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

Abstract

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...

Internal Solitary Waves in the Andaman Sea: New Insights from SAR Imagery

“…In this map, only the leading and strongest observations were accounted for, and the mean distance between mode-2 ISW-like packet envelopes in each image (λ) was also listed in Table 1, along with their corresponding number of observations (N WT ). It is worthwhile noting that these features consistently present interpacket distances of around 30 km, just as is observed in our case study in Figure 3b and in da Silva and Magalhaes [22]. Also, the composite map in Figure 1 reveals a well-organized IW field whose propagation envelope (in light red in Figure 1) is fairly along the tidal currents there, and suggests an origin along the shallow ridges to the west.…”

“…This is somewhat unexpected since the larger mode-1 ISW packets commonly observed in the Andaman Sea are known to be more frequently observed close to spring tides (see e.g., [13]). For consistency, one other image which is presented in da Silva and Magalhaes [22] is also presented in Table 1, and contains a very similar picture to that in Figure 3b.…”

“…Six SAR images that were acquired and processed from TerraSAR-X were selected as representative of the ISW activity in the Andaman Sea between 8 • and 11 • N. These are listed chronologically in Table 1, together with the Envisat-ASAR acquisition used in da Silva and Magalhaes [22], which is also especially important in this study (to be discussed in the next sections). All TerraSAR-X acquisitions were in ScanSAR-mode (with a nominal spatial resolution of 18 m), which means that the viewing strips were approximately 200 km wide and as long as 1000 km long in some cases.…”

Long lived second mode internal solitary waves in the Andaman Sea

“…A high-resolution non-hydrostatic and fully nonlinear setup is used in the MITgcm to simulate the internal wave field along the Ten-Degree Channel of the Andaman Sea. Previous studies 12,13 revealed the propagation of unusually long-lived mode-2 solitary-like waves along this particular stretch, whereas the usual large-scale mode-1 ISWs were never observed in satellite imagery. The model results are consistent with these observations since they too feature high-frequency mode-2 oscillations propagating along the pycnocline instead of the usual mode-1 solitary waves.…”

Metocean Criteria for Internal Solitary Waves Obtained from Numerical Models

“…The phase speeds estimated from satellite imagery by Tensubam et al [52] are in good agreement with phase speeds estimated from the Sturm-Liouville equation using climatological stratification and realistic bathymetry of the area. da Silva and Magalhaes [53] have estimated 2.37 m/s for the phase speed of mode-1 waves with interpacket distance of 106 km. Estimates from Sun [49] in the south Andaman Sea have provided values ranging from 2.35 to 2.65 m/s for the phase speed of long-lived mode-1 waves.…”