The Internal Waves Service Workshop What: This workshop gathered leading experts in satellite remote sensing, oceanography, and artificial intelligence to advance the development of the Internal Waves Service (IWS)-the first global, operational platform for automatic detection of internal solitary waves (ISWs) from synthetic aperture radar (SAR) imagery. The participants reviewed the current state of internal wave research, shared technical advances, and defined priorities for future collaboration. When: 3-4 April 2025 Where: Angra do Hero & iacute;smo, Azores, Portugal

Oceanic internal solitary waves (ISWs) play a pivotal role in ocean dynamics, climate regulation, nutrient transport, ecosystem health, and can impact anthropogenic structures such as offshore platforms and submarine navigation. Yet their observation has remained labor-intensive and geographically limited. Leveraging synthetic aperture radar (SAR) data, and the global coverage of Sentinel-1 Wave Mode, we have developed the Internal Waves Service (IWS), the world's first global, near-real-time, repository for ISW detection, indexing, mapping, and archiving. This unprecedented platform contributes to the unification of a previously segmented field of study, integrating Earth observation, advanced data pipelines, and AI-driven analytics into a single operational system. Using an XGBoost-based classifier, IWS processes SAR vignettes to automatically detect ISWs at planetary scale, generating a persistent, searchable dataset enriched with metadata. An interactive expert-validation interface enables continuous curation and iterative retraining, ensuring both scientific rigor and increased accuracy. Developed in collaboration with over twenty partner institutions worldwide, IWS is built to serve the research community, industry operators, and governmental agencies alike. By combining automation, global coverage, and open access, it establishes a new data science infrastructure for operational oceanography, climate research, and ecosystem modeling, delivering systematic monitoring of ISWs anywhere on a global ocean for the first time.

Wind and Internal Solitary Waves (ISWs) are well-known to mix the ocean's surface and inner stratification, but their combined effects appear to have not yet been investigated. A large ensemble of ISWs measured off the Portuguese Coast reveals that wind and ISWs may combine to increase turbulence and mixing beyond the linear combination of their individual contributions. It is found that low bulk Richardson numbers (Ri < (1)/(4)) and temperature inversions increase respectively by nearly four-fold and an order of magnitude, when comparing ISWs propagating under high and low-wind regimes. Furthermore, depth ranges where Ri < (1)/(4) associated with ISWs propagating with high winds are observed to extend at least between unperturbed thermocline and intermediate depths. Understanding how turbulence and mixing from ISWs and wind combine may have important implications ranging from parametrizations in ocean models to our understanding of biogeochemical processes modulated by diapycnal mixing.

This study presents a multi-satellite approach utilizing synthetic aperture radar (SAR) and optical sensors to distinguish the characteristics of internal solitary waves (ISWs) during different seasons in the Lombok Strait. SAR and optical sensors are employed to estimate the dynamic parameters (soliton number, wavelength, and phase speed) of ISWs during different seasons based on the detected ISW patterns. ISW characteristics in the Lombok Strait during two seasons were observed using Sentinel-1/SAR, GCOM-C/SGLI, and Terra/MODIS sensors. Results indicated that Sentinel-1/SAR detected a higher soliton number for the northward-propagating ISWs in the Lombok Strait during the north-west monsoon (NWM) than the south-east monsoon (SEM) period. The identified wavelengths of ISWs were wider during the SEM than during the NWM whenever two packets were detected in one image. Similar variations were observed by optical sensors (SGLI and MODIS). Estimation of ISW phase speed derived from multi-satellite images and the Korteweg-de Vries (KdV) equation showed that the phase speed was faster during the NWM than the SEM. These results correlated with the seasonal variation of thermocline depth and density differences between the two layers. Because of the bias caused by turbid water, a pattern of high chlorophyll-a was observed during the NWM period in the SGLI, MODIS, and VIIRS products. Propagation of turbid water to the north was assumed to be a result of the intense ISW activity in this area. Highly variable currents due to ISW activity in the Lombok Strait area were assumed to cause the anomalous sediment transport in this study. The multi-satellite observation data used in this study enhanced understanding of the influence of ISWs on coastal interactions in the Lombok Strait.

The decay of the low-mode internal tide due to the superharmonic energy cascade is investigated in a realistically forced global Hybrid Coordinate Ocean Model (HYCOM) simulation with 1/25 degrees (4 km) horizontal grid spacing. Time-mean and depth-integrated supertidal kinetic energy is found to be largest near low-latitude internal tide generation sites, such as the Bay of Bengal, Amazon Shelf, and Mascarene Ridge. The supertidal kinetic energy can make up to 50% of the total internal tide kinetic energy several hundred kilometers from the generation sites. As opposed to the tidal flux divergence, the supertidal flux divergence does not correlate with the barotropic to baroclinic energy conversion. Instead, the time-mean and depth-integrated supertidal flux divergence correlates with the nonlinear kinetic energy transfers from (sub)tidal to supertidal frequency bands as estimated with a novel coarse-graining approach. The regular spaced banding patterns of the surface-intensified nonlinear energy transfers are attributed to semidiurnal mode 1 and mode 2 internal waves that interfere constructively at the surface. This causes patches where both surface tidal KE and nonlinear energy transfers are elevated. The simulated internal tide off the Amazon shelf steepens significantly near these patches, generating solitary-like waves in good agreement with Synthetic Aperture Radar (SAR) imagery. Globally, we find that regions of high supertidal energy flux also show a high correlation with observed instances of solitary nonlinear internal waves.

The equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air-sea interactions and climate issues. Recently, Warner et al. revealed gravity currents appar-ently originating in tropical instability waves. Both phenomena have strong dissipation rates and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than 1 m. Here, we present Sentinel-3 satellite observations of internal solitary waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210 degrees to 265 degrees E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves' characteristics, such as the distance between wave crests, crest lengths, and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of kilometers. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents.

Satellite altimetry has been providing a continuous record of ocean measurements with numerous applications across the entire range of ocean sciences. A reference orbit has been used since 1992 with TOPEX/Poseidon, which was repeated in the Jason missions, and in the newly launched Sentinel-6 Michael Freilich (in November 2020) to continually monitor the trends of sea level rise and other properties of the sea surface. These multidecadal missions have evolved alongside major technological advances, whose measurements are unified into a single data record owing to continuous intercalibration and validation efforts. However, the new Sentinel-6 provides synthetic aperture radar (SAR) processing, which improves the along-track resolution of conventional altimeters from a few kilometres (e.g., for Jason-3) to about 300 m. This means a major leap in sampling towards higher frequencies of the ocean spectrum, which inevitably means reconciling the assumption of a uniform Brown surface between the footprints of the larger kilometre-scale conventional altimetry and those of the finer-scale SAR altimetry. To explore this issue, this study uses the vantage point of the Sentinel-6/Jason-3 tandem phase to compare simultaneous sea surface signatures of large-scale Internal Solitary Waves (ISWs) between SAR and conventional altimetry. These waves can modulate the sea surface into arrayed sections of increased and decreased roughness with horizontal scales up to 10 km, which inflict sharp transitions between increased and decreased backscatter in the radar altimeters. It is found that Sentinel-6 can provide more detailed structures of ISWs in standard level-2 products, when compared with those from the conventional Jason-3 (similarly to previous results reported from the SAR altimeter from Sentinel-3). However, a new and striking feature is found when comparing the radar backscatter between Sentinel-6 and Jason-3, which are in opposite phases in the ISWs. These intriguing results are discussed in light of the intrinsically different acquisition geometries of SAR and conventional altimeters as well as possible implications thereof.

In this study, the multi-polarization L-band scattering mechanisms of internal waves are investigated. Internal waves represent key geophysical factors for sea-air heat ex-change, playing an important role on the evolution of marine ecosystems. Hence, a better understanding of internal waves microwave scattering mechanisms can support the devel-opment of advanced tools based on polarimetric synthetic aperture radar imagery. The latter can be exploited to miti-gate the impact internal waves may have on offshore drilling operations and aquaculture. In this study, the analysis is performed using full-polarimetric L-band synthetic aperture radar images of internal waves ob-served under different radar imaging parameters and sea state. The analysis is also supported by ancillary information from external independent sources. Experiments show that the co-polarized backscattering of internal waves is remarkably impacted by non-polarized scattering mechanisms, with the wave crest signatures being more affected than the troughs.

In this study, the scattering mechanisms associated to internal waves (IWs) are investigated at L-band. IWs represent key geophysical factors for sea-air heat exchange and play a paramount role in the biological primary production and in the understanding of the evolution of climate ecosystem. In addition, a better understanding of IWs microwave scattering mechanisms can improve the modeling capability and, therefore, can boost the development on advanced synthetic aperture radar (SAR)-based added-value products to mitigate the risk for offshore drilling operations and aquaculture activities associated to IWs. The analysis of L-band multi-polarization SAR scattering of IWs under the influence of surface current straining is performed using a meaningful full-polarimetric Advanced Land Observing Satellite Phased Array type L-band 1 SAR data set collected over IWs observed under different imaging and wind conditions. Time and space co-located ancillary information is also available. Experimental results demonstrate that the non-polarized scattering mechanisms constitute a significant contribution to the total IW backscattering, especially in the case of surface current gradients owing to IWs (about 48-57%). It is also found that the non-polarized scattering contribution associated to IW concentrates along the wave crests, i.e. it is at least 60% larger than the one observed along the wave troughs. In addition, considering the IW traveling directions relative to that of the wind, the non-polarized scattering contribution associated to IWs is more remarkable at upwind direction while it is less significant at down/crosswind directions. The non-polarized scattering mechanisms also calls for a modulation induced by IWs which is much more significant,i.e. at least three times, that the one that characterizes the polarized scattering mechanism.

We address surface wave breaking caused by oceanic Internal Solitary Waves (ISWs) and how ISWs are manifested in the SAR altimeter onboard Sentinel-3A and -3B satellites by means of their effects in Significant Wave Height (SWH). Two different regions of the ocean are selected, namely the tropical Atlantic Ocean off the Amazon shelf and the Banda Sea in the Indian Ocean, where there are scenes of Sentinel-3 OLCI acquired simultaneously with an along-track SAR mode altimeter, which include signatures of large amplitude ISWs. New data of unfocused SAR (UF-SAR 20 Hz) and fully focused SAR (FF-SAR 160 Hz) modes are analyzed, which are retracked in full range and over a reduced range of bins (truncation carried out dynamically ten gates away from the estimated epoch position). At first order, in scales of 1–3 km, a strong decrease in the normalized radar cross section (NRCS) over the rough part of the ISWs is observed followed by a small increase in the smooth part relative to the unperturbed ocean background. A second order ISW signature, in scales of 20 km, is noted: the SWH is attenuated after the passage of an ISW, considering length scales of about 10 km before and after the ISW crest. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after the passage of an ISW, admittedly because intense meter-scale wave breaking results in surface wave energy dissipation. Furthermore, Sentinel-2 MSI images are analyzed and provide insights into this same phenomenon: white-capping resulting in a radiance increase at all (visible) wavelengths. Modulation of breaking waves owing to ISWs is demonstrated by estimates of the fraction of breaking waves in the presence of internal waves.

The study of dynamic features of the ocean, in which complex physical, chemical, and biological interactions evolve on multiple time scales, poses significant sampling challenges because the required spatial and temporal resolutions are not possible by ship or satellite studies alone. Satellite remote sensing captures only surface effects while expensive research vessels can only make discrete observations in finite periods of time. Our work with networked marine robotics in the aerial, surface, and underwater domains is at the vanguard of a new approach to scientific exploration and observation, which brings together several technologies to enable oceanographic vessels and robots to work in tandem, thus expanding the observational footprint of these vessels. We describe a scientific cruise in the Spring of 2018 in the open waters of the Pacific where we deployed a fleet of autonomous robots to demonstrate this approach for the synoptic observation of mesoscale and sub-mesoscale features of a frontal zone. We articulate the elements and methods to multi-vehicle coordination and challenges that lie ahead in ocean observation.

Timescales in the ocean can range from the transient turbulence to the long-term climate scales. Quantifying its changes involves establishing meaningful background states, but that can be challenging if an extensive array of wave phenomena is masking the ocean's variability. Internal Waves (IWs) are a fundamental part of these wave phenomena, which are now widely acknowledged as major contributors in ocean dynamics. In this study we use satellite imagery from Synthetic Aperture Radars (SAR) and moored temperature records to assess the variability scales and magnitudes of coastal IW systems propagating onshore off the western Portuguese shelf. The data shows significant variability in timescales of just a few days, and hence within the waves' typical propagation lifespan. A regional ocean circulation model is used together with linear theory to assess how mesoscale variability is contributing to the observed variability patterns observed in the IWs in the SAR and in situ data. According to linear theory, the IW variability is mostly owing to variable mesoscale currents, which off the Portuguese coast typically result from geostrophic flows, eddies and upwelling dynamics. IW variability owing to changes in stratification is found to amount to about half of that in background flows, but their effect can go unnoticed within the largest contributions from mesoscale currents. One extreme event is highlighted in which IWs from consecutive tidal cycles could possibly decouple from (i.e. not traceable back to) their originating tides in just a few days and hence within their typical lifespans.

Two ocean colour features in the western Iberian margin (western Europe) were described in present work. They were identified in 10 years (2002-2012) of MODIS-AQUA normalized water-leaving radiance (nLw) at 555 nm and their causes were investigated using complementary satellite, in-situ and model data. One feature was a band of high nLw555 (>0.8 mW cm-2 mu m- 1 sr- 1) with exceptionally large widths (>20 km) along the coast. Over the 10-year period, this wide coastal band of high nLw555 only occurred during winter, following better the increases in wave height, than the increases in precipitation and river discharge. The band seemed particularly associated with episodes of large wave heights (>6 m), under weak thermal stratification and northerly winds. Findings suggest that the band is the signature of exceptional offshore extensions of the more reflective turbid coastal waters, as a result of wave-induced sediment resuspension along the coast. The other feature consisted in areas of equally high nLw555, but separated from the coast by more than 40 km. These offshore areas of high nLw555 shared spectral signatures of phytoplankton, namely coccolithophores, typically formed during spring following water column re-stratification and reached larger areas after winters with deeper oceanic mixed layers. Less commonly, they also occurred from summer to autumn. Results indicate they are mainly the signature of enhanced coccolithophore abundances that also create turbid reflective waters due to their calcite plates. The used methodology separated the two features in time and therefore seems to allow for satellite monitoring of sediment resuspension and coccolithophore blooms off west Iberia and similar coastal environments, which is particularly relevant for detecting events of sediment transport, potentially linked to water quality problems, and long-term changes in phytoplankton composition.

This study investigates along-shelf propagating internal solitary waves (ISWs) on the Amazon Shelf using satellite observations and numerical modeling. These ISWs appear as streak-like patterns in satellite images along a narrow path within the North Brazilian Current (NBC), propagating against the current. The streak-like patterns of the along-shelf propagating ISWs are a result of the quasi-two-dimensional bathymetry below, suggesting the possibility of using sea surface imprints to detect the topographic features beneath. In this study, both the effects of the NBC and tidal current on ISW generation are considered over irregular seafloor. Near-critical conditions (internal Froude number close to 1) created by the joint effects of the NBC and the tides result in ISW generation, while the dominant subcritical conditions result in the upstream propagation of these ISWs. Both the NBC and tidal currents are needed to continuously generate ISWs. This study demonstrates that small-scale topographic features can result in the generation of large numbers of ISWs, which are expected to significantly contribute to ocean mixing and, potentially, sediment resuspension. The ISW-induced current also contributes to sea surface wave breaking as observed by satellites.

Internal waves (IWs) in the ocean span across a wide range of time and spatial scales and are now acknowledged as important sources of turbulence and mixing, with the largest observations having 200 m in amplitude and vertical velocities close to 0.5 m s −1 . Their origin is mostly tidal, but an increasing number of non-tidal generation mechanisms have also been observed. For instance, river plumes provide horizontally propagating density fronts, which were observed to generate IWs when transitioning from supercritical to subcritical flow. In this study, satellite imagery and autonomous underwater measurements are combined with numerical modeling to investigate IW generation from an initial subcritical density front originating at the Douro River plume (western Iberian coast). These unprecedented results may have important implications in near-shore dynamics since that suggest that rivers of moderate flow may play an important role in IW generation between fresh riverine and coastal waters.

Breaking surface waves play a key role in the exchange of momentum, heat, and gases between the atmosphere and the ocean. Waves break at the ocean's surface at high or medium wind speeds or in the absence of wind due to shoaling of the seafloor. However, surface waves also break due to interactions with internal solitary waves (ISWs). In this paper, we revisit surface wave breaking caused by ISWs and how ISWs are manifested in synthetic aperture radar (SAR) images acquired by the TerraSAR-X and Sentinel-1 satellites and in high-resolution radar altimeter data acquired by the SAR altimeter (SRAL) onboard the Sentinel-3A satellite. X-band TerraSAR-X images acquired at low wind speeds suggest that meter-scale surface breaking waves resulting from large-scale ISWs are associated with large modulations in backscatter at HH and VV polarizations that cannot be explained by present theories. Furthermore, Sentinel-1 C-band SAR satellite images acquired at moderate to high wind speeds also exhibit large radar signatures from surface wave breaking at VV and VH cross-polarizations. Finally, new observations from the Sentinel-3 SRAL altimeter show clear evidence of significant wave height (SWH) variations along the propagation paths of ISWs. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after an ISW passes, most likely because intense meter-scale wave breaking results in surface wave energy dissipation. In summary, these results show that surface wave breaking contributes significantly to radar remote sensing of ISWs.

In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both. In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.

Physical oceanography is increasingly relying on satellite remote sensing to survey the perpetually undersampled ocean, whereas the latest Synthetic Aperture Radars (SARs) are moving forward to provide a more continuous monitoring of the ocean. In this study we use a collection of SAR images to document the two-dimensional horizontal structure of Internal Solitary Waves (ISWs) propagating between two large submarine canyons off the Western Iberian Peninsula (between May and October 2018), which are observed to intersect approximately along the mid-shelf and originate a naturally-occurring interaction hotspot between different ISW packets. ISW interactions are well documented in theory and in laboratorial and numerical studies, but their observations in the real ocean are limited to airborne observations over the Strait of Georgia. The frequent SAR imagery of interacting ISWs in this region provides additional case studies to the literature, and we investigate if an energy proxy taken from their sea surface signatures can be used as an indicator for high-energy interaction events (e.g. when comparing with a non-interacting background). In particular, a quasi-synergetic event captured both in SAR and in a moored thermistor chain reveals that the often used weakly nonlinear theory for small-amplitude waves may underestimate the amplitudes measured in the waves’ interacting sections. ISWs provide the largest vertical displacements and velocities in the ocean. Understating how their vertical structure changes during wave-wave interactions may have important implications in the broader spectrum of ocean sciences, and SARs are shown in this study to be a first-approach tool to survey this frequent phenomenon in coastal regions.

Study of ocean processes is important to understanding climatic variability especially on the productive upper water-column. Ocean currents regulate the climate, it captures CO\(_2\) from the atmosphere and oxygen is generated by its plankton communities, all of which are part of the global environmental cycle which are being impacted by anthropogenic change. Much of the ocean, however, remains unexplored especially the bio-geochemical processes in the water-column which need to be examined at scale. Satellite remote sensing captures only surface effects while expensive research vessels can only make discrete observations in finite periods of time. Our work with networked marine robotics in the aerial, surface and underwater domains is at the vanguard of a new approach to scientific observation, which brings together technology to enable vessels and robots to work in tandem for capturing synoptic views of open ocean phenomena. We describe a cruise in the Spring of 2018 in the open waters of the Pacific where we employed a fleet of autonomous robots for simultaneous observations of mesoscale and sub-mesoscale features of an unexplored frontal zone. We articulate our approach to multi-vehicle coordination and challenges that lie ahead for research in this harsh domain.

In this paper we demonstrate, for the first time that, standard Level 2 SRAL altimeter data from Sentinel-3 can be used to estimate Internal Solitary Wave (ISW) amplitudes and surface velocities. A two-layer model is assumed based on the hydrostatic limit. The method is applied to Sentinel-3B data of the Banda Sea, where powerful ISWs emanate from a sill in the Ombai Strait in between Alor and Atauro Islands, one of the major passages of the Indonesian Through Flow. There, mean square slopes (<;s 2 >) of surface waves are calculated from the dual-band altimeter on board Sentinel-3, showing strong variations across ISW crests that are easily detected in the SRAL. Sea Level Anomalies, appear to be reliable and consistent with expected sea level elevations owing to the passage of solitons of depression (i.e., with isopycnals displaced downwards). Fifteen cycles of Sentinel-3B data were analyzed in the study region.