scholarly journals Modification of Parametric Subharmonic Instability in the Presence of Background Geostrophic Currents

2018 ◽  
Vol 45 (23) ◽  
Author(s):  
Wei Yang ◽  
Toshiyuki Hibiya ◽  
Yuki Tanaka ◽  
Liang Zhao ◽  
Hao Wei
Keyword(s):  
Geostrophic Currents ◽  
Parametric Subharmonic Instability

Disintegration of the K1 internal tide in the South China Sea due to parametric subharmonic instability

2021 ◽  
Author(s):  
Kun Liu ◽  
Zhongxiang Zhao
Keyword(s):  
South China Sea ◽  
South China ◽  
Internal Tide ◽  
The South China Sea ◽  
Internal Tides ◽  
Upper Ocean ◽  
The South ◽  
China Sea ◽  
Parametric Subharmonic Instability ◽  
Critical Latitude

<p>The disintegration of the equatorward-propagating K<sub>1</sub> internal tide in the South China Sea (SCS) by parametric subharmonic instability (PSI) at its critical latitude of 14.52ºN is investigated numerically. The multiple-source generation and long-range propagation of K<sub>1</sub> internal tides are successfully reproduced. Using equilibrium analysis, the internal wave field near the critical latitude is found to experience two quasi-steady states, between which the subharmonic waves develop constantly. The simulated subharmonic waves agree well with classic PSI theoretical prediction. The PSI-induced near-inertial waves are of half the K<sub>1</sub> frequency and dominantly high modes, the vertical scales ranging from 50 to 180 m in the upper ocean. From an energy perspective, PSI mainly occurs in the critical latitudinal zone from 13–15ºN. In this zone, the incident internal tide loses ~14% energy in the mature state of PSI. PSI triggers a mixing elevation of O(10<sup>-5</sup>–10<sup>-4</sup> m<sup>2</sup>/s) in the upper ocean at the critical latitude, which is several times larger than the background value. The contribution of PSI to the internal tide energy loss and associated enhanced mixing may differ regionally and is closely dependent on the intensity and duration of background internal tide. The results elucidate the far-field dissipation mechanism by PSI in connecting interior mixing with remotely generated K<sub>1</sub> internal tides in the Luzon Strait.</p>

scholarly journals Use of recent geoid models to estimate mean dynamic topography and geostrophic currents in South Atlantic and Brazil Malvinas confluence

2012 ◽  
Vol 60 (1) ◽  
pp. 41-48
Author(s):  
Alexandre Bernardino Lopes ◽  
Joseph Harari
Keyword(s):  
Numerical Model ◽  
Large Scale ◽  
Singular Spectrum Analysis ◽  
Satellite System ◽  
South Atlantic ◽  
Dynamic Topography ◽  
Geostrophic Currents ◽  
Mean Dynamic Topography ◽  
Level Model

The use of geoid models to estimate the Mean Dynamic Topography was stimulated with the launching of the GRACE satellite system, since its models present unprecedented precision and space-time resolution. In the present study, besides the DNSC08 mean sea level model, the following geoid models were used with the objective of computing the MDTs: EGM96, EIGEN-5C and EGM2008. In the method adopted, geostrophic currents for the South Atlantic were computed based on the MDTs. In this study it was found that the degree and order of the geoid models affect the determination of TDM and currents directly. The presence of noise in the MDT requires the use of efficient filtering techniques, such as the filter based on Singular Spectrum Analysis, which presents significant advantages in relation to conventional filters. Geostrophic currents resulting from geoid models were compared with the HYCOM hydrodynamic numerical model. In conclusion, results show that MDTs and respective geostrophic currents calculated with EIGEN-5C and EGM2008 models are similar to the results of the numerical model, especially regarding the main large scale features such as boundary currents and the retroflection at the Brazil-Malvinas Confluence.

scholarly journals 4DVAR HF radar assimilation South of Africa

2020 ◽  
Author(s):  
Xavier Couvelard ◽  
Christophe Messager ◽  
Pierrick Penven ◽  
Phillipe Lattes
Keyword(s):  
Complex Dynamics ◽  
Surface Current ◽  
Hf Radar ◽  
Detailed Knowledge ◽  
Oceanic Circulation ◽  
Geostrophic Currents ◽  
Area Of Interest ◽  
Covered Area ◽  
The One ◽  
Gas Fields

Abstract The oceanic circulation south of Africa is characterized by a complex dynamics with a strong variability due to the presence of the Agulhas current and numerous eddies. The area of interest of this paper, is also the location of several natural gas fields under seafloor which are targeted for drilling and exploitation.The complex and powerful ocean currents induce significant issues for ship operations at the surface as well as under the surface for deep sea operations. Therefore, the knowledge of the state of the currents and the ability to forecast them in a realistic manner could greatly enforce the safety of various marine operation. Following this objective an array of HF radar systems was deployed to allow a detailed knowledge of the Agulhas currents and its associated eddy activity. It is shown in this study that 4DVAR assimilation of HF radar allow to represent the surface circulation more realistically. Two kind of experiments have been performed, a one-month analysis and two days forecast. The one-month 4DVAR experiment have been compared to geostrophic currents issued from altimeters and highlight an important improvement of the geostrophic currents. Furthermore, despite the restricted size of the area covered with HF radar, we show that the solution is improved almost in the whole domain, mainly upstream and downstream of the HF radar's covered area. We also show that while benefits of the assimilation on the surface current intensity is significantly reduced in the first 6 hours of the forecast, the correction in direction persists after 48 hours.

Effect of background mean flow on PSI of internal wave beams

2019 ◽  
Vol 869 ◽  
Author(s):  
Boyu Fan ◽  
T. R. Akylas
Keyword(s):  
Internal Wave ◽  
Evolution Equations ◽  
Internal Gravity Wave ◽  
Necessary Condition ◽  
Finite Width ◽  
Asymptotic Model ◽  
Mean Flow ◽  
Short Scale ◽  
The Mean ◽  
Parametric Subharmonic Instability

An asymptotic model is developed for the parametric subharmonic instability (PSI) of finite-width nearly monochromatic internal gravity wave beams in the presence of a background constant horizontal mean flow. The subharmonic perturbations are taken to be short-scale wavepackets that may extract energy via resonant triad interactions while in contact with the underlying beam, and the mean flow is assumed to be small so that its advection effect on the perturbations is as important as dispersion, triad nonlinearity and viscous dissipation. In this ‘distinguished limit’, the perturbation dynamics are governed by the same evolution equations as those derived in Karimi & Akylas (J. Fluid Mech., vol. 757, 2014, pp. 381–402), except for a mean flow term that affects the group velocity of the perturbations and imposes an additional necessary condition for PSI, which stabilizes very short-scale perturbations. As a result, it is possible for a small amount of mean flow to weaken PSI dramatically.

Synergistic Use of Satellite and In-Situ Current Data to Improve the Characterisation of Seasonal Trends

2009 ◽  
Author(s):  
Liam Harrington-Missin ◽  
Mark Calverley ◽  
Gus Jeans
Keyword(s):  
Current Data ◽  
Seasonal Trend ◽  
Measured Signal ◽  
Geostrophic Current ◽  
Geostrophic Currents ◽  
Current Regime ◽  
Long Period ◽  
In Situ Data ◽  
One Year

The synergistic use of measured in-situ current data and altimetry derived geostrophic current data provides improved seasonal characterisation of the current regime, West of Shetland. In September 2007, considerable downtime was experienced by an offshore operator, West of Shetland, as a result of unexpectedly high currents persisting for a number of days. This downtime was unanticipated following conclusions derived from one year of in-situ measured data, which suggested a most favourable current regime during the months August to October. Ten years of altimetry derived geostrophic currents were utilised in conjunction with approximately 3 years of in-situ data to assess the validity of the reported seasonal trend. The altimetry derived geostrophic currents correlated well with the dominating long period signal extracted from the in-situ data. Seasonal comparison between the altimetry derived geostrophic currents and the total measured signal showed the previously available measurement year had a relatively benign September. Based on the 10 years of satellite data, the inter-annual variability of the current regime West of Shetland does not show any clear seasonal trend.

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