Ocean Eddies Research Articles

Characteristics of ocean mesoscale eddies in the Agulhas and Tasman Leakage regions from two eddy datasets

Mesoscale eddies play an essential role in the freshwater transport, ocean circulation and climate. Although there is no completely objective eddy dataset, observations can better reproduce the eddies in the ocean as data resolution is improved and eddy detection methods are developed to some extent. This study uses satellite altimeter data and compares the Mesoscale Eddy Trajectory Atlas Product (META) with the Global Ocean Mesoscale Eddy Atmospheric-Oceanic-Biological Interaction Observational Dataset (GOMEAD) to analyze the mesoscale eddy characteristics in the oceanic pathways in the Agulhas Leakage (AL) region at the intersection of the Indian and Atlantic Oceans, and the Tasman Leakage (TL) region between the Indian and Pacific Oceans. Here we show that, the META dataset is more applicable for the detection of small-radius eddies, but this product increases the risk of over-detection. The GOMEAD dataset offers precise eddy boundaries and is useful for identifying large diameter eddies. The characteristics of eddies in the AL and TL regions are analyzed, and the results indicate that GOMEAD is more suited for eddy detection and analysis in the AL, while the META dataset is better suited for studies of mesoscale eddies of the TL region. Choosing the appropriate dataset is important for understanding the role of mesoscale eddies of different areas in a changing climate.

The effect of normal and abnormal eddies on the mixed layer depth in the global ocean

Mesoscale eddies are broadly distributed over the global ocean and play a significant role in modulating the spatiotemporal evolution of mixed layer depth (MLD). The presence of abnormal eddies in the ocean has been shown; however, the precise quantification of the effect of eddies on MLD, given the case of abnormal eddies, has not been carried out thus far. Differently from the previous approach to identify abnormal eddies through sea surface temperature, we therefore, proposed a method to identify abnormal eddies, using potential density based on Argo data (Array for Real-time Geostrophic Oceanography) for 15 years from 2003 to 2017. Results showed that abnormal anticyclonic eddies (AAE) and abnormal cyclonic eddies (ACE) accounted for 21.67% and 20.17% of total matching anticyclonic eddies (TAE) and the total matching cyclonic eddies (TCE), respectively, in the global ocean. The proportions of abnormal eddies were relatively higher in tropical regions but lower in regions with the boundary current and strong eddy kinetic energy. The MLD changes caused by normal and abnormal eddies were estimated combining satellite altimetry data. The normal eddies were the total matching eddies with the removal of the abnormal eddies, separately called normal anticyclonic eddies (NAE) and normal cyclonic eddies (NCE). The overall influence of NAE (NCE) was more significant on MLD deepening (uplifting) than that of TAE (TCE). Globally, NAE (NCE) changed MLD deepening (uplifting) from ~66 m (~54 m) to ~67 m (~53 m) and exhibited a more pronounced change in the Indian Ocean sector of the Southern Ocean region, from ~111 m (~94 m) to ~115 m (~92 m) in the winter. AAE (ACE), exerted a relatively weak but opposite effect on MLD deepening (uplifting). In other words, the global average MLD caused by them shoaled (deepened) from ~66 m (~54 m) to ~59 m (~59 m), and the North Pacific Ocean shoaled (deepened) from ~61 m (~47 m) to ~49 m (~57 m) in winter. Given the above results, abnormal eddies should be accounted for when the impact of ocean eddies is evaluated on the global climate system.

Phytoplankton Blooms Triggered by Anticyclonic Eddy and Cyclonic Eddy during Tropical Cyclone Nada

Tropical cyclone (TC) Nada occurred in the southwestern Bay of Bengal (BOB) in the winter of 2016. With high photosynthetically available radiation (PAR) and minimal precipitation, phytoplankton blooms occurred from offshore to nearshore respectively. In this study, there was an pre-existed ocean eddy pair on the left and right sides of Nada’s path before its passage, we studied the different mechanisms of phytoplankton blooms caused by TC Nada and the ocean eddy pair with satellite observation, multisource reanalysis products, and Argo float data. An anticyclonic eddy near Sri Lanka was weakened during Nada and horizontal transport of chlorophyll-a (Chl-a) balanced the inhibition effect of anticyclone on Chl-a. Near the anticyclonic eddy’s periphery, an upwelling above the thermocline was enhanced by Nada which promoted the uplift of nutrient-rich deep-water. Meanwhile, the large cyclonic eddy in the northern Bay of Bengal was strengthened by Nada, with the vorticity increasing from 0.19 s^–1 to 0.28 s^–1. Significant inertial oscillation (~2 days period) happened in the subsurface layer, leading to strong vertical mixing and upwelling and subsequently causing the offshore surface Chl-a bloom. This study provides new insights to evaluate typhoons and ocean eddies that induce biological responses in the future.

Contemporary international Law:Regulating the upcoming fukushima radioactive wastewater discharge

In April 2021, the Japanese government decided to release tonnes of radioactive wastewater from the wrecked Fukushima nuclear power plant into the ocean, a decision that will likely to pose a direct threat to the marine environment of not only Japan, but also the jurisdictional waters of neighboring countries. Legal measures such as litigation against Japan has been an option for the neighboring countries, although they require more time than available, unless interim measure are ordered. Based on a multidisciplinary research approach that integrates open-source research data, authoritative data and conclusions, this paper shows that since the ocean is a complex and changeable system and radionuclides could be redistributed by the ocean currents and eddies, the radioactive wastewater discharge will cause or likely to cause contamination in fish products and fishing grounds and have negative impact on marine life and humans. In response to the widely known skepticism regarding the application and efficiency of international law concerning the 2011 Fukushima Daichii accident, this paper argues that the nuclear law, ocean law and general principles of international law do provide sufficient constraints on the forthcoming radioactive wastewater discharge. Among the IAEA's legal documents, the Convention on Nuclear Safety (CNS) and the Convention on Early Notification are the most applicable conventions to the Fukushima radioactive wastewater discharge. Treaty interpretation of the “at sea” element in the London Dumping Convention and its Protocol is crucial to determine the legitimacy of the planned oceanic discharge activity of Japan. In addition, legal characterization of sources of pollution, the place in which the oceanic discharge activity occurred and Japan's substantive and procedural obligation under the United Nations Convention on the Law of the Sea (UNCLOS) will also affect the treaty application in the forthcoming oceanic discharge. This paper proposes that for the international community as a whole, long term (regional and global cooperation) and short-term measures (inter-State cooperation) need to be taken into consideration in order to tackle Japan's decision as well as laying the framework for any similar discharges that could follow Japan's example.

How does 4DVar data assimilation affect the vertical representation of mesoscale eddies? A case study with observing system simulation experiments (OSSEs) using ROMS v3.9

Abstract. Accurate estimates and forecasts of ocean eddies in key regions such as western boundary currents are important for weather and climate, biology, navigation, and search and rescue. The dynamic nature of mesoscale eddies requires data assimilation to produce accurate eddy timings and locations in ocean model simulations. However, data assimilating models are rarely assessed below the surface due to a paucity of observations; hence it is not clear how data assimilation impacts the subsurface eddy structure. Here, we use a suite of observing system simulation experiments to show how the subsurface representation of eddies is changed within data assimilating simulations even when assimilating nearby observations. We examine in detail two possible manifestations of how the data assimilation process impacts three-dimensional eddy structure, namely, by producing overly active baroclinic instability and through inaccurate vertical mode structure. Therefore, in DA simulations, subsurface temperature structures can be too deep and too warm, particularly in dynamic eddy features. Our analyses demonstrate the need for further basic research in ocean data assimilation methodologies to improve the representation of the subsurface ocean structure.

Uncertainties in Altimetry Observations of Eddy Changes Induced by Tropical Cyclones

Abstract The effects of tropical cyclones (TCs) on preexisting eddies are generally quantified by comparing post-TC and pre-TC altimetry-based eddy amplitudes and radii. The dynamical and technical uncertainties in this quantification have been revealed by the altimetry-based and simulated eddy characteristics of five cyclonic ocean eddies (COEs) and two anticyclonic ocean eddies (AOEs). Although demonstrated by eddy cases, both the uncertainties should be universal in principle. The dynamical uncertainty primarily arises from the highly variable eddy characteristics associated with the post-TC quasigeostrophic evolutions driven by the inevitable pattern discrepancy between TC-injection and preexisting eddy’s potential vorticity (PV). The technical uncertainty is due to the artificial smoothness in the altimetry-based eddy characteristics produced by the mismatch between sparse data interpolation and sudden injection of TC-induced effects. Beyond the uncertainties, the amplitudes and radii of both the COEs and AOEs were damped directly by a rectilinear-track TC. After the TC passage, the COEs may strengthen again or remain in the damped state, depending on whether the COEs can absorb the TC-injected PV. By contrast, the AOEs remained in the damped state because the TC-injected positive PV cannot excite them to enhance and enlarge. More importantly, the above damped state of the perturbed COEs and AOEs may be the result of the developing geostrophic turbulence, not meaning the decay of the TC-induced effects. This fact, together with the dynamical and technical uncertainties, implies that the previously used quantification may significantly underestimate the TC-induced effects. Significance Statement Typhoons/hurricanes inject their effects on ocean eddies and further modulate the ocean circulation and climate by the accumulated effects. These effects are generally quantified by altimetry observations. Two uncertainties in this quantification are illustrated by using several eddy cases. The first uncertainty is caused by the eddy evolution, while the second is by the artificial smoothness in the altimetry-based eddy characteristics. These findings suggest that the effects of typhoons/hurricanes may be underestimated due to the two uncertainties and underscore that a new method based on physical understanding is necessary to quantify these effects.

Oceanic Eddy Identification Using Pyramid Split Attention U-Net With Remote Sensing Imagery

Oceanic eddy is the ubiquitous ocean flow phenomenon, which has been the key factor in the transportation of ocean energy and materials. Consequently, oceanographic understanding can be enhanced by the intelligent identification of eddy. The state-of-the-art deep learning technologies are gradually improving the identification methods. This letter proposes the pyramid split attention(PSA) eddy detection U-Net architecture (PSA-EDUNet) that targets oceanic eddy identification from ocean remote sensing imagery. As for the PSA-EDUNet, its inspiration comes from U-Net, which contains encoder and decoder parts, making the integration of inferior and senior features efficiently and ensuring the feature information will not be lost in large quantities through non-linear connection mode. Meanwhile, PAS module is introduced to enhance feature extraction. In terms of the fusion data, the sea surface feature is the main criterion of eddy identification, including sea surface temperature (SST) and sea level anomaly (SLA). The experiments are implemented on the Kuroshio Extension and the South Atlantic regions, the results demonstrate that the proposed method can outperform other methods, especially for eddy edges and small-scale eddies.

Diagnosing Scale-Dependent Energy Cycles in a High-Resolution Isopycnal Ocean Model

Abstract Energy exchanges between large-scale ocean currents and mesoscale eddies play an important role in setting the large-scale ocean circulation but are not fully captured in models. To better understand and quantify the ocean energy cycle, we apply along-isopycnal spatial filtering to output from an isopycnal 1/32° primitive equation model with idealized Atlantic and Southern Ocean geometry and topography. We diagnose the energy cycle in two frameworks: 1) a non-thickness-weighted framework, resulting in a Lorenz-like energy cycle, and 2) a thickness-weighted framework, resulting in the Bleck energy cycle. This paper shows that framework 2 is more useful for studying energy pathways when an isopycnal average is used. Next, we investigate the Bleck cycle as a function of filter scale. Baroclinic conversion generates mesoscale eddy kinetic energy over a wide range of scales and peaks near the deformation scale at high latitudes but below the deformation scale at low latitudes. Away from topography, an inverse cascade transfers kinetic energy from the mesoscales to larger scales. The upscale energy transfer peaks near the energy-containing scale at high latitudes but below the deformation scale at low latitudes. Regions downstream of topography are characterized by a downscale kinetic energy transfer, in which mesoscale eddies are generated through barotropic instability. The scale- and flow-dependent energy pathways diagnosed in this paper provide a basis for evaluating and developing scale- and flow-aware mesoscale eddy parameterizations. Significance Statement Blowing winds provide a major energy source for the large-scale ocean circulation. A substantial fraction of this energy is converted to smaller-scale eddies, which swirl through the ocean as sea cyclones. Ocean turbulence causes these eddies to transfer part of their energy back to the large-scale ocean currents. This ocean energy cycle is not fully simulated in numerical models, but it plays an important role in transporting heat, carbon, and nutrients throughout the world’s oceans. The purpose of this study is to quantify the ocean energy cycle by using fine-scale idealized numerical simulations of the Atlantic and Southern Oceans. Our results provide a basis for how to include unrepresented energy exchanges in coarse global climate models.

Geographic-dependent variational parameter estimation: A case study with a 2D ocean temperature model

Using observational information to tune uncertain physical parameters in an ocean model via a robust data assimilation method has great potential to reduce model bias and improve the quality of sea temperature analysis and prediction. However, how observational information should be used to optimize geographic-dependent parameters through four-dimensional variational (4DVAR) data assimilation, which is one of the most prevailing assimilation methods, has not been fully studied. In this study, a two-step 4DVAR method is proposed to enhance parameter correction when the assimilation model contains biased geographic-dependent parameters within a biased model framework. Here, the biased parameters are set to an oceanic eddy diffusion coefficient, K v , that plays an important role in modulating synoptic, seasonal and long-term changes in ocean heat content. Within a twin assimilation experiment framework, the temperature “observations” generated from sampling a “truth” model are assimilated into a biased model to investigate to what extent K v can be estimated using the 4DVAR method when K v remains geographic-dependent. The results show that the geographic-dependent K v distribution can be optimally estimated to further improve the sea temperature analysis performance compared with the state estimation only method. In addition, the model prediction performance is also discussed with optimally estimated parameters under various conditions of noisy and/or sparse ocean observations. These results provide some insights for the prediction of ocean temperature mixing and stratification in a 3D primitive ocean numerical model using 4DVAR data assimilation. • A two-step four-dimensional variation is proposed to enhance parameter correction. • Feasibility of Geographic-dependent parameter optimization (GPO) is explored. • Performance of the model prediction can be enhanced with GPO-estimated parameters.

Small object detection for ocean eddies using contextual information and attention mechanism

Small object detection for ocean eddies using contextual information and attention mechanism

Anticyclone Eddies Favor the Genesis of Off‐Season Tropical Cyclone in the Western North Pacific

AbstractMesoscale ocean eddies, which typically exhibit different heat and atmospheric conditions to their surroundings, significantly affect the path and intensity of tropical cyclones (TCs). By analyzing 27 years (1993–2019) of eddy and TC data, here we show that mesoscale eddies also play non‐negligible roles in the genesis of TCs (TCG). During the TC off‐season months of December to May, TCs generating over anticyclone eddies (AEs) clearly outnumber those over cyclone eddies (CEs) (14 vs. 6) in the western North Pacific, and featuring a further north location. Composite analysis also exhibits a higher genesis potential index () in AEs than that in CEs, indicative of a higher TCG probability over AEs especially under a relatively cooler oceanic background, which is mainly attributed to the eddy‐associated warming in the upper ocean. These results motivate future studies on better forecasting regional TC activities by considering the impacts of ocean eddies in air‐sea models.

What happened around an inverted V-shaped track turning of the tropical cyclone Madi?

The tropical cyclone (TC) Madi with an inverted V-shaped track happened over the Bay of Bengal (BoB) from Dec. 5 to Dec. 12, 2013. In this paper, the strong oceanic and atmospheric responses during the turning of the inverted V-shaped track of Madi were studied using the multisource satellite remote sensing and Argo float data. The results indicate that the response during the pre-turn phase (00:00 Dec.5 to 06:00 Dec.10) is more acute than that during the post-turn phase (06:00 Dec.10 to 18:00 Dec.12) of Madi. During the pre-turn phase, the cumulative precipitation reached 206 mm, the sea surface temperature (SST) decreased from 27.7 °C to 24.5 °C, and the maximum wind speed (MWS) increased from 7.71 m·s−1 to 43.72 m·s−1. At the turning corner on Dec.10, a symmetrical distribution of temperature drop, with greater precipitation on the left than that on the right. Meanwhile, the deep vertical wind shear was 17.13 m·s−1 with a northwestward moving cyclonic eddy. The barrier layer thickness was reduced from 25.82 m (24.88 m) to 23.8 m (12.38 m) during the pre-turn (post-turn) phase. Moreover, the potential vorticity (PV) caused by the ocean eddy disturbance is closely related to the intensity of TC. This study helps to estimate the sea-air interactions in the western BoB.

Influence of the interannual variability of the Kuroshio Extension on the Mediterranean trough in the cold season

The impacts of the Kuroshio Extension (KE) interannual fluctuation on the Mediterranean trough (MedT) and associated Euro-Mediterranean climate in wintertime are analyzed in this study. It appears that when the KE index is positive (corresponds to a stable KE state with sharp sea surface temperature front and subsided oceanic eddies), the MedT is observed to be stronger in the west of its climatological position, resulting in a colder condition to the west of the Black Sea and more precipitation in the East Mediterranean and Anatolia Plateau. The opposite atmospheric pattern holds when the KE index is negative (unstable KE state). The empirical orthogonal function analysis is performed on 500-hPa geopotential height to obtain the dominant modes of MedT. The results show the KE index is significantly correlated with the intensity and zonal displacement of MedT, with correlation coefficients of 0.57 and 0.40, respectively. The low-frequency Rossby wave activity and high-frequency eddies are revealed as the prominent contributors to this co-variability between the KE and MedT. The upward turbulent heat flux transport from the ocean to the atmosphere strengthened by the stable KE activates the Rossby waves propagating upward and westward to the MedT region, contributing to approximately 30% of the MedT deepening. Meanwhile, the KE-driven Rossby waves intensify the background baroclinicity to the north of the Mediterranean Sea and thus enhance the transient eddy activity, consequently leading to another 20% of the MedT amplification via the eddy–mean flow interaction. The findings highlight the impacts of the KE state on the meteorological conditions in the Euro-Mediterranean region, which can potentially optimize the forecast accuracies of the MedT and associated Euro-Mediterranean climate variability.

Submesoscale oceanic eddy detection in SAR images using context and edge association network

Oceanic eddies have a non-negligible impact on ocean energy transfer, nutrient distribution, and biological migration in global oceans. The fine detection of oceanic eddies is significant for the development of marine science. Remarkable achievements of eddy recognition were achieved by mining the satellite altimeter data and its derived data. However, due to the limited spatial resolution of the altimeters, it is difficult to detect the submesoscale oceanic eddies with radial dimensions less than 10 km. Different from the previous works, the context and edge association network (CEA-Net) is proposed to identify submesoscale oceanic eddies with high spatial resolution Sentinel-1 data. The edge information fusion module (EIFM) is designed to associate the context and edge feature more accurately and efficiently. Furthermore, a multi-scale eddy detection strategy is proposed and applied to Sentinel-1 interferometric wide swath data to solve the scale problem of oceanic eddy detection. Specifically, a manually interpreted dataset, SAR-Eddy 2019, was constructed to address the dilemma of insufficient datasets for submesoscale oceanic eddy detection. The experimental results demonstrate that CEA-Net can outperform other mainstream models with the highest mAP reaching 85.47% with SAR-Eddy 2019 dataset. The CEA-Net proposed in this research provides important significance for the study of submesoscale oceanic eddies.

Observation of a mesoscale warm eddy impacts acoustic propagation in the slope of the South China Sea

Acoustic rays are modified while propagating through oceanic eddies. However, due to the lack of field synchronous observation, the impact of mesoscale eddy on the acoustic propagation is less clarified. To address the issue, an eddy-acoustic synchronous observation (EASO) field experiment for a mesoscale warm eddy was carried out in the slope of the South China Sea (SCS) in October, 2021. During the field experiment, a total of 105 conductivity-temperature-depth (CTD) stations, as well as a zonal acoustic survey line through the center of the warm eddy, were obtained. The vertical structures of temperature and salinity indicate that the warm eddy is surface-intensified with temperature and salinity cores confined within depths from 70 m to 200 m and 10 m to 70 m, respectively. The acoustic observation shows two obvious convergency zones (CZs) at about 39 km and 92 km in the eastern half acoustic line, and one convergency zones (CZ) at about 25 km in the western half acoustic line. By comparing with the none eddy circumstance, the respective impacts of the topography and warm eddy are quantitatively analyzed with a ray-tracing model. The results indicate that the topography shortens the horizontal span of the CZ by 11.4 km, while the warm eddy lengthens it by 1.7 km. Additionally, the warm eddy shallows the depth and broadens the width of the CZ by 32 m and 1.4 km, respectively. The anisotropy of 3D sound fields jointly influenced by the warm eddy and the local topography show that the distance differences of the first CZs in different horizontal directions can be as long as 31 km.

Search and tracking strategy of autonomous surface underwater vehicle in oceanic eddies based on deep reinforcement learning

Due to dynamic changes and instability of oceanic eddies, continuous tracking and sampling using mobile platforms is a challenging field. Aiming at the requirements of accurate observation of mesoscale eddies, this paper studies the problem of searching and tracking the eddy center in a mapless environment with an underactuated autonomous surface underwater vehicle (ASUV) and proposes a path planning method based on the deep reinforcement learning (DRL). Firstly, the existing observation methods are summarized, and the dynamic tracking framework of mesoscale eddies is established. Then, the DRL and long short-term memory (LSTM) are combined to train end-to-end and real-time planning strategies in an eddy environment. Finally, a high-fidelity simulation platform in the eddy environment is built, and actual data from the Kuroshio Extension region is used to verify the effectiveness and feasibility of the strategies. The experimental results show that the ASUV with the proposed strategies can realize the autonomous search and tracking of the eddy center, have good stability and real-time performance, and the tracking error is always within an acceptable range.

A Numerical Study on the Role of Mesoscale Cold-Core Eddies in Modulating the Upper-Ocean Responses to Typhoon Trami (2018)

Abstract With two groups of numerical experiments with and without the cold-core eddy (CCE), the impacts of the CCE on the upper-ocean responses to Typhoon Trami (2018) were investigated using a coupled atmosphere–ocean model. It is commonly accepted that the CCE promotes the sea surface cooling (SSC) primally through the enhanced vertical mixing, while the contributions from the wind-driven advection and the near-inertial advection to the differences in the sea surface temperature (dSST) were underestimated. This study found that the presence of CCE contributed to the stronger along-track cold advection, which dominated the increase in the SSC near the radius of maximum wind (RMW) to the right of Trami’s track, and the stronger cross-track warm advection was acting to prevent the cooling induced by the vertical mixing. During the relaxation stage, the stronger near-inertial advection within the CCE accounted largely for the amplification and the redistribution of the dSST. As for the dynamic responses, the enhanced upwelling and downwelling within the CCE explained the larger cooling and warming in the subsurface temperature oscillations. The wind-driven acceleration of the currents in the mixing layer was larger during the typhoon–eddy interaction so that the CCE became an efficient mixer, thus contributing to the rapid surfacing of the cold water and the ensuing stronger wind-driven advection. These results highlight the importance of the advection processes in the modulating effect of the CCE. Therefore, 3D ocean models are needed to incorporate the mesoscale features of the oceanic eddies for realistically reproducing the upper-ocean responses to tropical cyclones (TCs).

Enhancement of Oceanic Eddy Activity by Fine‐Scale Orographic Winds Drives High Productivity, Low Oxygen, and Low pH Conditions in the Santa Barbara Channel

AbstractThe Santa Barbara Channel is one of the most productive regions of the California Current System. Yet, the physical processes that sustain this high productivity remain unclear. We use a high‐resolution physical‐biogeochemical model to show that submesoscale eddies generated by islands are energized by orographic effects on the wind, with significant impacts on nutrient, carbon, and oxygen cycles. These eddies are modulated by two co‐occurring air‐sea‐land interactions: transfer of wind energy to ocean currents that intensifies ocean eddies, and a wind‐current feedback that tends to dampen them. Here we show that the dampening is overwhelmed by fine scale wind patterns induced by the presence of surrounding capes and islands. The fine‐scale winds cause an additional transfer of momentum from the atmosphere to the ocean that energizes submesoscale eddies. This drives upward doming of isopycnals in the center of the channel, allowing a more efficient injection of nutrients to the surface, and triggering intense phytoplankton blooms that nearly double productivity relative to the case without fine‐scale winds. The intensification of the doming effect by the wind‐curl and submesoscale eddies pumps deep low oxygen, acidic waters to the center of the cyclonic eddies. These eddies are then transported away from the Channel into the California Current, where they impact a wider area along the central coast, with potential ecological consequences. Our study highlights the important role of air‐sea‐land interactions in modulating coastal processes, and suggests that submesoscale resolving models are required to correctly represent coastal processes and their ecological impacts.

Growth of ocean thermal energy conversion resources under greenhouse warming regulated by oceanic eddies

The concept of utilizing a large temperature difference (>20 °C) between the surface and deep seawater to generate electricity, known as the ocean thermal energy conversion (OTEC), provides a renewable solution to fueling our future. However, it remains poorly assessed how the OTEC resources will respond to future climate change. Here, we find that the global OTEC power potential is projected to increase by 46% around the end of this century under a high carbon emission scenario, compared to its present-day level. The augmented OTEC power potential due to the rising sea surface temperature is partially offset by the deep ocean warming. The offsetting effect is more evident in the Atlantic Ocean than Pacific and Indian Oceans. This is mainly attributed to the weakening of mesoscale eddy-induced upward heat transport, suggesting an important role of mesoscale eddies in regulating the response of thermal stratification and OTEC power potential to greenhouse warming.

Observing the spread of Agulhas Leakage into the Western South Atlantic by tracking mode waters within ocean rings

The Agulhas rings transport warm and salty waters that feed the surface limb of the Atlantic Meridional Overturning Circulation. Some studies have focused on the conveying capacity of ocean eddies, and recently, the role of the Agulhas rings in advecting water masses and organisms has been explored. Here we show evidence that the Agulhas rings are responsible for the advection of mode waters from the Cape Basin to the western side of the Atlantic. We analyzed more than 3,200 temperature profiles and 2,400 salinity profiles from historical databases collocated with 52 long-lived Agulhas rings tracked from 1993 through 2016. An automated algorithm was used to identify thermostads in the profiles acquired within the rings. The data revealed mode water layers trapped inside 88% of the rings. The joint distribution of temperature and salinity indicated two types of mode waters in the range 16.2 ± 0.6°C, 35.6 ± 0.1 (Type I) and 12.9 ± 0.7°C, 35.2 ± 0.1 (Type II). The majority (67%) of the rings carrying mode waters had both types detected inside. Moreover, considering only those rings sampled west of the Mid-Atlantic Ridge, we found that 45% of them advected mode waters to the western basin. Therefore, our results demonstrate that, despite the long journey, interaction with the bottom topography and other vortices, ocean-atmosphere exchanges, and decay, the Agulhas rings are responsible for spreading mode waters initially available at the Cape Basin throughout the South Atlantic, contributing to a positive anomaly in temperature and salinity along the eddy corridor joining the Cape Basin to the Brazil Basin.