Internal solitary waves (ISWs) have considerable energy to drive the mixing of water masses in the Sulu Sea. The propagation speed is a critical parameter in demonstrating the dynamic characteristics of ISWs. We collected 1354 groups of ISWs’ speeds from tandem satellite remote sensing images with short temporal intervals and analyzed their spatial and multi-scale temporal variations in the Sulu Sea. The wave speeds increase exponentially with water depth with a power of 0.26. The fortnightly spring/neap tidal currents cause daily variations of wave speeds up to 30%. In addition to the well-recognized stratification that leads to monthly variations of wave speed, the seasonal circulations lead to a maximum decrease of wave speeds by 0.27 m/s. With respect to interannual variations, the wave speeds increase in La Niña years and decrease in El Niño years, caused by the climatic modulation of ocean stratification.

Attenuation of ocean waves by ice is a crucial process of the interaction between waves and sea ice in marginal ice zone (MIZ), while such interaction can contribute to the retreating of sea ice in the Arctic. Based on the retrieved two-dimensional ocean wave spectra by spaceborne SAR, we investigated the attenuation of ocean waves in the MIZ in Svalbard and Greenland. The results show that the energy attenuation rate ranges from \(0.126\times10^{-4}\)/m to  \(0.618\times10^{-4}\)/m . Quantitative analysis suggests that the attenuation rate is significantly related to wave height and peak wave period of coming waves. It is further found that the waves decay faster in the area with ice thickness exceeding 0.5 m. We compared the derived wave attenuation rates in the present study with those in previous studies based on in situ measurements, which reveals that waves are becoming less attenuated by sea ice in the Arctic in recent decades.

Nonlinear internal waves (NLIWs) are prevalent on the northern continental shelf south of Hainan Island in the South China Sea. However, our understanding of these NLIWs is still in a preliminary state. In the study, through a synergistic analysis of satellite observations, in situ measurements and numerical simulations, we take a step toward fully elucidating the generation and propagation process of the NLIWs on the northern continental shelf south of Hainan Island. Both mooring and satellite observations suggest that the Xisha Islands remotely generate the NLIWs on the northern continental shelf south of Hainan Island; moreover, more than one source exists in the Xisha Islands. Based on the realistic topography, stratification and tidal forcing, two-dimensional numerical simulations using the MITgcm reveal that the NLIWs observed on the continental shelf can be generated from different sills in Xisha Islands through the internal tidal beam or mixed tidal lee wave regime. Furthermore, the internal tide energy flux estimation suggests that the locally generated internal tides emanating from the continental shelf break play an essential role in the baroclinic energy field of the deep sea while their influence on the baroclinic energy field of the mid-continental shelf can be almost negligible.

The generation of shoreward nonlinear internal waves (NLIWs) on the continental shelf south of the Hainan Island (SHI) is investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. Two types of shoreward NLIWs are identified from SAR images according to their distinct geographic distribution. One type of NLIWs, named Type-N NLIWs, is distributed on the northern SHI, and the other one is named Type-S NLIWs, distributed on the southern SHI. The SAR-observed wave occurrence frequency during the spring and neap tides, combined with the calculated body force, suggests that the Type-N NLIWs originate from the Xisha Islands, whereas the Type-S NLIWs originate from both the Xisha Islands and the continental shelf break, and the shelf break has a larger contribution. The synergistic analyses of the internal tidal ray path, gamma parameter and earliest SAR-observed NLIWs reveal that the Type-N NLIWs are excited by the impingement of the diurnal internal tidal beams emanating from the Xisha Islands on the near-surface pycnocline close to the continental shelf. Based on the realistic shelf-slope topography and tidal forcing, the two-dimensional numerical simulations using the MITgcm suggest that the Type-S NLIWs result from the nonlinear disintegration of a mode-1 diurnal internal tide which develops from a lee wave formed at the continental shelf break. Furthermore, the sensitive numerical experiments show that the background current can greatly affect the nonlinear evolution of the internal waves generated at the shelf break.

The twin Sentinel-1 (S1) satellites have been extensively acquiring synthetic aperture radar (SAR) data in the Arctic, providing the unique opportunity to obtain ocean dynamic parameters with both high spatial resolution and wide swath coverage in the marginal ice zone (MIZ). In this paper, we proposed a method for retrieving the ocean significant wave height (SWH) from S1 SAR data in horizontal-horizontal (HH) polarization based on a backpropagation neural network (BPNN). A total of 4,273 scenes from S1 extra wide swath mode data acquired in the Arctic were collocated with data from four radar altimeters (RA), yielding 126,128 collocated data pairs. These data were separated into training and testing datasets to develop a BPNN model for retrieving SWH. Comparing the S1 retrieved SWH using the testing dataset with the RA SWH yielded a bias of 0.17 m, a root-mean-square error of 0.71 m and a scatter index of 23.05% for SWH less than 10 m. The S1 retrieved SWH were further compared with CFOSAT/SWIM data acquired in the Arctic between August 2019 and May 2020 to validate the SWIM performance on wave measurements at different beams.

Interaction between ocean waves and sea ice may play an important role in sea ice retreat in the Arctic. However, it is difficult to quantify the change of ocean waves propagating in ice as nearly no available measurements. Although SAR has shown the capability of imaging ocean waves in ice-covered areas, there are few attempts to retrieve two-dimensional ocean wave spectra (OWS) by SAR. In this study, we applied the previously developed nonlinear inversion scheme, i.e., the MPI scheme, to retrieve OWS by the Sentinel-1 SAR data acquired in the Barents Sea, where waves penetrate deeply in ice. We compared the retrieved spectra by different combinations of modulation transfer functions (MTFs) used in the MPI scheme, i.e., the same MTFs as those used in retrievals in open water, neglecting both the hydrodynamic and tilt modulations in the MTFs, and neglecting the hydrodynamic modulation but remaining the tilt modulation (a new one fitted in this study for HH-polarized SAR data over ice) in the MTFs. As no in situ measurements (e.g., by directional buoys) available, we compared the simulated SAR image spectra based on the retrievals with the observational SAR image spectra to quantify their respective performances. The comparisons suggest that neglecting hydrodynamic modulation can significantly improve the retrievals. Remaining tilt modulation can further improve the retrievals, particularly for range-travelling waves. The study enhances the understanding of principles of SAR imaging waves in ice and provides basics for retrievals of ocean wave spectra by SAR data in ice-covered areas.