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1. Development of the Yearly Mode-1 M2 Internal Tide Model in 2019

   https://doi.org/10.1175/JTECH-D-21-0116.1  

   Zhao, Zhongxiang ( April 2022 , Journal of Atmospheric and Oceanic Technology)

   The yearly mode-1 M₂internal tide model in 2019 is constructed using sea surface height measurements made by six concurrent satellite altimetry missions:Jason-3,Sentinel-3A,Sentinel-3B,CryoSat-2,Haiyang-2A, andSARAL/AltiKa. The model is developed following a three-step procedure consisting of two rounds of plane wave analysis with a spatial bandpass filter in between. Prior mesoscale correction is made on the altimeter data using AVISO gridded mesoscale fields. The model is labeled Y2019, because it represents the 1-yr-coherent internal tide field in 2019. In contrast, the model developed using altimeter data from 1992 to 2017 is labeled MY25, because it represents the multiyear-coherent internal tide field in 25 years. Thanks to the new mapping technique, model errors in Y2019 are as low as those in MY25. Evaluation using independent altimeter data confirms that Y2019 reduces slightly less variance (∼6%) than MY25. Further analysis reveals that the altimeter data from five missions (withoutJason-3) can yield an internal tide model of almost the same quality. Comparing Y2019 and MY25 shows that mode-1 M₂internal tides are subject to significant interannual variability in both amplitude and phase, and their interannual variations are a function of location. Along southward internal tides from Amukta Pass, the energy flux in Y2019 is 2 times larger and the phase speed is about 1.1% faster. This mapping technique has been applied successfully to 2017 and 2018. This work demonstrates that yearly internal tides can be observed by concurrent altimetry missions and their interannual variations can be determined.

   This work is motivated to study the interannual variations of internal tides using observation-based yearly internal tide models from satellite altimetry. Previous satellite observations of internal tides are usually based on 25 years of altimeter data from 1993 to 2017. The yearly subsetted altimeter data are short, so that the resultant yearly models are overwhelmed by noise. A new mapping technique is developed and demonstrated in this paper. It paves a path to study the interannual and decadal variations of internal tides on a global scale and monitor the global ocean changes by tracking long-range internal tides.

    

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2. Southward Internal Tides in the Northeastern South China Sea

   https://doi.org/10.1029/2020JC016554  

   Zhao, Zhongxiang ( October 2020 , Journal of Geophysical Research: Oceans)

   The M₂internal tides in the northeastern South China Sea are studied using satellite altimeter data from 1992–2018. By an improved mapping technique that combines plane wave analysis and two‐dimensional spatial filtering, multiple internal tides are separately extracted with weak internal tides becoming detectable. The satellite results reveal for the first time a 300‐km‐long southward M₂internal tidal beam in the northeastern South China Sea. The generation source is on the steep continental slope at the southern entrance to the Taiwan Strait. It ranges from 118–120°E along 22°N. Combining satellite‐observed internal solitary waves and internal tides, it is found that the onshore radiation evolves into nonlinear solitary waves and the offshore radiation in the form of linear internal tides. Based on the 26‐year‐coherent satellite results, the integrated southward energy flux is 0.18 GW, about 10% of the westward energy flux from the Luzon Strait. In the northeastern South China Sea, the westward and southward internal tides form a multiwave interference field, which features significant spatial variations in the magnitude and direction of energy flux. Further analyses reveal that the steep continental slope radiates southward semidiurnal M₂and S₂internal tides, but not diurnal K₁and O₁internal tides.

    

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3. Decomposition of the Multimodal Multidirectional M 2 Internal Tide Field

   https://doi.org/10.1175/JTECH-D-19-0022.1  

   Zhao, Zhongxiang ; Wang, Jinbo ; Menemenlis, Dimitris ; Fu, Lee-Lueng ; Chen, Shuiming ; Qiu, Bo ( June 2019 , Journal of Atmospheric and Oceanic Technology)

   The M₂internal tide field contains waves of various baroclinic modes and various horizontal propagation directions. This paper presents a technique for decomposing the sea surface height (SSH) field of the multimodal multidirectional internal tide. The technique consists of two steps: first, different baroclinic modes are decomposed by two-dimensional (2D) spatial filtering, utilizing their different horizontal wavelengths; second, multidirectional waves in each mode are decomposed by 2D plane wave analysis. The decomposition technique is demonstrated using the M₂internal tide field simulated by the MITgcm. This paper focuses on a region lying off the U.S. West Coast ranging 20°–50°N, 220°–245°E. The lowest three baroclinic modes are separately resolved from the internal tide field; each mode is further decomposed into five waves of arbitrary propagation directions in the horizontal. The decomposed fields yield unprecedented details on the internal tide’s generation and propagation, which cannot be observed in the harmonically fitted field. The results reveal that the mode-1 M₂internal tide in the study region is dominantly from the Hawaiian Ridge to the west but also generated locally at the Mendocino Ridge and continental slope. The mode-2 and mode-3 M₂internal tides are generated at isolated seamounts, as well as at the Mendocino Ridge and continental slope. The Mendocino Ridge radiates both southbound and northbound M₂internal tides for all three modes. Their propagation distances decrease with increasing mode number: mode-1 waves can travel over 2000 km, while mode-3 waves can only be tracked for 300 km. The decomposition technique may be extended to other tidal constituents and to the global ocean.

    

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4. Phase‐Accurate Internal Tides in a Global Ocean Forecast Model: Potential Applications for Nadir and Wide‐Swath Altimetry

   https://doi.org/10.1029/2023GL107232  

   Yadidya, Badarvada ; Arbic, Brian K. ; Shriver, Jay F. ; Nelson, Arin D. ; Zaron, Edward D. ; Buijsman, Maarten C. ; Thakur, Ritabrata ( February 2024 , Geophysical Research Letters)

   Internal tides (ITs) play a critical role in ocean mixing, and have strong signatures in ocean observations. Here, global IT sea surface height (SSH) in nadir altimetry is compared with an ocean forecast model that assimilates de‐tided SSH from nadir altimetry. The forecast model removes IT SSH variance from nadir altimetry at skill levels comparable to those achieved with empirical analysis of nadir altimetry. Accurate removal of IT SSH is needed to fully reveal lower‐frequency mesoscale eddies and currents in altimeter data. Analysis windows of order 30–120 days, made possible by the frequent (hourly) outputs of the forecast model, remove more IT SSH variance than longer windows. Forecast models offer a promising new approach for global internal tide mapping and altimetry correction. Because they provide information on the full water column, forecast models can also help to improve understanding of the underlying dynamics of ITs.

    

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5. On Internal Tides Driving Residual Currents and Upwelling on an Island

   https://doi.org/10.1029/2021JC018261  

   Rogers, Justin S. ; Mayer, Frederick T. ; Davis, Kristen A. ; Fringer, Oliver B. ( June 2022 , Journal of Geophysical Research: Oceans)

   We present a modeling study of an idealized island with sloping sides in a stratified ocean with internal tides propagating through the domain, which transform into nonlinear internal waves as they encounter shallow depths. In the base condition, representative of a mid‐latitude island with relatively steep super critical slopes, the nonlinear internal waves shoaling on the island wrap around the island, scatter, and create an asymmetric wavefield. Radiation stress gradients from this wavefield and rotation are balanced by pressure gradients and turbulent vertical diffusion in the momentum equations which drive residual mean currents around the island. We explore the effects of different physical parameters including no rotation, higher Froude number, subcritical slope, addition of barotropic tides, larger excursion number, and addition of large‐scale mean currents. All simulations showed formation of residual currents of varying intensity, upwelled deep waters in shallow regions, and cooler waters on the side of the island consistent with the direction of incoming internal tides. While the no rotation and strong mean flow conditions created some upwelling, of all the modeled conditions, the subcritical slope has the greatest potential for creating favorable conditions for benthic organisms through enhanced upwelling. Areas of the world's oceans with stratified waters sufficient to sustain internal tide motions, strong internal tide energy, and local subcritical bottom slopes are likely to have cooler waters and more upwelled deep waters than the surrounding areas, and potentially serve as thermal refugia from future ocean warming.

    

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