A note on radar altimeter signatures of Internal Solitary Waves in the ocean

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. similar to 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 Jason2 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-frequency sub-mesoscale dynamics

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

Synthetic aperture radar (SAR) imagery from the Amazon shelf break region in the tropical west Atlantic reveals for the first time the two-dimensional horizontal structure of an intense Internal Solitary Wave (ISW) field, whose first surface manifestations are detected several hundred kilometres away from the nearest forcing bathymetry. Composite maps and an energy budget analysis (provided from the Hybrid Coordinate Ocean Model - HYCOM) help to identify two major ISW pathways emanating from the steep slopes of a small promontory (or headland) near 44 degrees W and 0 degrees N, which are seen to extend for over 500 km into the open ocean. Further analysis in the SAR reveals propagation speeds above 3 ms(-1), which are amongst the fastest ever recorded. The main characteristics of the ISWs are further discussed based on a statistical analysis, and seasonal variability is found for one of the ISW sources. This seasonal variability is discussed in light of the North Equatorial Counter Current. The remote appearance of the ISW sea surface manifestations is explained by a late disintegration of the internal tide (IT), which is further investigated based on the SAR data and climatological monthly means (for stratification and currents). Acknowledging the possibility of a late disintegration of the IT may help explain the remote-sensing views of other ISWs in the world's oceans

SAR IMAGING OF WAVE TAILS: RECOGNITION OF SECOND MODE INTERNAL WAVE PATTERNS AND SOME MECHANISMS OF THEIR FORMATION

Mode-2 internal waves are usually not as energetic as larger mode-1 Internal Solitary Waves (ISWs), but they have attracted a great deal of attention in recent years because they have been identified as playing a significant role in mixing shelf waters [1]. This mixing is particularly effective for mode-2 ISWs because the location of these waves in the middle of the pycnocline plays an important role in eroding the barrier between the base of the surface mixed layer and the stratified deep layer below. An urgent problem in physical oceanography is therefore to account for the magnitude and distribution of ISW-driven mixing, including mode-2 ISWs. Several generation mechanisms of mode-2 ISWs have been identified. These include: (1) mode-1 ISWs propagating onshore (shoaling) and entering the breaking instability stage, or propagating over a steep sill; (2) a mode-1 ISW propagating offshore (antishoaling) over steep slopes of the shelf break, and undergoing modal transformation; (3) intrusion of the whole head of a gravity current into a three-layer fluid; (4) impingement of an internal tidal beam on the pycnocline, itself emanating from critical bathymetry; (5) nonlinear disintegration of internal tide modes; (6) lee wave mechanism. In this paper we provide methods to identify internal wave features denominated Wave Tails in SAR images of the ocean surface, which are many times associated with second mode internal waves. The SAR case studies that are presented portray evidence of the aforementioned generation mechanisms, and we further discuss possible methods to discriminate between the various types of mode-2 ISWs in SAR images, that emerge from these physical mechanisms. Some of the SAR images correspond to numerical simulations with the MITgcm in fully nonlinear and nonhydrostatic mode and in a 2D configuration with realistic stratification, bathymetry and other environmental conditions. Results of a global survey with some of these observations are presented, including: the Mascarene Ridge of the Indian Ocean; South China Sea; Andaman Sea; tropical Atlantic off the Amazon shelf break, Bay of Biscay of the western European margin; etc. The survey included the following SAR missions: ERS-1/2; Envisat and TerraSAR-X

Radar probing of surfactant films on the water surface using dual co-polarized SAR

Microwave radar is a very perspective tool for all-weather monitoring of film slicks which appear in radar imagery of the water surface as areas of reduced backscattering due to damping of short wind waves. Information about the backscatter variations obtained from single band/one polarization radar seems to be insufficient for film characterization, so, new capabilities of multi-polarization radar for monitoring of film slicks have been actively discussed in the literature. In this paper results of new field experiments on remote sensing of film slicks using dual co-polarized radars: a satellite X-band TerraSAR-X and recently designed at IAP RAS a Multifrequency Radar Complex - three-band scatterometer operating in X-/C-/S-bands and mounted onboard a ship are presented. Along with backscattering depression the variations of polarized (Bragg) and non polarized radar backscatter components in slicks were analyzed. It is obtained that VV-to-HH backscatter ratio is smaller than the ratio predicted by a Bragg (two-scale) model thus indicating that additional, non polarized (NP), component also contributes to the total radar backscatter. Assuming the radar backscatter to be a sum of polarized (Bragg) and NP components the latter was eliminated from the total radar backscatter, and contrasts for the Bragg and NP components were obtained. The contrasts for the polarized component allowed us to estimate damping of gravity-capillary wind waves at Bragg wavelengths in slick and to give more accurate comparison with models of wave damping due to elastic film