Sea Surface Currents Research Articles

Downscaling of ocean fields by fusion of heterogeneous observations using Deep Learning algorithms

We present a deep learning method to downscale low-resolution geophysical fields by merging them with high-resolution data. The downscaling was performed using an ensemble of convolutional neural networks (CNNs), whose prediction values are the average values of the outputs of 20 CNNs. Academic experiments were conducted on simulated ocean data in the Gulf Stream region, given by the outputs of the NATL60 model. The CNNs forced with low-resolution (120 × 120 km) sea surface high (SSH) data and mesoscale resolution (12 × 12 km) sea surface temperature (SST) data allowed us to obtain mesoscale resolution sea surface currents with good accuracy. Sensitivity experiments have shown that taking SST into account significantly increases the accuracy of the high-resolution velocity retrieval, even when noise is added to the SSH data. The velocity information embedded in the transport equation modeling the SST advection is taken into account by the CNN, which greatly increases the resolution of ocean currents provided by SSH. In the present work, we only consider spatial downscaling by assuming that SSH and SST are daily observations. The method we developed is generic and can be used to improve the resolution of a wide variety of large-scale fields by merging them with high-resolution fields.

Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand

In this study, a high-resolution one-way nested, hindcast ROMS model was developed to analyse the coastal circulation and Lagrangian statistics within the Bay of Plenty (BoP) region in Aotearoa, New Zealand. The Bay of Plenty Model (BoPM) was statistically evaluated against a set of multiple remote sensing and in situ observations from 2003–2004, forming the analysis period for this study. Overall, the BoPM possesses good skill reproducing ocean water temperature, salinity, sea level and water column velocity over tidal and non-tidal timescales (Willmott skill ¿0.8 for most variables). Root-mean-squared errors of <1 °C for ocean water temperature, ≈0.15 for salinity (an exception present during winter), <8 cm s−1 for water column velocity and 0.09 m for non-tidal sea surface height are achieved. Over the 2-year period, nearshore modelled sea surface currents are correlated to local wind forcing on the western and central region of the BoP consistent with coastal wind-driven upwelling dynamics. Up to 30% of the cross-shelf current variability is explained by along-shore wind stress, consistent with previous observational studies. Meanwhile, the eastern region has no significant correlation to the along-shore winds, suggesting other forcings must be considered. Circulation patterns and Lagrangian statistics under two distinct atmospheric conditions over the two years were analysed: January, with predominantly moderate upwelling-favourable winds and July, with highly variable and stronger winds. January conditions show an eastward flowing large-scale boundary current, the East Auckland Current (EAUC), and a quasi-stationary eddy, the East Cape Eddy (ECE), close to the shelf break, while July conditions show the EAUC and ECE located further from shore. A series of particle release experiments from inner-, mid-shelf, and shelf break locations are used to identify trajectory and dispersion variability between western, central, and eastern sections of the BoP under January and July conditions. Particles released under January conditions tend to flow eastward following the EAUC. Under July conditions, inner- and mid-shelf releases converge towards the central BoP, where bathymetric changes and islands create pathways for particles to exit the continental shelf. Relative dispersion (R2) under January conditions shows a ballistic dispersion regime (R2≈t2) over the first 10 days followed by a diffusive regime (R2≈t1) after 15 days. Under July conditions, a ballistic-like regime is maintained throughout a 30-day period, likely due to strong mixing in the BoP associated with submesoscale processes. Particles released in proximity to East Cape headland that remain in the water column for >5 days show higher dispersion relative to releases from the central and western regions of the BoP. However, 70%–90% of particles released on the eastern inner-shelf strand to the coastline in <5 days. This suggests that the eastern region can act as a retention zone, constraining the particles towards the shore. Drift duration increases for particles released further from the shore, where offshore mesoscale currents influence them more, and islands become important receptor locations.

Retrieval of Sea Surface Currents and Directional Wave Spectra by 24 GHz FMCW MIMO Radar

This article investigates the capabilities of 24 GHz frequency-modulated continuous-wave (FMCW) multiple-input–multiple-output (MIMO) radar technology to retrieve sea surface currents and directional wave spectra. A procedure based on the dispersion relation, which was previously applied to process X-band marine radar data, is here exploited. The estimation performance of the radar sensor is first assessed by numerical tests in the case of synthetic sea wave fields with known characteristics in terms of wave direction and surface currents. Finally, the estimation procedure is assessed on real data collected at San Vincenzo quay in the port area of Naples, Italy. The achieved results are encouraging and highlight that 24 GHz FMCW MIMO radar is a viable technology for sea wave monitoring.

Multimodal 4DVarNets for the Reconstruction of Sea Surface Dynamics From SST-SSH Synergies

The space-time reconstruction of sea surface dynamics from satellite observations is a challenging inverse problem due to the associated irregular sampling. Satellite altimetry provides a direct observation of the sea surface height (SSH), which relates to the divergence-free component of sea surface currents. The associated sampling pattern prevents operational schemes from retrieving fine-scale dynamics, typically below 10 days. By contrast, other satellite sensors provide higher-resolution observations of sea surface tracers such as sea surface temperature (SST). Multimodal inversion schemes then arise as appealing approaches. Though theoretical evidence supports the existence of an explicit relationship between sea surface temperature and sea surface dynamics under specific dynamical regimes, the generalization to the variety of upper ocean dynamical regimes is complex. Here, we investigate this issue from a physics-informed learning perspective. We introduce a trainable multimodal inversion scheme for the reconstruction of sea surface dynamics from multi-source satellite-derived observations, namely satellite-derived SSH and SST data. The proposed multimodal 4DVarNet schemes combine a variational formulation involving trainable observation and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> terms with a trainable gradient-based solver. An observing system simulation experiment for a Gulf Stream region supports the relevance of our approach compared with state-of-the-art schemes. We report a relative improvement greater than 60% compared with the operational altimetry product in terms of root mean square error and resolved space-time scales. We discuss further the potential and the limitations of the proposed approach for the reconstruction and forecasting of geophysical dynamics from irregularly-sampled satellite observations.

Retrieval of a Real‐Time Sea Surface Vector Field From SAR Doppler Centroid: 1. Ekman Current Retrieval

AbstractCurrent direction retrieval is a significant part of the Doppler Centroid Technique (DCT) for remote sensing of the sea surface vector field, and it remains an unsolved problem that has prevented its wide application for more than 15 years. In this paper, we review and update the core concept of the DCT, through which the Ekman scheme for current direction retrieval is obtained. In addition, the real sea surface vector field at approximately 24°N with weak background flow during Hurricane Sam is retrieved. We also prove that the measured velocity of the DCT is just the magnitude of the sea surface current vector, and thus, the DCT is the most promising technique for remote sensing of sea surface current dynamics.

Development of a prey-predator species distribution model for a large piscivorous fish: A case study for Japanese Spanish mackerel Scomberomorus niphonius and Japanese anchovy Engraulis japonicus

One of the major challenges of species distribution models (SDMs) is incorporating the influence of prey on the predator. An ensemble modeling approach, the prey-predator species distributabion model (PPSDM), uses environmental variables (sea surface temperature, bathymetry, sea surface salinity, and sea surface current) and a biotic variable (predicted prey distribution: Japanese anchovy) to predict a piscivorous fish (Japanese Spanish mackerel) habitat in the China Seas. PPSDM comprised selected preferable single algorithms from nine algorithms, including tree-based models, machine learning models, regression models, an envelope-style method, and a classification model. Compared with that obtained when using habitat without the Japanese anchovy distribution, the regression models, machine learning models and a classification model have significant improvements. However, tree-based models have only slightly improved. The PPSDM results well reflected the ontogenetic migration of Japanese Spanish mackerel. We found that the wintering ground was distributed in regions with temperatures between 11 and 17 °C and salinities between 33.6 and 34.1. The bottom topography was also closely correlated with Japanese Spanish mackerel, and the suitable habitat was at 50–100 m isobaths as topography determined the position of isotherms (11, 17 °C), salinity isolines (33.6, 34.1), and fronts (temperature, salinity) in winter. The relationships among different climatic indices, temperature, and Japanese Spanish mackerel were analyzed. We found that the East Asian monsoon significantly influenced the Japanese Spanish mackerel habitat by affecting temperature variation. In the future, the suitable habitat of Japanese Spanish mackerel is projected to decrease by 21%, 28%, 36%, and 42% under RCP2.6, RCP4.5, RCP6.0, and RCP8.5, respectively. The Japanese Spanish mackerel in the northern Yellow Sea is projected to experience the ‘cul de sac’ effect, which would drive this species toward local extinction. The Japanese Spanish mackerel in the China Seas will probably shift northeastward through the Tsushima Strait, and the Sea of Japan may form the main habitat at the end of the century.

Construction of sea surface current vectors using a single HF radar: A theoretical study.

A technique called the stream and potential function method (SPFM) is presented to retrieve the sea surface current vector field from the radial-component measurements of only one high-frequency (HF) radar. SPFM estimates the surface current vector field jointly based on hydrodynamic constraints and a two-dimensional (2D) ocean current model. The current vector field is assumed to comprise two parts: nondivergent vortex flow and irrotational divergent flow; this guarantees that both the stream function and the potential function are considered. Physically, SPFM is embedded in a more physically consistent hydrodynamic framework that enables the spatiotemporal distribution characteristics of the current vector field at the sea surface to be effectively captured by a single HF radar. The evaluation of SPFM with HYCOM surface current dataset verifies that this approach is more reliable than the stream function method (SFM).

Deep phylogeographic splits and limited mixing by sea surface currents govern genetic population structure in the mangrove genus Lumnitzera (Combretaceae) across the Indonesian Archipelago

AbstractThe Indonesian Archipelago accommodates the largest mangrove area in Southeast Asia and possesses the world's richest composition of mangrove species. The archipelago comprises areas of the biogeographic regions Sunda and Wallacea, separated by Wallace's line. Here, we used the true mangrove species Lumnitzera littorea and Lumnitzera racemosa as a study case for understanding the effects of phylogeographic history, sea surface currents, and geographical distance on genetic diversity and genetic structure. We sampled 14 populations of L. littorea (N = 106) and 21 populations of L. racemosa (N = 152) from Indonesia and used 3122 and 3048 SNP loci, respectively, genotyped using the ddRADseq approach. We assessed genetic diversity, genetic structure, and effective dispersal of the populations and related them to geographical distance and sea surface currents. Our study revealed low levels of genetic variation at the population level in Lumnitzera. Pronounced genetic differentiation between populations indicated two phylogroups in both species. While in L. littorea the two phylogroups were largely separated by Wallace's line, L. racemosa showed a northwest vs. southeast pattern with strong mixture in Wallacea. Our findings provide novel insights into the phylogeography of the mangrove genus Lumnitzera and the role of sea surface currents in the Indonesian Archipelago.

Long-term mean circulation in the Japan Sea as reproduced by multiple eddy-resolving ocean circulation models

The capacity of four eddy-resolving ocean circulation models—HYCOM (HYbrid Coordinate Ocean Model), MRI.COM (Meteorological Research Institute Community Ocean Model), OFES (Ocean General Circulation Model for the Earth Simulator), and NEMO (Nucleus for European Modeling of the Ocean)—to simulate the long-term mean hydrographic conditions and circulation patterns in the Japan Sea is investigated in this study. The assessment of this study includes the evaluation of mean vertical profiles and time series of temperature and salinity at the representative monitoring stations. Different model products from 1993 to 2015 are compared with in situ measurements provided by historical cruises and monitoring stations. After that, we compared the observed and simulated surface current velocities over the basin and volume transports through the key straits in the Japan Sea. Simulated current velocities are validated against 15 years of Acoustic Doppler Current Profiler (ADCP) measurements near the longshore and offshore branches of the East Korea Warm Current (EKWC). Furthermore, the atmospheric forcing data of the four ocean circulation models are validated against the satellite wind product. We found that the vertical profiles and long-term variations of temperature and salinity reproduced by MRI.COM and HYCOM are closer to in situ measurements. All models simulate temperature well in upper ocean, but salinity simulations are of lower quality from OFES and NEMO at several stations. Simulated current velocities predominantly lie within the standard deviation of ADCP measurements at two locations. However, the sea surface currents are underestimated by four models compared with Drifter data. Although simulated hydrographic profiles agree well with in situ observations, the mean circulation patterns greatly differ between the models, which highlight the need for additional evaluation and corrections based on the long-term current measurements. Because of the lack of ocean current measurements, only the baroclinic velocities simulated by each model are reliable. The substantial part of the differences in barotropic velocities among the simulate result of four models is explained by the differing wind velocities from the corresponding atmospheric forcing datasets.

Study on the Elimination Method of Wind Field Influence in Retrieving a Sea Surface Current Field.

An along-the-track interferometric synthetic aperture radar (ATI-SAR) system can estimate the radial velocity of a moving target on the ground and on a sea surface current. This acquires the interference phase by combining two composite SAR images obtained by two antennas spatially separated along the direction of movement of the platform. The key to retrieving the sea surface current is to remove the interference of sea surface waves, wind-generated current, and Bragg phase velocity in the interference Doppler velocity. Previous methods removed the surface waves, Bragg phase velocity, and other interferences based on externally-assisted wind fields (e.g., ECMWF), using the M4S or other models. However, the wind fields obtained from ECMWF and other external information are often average results of a large temporal and spatial scale, while the images obtained from SAR are high-resolution images of sea surface transients, which are quite different in time and space. This paper takes the SAR image data of the Gaofen-3 satellite as the research object and employs an SAR-based wind field retrieval method to obtain an SAR-observed transient wind field. Combined with the CDOP model, the interference of Doppler velocities, such as the sea surface wave, wind-generated current, and Bragg wave phase velocity, was calculated and subtracted from the Doppler velocity, to obtain the sea surface velocity result. Then, the current field measured by the shore-based HF radar was compared with that obtained by correcting the ATI Doppler velocity based on the SAR retrieved wind field and the ECMWF wind field. The comparison of results indicated that the wind field correction result based on the SAR retrieved wind field was closer to the current field measured by the shore-based HF radar than the wind field correction result based on the ECMWF wind field.

Estuary boundary based on waters surface salinity in cimandiri estuary and cisadane estuary, indonesia.

Estuary is an exclusive system and have an important role but also have vulnerabilities that can threaten life in the estuary region, so it is necessary to have knowledge of estuarine boundaries so that conservation can be maintained. This study aims to analyze the boundaries of the Cimandiri Estuary and the Cisadane Estuary based on surface water salinity, and analyze the relationship of estuarine surface water salinity with rainfall, sea surface currents and tides in the Cimandiri Estuary and Cisadane Estuary. The variables used in this study are salinity, rainfall, sea surface currents, and tides. The research method use the Cimandiri Algorithm by processing OLI Landsat 8 Imagery in 2018 and 2019. The results of this study are estuary boundaries divided into 3 estuary boundary classes specifically Mexo-oligohaline (0.5-5 ‰), Mexo-mesohaline (5-18 ‰), and Mexo-polyhaline (18-30 ‰). Cimandiri’s salinity value is strongly affected by season, then by tides and last by the currents, it is because the Cimandiri Estuary has a deep waters. Meanwhile in the Cisadane Estuary, the salinity value is strongly influenced by season, then by the currents and tides, and also influenced by freshwater from land, it is because the Cisadane Estuary has a shallow water.

Impact of the climate on fishing locations of fish Larvae in Palabuhanratu bay

The phenomenon of climate change influences primary production, food chain interactions, and distribution of fisheries species. Sukabumi Regency, especially Palabuhanratu Bay, is a fishing ground for fish larvae. Fish larvae are an essential phase in supporting the existence of fish resources. The study aims to determine the spatial distribution of fish larvae locations in Palabuhanratu Bay and analyze the relationship between the phenomenon of climate change and fish larvae locations. The variables are rainfall, salinity, sea surface temperature, currents, and tides. The phenomenon of climate change using rainfall data years 2000-2019, Landsat 5 TM and Landsat 8 OLI/TIRS. Salinity estimation generated from Landsat 5 TM and Landsat 8 OLI/TIRS using the Cimandiri Algorithm. The Split-Window Algorithm with the sea surface temperature value. The results found fishing larvae located around the estuary and the beach towards the sea along the Palabuhanratu Bay. The biggest catches from these fish larvae are in Cimandiri estuary and Citepus estuary. This research concluded that rainfall affects the catches of fish larvae because it is directly related to the oceanographic condition of the bay waters.

On the Feedback Between Air‐Sea Turbulent Momentum Flux and Oceanic Submesoscale Processes

AbstractAccurate representation of air‐sea interaction is crucial to numerical prediction of the ocean, weather, and climate. Sea surface temperature (SST) gradients and surface currents in the oceanic mesoscale regime are known to have significant influence on air‐sea fluxes of momentum. Studies based on high‐resolution numerical models and observations reveal that SST gradients and surface currents in the submesoscale regime are much stronger than those in the mesoscale. However, the feedback between the submesoscale processes and the air‐sea turbulent fluxes is not well understood. To quantitatively assess the responses between air‐sea flux of momentum and submesoscale processes, a non‐hydrostatic ocean model is implemented in this study. The inclusion of SST gradients and surface currents in air‐sea bulk fluxes are argued to be significant for modeling accurate wind stress in the submesoscale regime. Taking both into account, this study shows that the linear relationship between wind stress curl/divergence and crosswind/downwind SST gradients existing in the mesoscale regime is not obvious in the submesoscale. Instead, a linear relationship between wind stress curl/divergence and surface current curl/divergence is revealed in the submesoscale. Furthermore, the magnitude of wind stress curl introduced by submesoscale processes is much greater than that presented by mesoscale processes. Another key finding is that tracer subduction and potential vorticity distribution in the submesoscale is susceptible to submesoscale‐modified air‐sea turbulent momentum flux. This study serves as a starting point in investigating the feedbacks between atmospheric and oceanic submesoscale processes.

Improving the Modeling of Sea Surface Currents in the Persian Gulf and the Oman Sea Using Data Assimilation of Satellite Altimetry and Hydrographic Observations

Sea surface currents are often modeled using numerical models without adequately addressing the issue of model calibration at the regional scale. The aim of this study is to calibrate the MIKE 21 numerical ocean model for the Persian Gulf and the Oman Sea to improve the sea surface currents obtained from the model. The calibration was performed through data assimilation of the model with altimetry and hydrographic observations using variational data assimilation, where the weights of the objective functions were defined based on the type of observations and optimized using metaheuristic optimization methods. According to the results, the calibration of the model generally led the model results closer to the observations. This was reflected in an improvement of about 0.09 m/s in the obtained sea surface currents. It also allowed for more accurate evaluations of model parameters, such as Smagorinsky and Manning coefficients. Moreover, the root mean square error values between the satellite altimetry observations at control stations and the assimilated model varied between 0.058 and 0.085 m. We further showed that the kinetic energy produced by sea surface currents could be used for generating electricity in the Oman Sea and near Jask harbor.

A Novel Multi-Candidate Multi-Correlation Coefficient Algorithm for GOCI-Derived Sea-Surface Current Vector with OSU Tidal Model

The maximum cross-coefficient (MCC) algorithm based on the template matching technique is a typical algorithm for obtaining the sea-surface currents (SSCs) in marginal seas. However, this algorithm has mismatches between images in highly turbid water. In this study, we implemented the MCC algorithm to Geostationary Ocean Color Imager-derived total suspended matter to obtain the SSCs in the Yellow Sea and the East China Sea. We propose a novel vector optimization algorithm, which is combined with the accurate estimate of tidal ellipses from the OSU tidal model. This method considers the three greatest candidate acquisitions from multi-correlation coefficients as potential vectors. The rotation direction of the vector within the tidal oscillation is used to identify and substitute for the spurious vector. The obtained average speed of SSC reached 0.60 m/s, which was close to the buoy-measured average speed of 0.58 m/s. Compared with the existing spurious vector eliminating method, the average angular error was improved by 20%, and the average relative amplitude error was improved by 4% in our case study. On the basis of ensuring data integrity, the inversion accuracy was improved.

Prediction of the dynamic distribution for Eucheuma denticulatum (Rhodophyta, Solieriaceae) under climate change in the Indo-Pacific Ocean

Eucheuma is one of the most important commercial red seaweeds in Southeast Asia, and plays an important role in the global seaweed aquaculture. It is expected to exhibit great responses to ocean warming. Here, we used maximum entropy species distribution models (SDMs) to estimate the suitable habitat of Eucheuma denticulatum under present conditions, and to predict the future range dynamics under the four representative concentration pathway (RCP) scenarios. The best marine environmental factors for E. denticulatum distribution modeling were distance to shore, sea surface temperature and currents velocity. Our results showed that E. denticulatum’ distributions would contract in the Central Indo-Pacific Ocean, especially the regions of the Sunda Shelf, while expanding poleward along the south coast of Australia in 2100. Our study provided important knowledge for the prediction of the tropical seaweed distribution, conservation and sustainable developments of E. denticulatum in the future.

Atlas of surface currents in the Mediterranean and Canary–Iberian–Biscay waters

ABSTRACT Sea surface currents probably are the most relevant essential ocean variable affecting diverse societal challenges concerning the marine environmental (as, for example, safe and efficient navigation, marine pollution and ecological connectivity). This work introduces a climatological Atlas (monthly resolution) of currents in the Mediterranean and Canary–Iberian–Biscay basins, based on today's state of the art reanalyses of the ocean circulation. The focus is on surface and subsurface reanalyses (here understood as and m, respectively) provided by the Copernicus Marine Environment Monitoring Service (CMEMS). The climatological values are computed from the median of the empirical probability density functions and the Atlas also includes the variance matrix and a bimodality index to have quantitative information on their variability. For both domains, the subsurface climatological fields are reasonably consistent with circulation schemes proposed in the previous literature but clearly improving the time and space resolution of the emerging patterns. For the Canary–Iberian–Biscay domain, the monthly climatological surface currents capture accurately the characteristic seasonal signal and its transition between a favourable and non-favourable upwelling regime. In the Mediterranean basin, differences between the near-surface and the 15 m velocity fields suggest a non-negligible role of winds over the variability of the uppermost ocean layer, specially in the Eastern Mediterranean basin. This is, up to our knowledge, the first time that such near-surface climatological patterns are computed. It has been found that, in general, the resulting patterns agree with surface drifter trajectories. In several regions, interannual variability foster bimodal and multimodal probability distributions. The Atlas has been conceived with the purpose of providing a first quantitative assessment on the surface circulation, thus being a complementary tool of real-time ocean forecasting systems. The Atlas is distributed following the FAIR principles and is accompanied with a digital version, with enhanced visualization capabilities for both research and assessment.

Coastal oceanographic conditions during a cold front passage over a fish spawning aggregation site located in the Mesoamerican Reef

In this study, an observational campaign that included measurements of coastal currents and thermohaline structure on a fish aggregation reef site during 11–17 March 2017 is analyzed. For the period of the present study, we observed the response of the coastal oceanographic conditions to meteorological forcing. In particular, we described the effects of the cold front passage (locally called a “Norte”) on the water column over a fish aggregation reef site. Before the passage of the Norte, the site was exposed to NE - SE Trade winds, stratification of the water column and sea surface currents moved mainly northward in the alongshore component and offshore in the cross-shore component. However, during the passage of the cold front or Norte, there were changes in the wind field, temperature and atmospheric pressure. Norte conditions were associated with strong northerly winds and with a continuous drop in air temperature. This atmospheric forcing resulted in an evolution of the oceanographic conditions at the reef site from that observed during Trade winds. It changed from stratified to a well-mixed water column, and then dense water on the shallow reef area resulted by cooling and evaporation due to the extreme weather condition during Nortes. This dense water descends down the inner-shelf with density contrast of 0.52 kg/m3 producing a near bottom offshore flow (below 15 m depth). Thus, weather conditions have a determinant role in coastal currents and in the thermohaline structure in the fish aggregation reef site.

Submesoscale Currents from UAV: An Experiment over Small-Scale Eddies in the Coastal Black Sea

A commercial unmanned aerial vehicle (UAV) is used for coastal submesoscale current estimation. The measurements were conducted in the Black Sea coastal area with a DJI Mavic quadcopter operated in self-stabilized mode at different look geometry (200–500-m altitude, 0–30∘ incidence angle). The results of four flights during 2020–2021 are reported. Some scenes captured a train of or individual eddies, generated by a current flowing around a topographic obstacle (pier). The eddies were optically visible due to the mixing of clear and turbid waters in the experiment area. Wave dispersion analysis (WDA), based on dispersion shell signature recognition, is used to estimate the sea surface current in the upper 0.5-m-thick layer. The WDA-derived current maps are consistent with visible eddy manifestations. The alternative method, based on 4D-variational assimilation (4DVAR), agrees well with WDA and can complement it in calm wind conditions when waves are too short to be resolved by the UAV sensor. The error of reconstructed velocity due to the uncontrolled UAV motions is assessed from referencing to static land control points. At a 500-m altitude and 7–10 m s−1 wind speed (reported by a local weather station for 10-m height), the UAV drift velocity, or the bias of the current velocity estimate, is about 0.1 m s−1, but can be reduced to 0.05 m s−1 if the first 10 s of the UAV self-stabilization period are excluded from the analysis. The observed anticyclonic eddies (200–400 m in diameter with 0.15–0.30 m s−1 orbital velocity) have an unexpectedly high Rossby number, Ro∼15, suggesting the importance of nonlinear centrifugal force for such eddies and their significant role in coastal vertical transport.

Gain-scheduled steering control for a wave-propelled unmanned surface vehicle

The persistent observation of oceanographic phenomena is nowadays hampered by technological limitations, that prevent marine robotic platforms to autonomously execute scientific surveys for extended periods of time. The propulsion and course-over-ground (COG) of wave-propelled unmanned surface vehicles (USVs) rely primarily on the forces exerted by the environment and, for this reason, ensuring an intended navigational behavior at sea is a challenging task. In this article we discuss the design and experimental validation of a gain-scheduled steering control system that governs the course of a wave-propelled USV subject to different sea surface currents. A theoretical investigation of the vehicle nonlinear dynamics identifies in a scalar scheduling variable γ the major source of model nonlinearities due to vehicle’s and ocean current velocities. The impact of such nonlinearities on the control of the course-over-ground is investigated with a quasi-linear model analysis in the frequency domain. This provides a basis for the design of a gain-scheduled steering controller, whose theoretical assumptions are supported and validated by extensive simulations and experimental results. Moreover, we show that scheduling of the autopilot gains can still be achieved despite the lack of sea current measurements onboard the USV. Finally, it is demonstrated that the navigation and course-keeping performances improve significantly when the gain-scheduled controller is employed, as compared to the nominal fixed-gain autopilot.The presented control system is assessed with simulations and validated on a series of field experiences meant to quantify the autopilot robustness and the vehicle course-keeping capabilities.