Effect of the North Equatorial Counter Current on the generation and  propagation of internal solitary waves off the Amazon shelf (SAR  observations)

Magalhaes, J.M.; Silva, J. C. B. da; Buijsman, Maarten C.; Garcia, Carlos Alberto Eiras

doi:10.5194/os-12-243-2016

Cited by 41 publications

(61 citation statements)

References 60 publications

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“…Therefore, large interfacial ITs may form in the thermocline directly above bottom topography and evolve (through nonlinear processes) to higher-frequency ISW packets [13] . Another possibility is that the IT energy may propagate obliquely with a slope to the horizontal, giving rise to a second generation mechanism known as "local generation" (see e.g.…”

Section: Methodsmentioning

confidence: 99%

Internal solitons in the Andaman Sea: a new look at an old problem

Silva¹,

Magalhaes²

2016

SPIE Proceedings

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When Osborne and Burch [1] reported their observations of large-amplitude, long internal waves in the Andaman Sea that conform with theoretical results from the physics of nonlinear waves, a new research field on ocean waves was immediately set out. They described their findings in the frame of shallow-water solitary waves governed by the K-dV equation, which occur because of a balance between nonlinear cohesive and linear dispersive forces in a fluid. It was concluded that the internal waves in the Andaman Sea were solitons and that they evolved either from an initial waveform (over approximately constant water depth) or by a fission process (over variable water depth). Since then, there has been a great deal of progress in our understanding of Internal Solitary Waves (ISWs), or solitons in the ocean, particularly making use of satellite Synthetic Aperture Radar (SAR) systems. While two layer models such as those used by Osborne and Burch [1] allow for propagation of fundamental mode (i.e. mode-1) ISWs, continuous stratification permits the existence of higher mode internal waves. It happens that the Andaman Sea stratification is characterized by two (or more) maxima in the vertical profile of the buoyancy frequency N(z), i.e. a double pycnocline, hence prone to the existence of mode-2 (or higher) internal waves. In this paper we report solitary-like internal waves with mode-2 vertical structure co-existing with the large well know mode-1 solitons. The mode-2 waves are identified in satellite SAR images (e.g. TerraSAR-X, Envisat, etc.) because of their distinct surface signature. While the SAR image intensity of mode-1 waves is characterized by bright, enhanced backscatter preceding dark reduced backscatter along the nonlinear internal wave propagation direction (in agreement with Alpers, 1985 [2] ), for mode-2 solitary wave structures, the polarity of the SAR signature is reversed and thus a dark reduced backscatter crest precedes a bright, enhanced backscatter feature in the propagation direction of the wave. The polarity of these mode-2 signatures changes because the location of the surface convergent and divergent zones is reversed in relation to mode-1 ISWs. Mode-2 ISWs are identified in many locations of the Andaman Sea, but here we focus on ISWs along the Ten Degree Channel which occur along-side large mode-1 ISWs. We discuss possible generation locations and mechanisms for both mode-1 and mode-2 ISWs along this stretch of the Andaman Sea, recurring to modeling of the ray pathways of internal tidal energy propagation, and the P. G. Baines [3] barotropic body force, which drives the generation of internal tides near the shallow water areas between the Andaman and Nicobar Islands. We consider three possible explanations for mode-2 solitary wave generation in the Andaman Sea: (1) impingement of an internal tidal beam on the pycnocline, itself emanating from critical bathymetry; (2) nonlinear disintegration of internal tide modes; (3) the lee wave forming mechanism to the west of a ridge during westw...

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Section: Methodsmentioning

confidence: 99%

Internal solitons in the Andaman Sea: a new look at an old problem

Silva¹,

Magalhaes²

2016

SPIE Proceedings

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“…First, the internal tide generation (barotropic to baroclinic conversion) may vary in time with the tidal forcing due to local changes in stratification and/or remotely generated incoherent internal tides [ Chavanne et al ., ; Nash et al ., ; Kelly et al ., ; Pickering et al ., ; Kerry et al ., ]. After generation, the internal tides propagate through mesoscale flows, and the associated spatial and temporal variability in stratification, currents, and vorticity may cause time variable refraction of the internal gravity wave fields [ Park and Watts , ; Rainville and Pinkel , ; Ward and Dewar , ; Zaron and Egbert , ; Dunphy and Lamb , ; Ponte and Klein , ; Kelly and Lermusiaux , ; Kelly et al ., ; Kerry et al ., ; Magalhaes et al ., ]. Hence, the internal tides become phase incoherent with the tidal forcing at a given location in the ocean.…”

Section: Introductionmentioning

confidence: 99%

“…Magalhaes et al . [] attributed the seasonal variability of the internal tides from the Amazon shelf break, observed in satellite imagery, to the seasonal variability of the North Equatorial Counter Current.…”

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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“…On September 22, 2012, Jason-2 overpassed the equatorial Atlantic Ocean (Pass 176; Cycle 155) at 21:30 UTC, to the North of the Amazon shelf break where large amplitude internal solitary waves are generated (see Magalhaes et al [25] ; their Figure 3a). A MODIS Aqua image acquired at 16:10 UTC over the same region reveals several large ISW trains I a+iw,d+vi+, 1,y,akt41, propagating to the northeast, whose wave fronts extend for at least 300 km approximately across the direction of the altimeter track (see red line in Fig.…”

Section: Amazon Tropical Atlantic Oceanmentioning

confidence: 99%

“…2a were corrected for the time gap between the two satellite observations, since ISWs propagate with velocities as high as 3.4 m/s off the Amazon shelf (see Magalhaes et al [25] ). Assuming a phase speed of 3.4 m/s and a wave propagation direction of 045 °TN, during the 5h20min between MODIS overpass and the Jason-2 overpass, the waves (and their surface manifestations) moved 0.413° in longitude to the East and 0.413° to the North.…”

Section: Amazon Tropical Atlantic Oceanmentioning

confidence: 99%

A note on radar altimeter signatures of internal solitary waves in the ocean

Silva¹,

Cerqueira²

2016

SPIE Proceedings

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It is well known that Internal Waves of tidal frequency (i.e. Internal Tides) are successfully detected in seasurface height (SSH) by satellite altimetry [1]. Shorter period Internal Solitary Waves (ISWs), whose periods are an order of magnitude smaller than tidal internal waves, are however generally assumed too small to be detected with standard altimeters (at low sampling rates, i.e. 1 Hz). This is because the Radar Altimeter (RA) footprint is somewhat larger, or of similar size at best, than the ISWs typical wavelengths. Here it will be demonstrated that new generation high sampling rate satellite altimetry data (i.e. ~20 Hz) hold a variety of short-period signatures that are consistent with surface manifestations of ISWs in the ocean. Our observational method is based on satellite synergy with imaging sensors such as Synthetic Aperture Radar (SAR) and other high-resolution optical sensors (e.g. 250m resolution MODIS images) with which ISWs are unambiguously recognized. A first order commonly accepted ISW radar imaging mechanism is based on hydrodynamic modulation models [2] [3] in which the straining of surface waves due to ISW orbital currents is known to cause modulation of decimeter-scale surface waves, which have group velocities close to the IW phase velocity. This effect can be readily demonstrated by measurements of wind wave slope variances associated with short-period ISWs, as accomplished in the pioneer work of Hughes and Grant [4] . Mean square slope can be estimated from nadir looking RAs using a geometric optics (specular) scattering model [5][6] [7] , and directly obtained from normalized backscatter (sigma0) along-track records. We use differential scattering from the dual-band (Ku-and C-bands) microwave pulses of the Jason-2 high-rate RA to isolate the contribution of small-scale surface waves to mean square slope. The differenced altimeter mean square slope estimate, derived for the nominal wave number range 40-100 rad/m, is then used to detect ISWs in records of along-track high sampling rate RAs. The RA signatures of these ISWs are also apparent in radar backscattered pulse waveforms from the original Sensor Geophysical Data Records (SGDR), in high resolution (20-Hz) data. The shape of these waveforms is modified by the ISWs with respect to waveforms unperturbed by short-period internal waves. Hence, a new method for identification of ISWs in high-rate RA records that combines along-track differenced mean square slopes across ISW crests and waveform shape variation is put forward in this paper. Validation of the method is warranted with quasi-coincident (in time and space) SAR images of ISWs in various deep ocean regions, such as the Andaman Sea, the Mascarene Ridge of the Indian Ocean and the North Atlantic tropical ocean. The practical significance of this new method is related to the anticipated SWOT wide-swath altimeter mission as well as the recently launched Sentinel-3A SARAL, for which removal of internal wave signals may be of critical importance for observing other high-...

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Effect of the North Equatorial Counter Current on the generation and propagation of internal solitary waves off the Amazon shelf (SAR observations)

Cited by 41 publications

References 60 publications

Internal solitons in the Andaman Sea: a new look at an old problem

Internal solitons in the Andaman Sea: a new look at an old problem

Semidiurnal internal tide incoherence in the equatorial Pacific

A note on radar altimeter signatures of internal solitary waves in the ocean

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