Distributed optical fibre sensing (DOFS) is a technology that enables continuous, real-time measurements of a range of environmental parameters along the length of a fibre optic cable. In this article, we review the recently emerged applications of DOFS techniques in physical oceanography and offer a perspective on the technology's potential for future growth within the field. The introduction of DOFS to physical oceanography is contextualised with a brief history of the technology, which spun off primarily from the use of seafloor-laid optical fibres for telecommunications purposes. The key components and underpinning physics of a DOFS system are outlined and, on their basis, the suite of variables that are observable with DOFS are discussed. The implementation factors of DOFS, which include trade-offs between measurement accuracy and spatio-temporal resolutions and ranges, are also examined. The physical oceanographic applications of DOFS to date are then illustrated with case examples of four distinct DOFS techniques: distributed temperature sensing (DTS), which can provide ocean temperature observations; distributed static strain sensing (DSS) and distributed acoustic sensing (DAS), which are sensitive to temperature, cable strain and strain-associated variables, such as pressure and ocean velocity; and ultra-long-range observations of ocean currents with optical interferometry. The forthcoming prospects of DOFS in physical oceanography are considered, and are proposed to include new fibre optic-based approaches to sense ocean salinity and measure through the water column. We conclude with reflections on the future role of DOFS within the Global Ocean Observing System, and highlight the opportunities provided by the existing world-wide network of seafloor-laid optical fibres.
The Red River (RR) plume region of the Gulf of Tonkin (GoT) plays an important role in driving coastal dynamic variability and regulating sediment and nutrient transport and budgets, and is therefore vital for coastal ecosystems and maritime activities. High-frequency radar (HFR) surface current measurements were used to characterize surface circulation and assess passive-tracer dispersion from August to December 2024, improving understanding of particle transport, dispersal, and fate in this region. The coastal circulation in the region, found to be strongly influenced by winds, tidal forcing, riverine input, and coastal bathymetry, exhibited a large spatio-temporal variability during the analysis period with the occurrence of small-scale structures, i.e., submesoscale eddies. The dispersion under varying forcing conditions and an extreme event – the typhoon Yagi – was analyzed by particle tracking and Lagrangian diagnostics. The results revealed that the dispersion within the RR plume region predominantly approached a Richardson super-diffusive regime after 24 h of tracking. Under the influence of typhoon Yagi, the dispersion was approximated by a ballistic regime after 12 h of tracking, with the spreading rate 10 times faster than that during normal conditions. In addition, the presence of Lagrangian Coherent Structures (LCSs), i.e., eddies next to the river outflow jets, coastal plume fronts, and zones of surface current convergence and divergence in the vicinity of river outlets, significantly influenced the dispersion behavior of tracers in the RR plume region. Overall, this study provides new insights into how coastal circulation and material dispersal in the RR plume respond across a wide range of weather conditions, including extreme events.
An example for (a) is a digital elevation model (DEM) purely based on measurement points collected by you and derived by using a software product. If you use an existing map layer from another originator as a basis for significantly enriching the map with your own content, this would be an example for case (b). Mesoscale eddies play a critical role in mediating meridional transport across the Antarctic Circumpolar Current (ACC), yet the dynamics of cross-frontal eddies (CFEs) and their energy exchanges with frontal jets remain inadequately quantified. This study presents a systematic analysis of CFE characteristics, kinetic energy evolution, and thermohaline transport effects in the Pacific sector of the Southern Ocean, utilizing 23 years (2000–2022) of satellite altimetry and Argo float data. Our results reveal a fundamental polarity- and direction-dependent asymmetry in CFE dynamics. Equatorward-propagating cyclonic eddies (CEs) dominate CFE activity, followed by poleward-moving anticyclonic eddies (AEs). These dominant CFE types exhibit superior energetic characteristics, including significantly higher eddy kinetic energy (EKE) and stronger nonlinearity compared to their counterparts. Complete CFEs experience polarity- and direction-selective energy gains during frontal crossing, with equatorward CEs and poleward AEs extracting energy from eastward frontal jets, while their counterparts lose energy. This energization mechanism has intensified over the past two decades, with both polarity CFEs showing substantial EKE increases that substantially exceed previous basin-scale estimates. Hydrographic analysis demonstrates that CEs and AEs transport distinct water masses across frontal boundaries, creating sharp thermohaline contrasts within interfrontal zones. Our findings establish CFEs as crucial regulators that buffer wind- and warming-induced baroclinicity increases through compensatory heat transport, thereby maintaining the Southern Ocean's thermal equilibrium and modulating the ACC's response to external forcing in a changing climate.
The Barents Sea is a primary gateway for Atlantic Water entering the central Arctic Ocean and ubiquitous water-mass transformation on the Barents shelf is key for mitigating increases in heat transport to the central Arctic through the St. Anna Trough. Using a mesoscale-permitting reanalysis spanning 40 years, we derive the first long-term estimate of heat transport through the St. Anna trough, finding that it has increased by 0.11 TW yr−1TW yr−1
The Black Sea regional reanalysis serves as an essential tool for understanding the Black Sea's response to climate variability and advancing regional ocean monitoring efforts. In particular, the Black Sea reanalysis (BLK-REA) is built with high spatial resolution,
Marine heatwaves (MHWs) can have devastating and lasting impacts on marine ecosystems. We investigated past MHW characteristics around 12 southwestern Pacific Island countries and territories (PICTs) using two observed sea surface temperature products and an ocean reanalysis product. PICTs are highly dependent on their marine resources for their livelihoods: a better understanding of MHW characteristics is needed for planning and adaptation to risks associated with MHWs. Our research builds on previous studies where MHWs have been detected and described using a point-based definition. We first revisit past MHW characteristics based on their spatial extent, vertical extent and seasonality. We show that filtering MHWs by size (spatial extent) and seasonality can greatly affect their characterisation and help trace their physical drivers. We then characterise past events inside each Exclusive Economic Zone (EEZ) and at the coast with MHW indices tailored to benefit Pacific Island stakeholders. We consider two types of events: large-scale events, covering a large part of the EEZ, likely to affect pelagic fisheries, and events affecting coastal zones and ecosystems. We distinguish between events occurring in the hot season (November to April), and in the cold season (May to October). We show that all 12 PICTs experienced MHWs in the past 30 years that are getting more frequent with greater spatial extents, longer durations, explained by the long-term warming trend in sea surface temperature, but with lower maximum intensity. New Caledonia, Vanuatu, Fiji and Tonga appear to be more exposed to MHWs with longer duration, higher maximum intensity, and deeper extent compared to other countries.
We present transport estimates of Atlantic Water (AW) and Recirculating Atlantic Water (RAW) across a zonal transect at 77°15′ N using repeated ocean glider observations. Over three missions during autumn and winter of 2020 to 2022, 22 high-resolution sections were collected, enabling detailed characterization of circulation branches and volume transport. On average, the West Spitsbergen Current (WSC) and the Front Current each transport approximately 2.5 SvΘ>2 °C,
Marine heatwaves (MHWs), have doubled in frequency globally in recent decades and are becoming longer, more intense, and increasingly disruptive to marine ecosystems. However, despite their growing ecological and biogeochemical importance, major productive coastal systems remain understudied, particularly in the Southern Hemisphere. Here, we provide the first comprehensive characterization of MHWs across the Patagonian Shelf (PS), one of the most biologically productive marine regions on Earth, using 40 years of satellite-derived daily sea surface temperature (SST) data. We first assess how the choice of MHW detection method (fixed versus moving climatology) and SST-dataset selection affect MHW metrics. Then we quantify MHW frequency, intensity, duration, and long-term trends, revealing that the PS experiences on average 1.9 ± 2 MHWs yr−1d yr−1± 0.3 °C. We show that MHW activity varies substantially across the region, with the northern sector and the outer shelf experiencing the most frequent and intense events (>2 events yr−1>2 °C). A notable increase in MHW days (+5–10 d per decade) is observed in the northern PS, whereas no significant trends are observed to the south (i.e., south of 48° S). These trends are consistent with background warming of the ocean in this region, suggesting a mechanistic link, whereby long-term warming enhances the likelihood of MHWs occurrence and duration. We further demonstrate that a component of MHW variability can be attributed to the El Niño Southern Oscillation, which exerts a stronger influence on the intensity of thermal anomalies than on the cumulative duration of the events. Together, these findings constitute the first comprehensive assessment of MHWs on the PS and provide essential insight for anticipating their ecological and climatic impacts in one of the Southern Hemisphere's key marine ecosystems.
The curvature of streamlines plays a significant dynamical and structural role in meandering currents. At scales comparable to the deformation radius, the motion of eddies is governed by a balance between the pressure gradient force, the Coriolis force, and the centrifugal force. For mesoscale eddies, the nonlinear term induced by the local curvature of streamlines is non-negligible. This study compares the statistical and dynamical parameters of mesoscale eddies under geostrophic and cyclogeostrophic balances by examining five energetic North Pacific regions: the Aleutian Islands, Kuroshio Extension, South China Sea, California Coastal Current, and Hawaiian Islands. The comparison shows that cyclogeostrophic EKE is lower than geostrophic EKE for cyclonic eddies, whereas it is higher for anticyclonic eddies, particularly in energetic, low-latitude regions. The total number of eddies detected under the cyclogeostrophic balance is 35.65 % lower than under the geostrophic balance. However, the frequency distributions of eddy radii in both frameworks show a right-skewed normal distribution. Detection under the cyclogeostrophic balance tends to eddies with larger radii and shorter lifespans. The velocity difference between the two balances for eddies increases with decreasing latitude. Similarly, case studies indicate that anticyclonic eddies exhibit more pronounced variability in their energy evolution under the influence of streamline curvature, making them more prone to dissipation in low-latitude seas.
The thermodynamic concepts that are used in physical oceanography are reviewed, including how the First Law of Thermodynamics is derived, and introducing the several different types of salinity. Different temperature-like variables are discussed, leading to potential enthalpy and Conservative Temperature because of the need to accurately quantify the ocean's role in transporting heat. A key aspect of a thermodynamic variable is the extent of its non-conservation when mixing occurs at a given pressure. Methods are presented that quantify the amount of this non-conservation of several thermodynamic variables, and these are illustrated in the context of the global ocean. There has been confusion in the literature about the meaning of the salinity and temperature variables carried by ocean models, and here we explain why even in older ocean models that use the EOS-80 equation of state (rather than TEOS-10), the model's salinity is Preformed Salinity and the model's temperature variable is Conservative Temperature. The thermodynamic reasoning that leads to the concept of neutral surfaces is reviewed, along with thermobaricity, cabbeling, the dianeutral motion caused by the ill-defined nature of neutral surfaces, and Neutral Surface Planetary Potential Vorticity.
This study investigates the influence of tides on chlorophyll a (CHL) variability in the Brazilian Equatorial Margin using daily GlobColour and MODIS-Aqua CHL data from 2005 to 2021. The impact of the tides is assessed by comparing the spring with the neap tide signals (fortnightly signal, 14.7 d). Results show that, on the shallow Amazon shelf, significant fortnightly CHL variability is likely primarily driven by barotropic tide-induced friction on the shelf that produces significant vertical mixing. On the northwestern shelf, where the Amazon River plume prevails, CHL levels are higher during neap tides, resulting in a negative spring–neap tide CHL difference (GlobColour: −50 %; MODIS-Aqua: −84 %). Conversely, on the northeastern shelf, characterized by low-turbidity waters, CHL levels are higher during spring tides, leading to a positive spring–neap tide CHL difference (GlobColour: +30 %; MODIS-Aqua: +70 %). Offshore, baroclinic tides, also known as internal tides (ITs), seem to enhance the CHL along their pathways with a spatial structure of a wave-like pattern. The positive CHL peaks are spaced by mode-2 wavelengths (about 68 km), with peak values reaching +3.3 % (GlobColour) and +9.0 % (MODIS-Aqua). Analysis shows that the CHL wave-like pattern suggests contributions from mode-1 and mode-2 internal tides, with mode-2 components having higher spectral coherence with the original signal. A 1–3 d lag between higher CHL variability and tidal potential may indicate delayed nutrient mixing post-spring–neap tides. The effects of ITs on CHL are more pronounced than on sea surface temperature.
We provide best practices for estimating the dissipation rate of turbulent kinetic energy, ε, from velocity measurements in an oceanographic context. These recommendations were developed as part of the Scientific Committee on Oceanographic Research (SCOR) Working Group #160 “Analyzing ocean turbulence observations to quantify mixing”. The recommendations here focus on velocity measurements that enable fitting the inertial subrange of wavenumber velocity spectra. The method examines the measurable range for this method of dissipation rates in the ocean, seas, and other natural waters. The recommendations are intended to be platform-independent since the velocities may be measured using bottom-mounted platforms, platforms mounted beneath the ice, or platforms directly on mooring lines once the data is motion-decontaminated. The procedure for preparing the data for spectral estimation is discussed in detail, along with the quality control metrics that should accompany each estimate of ε
Cold-water corals form complex three-dimensional structures on the seafloor, providing habitat for numerous species, and act as a carbon cycling hotspot in the deep-sea. The distribution of these important ecosystems is often predicted by statistical habitat suitability models, using variables such as terrain characteristics, temperature, salinity, and surface productivity. While useful, these models do not provide a mechanistic understanding of the processes that facilitate cold-water coral occurrence, and how this may change in the future. Here, we present the results of a mechanistic process-based model in which coral biomass and respiration are predicted based on hydrodynamics, organic matter transport and coral physiology. The model domain comprises the cold-water coral mounds of south-east Rockall Bank in the north-east Atlantic Ocean. Hydrodynamic forcing is provided by a high-resolution Regional Ocean Modelling System (ROMS) model, which drives the transport of reactive suspended particulate organic matter in the region. The physiological cold-water coral model, with coral food uptake, assimilation, and respiration as key variables and with model parameters estimated from available experimental reports, is coupled to the reactive transport model of suspended particulate organic matter. Cold-water coral biomass was mainly predicted on coral mounds and ridges in the area. Model predictions agree with coral reef biomass and respiration observations in the study area and coral occurrences agree with predictions from previously published habitat suitability models. Filter feeding activity by cold-water corals proved to strongly deplete food particles in the bottom waters. Replenishment of food particles by tidal currents was therefore vital for cold-water coral growth. This mechanistic modelling approach has the advantage over statistical and machine learning-based predictions that it can be used to obtain an understanding of the effect of changing environmental conditions such as ocean temperature, surface production export, or ocean currents on cold-water coral biomass distribution and can be applied to other study areas and/or species.
The Loop Current (LC) and its associated eddies, known as Loop Current Eddies (LCEs), are key oceanic features in the Gulf of Mexico. Using a statistical analysis of 29 years of satellite altimeter data (1993–2021), we show that more than half of the LCEs that detach from the LC reattach within 30 d and that only 42 % truly separate from the LC and move westward in the Gulf. Our observational analysis also shows that (i) before a detachment can occur, the LC needs to extend far enough north in the Gulf to reach the Mississippi fan (∼27.5° N); and (ii) the ratio of separations to reattachments depends on latitude, with detachments being more prone to reattach if they occur south of 25° N and to separate if they occur north of 25° N. In case (iii), cyclonic eddies are consistently present during the detachment process, with one cyclonic eddy on the eastern side of the LC if the LCE is to reattach, and one on each side if the LCE is to separate. When cyclonic eddies occur on both sides of the LC, their co-occurrence in the LC bottleneck zone forms a large cyclonic structure. This large cyclonic structure is often observed during separation events, and when it is absent, LCEs are more likely to reattach, indicating a potential role in modulating the LC extension into the Gulf of Mexico. Sometimes, it can restrict LC growth on time scales of several months. The observed associations between cyclonic eddies and LCE detachments provide a statistical framework that could help anticipate separation events.
Marine debris is responsible for major problems in our oceans, causing serious environmental degradation, detrimental health effects and economic losses in sectors related to the marine environment. In this work, we examine how plastics released by the Ulla river at the estuary's extreme affect the transport, accumulation, and beaching of floating particles in the Ría de Arousa, an estuary on the northwest coast of the Iberian Peninsula, as a result of wind force. Using Lagrangian simulations of particle tracking under different wind drag coefficients (1 %, 3 % and 5 %), we evaluate the spatial and seasonal patterns of particle concentration, residence time and deposition on the coast. Our results show that wind plays a crucial role in modulating particle behavior. Low wind-driven conditions favor greater near-shore accumulation and longer residence times, especially in the northern and inner regions of the estuary. As wind influence increases, particle dispersion intensifies, leading to lower overall accumulation and weakening of correlations between river discharge and coastal deposition. Seasonal differences are also studied, with higher concentrations observed in the north during winter and in the south during summer.
The EUCFe cruise (RV Kilo Moana, 2006) was designed to characterize sources of Fe to the western equatorial Pacific and its transport by the Equatorial Undercurrent (EUC), a narrow and fast eastward current flowing along the equator, to the eastern equatorial Pacific High Nutrient Low Chlorophyll (HNLC) region. This study presents seawater dissolved (DFe) and particulate (PFe) iron concentrations and isotopic compositions (δ56DFe and δ56PFe) from 15 stations in the equatorial band (2° N–2° S) between Papua New Guinea and 140° W, over more than 8500 km along the equator and in the upper 1000 m of the water column.δ56DFe and δ56PFe ranged from −0.22 ‰ to +0.79 ± 0.07 ‰ and from −0.52 ‰ to +0.43 ± 0.07 ‰, respectively (relative to IRMM-14, 95 % confidence interval). Source signatures, biogeochemical processes and transport all contribute to these observations. Two distinct areas, one under continental influence (the western equatorial Pacific) and an open ocean region (the central equatorial Pacific), emerged from the data. In the area under continental influence, high PFe concentrations along with δ56DFe values systematically heavier than that of δ56PFe indicated an equilibrium fractionation and the co-occurrence of chemical fluxes from both phases toward the other. This exchange occurs through non-reductive processes, as previously proposed from three of the eight stations of this area (Labatut et al., 2014) and extends up to 1200 km from the coast. In the open ocean area, preservation of a DFe isotopic signature of ∼ +0.36 ‰ within the EUC, from Papua New Guinea to the central equatorial Pacific (7800 km), confirmed the origin of the DFe carried within this current toward the HNLC region. At the same depth, bordering the EUC at 2° N and 2° S at 140° W, light isotopic signatures suggested that iron was originating from the eastern Pacific oxygen minimum zones. These light signatures were also observed in deeper central waters, between 200 and 500 m. Our data did not allow conclusions about fractionation during uptake by phytoplankton, but indicated that any fractionation, if present, must be small, no larger than a few tenths of a per mil.
The sensitivity of Baltic Sea salinities to changed freshwater forcing and other forcing factors have been debated during the last decades, since changed salinities would have large impacts on the marine ecosystems, and since this parameter still shows a high degree of uncertainty in regional climate projections. In this study, we performed a sensitivity experiment where freshwater forcing and salinities at the outer boundaries of the North Sea were perturbed in a systematic way in order to obtain a second-order Taylor polynomial of the statistical steady state mean salinity. The polynomial was constructed based on perturbations of a 57 year long hindcast run for the period 1961–2017 with a regional ocean model covering the North Sea and the Baltic Sea. The results show that the Baltic Sea is highly sensitive to freshwater forcing, and that about one third of the boundary salinity change propagates into the Baltic Sea. The results are also analysed in terms of a total exchange flow analysis in the entrance region, and it is found that the Baltic Sea salinity sensitivity to freshwater forcing to a large degree can be explained by increased freshwater input causing (1) dilution inside the Baltic Sea, (2) decreased inflows caused by changes to the mean sea level gradient in the entrance region, and (3) reduced inflow salinities due to recirculation of outflowing Baltic Sea water in the entrance region where the inflow water consists of about two parts outflowing Baltic water and one part North Sea water. Besides providing new understanding of the processes that govern the Baltic Sea salinity sensitivity to freshwater forcing, the results of this study provide means of quickly assessing Baltic Sea salinity changes based on changes of North-East Atlantic salinities and Baltic Sea freshwater forcing.
Tidal storm surges can result in significant inundation and damage if sea defences are insufficiently robust. Coastal planners need to know the risk of flooding so that sea defences and coastal developments can be specified and located appropriately. Since the original work on extreme value statistics by Gumbel (1954), several alternatives have been proposed for evaluating the risk of tidal inundation, with the Skew Surge Joint Probability Method (SSJPM) gaining popularity. However, SSJPM is complex and cannot always be applied generally. Guided by the search for a general method having wide application and amenable to automation, this paper re-examines the original approach of Gumbel and proposes a simple modification for combined peak selection and declustering; it is termed here TMAX, since it selects one maximum per tidal day. In comparison with the method of Gumbel (1954) using annual maxima (later termed AMAX), the TMAX method offers more efficient use of extreme data events and in addition, simpler handling of missing data. The results of the TMAX method are compared with those of a recent UK study using the SSJPM method at the same United Kingdom coastal locations. The broadly applicable TMAX method has the potential to offer more widespread calculation of flood return period, thereby improving strategies regarding coastal management and resilience.
The Caribbean Through-Flow (CTF) provides a key pathway linking the North Atlantic Subtropical Gyre and the upper limb of the Atlantic Meridional Overturning Circulation. Yet, its internal structure and variability remain poorly resolved. Autonomous underwater gliders offer a unique capability to address this gap by collecting high-resolution hydrographic and velocity observations in regions where sampling is sparse. Here, data from a glider that operated for >90 d along 69° W in summer 2024 were analyzed to investigate mesoscale-driven variability in the CTF. Two consecutive occupations of this ∼600 km trans-Caribbean section revealed a sharp decline in zonal transport from −17.64−9.22 Sv, coinciding with a shift in mesoscale activity. Magnitude and variability in the vertical shear of the subsurface currents and dynamic height anomaly calculations from the glider data showed a shift from flow largely in geostrophic balance during Transect #1 to increased mesoscale influence during Transect #2. Satellite altimetry spanning the full deployment suggested this shift was driven by a cyclonic eddy that passed through the northern half of the section between the timing of the two transects. Despite the large changes in transport between transect occupations, water mass analysis showed that the relative contributions from North and South Atlantic water masses remained nearly constant. Direct sampling of an anticyclonic eddy during a partial Transect #3 revealed strong temperature and salinity anomalies in the upper 200 m. These findings highlight how glider observations can resolve key features and processes governing variability in this critical inter-basin pathway and improve understanding of mesoscale influences on large-scale circulation.
Wind driven circulation of a uniform density fluid on a linearly sloping continental shelf is studied by employing the Lagrangian equations of motion forced by periodic rotary wind stress. The analysis yields explicit approximate expressions for the water column trajectories in the longshore and cross-shore directions, and these expressions are verified by numerical integration of the governing nonlinear equations. The periodic rotary wind stress generates a steady longshore drift directed with land to its left when the wind rotates counterclockwise at sub-inertial frequencies and with land to its right in all other frequencies. Counterclockwise rotation of the wind at the local inertial frequency results in a strong resonance manifested in very fast longshore drift.