The Global Mode‐2 M<sub>2</sub> Internal Tide

Zhao, Zhenwei

doi:10.1029/2018jc014475

Cited by 29 publications

(22 citation statements)

References 84 publications

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“…Satellite altimetry gives a good estimate of the multiyear mean of the coherent part of the internal tide flux which in many areas is most likely a lower‐bound estimate for the in situ fluxes. The data processing additionally leads to a suppression of the east‐west‐oriented waves due to the generally north‐south‐oriented satellite tracks (de Lavergne et al, ; Zhao, ; Zhao et al, ), and the spatial resolution of the satellite‐based energy fluxes is limited due to the spatial coverage of the satellites. Internal tide parameters from satellite altimetry are determined by fitting plane waves to time series of sea surface height in windows of 160 km × 160 km on a 0.1°×0.1° grid.…”

Section: Methodsmentioning

confidence: 99%

“…10.1029/2019JC015156 additionally leads to a suppression of the east-west-oriented waves due to the generally north-south-oriented satellite tracks (de Lavergne et al, 2019;Zhao, 2018;Zhao et al, 2016), and the spatial resolution of the satellite-based energy fluxes is limited due to the spatial coverage of the satellites. Internal tide parameters from satellite altimetry are determined by fitting plane waves to time series of sea surface height in windows of 160 km × 160 km on a 0.1 • × 0.1 • grid.…”

Section: Journal Of Geophysical Research: Oceansmentioning

confidence: 99%

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Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

et al. 2019

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Low‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m−1 near the generation sites to 0.5 kW m−1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m−2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

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Section: Methodsmentioning

confidence: 99%

Section: Journal Of Geophysical Research: Oceansmentioning

confidence: 99%

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

et al. 2019

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“…Both were not observed here and thus further modeling is required. Although global mapping suggests mode-2 internal tides to emanate from (non-specified) gentler slopes than those of mode-1 (Zhao, 2018), the quasimode-2 motions observed here seemed more associated with local internal tide beams (LeBlond and Mysak, 1978) as they did not emanate from the Caribbean Arc to the southwest of Saba Bank (Zhao, 2018).…”

Section: Internal Wave Observations Off Saba Bankmentioning

confidence: 57%

Internal Wave Observations Off Saba Bank

2019

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The deep sloping sides of Saba Bank, the largest submarine atoll in the Atlantic Ocean, show quite different internal wave characteristics. To measure these characteristics, two 350 m long arrays consisting of primary a high-resolution temperature T-sensor string and secondary an acoustic Doppler current profiler were moored around 500 m water depth at the northern and southern flanks of Saba Bank for 23 days. We observed that the surrounding density stratified waters supported large internal tides and episodically large turbulent exchange in up to 50 m tall overturns. However, an inertial subrange was observed at frequencies/wavenumbers smaller than the mean buoyancy scales but not at larger than buoyancy scales, while near-bottom non-linear turbulent bores were absent. The latter reflect more open-ocean than steep sloping topography internal wave turbulence. Both the Banks' north-side and south-side slopes are locally steeper 'supercritical' than internal tide slope angles. However, the three times weaker north-side slope showed quasi-mode-2 semidiurnal internal tides, not high-frequency solitary waves occurring every 12 h, over the range of observations, centered with dominant nearinertial shear around 150 m above the bottom. They generated the largest turbulence when touching the bottom and providing off-bank flowing turbid waters. In contrast, the steeper south-side slope showed quasi-mode-1 internal tides occasionally having excursions > 100 m crest-trough, with weak inertial shear and smallest buoyancy scale turbulence periodicity occurring near the bottom and about half-way the water column, below abundant coral reefs in shallow <20 m deep waters.

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“…The barotropic tide and loading tide are corrected using the global ocean tide model GOT4.8 (Ray, 2013). Data from these missions have been used for mapping internal tides in a number of previous studies (Ray & Zaron, 2016;Zaron, 2019;Zhao, 2018;Zhao et al, 2016). The SSH data used in Zhao et al (2016) and this study are indicated by two boxes in Figure 1.…”

Section: Satellite Altimeter Datamentioning

confidence: 99%

“…Fortunately, the constellation of multiple exact-repeat-track satellite missions has more ground tracks; therefore, multisatellite altimetry makes it possible to map internal tide in the horizontal two-dimensional (2-D) field (see details in section 2). In recent years, several altimeter-based empirical global internal tide models have been developed using different mapping techniques (Dushaw, 2015;Ray & Zaron, 2016;Zaron, 2019;Zhao, 2018;Zhao et al, 2016). Egbert and Ray (2017) and Ray and Egbert (2018) summarized recent progresses and existing issues in mapping internal tides from satellite altimetry.…”

Section: Introductionmentioning

confidence: 99%

Mapping Internal Tides From Satellite Altimetry Without Blind Directions

Zhao

2019

JGR Oceans

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Satellite altimetry is one practical technique for observing internal tides on the global scale.However, it is a great challenge to extract weak internal tide signals. This paper presents a new technique for mapping internal tides from satellite altimeter data. Along-track high-pass filtering is needed to remove long-wavelength nontidal noise and the barotropic tidal residual; however, the filter also removes internal tides having large angles with respect to satellite ground tracks. It thus causes blind directions in mapping internal tides from satellite altimetry: Generally west-east propagating internal tides are missed. The new technique addresses the blind-direction issue by replacing the problematic one-dimensional (1-D) high-pass filter with a two-dimensional (2-D) band-pass filter. This mapping technique is able to retrieve ubiquitous westbound and eastbound internal tides not captured in previous estimates. Long-range westbound and eastbound waves travel over thousands of km from numerous generation sites such as the Emperor seamount chain, the Hawaiian Ridge, and the Kermadec trench. Evaluation using independent Cryosat-2 data reveals that the new internal tide model may reduce more SSH variance than a model built in 2016 does in regions of strong internal tides. However, this mapping technique makes no improvement in strong boundary current regions, due to the dominance of mesoscale motions. Moreover, the new internal tide model contains leaked noise from westward propagating tropical instability waves (TIWs), which can be suppressed by prior along-track high-pass filtering. This paper suggests that better internal tide models may be constructed using both 1-D and 2-D filters with optimized parameters. Plain Language SummaryInternal tides are ubiquitous in the ocean and play an important role in various ocean processes. It is a significant task in the community to construct internal tide models. Satellite altimetry is the most important technique for observing global internal tides. However, it is a great challenge to extract weak internal tides, mainly due to the low spatiotemporal sampling rate of satellite altimetry. Fortunately, dedicated mapping techniques make the best use of satellite altimeter data to construct better and better internal tide models. In this paper, I make improvements by addressing an issue in previous techniques. Along-track high-pass filtering is widely used in previous studies to remove long-wavelength nontidal noise and barotropic tidal residual. Unfortunately, the filter also removes internal tides having large angles with respect to satellite ground tracks and misses west-east propagating internal tidal waves. The new internal tide model developed by the new technique improves over previous models in retrieving westbound and eastbound internal tides. The results are confirmed using independent Cryosat-2 data. The new model shows that west-east propagating internal tides occur throughout the ocean and can travel up to 3,800 km from Hawaii to California.

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The Global Mode‐2 M₂ Internal Tide

Cited by 29 publications

References 84 publications

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

Internal Wave Observations Off Saba Bank

Mapping Internal Tides From Satellite Altimetry Without Blind Directions

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The Global Mode‐2 M2 Internal Tide

Cited by 29 publications

References 84 publications

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

Internal Wave Observations Off Saba Bank

Mapping Internal Tides From Satellite Altimetry Without Blind Directions

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The Global Mode‐2 M₂ Internal Tide