Search Results (31)

This study experimentally evaluates the survivability and hydrodynamic performance of a moored oscillating water column (OWC) wave energy converter (WEC) subjected to extreme cyclonic wave conditions emulating tropical cyclone Oma (2019). Laboratory tests recreated realistic cyclonic sea states using focused wave groups through the NewWave theory, combining singular and embedded focused waves within irregular seas to simulate extreme crests, troughs, and transient slamming events. Three mooring systems, including catenary, vertical-taut, and taut with 45° tendons, were tested to quantify their influence on structural response, chamber pressures, mooring tensions, and motion dynamics. The results revealed a critical trade-off: mooring configurations optimised for energy capture efficiency (e.g., taut systems) exhibited reduced survivability during extreme waves, while survivability-focused designs (e.g., catenary) compromised operational performance. Slamming pressures and transient loads were highly sensitive to wave group and mooring stiffness, with vertical taut systems experiencing the largest peak tensions. By integrating localised slamming pressure data with global mooring load measurements, this work provides a novel framework for balancing energy production and storm resilience in OWC design. Full article

Floating Offshore Wind Turbines (FOWTs) are gaining traction as a solution for harnessing wind energy in deepwater regions where traditional fixed-bottom turbines may not be viable due to water depth. This paper investigates the feasibility and optimization of a floating wind farm in a tropical cyclone (typhoon) region, using the IEA 15 MW turbine and semi-submersible floaters. Because of the extreme environment, the FOWT’s mooring system requires nine catenary chains in a 3 × 3 pattern, which has a large footprint. One challenge in the wind farm design is fitting the FOWTs in a limited area and minimizing wake effects. This research compares a linear layout and an offset grid layout, focusing on the effects of spacing and wake dynamics. The results show that while the linear layout maintains optimal power generation without energy loss, the offset grid layout, although resulting in 2% energy loss, offers greater spatial efficiency for larger-scale projects. The findings highlight the importance of balancing energy efficiency with spatial optimization, particularly for large offshore wind farms. This study explores the use of the Gauss–Curl hybrid model in wake modeling, and the methodology employed provides insights into FOWT placement and mooring system arrangement. The result concludes that a wind farm containing twelve (12) units of 15 MW wind turbines can achieve the 7.0 MW/km² power generation density required by a regulatory government agency. It proves the technical feasibility of a wind farm congested with large mooring systems in a tropical cyclone region. Full article

Temporal variations in diurnal internal tides (ITs) and near-inertial waves (NIWs) in the southern South China Sea (SCS) are characterized, based on two 13-month moored current observations. Diurnal ITs, dominated by O₁ and K₁, are found to exhibit spring–neap cycles of about 14 days and significant seasonal variations. The incoherent components explain 54% and 56% of the total energy in the diurnal band, which further complicates its temporal variabilities. As for NIWs, wind energy input serves as the primary energy source and three strong events are observed. Tropical cyclone RAI passed through two moorings during the event 1 period, and triggered a peak near-inertial kinetic energy of 19.55 J m⁻³ (18.82 J m⁻³) at two moorings. After generation, the NIWs propagated downward to around 300 m, becoming the most intense event observed at DA2. In contrast, the NIWs response to tropical cyclone NOCK’s passage during event 3 was relatively weaker. The near-inertial KE generated by NOCK was confined to depths shallower than 150 m, with the average near-inertial KE being only 85% (52%) of that during event 1 for two moorings, despite the near-inertial energy input from NOCK being nearly 400% that of RAI. The modulation of background vorticity is considered the primary factor resulting in the difference in intensity of two NIW events. The penetrating depth of NIWs under the modulation of anticyclonic eddies was more than twice that under the cyclonic eddies. Furthermore, the strongest NIWs during event 2 that were observed below 350 m at mooring 2 (183% stronger than average) were also related to a strong anticyclonic eddy. Full article

Evaluating sea surface temperature (SST) products is essential before their application in marine environmental monitoring and related studies. SSTs from the in situ SST Quality Monitor (iQuam) system, Advanced Microwave Scanning Radiometer 2 (AMSR2) aboard the Global Change Observation Mission 1st-Water, and the Microwave Radiation Imager (MWRI) aboard the Chinese Fengyun-3D satellite are intercompared utilizing extended triple collocation (ETC) and direct comparison methods. Additionally, error characteristic variations with respect to time, latitude, SST, sea surface wind speed, columnar water vapor, and columnar cloud liquid water are analyzed comprehensively. In contrast to the prevailing focus on SST validation accuracy, the random errors and the capability to detect SST variations are also evaluated in this study. The result of ETC analysis indicates that iQuam SST from ships exhibits the highest random error, above 0.83 °C, whereas tropical mooring SST displays the lowest random error, below 0.28 °C. SST measurements from drifters, tropical moorings, Argo floats, and high-resolution drifters, which possess random errors of less than 0.35 °C, are recommended for validating remotely sensed SST. The ability of iQuam, AMSR2, and MWRI to detect SST variations diminishes significantly in ocean areas between 0°N and 20°N latitude and latitudes greater than 50°N and 50°S. AMSR2 and iQuam demonstrate similar random errors and capabilities for detecting SST variations, whereas MWRI shows a high random error and weak capability. In comparison to iQuam SST, AMSR2 exhibits a root-mean-square error (RMSE) of about 0.51 °C with a bias of −0.05 °C, while MWRI shows an RMSE of about 1.26 °C with a bias of −0.14 °C. Full article

The ocean surface current influences the roughness of the sea surface, subsequently affecting the scatterometer’s measurement of wind speed. In this study, the effect of surface currents on ASCAT-retrieved winds is investigated based on in-situ observations of both surface winds and currents from 40 open ocean moored buoys in the tropical and mid-latitude oceans. A total of 28,803 data triplets, consisting of buoy-observed wind vectors, current vectors, and ASCAT Level 2 wind vectors, were collected from the dataset spanning over 10 years. It is found that the bias between scatterometer-retrieved wind speed and buoy-observed wind speed is negatively correlated with the ocean surface current speed. The wind speed bias is approximately 0.96 times the magnitude of the downwind surface current. The root-mean-square error between the ASCAT wind speeds and buoy observations is reduced by about 15% if rectification with ocean surface currents is involved. Therefore, it is essential to incorporate surface current information into wind speed calibration, particularly in regions with strong surface currents. Full article

The main purpose of our work is to investigate the performance of wave breaking and its effect on wave retrieval in data acquired from the Chinese Gaofen-3 (GF-3) synthetic aperture radar (SAR) at C-band and the German TerraSAR-X (TS-X) at X-band. The SAR images available for this study included 140 GF-3 images acquired in quad-polarization strip (QPS) mode and 50 dual-polarized (vertical-vertical (VV) and horizontal-horizontal (HH)) TS-X images acquired in stripmap (SM) mode. Moreover, these images were collocated with the waves simulated by the numeric WAVEWATCH-III (WW3) (version 5.16) model and HYbrid Coordinate Ocean Model (HYCOM) current. In particular, a few images covered the moored buoys monitored by the National Data Buoy Center (NDBC) of the National Oceanic and Atmospheric Administration (NOAA). The comparison between the WW3-simulated results and the significant wave heights (SWHs) from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data (ERA-5) showed that the correlation coefficient (COR) was 0.4–0.6 with a root mean squared error (RMSE) of about 0.2 m at SWHs of 0–4 m. The winds were inverted using VV-polarized geophysical model functions (GMFs), e.g., CSARMOD-GF for the GF-3 images and XMOD2 for the TS-X images. The Bragg resonant roughness in the normalized radar cross section (NRCS) was simulated using a radar backscattering model and the SAR-derived wind, WW3-simulated wave parameters, and HYCOM current. Then, the contribution of the non-polarized (NP) wave breaking to the SAR data was estimated by the VV-polarized NRCS, the HH-polarized NRCS, and the polarization ratio (PR) of the co-polarized Bragg resonant components in the NRCS. Because co-polarized Bragg resonant components in the NRCSs have poor results, due to the saturation for wind speeds greater than 20 m/s, the analysis of wave breaking is excluded at such conditions. The results revealed that the backscattering signal in the C-band was more sensitive to wave breaking than the backscattering signal in the X-band. Interestingly, the ratio had a linear correlation with wind speed. Moreover, the variation in the bias (inverted SWH minus WW3 simulation) showed that the bias increased as the wind speed (>8 m/s) and whitecap coverage (>0.005) increased. Following this rationale, wave retrieval during tropical cyclones should consider the influence of wave breaking. Full article

Sea surface salinity (SSS) is one of the Essential Climate Variables (ECVs) as defined by the Global Climate Observing System (GCOS). Acquiring high-quality SSS datasets with high spatial-temporal resolution is crucial for research on the hydrological cycle and the earth climate. This study assessed the quality of SSS data provided by five high-resolution ocean reanalysis products, including the Hybrid Coordinate Ocean Model (HYCOM) 1/12° global reanalysis, the Copernicus Global 1/12° Oceanic and Sea Ice GLORYS12 Reanalysis, the Simple Ocean Data Assimilation (SODA) reanalysis, the ECMWF Oceanic Reanalysis System 5 (ORAS5) product and the Estimating the Circulation and Climate of the Ocean Phase II (ECCO2) reanalysis. Regional comparison in the Mediterranean Sea shows that reanalysis largely depicts the accurate spatial SSS structure away from river mouths and coastal areas but slightly underestimates the mean SSS values. Better SSS reanalysis performance is found in the Levantine Sea while larger SSS uncertainties are found in the Adriatic Sea and the Aegean Sea. The global comparison with CMEMS level-4 (L4) SSS shows generally consistent large-scale structures. The mean ΔSSS between monthly gridded reanalysis data and in situ analyzed data is −0.1 PSU in the open seas between 40° S and 40° N with the mean Root Mean Square Deviation (RMSD) generally smaller than 0.3 PSU and the majority of correlation coefficients higher than 0.5. A comparison with collocated buoy salinity shows that reanalysis products well capture the SSS variations at the locations of tropical moored buoy arrays at weekly scale. Among all of the five products, the data quality of HYCOM reanalysis SSS is highest in marginal sea, GLORYS12 has the best performance in the global ocean especially in tropical regions. Comparatively, ECCO2 has the overall worst performance to reproduce SSS states and variations by showing the largest discrepancies with CMEMS L4 SSS. Full article

Extreme weather events such as hurricanes are expected to become more severe with the human-induced increase in average global temperatures, exacerbating the risk of major damage. Efforts to predict these events typically require detailed hydrodynamic data that are difficult to collect in the field. Here, nearshore data collected with three ADCP moorings were used to describe the hydrodynamics induced by Hurricane Harvey along the southern Texas coast. Wave spectra and nearshore current variations were analyzed along the hurricane’s trajectory and compared to other offshore locations. The results indicate that winds intensified along the coast as Harvey approached the Port Aransas coastline. Southerly wind stresses of ~−0.9 Nm⁻² generated ~2 ms⁻¹ depth-averaged flows towards the southwest close to landfall in the north, while flows of ~1 ms⁻¹ and <1 ms⁻¹ were measured in the center and the south of the study site, respectively. The hydrodynamics induced by the hurricane were compared to those induced by an intense synoptic-scale cold front (CF). Both events generated southward-directed alongshore wind stresses of similar magnitudes (${\tau }_y$ ~−0.4 Nm⁻²) that caused similar depth-averaged flows (0.5 to 0.7 ms⁻¹) and wave energy conditions (H_s of ~4 m) in the south. Harvey caused extremely energetic conditions close to landfall in the north compared to the CF; depth-averaged flows and H_s of 2 ms⁻¹ and 10 m were induced by Harvey, as opposed to 0.6 ms⁻¹ and 4 m by the CF, respectively. While intense currents (>1 ms⁻¹) and waves (H_s > 4 m) lasted for less than a day during Harvey, these persisted a few days longer during the CF. This study highlights the relevant role of synoptic-scale cold fronts in modulating the nearshore hydrodynamics, which occur more frequently than tropical cyclones in the northwestern Gulf of Mexico. Full article

Wind speed reconciliation across different wind sources is critically needed for extending available satellite wind records in Tropical Cyclones. The deviations between wind references of extremes, such as the moored buoy data and dropsonde wind estimates for guidance on geophysical model function development, are one of the main causes of wind speed differences for wind products, for instance, the overestimation of Synthetic Aperture Radars (SARs) relative to ASCAT winds. The study proposes a new wind speed adjustment to achieve mutual adjustment between ASCAT CMOD7 winds and simultaneous SAR wind speeds. The so-called CMOD7D-v2 adjustment is constructed based on the statistical analysis of SAR and ASCAT Tropical Cyclone acquisitions between 2016 and 2021, showing a satisfactory performance in wind speed reconciliation for winds with speeds higher than 14 m/s. Furthermore, the error characteristics of the CMOD7D-v2 adjustment for Tropical Cyclone winds are analyzed using the Triple Collocation analysis technique. The analysis results show that the proposed wind adjustment can reduce ASCAT wind errors by around 16.0% when adjusting ASCAT winds to SAR wind speeds. In particular, when downscaling SAR winds, the improvement in ASCAT wind errors can be up to 42.3%, effectively alleviating wind speed differences across wind sources. Furthermore, to avoid the impacts of large footprints by ASCAT sensors, wind speeds retrieved from SAR VV signals (acting as a substitute for ASCAT winds) are adjusted accordingly and compared against SAR dual-polarized winds and collocated Stepped Frequency Microwave Radiometer (SFMR) observations. We find that the bias values of adjusted winds are lower than products from other adjustment schemes by around 5 m/s at the most extreme values. These promising results verify the plausibility of the CMOD7D-v2 adjustment, which is conducive to SAR and ASCAT wind speed comparisons and extreme wind analysis in Tropical Cyclone cases. Full article

Validating Sea Surface Salinity (SSS) data has become a key component of the Soil Moisture Ocean Salinity (SMOS) satellite mission. In this study, the gridded SMOS SSS products are compared with in situ SSS data from analyzed products, a ship-based thermosalinograph and a tropical moored buoy array. The comparison was conducted at different spatial and temporal scales. A regional comparison in the Baltic Sea shows that SMOS slightly underestimates the mean SSS values. The influence of river discharge overrides the temperature in the Baltic Sea, bringing larger biases near river mouths in warm seasons. The global comparison with two Optimal Interpolated (OI) gridded in situ products shows consistent large-scale structures. Excluding regions with large SSS biases, the mean ΔSSS between monthly gridded SMOS data and OI in situ data is −0.01 PSU in most open sea areas between 60°S and 60°N, with a mean Root Mean Square Deviation (RMSD) of 0.2 PSU and a mean correlation coefficient of 0.50. An interannual tendency of mean ΔSSS shifting from negative to positive between satellite SSS and in situ SSS has been identified in tropical to mid-latitude seas, especially across the tropical eastern Pacific Ocean. A comparison with collocated buoy salinity shows that on weekly and interannual scales, the SMOS Level 3 (L3) product well captures the SSS variations at the locations of tropical moored buoy arrays and shows similar performance with in situ gridded products. Excluding suspicious buoys, the synergetic analysis of SMOS, SMAP and gridded in situ products is capable of identifying the erroneous data, implying that satellite SSS has the potential to act as a real-time 27 Quality Control (QC) for buoy data. Full article

The 3rd full-mission reanalysis (RAN3) of global sea surface temperature (SST) with a 750 m resolution at nadir is available from VIIRS instruments flown onboard two JPSS satellites: NPP (February 2012–present) and N20 (January 2018–present). Two SSTs, ‘subskin’ (sensitive to skin SST) and ‘depth’ (proxy for in situ SST at depth of 20 cm), were produced from brightness temperatures (BTs) in the VIIRS bands centered at 8.6, 11 and 12 µm during the daytime and an additional 3.7 µm band at night, using the NOAA Advanced Clear Sky Processor for Ocean (ACSPO) enterprise SST system. The RAN3 dataset is fully archived at NASA JPL PO.DAAC and NOAA CoastWatch, and routinely supplemented in near real time (NRT) with a latency of a few hours. Delayed mode (DM) processing with a 2 months latency follows NRT, resulting in a more uniform science quality SST record. This paper documents and evaluates the performance of the VIIRS RAN3 dataset. Comparisons with in situ SSTs from drifters and tropical moorings (D+TM) as well as Argo floats (AFs) (both available from the NOAA iQuam system) show good agreement, generally within the NOAA specifications for accuracy (±0.2 K) and precision (0.6 K), in a clear-sky domain covering 18–20% of the global ocean. The nighttime SSTs compare with in situ data more closely, as expected due to the reduced diurnal thermocline. The daytime SSTs are also generally within NOAA specs but show some differences between the (D+TM) and AF validations as well as residual drift on the order of −0.1 K/decade. BT comparisons between two VIIRSs and MODIS-Aqua show good consistency in the 3.7 and 12 µm bands. The 11 µm band, while consistent between NPP and N20, shows residual drift with respect to MODIS-Aqua. Similar analyses of the 8.6 µm band are inconclusive, as the performance of the MODIS band 29 centered at 8.6 µm is degraded and unstable in time and cannot be used for comparisons. Full article

The 40+ years-long sea surface temperature (SST) dataset from 4 km Global Area Coverage (GAC) data of the Advanced Very High-Resolution Radiometers (AVHRR/2s and/3s) flown onboard ten NOAA satellites (N07/09/11/12/14/15/16/17/18/19) has been created under the NOAA AVHRR GAC SST Reanalysis 2 (RAN2) Project. The data were reprocessed with the NOAA Advanced Clear Sky Processor for Ocean (ACSPO) enterprise SST system. Two SST products are reported in the full ~3000 km AVHRR swath: ‘subskin’ (highly sensitive to true skin SST, but debiased with respect to in situ SST) and ‘depth’ (a closer proxy for in situ data, but with reduced sensitivity). The reprocessing methodology aims at close consistency of satellite SSTs with in situ SSTs, in an optimal retrieval domain. Long-term orbital and calibration trends were compensated by daily recalculation of regression coefficients using matchups with drifters and tropical moored buoys (supplemented by ships for N07/09), collected within limited time windows centered at the processed day. The nighttime Sun impingements on the sensor black body were mitigated by correcting the L1b calibration coefficients. The Earth view pixels contaminated with a stray light were excluded. Massive cold SST outliers caused by volcanic aerosols following three major eruptions were filtered out by a modified, more conservative ACSPO clear-sky mask. The RAN2 SSTs are available in three formats: swath L2P (144 10-min granules per 24 h interval) and two 0.02° gridded (uncollated L3U, also 144 granules/24 h; and collated L3C, two global maps per 24 h, one for day and one for the night). This paper evaluates the RAN2 SST dataset, with a focus on the L3C product and compares it with two other available AVHRR GAC L3C SST datasets, NOAA Pathfinder v5.3 and ESA Climate Change Initiative v2.1. Among the three datasets, the RAN2 covers the global ocean more completely and shows reduced regional and temporal biases, improved stability and consistency between different satellites, and in situ SSTs. Full article

Projections of future climate are sensitive to the representation of upper-ocean diurnal variability, including the diurnal cycle of winds. Two different methods suitable for time series with missing data are used here to characterize how observed diurnal winds vary over the year. One is based on diurnal composites of mooring data, and the other is based on harmonic analysis via a least squares fit and is able to isolate annual (i.e., 1 cycle per year) modulation of diurnal variability. Results show that the diurnal amplitude in meridional winds is larger than in zonal winds and peaks in the tropical Pacific, where diurnal variability in zonal winds is overall weaker compared to other basins. Furthermore, the amplitude and phasing of diurnal winds in the tropical oceans are not uniform in time, with overall larger differences through the year in the meridional component of tropical winds. Estimating the annual modulation of the diurnal signal implies resolving both the diurnal energy peak and also the modulation of this peak by the annual cycle. This leads to a recommendation for sampling at least 6 times per day and for a duration of at least 3 years. Full article

The Indian Ocean dipole (IOD) is one of the main modes characterizing the interannual variability of the large-scale ocean–atmosphere interaction in the equatorial zone of the World Ocean. A dipole manifests itself as an out-of-phase interannual fluctuation of the ocean–atmosphere characteristics in the western and eastern parts of the equatorial–tropical zone of the Indian Ocean. IOD can be a consequence of the ENSO (El Niño–Southern Oscillation) events in the Pacific Ocean, or it can be independent of them and arise due to the Indian Ocean inherent processes. Earlier, it was suggested that the generation of the long planetary waves in the Indian Ocean by the ENSO events is one of the mechanisms of the ENSO impact on the IOD. However, quite often, such a mechanism is not the case and IOD is generated itself as an independent Indian Ocean mode. We hypothesized that this generation is due to the growing oceanic disturbances, as a result of instability of the system of Indian Ocean zonal currents in the vicinity of the critical layer, in which the phase velocity of Rossby waves is equal to the average velocity of the zonal currents. In the present work, the study of the features of the formation of the critical layer in the equatorial–tropical zone of the Indian Ocean is continued using different oceanic re-analyses and standard theory of the Rossby waves. As a result of comparison of different re-analyses data with the RAMA (The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) measurements, the operative re-analysis ORAS5 output of European Centre for Medium-Range Weather Forecasts (ECMWF) on potential temperature, salinity, and the zonal component of the currents’ velocity for the period 1979–2018 was used. Monthly profiles of potential temperature, salinity, and the zonal component of the currents’ velocity were selected from the ORAS5 archive for the sections situated between 7.5–15.5° S and 50–100° E. From these data and for each month, using the standard theory of planetary waves, the phase velocity of the lowest baroclinic mode of the Rossby long waves was calculated and the critical layers were determined. For each critical layer, its length was calculated. The obtained time series of the length of the critical layers were compared to the variability of dipole mode index (DMI). It is shown that the majority of the cases of the IOD generation as inherent (independent on the Pacific processes) mode were accompanied by the critical layer formation in the region of interest. Usually, the critical layers occur in spring, one to two months before the onset of the positive IOD events. This indicates that the presence of instability in the system of the zonal currents can be a reason for the generation of IOD and the asymmetry of the amplitude of the dipole mode index between positive and negative events. During the extremely intense ENSO event of 1997–1998, which was accompanied by the strong IOD event, the critical layer in the equatorial–tropical zone of the Indian Ocean was absent. This ENSO event generated the oceanic planetary waves at the eastern edge of the Indian Ocean. Therefore, it is shown that the above mechanism of the ENSO–IOD interaction is a reality. Full article

The Pilot-Mission Exploitation Platform (Pi-MEP) for salinity is an ESA initiative originally meant to support and widen the uptake of Soil Moisture and Ocean Salinity (SMOS) mission data over the ocean. Starting in 2017, the project aims at setting up a computational web-based platform focusing on satellite sea surface salinity data, supporting studies on enhanced validation and scientific process over the ocean. It has been designed in close collaboration with a dedicated science advisory group in order to achieve three main objectives: gathering all the data required to exploit satellite sea surface salinity data, systematically producing a wide range of metrics for comparing and monitoring sea surface salinity products’ quality, and providing user-friendly tools to explore, visualize and exploit both the collected products and the results of the automated analyses. The Salinity Pi-MEP is becoming a reference hub for the validation of satellite sea surface salinity missions by providing valuable information on satellite products (SMOS, Aquarius, SMAP), an extensive in situ database (e.g., Argo, thermosalinographs, moorings, drifters) and additional thematic datasets (precipitation, evaporation, currents, sea level anomalies, sea surface temperature, etc.). Co-localized databases between satellite products and in situ datasets are systematically generated together with validation analysis reports for 30 predefined regions. The data and reports are made fully accessible through the web interface of the platform. The datasets, validation metrics and tools (automatic, user-driven) of the platform are described in detail in this paper. Several dedicated scienctific case studies involving satellite SSS data are also systematically monitored by the platform, including major river plumes, mesoscale signatures in boundary currents, high latitudes, semi-enclosed seas, and the high-precipitation region of the eastern tropical Pacific. Since 2019, a partnership in the Salinity Pi-MEP project has been agreed between ESA and NASA to enlarge focus to encompass the entire set of satellite salinity sensors. The two agencies are now working together to widen the platform features on several technical aspects, such as triple-collocation software implementation, additional match-up collocation criteria and sustained exploitation of data from the SPURS campaigns. Full article