.05 and 0.43 cm2, respectively, for the semidiurnal band, and (3) a map of the steric supertidal contributions, which are dominated by the internal gravity wave continuum, with an average variance of 0.15 cm2. Stationary internal tides (which are predictable), nonstationary internal tides (which will be harder to predict), and nontidal internal gravity waves (which will be very difficult to predict) may all be important sources of high‐frequency “noise” that could mask lower frequency phenomena in SSH measurements made by the SWOT mission.

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0.05 {cm}^{2}

in the diurnal band and

0.43 {cm}^{2}

in the semidiurnal band (Table ), the latter being comparable to the

\sim 0.33 {cm}^{2}

estimated from Zaron [].…”

Section: Resultsmentioning

confidence: 99%

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

et al. 2017

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“…Outside the Pacific, the equatorial currents in the Atlantic and Indian Oceans also cause a strong decoherence of the internal tides radiating from, e.g., the Amazon shelf and the Andaman and Nicobar Islands, respectively. Consistent with the results described here, the prominence of the equator in maps of incoherent internal tides can also be seen in maps made from satellite altimetric [ Zaron , ] and modeled [ Savage et al ., ] sea surface heights.…”

Section: Discussionmentioning

confidence: 99%

“…These incoherent motions have been attributed to internal tides that are not coherent with the tidal forcing. Zaron [] argues that up to 44% of the total semidiurnal internal tide signal in the world's oceans is incoherent. Because of their

O (10

day) sampling times, satellite altimeters do not easily allow for estimation of the incoherent tide amplitude at a particular location in the ocean.…”

Section: Introductionmentioning

confidence: 99%

Semidiurnal internal tide incoherence in the equatorial Pacific

et al. 2017

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The jets in the equatorial Pacific Ocean of a realistically forced global circulation model with a horizontal resolution of 1/12.5° cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian Islands and Hawaii. This loss of coherence is quantified with a baroclinic energy analysis, in which the semidiurnal‐band terms are separated into coherent, incoherent, and cross terms. For time scales longer than a year, the coherent energy flux approaches zero values at the equator, while the total flux is ∼500 W/m. The time variability of the incoherent energy flux is compared with the internal‐tide travel‐time variability, which is based on along‐beam integrated phase speeds computed with the Taylor‐Goldstein equation. The variability of monthly mean Taylor‐Goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane‐wave fit technique. On monthly time scales, the loss of phase coherence in the equatorward beams from the French Polynesian Islands is attributed to the time variability in the vertically sheared background flow associated with the jets and tropical instability waves. On an annual time scale, the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence. In the model simulations, low‐frequency equatorial jets do not noticeably enhance the dissipation of the internal tide, but merely decohere and scatter it.

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“…First, it contains the phase‐locked component only and misses the time‐varying components of various time scales. Previous studies reveal that, in the equatorial Pacific and Indian Oceans, the nonstationary component of the mode‐1 M 2 internal tide may be larger than its phase‐locked stationary component (Zaron, ). It will be interesting to compare the stationarity of mode‐1 and mode‐2 M 2 internal tides in the world oceans.…”

Section: Discussionmentioning

confidence: 99%

The Global Mode‐2 M₂ Internal Tide

Zhao

2018

JGR Oceans

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The mode‐2 M2 internal tide is observed from satellite altimetry. It is extracted in two steps: First, the mode‐2 component is separated from modes 1 and 3 by a bandpass filter with cutoff wavelengths of [0.85 1.35]×λ2, where λ2 is the mode‐2 wavelength; and second, three mode‐2 internal tidal waves are extracted by fitting plane waves in each 120 km by 120 km window. The satellite‐observed mode‐2 M2 internal tide underestimates its strength: It contains the phase‐locked component only, missing the time‐varying component; it contains the southbound/northbound component only, missing the eastbound/westbound component. The satellite results provide rich information on the global mode‐2 M2 internal tide. The results show that the mode‐2 M2 internal tide is ubiquitous in the ocean, and its sea surface height amplitudes are O (1 mm). The spatial patterns of mode‐1 and mode‐2 internal tides are very different. Mode 1 mainly originates at steep topographic features such as submarine ridges, but mode 2 is also generated at gentler topographic features such as abyssal seamounts and fracture zones. Mode‐1 beams propagate O (1,000 km), while mode‐2 beams can be tracked for O (100 km). Depth‐integrated energy and flux are calculated using sea surface height amplitudes and conversion functions built from the World Ocean Atlas 2013. The globally integrated energy of the mode‐2 M2 internal tide approximates to 8 PJ, about 22% of mode 1 (36.4 PJ). This work suggests that higher‐mode internal tides play an important role in the tidal energetics, in particular, over abyssal seamounts and fracture zones.

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Mapping the nonstationary internal tide with satellite altimetry

Cited by 74 publications

References 53 publications

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

Semidiurnal internal tide incoherence in the equatorial Pacific

The Global Mode‐2 M₂ Internal Tide

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Mapping the nonstationary internal tide with satellite altimetry

Cited by 74 publications

References 53 publications

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

Semidiurnal internal tide incoherence in the equatorial Pacific

The Global Mode‐2 M2 Internal Tide

Contact Info

Product

Resources

About

The Global Mode‐2 M₂ Internal Tide