Edge Of Eddies Research Articles

Oceanic eddy with submesoscale edge drives intense air-sea exchanges and beyond

Oceanic mesoscale eddies influence air-sea interaction and atmosphere dynamics through ventilating heat and moisture upward. However, whether the sea surface temperature (SST) gradient on the eddy edge could affect the heat and moisture release is still unknown because of the limited observations and coarse-resolution climate models. Using high-resolution atmospheric simulations, this study compares the atmospheric response to the mesoscale (~ 40 km) and submesoscale (~ 4 km) SST gradients at the edge of an eddy. Results show that submesoscale SST gradient drives stronger surface heat and moisture fluxes, enhancing the vertical mixing intensity by 2–3 times within and above the marine atmospheric boundary layer. As a result, one local precipitation event is found to be an order of magnitude larger overlying the eddy. Our findings highlight the importance of resolving oceanic submesoscale features for accurately predicting atmosphere dynamics and precipitation over the ocean.

Nitrogen fixation in the North Atlantic supported by Gulf Stream eddy-borne diazotrophs

Mesoscale oceanic eddies contribute to the redistribution of resources needed for plankton to thrive. However, due to their fluid-trapping capacity, they can also isolate plankton communities, subjecting them to rapidly changing environmental conditions. Diazotrophs, which fix dinitrogen (N2), are key members of the plankton community, providing reactive nitrogen, particularly in large nutrient-depleted regions such as subtropical gyres. However, there is still limited knowledge about how mesoscale structures characterized by specific local environmental conditions can affect the distribution and metabolic response of diazotrophs when compared with the large-scale dynamics of an oceanic region. Here we investigated genetic diazotroph diversity and N2 fixation rates in a transect across the Gulf Stream and two associated eddies, a region with intense mesoscale activity known for its important role in nutrient transport into the North Atlantic Gyre. We show that eddy edges are hotspots for diazotroph activity with potential community connectivity between eddies. Using a long-term mesoscale eddy database, we quantified N2 fixation rates as up to 17 times higher within eddies than in ambient waters, overall providing ~21 µmol N m−2 yr−1 to the region. Our results indicate that mesoscale eddies are hotspots of reactive nitrogen production within the broader marine nitrogen cycle.

Response of Cyclonic Eddies to Typhoon Surigae and Their Weakening Effect on the Kuroshio Current in the Western North Pacific Ocean

This study investigated the dynamic and thermal responses of cyclonic eddies (CEs) to Typhoon Surigae in the western North Pacific Ocean using satellite data and a coupled ocean–atmosphere model. Observations and simulations revealed that the typhoon enhanced the two preexisting CEs (C1 and C2). After the typhoon passed the two eddies, the sea surface height (SSH) lowered and the eddy velocity increased above 200 m. C1 was stretched with elliptical deformation accompanied by an SSH trough and jets on the sides of the typhoon track at the eddy edge. The comparative experiments indicated that the typhoon caused the SSH of C1 and C2 to lower by 53.52% and 25.14% compared to conditions without the typhoon, respectively, and the kinetic energy of C1 and C2 to increase by 12 times and 65.76%, respectively. The positive vorticity anomaly input from the typhoon to the CEs was the main mechanism for the enhancement of the CEs. The enhanced CEs modulated the typhoon-induced sea surface temperature (SST) cooling, causing the temperature within the eddies to decrease by upwelling and mixing, and the SST cooling became significant at the center of the CEs and propagated westward with the eddies. This study also revealed that typhoons can significantly perturb eddy dynamic structures by enhancing or generating cyclonic cold eddies and eradicating anticyclonic eddies, thereby weakening the Kuroshio Current transport via eddy–Kuroshio interactions.

Tracking Loop Current Eddies in the Gulf of Mexico Using Satellite-Derived Chlorophyll-a

During the period of 2018–2022, there were six named Loop Current Eddy (LCE) shedding events in the central Gulf of Mexico (GoM). LCEs form when a large anticyclonic eddy (AE) separates from the main Loop Current (LC) and propagates westward. In doing so, each LCE traps and advects warmer, saltier waters with lower Chlorophyll-a (Chl-a) concentrations than the surrounding Gulf waters. This difference in water mass permits the study of the effectiveness of using Chl-a from satellite-derived ocean color to identify LCEs in the GoM. In this work, we apply an eddy-tracking algorithm to Chl-a to detect LCEs, which we have validated against the traditional sea surface height-(SSH) based eddy-tracking approach with three datasets. We apply a closed-contour eddy-tracking algorithm to the SSH of two model products (HYbrid Coordination Ocean Model; HYCOM and Nucleus for European Modelling of the Ocean; NEMO) and absolute dynamic topography (ADT) from altimetry, as well as satellite-derived Chl-a data to identify the six named LCEs from 2018 to 2022. We find that Chl-a best characterizes LCEs in the summertime due to a basin-wide increase in the horizontal gradient of Chl-a, which permits a more clearly defined eddy edge. This study demonstrates that Chl-a can be effectively used to identify and track LC and LCEs in the GoM, serving as a promising source of information for regional data assimilative models.

Three-dimensional characteristics of mesoscale eddies in the western boundary current region of the Bay of Bengal using ROMS-NPZD

Eddies play an important role in transporting and redistributing the heat, salt, and biological parameters in the global ocean. In this study, three-dimensional physio-biochemical characteristics of an anticyclonic (AE) and a cyclonic eddy (CE) associated with the poleward western boundary current (WBC) in the Bay of Bengal (BoB) are analyzed using a coupled bio-physical ocean model (ROMS-NPZD). Due to eddy-induced upwelling (downwelling) associated with the CE (AE), monopole patterns of temperature and salinity anomaly around the eddy center are occurred with extremum values at 100 – 150 m and 50 – 100 m depth, respectively. The upward (downward) curvature with a tip at eddy centers is observed in the isothermal layer depth, 20℃ isotherm (D20) nutricline, and oxycline. The depth of D20 deepens and shallows at the center from the edge by ∼60 m and ∼40 m for AE and CE, respectively. A relatively thick barrier layer around the center of AE is noted as compared to the CE. Within eddy interior, temperature and salinity tendencies of horizontal advection show a strong dipole pattern with extremum at eddy edges. The AE currents show positive tendencies in horizontal advection of temperature (salinity) at its northeastern (southwestern) side, which is totally opposite to that of CE. Intense horizontal advections in the adjacent flanks of eddies to the eastward jet of the WBC are due to their strong interactions. Vertical advection shows a quadrupole structure with alternating positive and negative cells within the eddies following the distorted vertical velocity field. Diffusion terms within eddies are quite small compared to the advection. Abundances of chlorophyll-a (∼1.2 mg/m3) at the base of the mixed layer around the CE center are absent for the AE.

Enhanced Mixing at the Edges of Mesoscale Eddies Observed From High‐Resolution Seismic Data in the Western Arctic Ocean

AbstractIn recent years, the Arctic Ocean has experienced a dramatic decrease in sea ice cover, leading to enhanced turbulent mixing and eddy activity. Due to the scarcity of high‐resolution observations, our understanding of turbulent mixing in the Arctic is incomplete. Using three seismic reflection transects with high spatial resolution (∼O (10) m) and concurrent ADCP‐derived current velocity, this study identified 11 mesoscale eddies on the ice‐free Chukchi Borderlands of the western Arctic Ocean. 82% of them are intra‐halocline anticyclones. The total length of horizontal scales of these 11 eddies occupies ∼40% of the total length of three seismic lines. Diapycnal diffusivities estimated from seismic data were enhanced and can be up to 10−4 m2 s−1 at the edges of these eddies, compared with the average values (∼10−6 m2 s−1) at the Chukchi Borderlands. Seismic‐estimated diffusivities were validated by fine‐scale parameterization using ADCP‐derived velocity data and historical hydrographic data. Enhanced diffusivities at the edges of eddies may be attributed to shear instabilities at the top and bottom edges and to submesoscale motions at the lateral edges of these eddies. We highlight the enhanced mixing at the edges of eddies in the halocline can increase the upward heat flux. This upward heat flux can transfer the heat of the warm Atlantic water below to the surface, which may further promote the melting of surface sea ice.

EFGAN: An automatic GAN-based methodology for mining eddy-front coupling with fused remote sensing data

Oceanic eddies and fronts are plentiful in the global oceans. There are abundant frontal processes at the edges of eddies, meanwhile, eddies can be formed due to instability along the fronts. Moreover, they can both induce submesoscale processes and strong vertical motions. Consequently, it is crucial to understand the correlation between eddies and fronts, along with their spatiotemporal distribution characteristics, for the transport of ocean energy and material and marine ecosystems. Generally, using remote sensing to detect eddies and fronts is based on geometrics, physical parameters, and handcrafted features. Existing approaches are inaccurate due to the highly dynamic nature of eddies and fronts as well as the lack of physical mechanisms between them. This paper proposes an adversarial approach for mining eddy-front coupling, dubbed the eddy-front generative adversarial network (EFGAN). In EFGAN, the generator follows the encoder–decoder structure and consists of a data encoder, a feature decoder, and a multi-task generation module. The rich contextual and semantic information on eddy features and frontal structure, which are the physical constraints for EFGAN, can be extracted from the fusion of satellite sea level anomaly (SLA) and sea surface temperature (SST) data, generating styled eddy-front coupling images with the accompaniment of the mask of eddy-front categories and styled fronts. To tackle the issue of inconsistency in the category of a single eddy, an instance consistency loss is designed to make the category of each individual eddy as consistent as possible, thus ensuring that the correlation of eddy-front belongs to a single category. Furthermore, multiple loss functions are combined to jointly guide network training to improve stability. Extensive experiments demonstrate that EFGAN outperforms state-of-the-art generative adversarial network-based models. The spatial distribution indicates that frontal eddies are more prevalent at the confluence where warm and cold currents meet. In the Kuroshio Extension region, there are concentrated distribution areas of frontal eddies that are strong in the boreal spring and summer, while eddy-induced fronts are highly active in the boreal winter and spring but weak in autumn.

Subsurface imbalance stimulated in a mesoscale eddy. Part I: Observations

This study presents observational evidence of a stimulated imbalance below the mixed layer from a fast-tow cruise survey with approximately 4 km resolution that traverses a cyclonic mesoscale eddy in the central South China Sea. The observed physical and biochemical properties reveal both geostrophic and ageostrophic physics in the mesoscale eddy. In addition to the well-known eddy-induced features, banded structures of temperature and chlorophyll anomaly across the section indicate unbalanced motions. These features are notably active near the eddy's edge and show larger amplitudes of variance than those induced by the mesoscale eddy. The results also show remarkable unbalanced variances of temperature and salinity, as well as lateral temperature-salinity compensation on isopycnals, likely due to stirring associated with internal waves. Spectral analysis suggests that there exist significant unbalanced variances of temperature and chlorophyll at ∼13 km. Our further analysis suggests that symmetric instability may have been stimulated due to strong lateral buoyancy gradients and vertical shear of velocity, which may account for the imbalance signature in the pycnocline. The observed imbalances point to the instability due to the interaction between mesoscale, submesoscale, and internal wave dynamics, which may play an important role in catalyzing the forward energy cascade in the ocean.

Bio‐Optical, Physical, and Chemical Properties of a Loop Current Eddy in the Gulf of Mexico

AbstractMulti‐sensor data collected with in situ and satellite instruments during August 2015 were used to understand how the three‐dimensional bio‐optical properties of a Loop Current Eddy (LCE) in the Gulf of Mexico (GoM) contrast those of the background waters, and how these properties are related to physical and chemical properties. With a surface radius of ∼150 km and vertical extension to 1,400–1,500 m, the LCE was found to have highly stratified waters in two layers, with one lying just below the mixed layer (16 m) and the other coinciding with the pycnocline (∼120–200 m within the eddy). Strong contrasts were found in the bio‐optical properties (chlorophyll‐a concentration, absorption of particulate and dissolved matters, particulate backscattering, and beam‐c attenuation) across the eddy core, eddy edge, and surrounding waters. Absorption coefficients (400 nm) of surface particulate and dissolved matters were ∼4 times higher in the surrounding waters than in the eddy core, while surface reflectance (400 nm) in the eddy core was ∼7 times higher than in the surrounding waters. The magnitude of deep chlorophyll maximum (DCM) was comparable (0.3–0.33 mg/m3) in all waters, but the depth of DCM in the eddy core (∼115 m) was much deeper than in the surrounding waters (60–75 m). These contrasts were found to correspond to different water masses with different physical (temperature, density, and buoyancy frequency) and chemical properties (salinity and dissolved oxygen concentration), where physical processes (river plume advection and eddy‐induced downwelling) appeared to drive the changes in bio‐optical properties.

Particulate organic carbon in the deep-water region of the Gulf of Mexico

Ocean eddies play a major role in lateral and vertical mixing processes of particulate organic carbon (POC), as well as in the transport of heat, salinity, and biogeochemical tracers. In the open waters of the Gulf of Mexico (GoM), however, there are limited observations to characterize how these mesoscale structures affect the spatial distribution of POC in the upper water column, which is important for organic matter cycling and export. We present the distribution patterns of POC relative to mesoscale features throughout the water column in the deep-water region of the GoM during three oceanographic cruises held during the summer months of 2015, 2016, and 2017. Samples were collected under well-stratified upper ocean conditions, which allowed us to assess the spatial and temporal distribution of POC as a function of non-steric sea surface height, density, apparent oxygen utilization, and chlorophyll fluorescence. We further explored the variability of integrated surface layer POC concentrations at stations located within the cores and the edges of cyclonic and anticyclonic eddies, and those collected outside these structures. Although our results indicate mesoscale eddies modulate several important physical and biogeochemical variables and POC concentrations in the upper ocean, these features do not fully explain the spatial distribution of POC concentrations throughout the deep-water region of the GoM. Relatively lower POC concentrations were observed in the border of the cyclonic and the center of the anticyclonic eddies, in contrast to the relatively higher POC concentrations at the center of the cyclonic and the border of anticyclonic eddies. We observed high variability in POC concentration variability outside mesoscale structures, which may be attributed to other processes such as upwelling over the shelves, and the contribution by rivers during the summer especially in the northern and southern GoM.

Intelligent identification of oceanic eddies in remote sensing data via Dual-Pyramid UNet

Oceanic eddies are an omnipresent phenomenon of seawater flow and critical in transporting oceanic energy and material. Consequently, mastering and comprehending the characteristics of ocean eddies through detecting and recognizing eddies contributes to the understanding of oceanography. In traditional oceanography, a series of methods to identify eddies with physical or geometric characteristics have been developed. Deep learning frameworks have recently been applied in the eddy detection field. In this paper, a Dual-Pyramid UNet architecture that combines a pyramid split attention (PSA) module and atrous spatial pyramid pooling (ASPP) is proposed to identify oceanic eddies from remote sensing data. The encoder and decoder parts can effectively integrate low-level and high-level features, thus ensuring that feature information is not lost in large quantities after the nonlinear connection mode. In addition, the PSA and ASPP modules are introduced into the encoding, decoding, and skip connections to enhance feature extraction. Experiments were implemented in two typical study areas—the North Atlantic and South Atlantic. The recognition results demonstrate that Dual-Pyramid UNet can outperform four other competitive AI-based methods, especially for eddy edges and small-scale eddies.摘要海洋涡旋是大洋中重要的组成部分, 对海洋能量和物质的输送至关重要. 海洋涡旋的检测和表征无论是对于海洋气象学, 海洋声学还是海洋生物学等领域都具有重要的研究价值. 本文基于UNet架构, 并结合金字塔分割注意力(PSA)模块和空洞空间卷积池化金字塔(ASPP)构造了Dual-Pyramid UNet模型, 以平面异常和海表面温度数据中进行海洋涡旋的识别. 实验在北大西洋和南大西洋两个涡旋活跃区域进行并选用多个评价指标对识别结果进行评价以证明模型的优异性能.

Spatial and seasonal variability of horizontal temperature fronts in the Mozambique Channel for both epipelagic and mesopelagic realms

IntroductionOcean fronts are moving ephemeral biological hotspots forming at the interface of cooler and warmer waters. In the open ocean, this is where marine organisms, ranging from plankton to mesopelagic fish up to megafauna, gather and where most fishing activities concentrate. Fronts are critical ecosystems so that understanding their spatio-temporal variability is essential not only for conservation goals but also to ensure sustainable fisheries. The Mozambique Channel (MC) is an ideal laboratory to study ocean front variability due to its energetic flow at sub-to-mesoscales, its high biodiversity and the currently debated conservation initiatives. Meanwhile, fronts detection relying solely on remotely-sensed Sea Surface Temperature (SST) cannot access aspects of the subsurface frontal activity that may be relevant for understanding ecosystem dynamics.MethodIn this study, we used the Belkin and O’Reilly Algorithm on remotely-sensed SST and hindcasts of a high-resolution nested ocean model to investigate the spatial and seasonal variability of temperature fronts at different depths in the MC.ResultsWe find that the seasonally varying spatial patterns of frontal activity can be interpreted as resulting from main features of the mean circulation in the MC region. In particular, horizontally, temperature fronts are intense and frequent along continental shelves, in islands’ wakes, at the edge of eddies, and in the pathways of both North-East Madagascar Current (NEMC) and South-East Madagascar Current (SEMC). In austral summer, thermal fronts in the MC are mainly associated with the Angoche upwelling and seasonal variability of the Mozambique current. In austral winter, thermal fronts in the MC are more intense when the NEMC and the Seychelles-Chagos and South Madagascar upwelling cells intensify. Vertically, the intensity of temperature fronts peaks in the vicinity of the mean thermocline.DiscussionConsidering the marked seasonality of frontal activity evidenced here and the dynamical connections of the MC circulation with equatorial variability, our study calls for addressing longer timescales of variability to investigate how ocean ecosystem/front interactions will evolve with climate change.

Modulation Effects of Mesoscale Eddies on Sea Surface Wave Fields in the South China Sea Derived From a Wave Spectrometer Onboard the China‐France Ocean Satellite

AbstractThis study aims to examine the modulation effects of mesoscale eddies on sea surface wave fields in the South China Sea using data measured by a wave spectrometer named Surface Wave Investigation and Monitoring onboard the China‐France Ocean Satellite from July 2019 to August 2021. The statistical results show that the significant wave heights (SWHs) decrease (increase) from inside to outside of warm (cold) eddies, while the wavelengths have minimum values at the eddy edge in all cases. In cases of strong warm eddies, the maximum variations in SWH, wavelength, and wave direction caused by mesoscale eddy modulation reach up to 18%, 24%, and 28.4°, respectively. The eddy modulation on wave parameters is stronger at low sea states. The correlation analysis reveals that the wave parameter variations across eddies result from the eddy current shear and the direction difference between the eddy current and the waves. The dynamic analysis indicates that the deformation term of the eddy current is a dominant term affecting the SWH at the eddy edge, and the eddy current divergence and its product with wave direction determine the wavelength variation. The eddy vorticity deflects the wave direction by approximately 2°–3°, and the wave‐eddy angle and eddy‐modulated wind variation also contribute to the wave direction variation. The background current‐wave interaction is negligible in the eddy cases studied.

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.

Spatial Structure of Vertical Motions and Associated Heat Flux Induced by Mesoscale Eddies in the Upper Kuroshio‐Oyashio Extension

AbstractVertical motions induced by mesoscale eddies in the upper ocean play a vital role in the heat transport over the global ocean. However, the spatial structure of vertical eddy velocity and associated heat flux remains unclear. This study addresses this issue in the Kuroshio‐Oyashio Extension based on an eddy‐rich Community Earth System Model simulation. Adopting a mesoscale eddy composite analysis, we show that the vertical eddy velocity is enhanced at edges of mesoscale eddies with a maximum magnitude of 0.8 m day−1 in the upper ocean for anticyclonic eddies (AEs) and 0.5 m day−1 for cyclonic eddies (CEs). The associated vertical heat flux is upward, reaching 52 and 32 W m−2 along AEs' and CEs' periphery, respectively. Diagnostic analysis suggests that the enhanced vertical eddy motions at eddy edges are primarily attributed to the ageostrophic secondary circulation (ASC) under the turbulent thermal wind balance that accounts for more than 70% of the vertical eddy velocity and 80% of vertical eddy heat flux in the upper ocean. This ASC is stronger in winter than summer due to the intense turbulent mixing induced by strong surface cooling and wind stirring in winter. Accordingly, the vertical eddy velocity and associated heat flux exhibit a distinct seasonal cycle.

Foraging behavior of a mesopelagic predator, the northern elephant seal, in northeastern Pacific eddies

The role of mesoscale features in structuring trophic transfer in the mesopelagic zone is poorly understood. Deploying sensors on marine animals, or “biologging,” is a powerful tool to infer the organism's behavior and simultaneously collect high-resolution oceanographic data to describe physical-biological interactions. We investigated whether mesoscale eddies are used by a mesopelagic predator, the northern elephant seal (Mirounga angustirostris), and if so, what mechanisms might create beneficial foraging conditions in association with eddies. We hypothesized seals would increase their foraging behavior in both cyclonic and anticyclonic eddies due to nutrient enhancement and physical aggregation of prey and that seals would dive deeper in anticyclonic eddies in response to a deeper prey field. We used tracking data and continuous in situ temperature measurements from 221 adult female northern elephant seals collected between 2004 and 2019. These predators primarily targeted myctophid fishes and squid throughout the northeast Pacific mesopelagic zone, foraging between approximately 400–800 m. Eddy encounters were identified using remotely sensed sea level anomaly data and confirmed visually with collocated sea level anomaly, in situ temperature, and in situ temperature anomaly. Over more than 30,000 days of data and >3 million temperature casts collected by seals, we found 129 high confidence encounters with anticyclonic eddies and 83 with cyclonic eddies. Overall, seals traveled more slowly and in a less directed manner while associated with eddies, particularly anticyclonic eddies, suggesting increased foraging behavior, especially in the California Current. Elephant seals spent more time at the edges of cyclonic eddies than their center. In contrast, they utilized both the interior and edge of anticyclonic eddies. This suggests that the aggregation of prey at the frontal region of an eddy is an important mechanism, whereas nutrient upwelling associated with an eddy play a more minor role in enhancing the seals' prey field. Seal foraging behavior was not influenced by eddy age, size, amplitude, or rotational speed. The large sample size in this study showed considerable variation between individual behavioral responses, suggesting caution when extrapolating individual behavior to a population level. Our data show that both cyclonic and anticyclonic eddies can affect the seals' prey field as demonstrated by enhanced foraging behavior. Still, the variation in behavioral responses resulting from individual foraging strategies, eddy histories, and possible spatiotemporal mismatches between eddy physics and biological responses relevant to mesopelagic predators merit further investigation.

The Influence of Agulhas Leakage on Primary Production and Nitrogen Cycling in the Southeastern Atlantic Ocean

AbstractThe well‐established importance of Agulhas Current “leakage” for Indian‐Atlantic Ocean exchange of heat and salt raises the question of the relevance of Agulhas eddies for carbon and nitrogen cycling. We measured net primary production (NPP), nitrate and ammonium uptake, and nitrification along a transect of the Cape Basin (CB) in winter 2017, including across an anticyclonic Agulhas eddy sampled at high‐resolution. Euphotic zone‐integrated rates of NPP and nitrogen uptake were more than three‐fold lower at the eddy center than at its edges, with intermediate rates measured outside the eddy. Additionally, proportionally more nitrate was consumed at the eddy edges than at its center. Mixed‐layer integrated nitrification rates were highest at the eddy‐center, with lower rates at the edges and in the surrounding basin. Accounting for phytoplankton consumption of newly nitrified nitrate in the eddy decreased the carbon export potential inferred from nitrate uptake (i.e., the f‐ratio) by 66% ± 37% (vs. a 14% ± 22% decrease across the rest of the transect). At most stations, NPP and total measured nitrogen uptake were decoupled relative to stoichiometric expectations, with roughly a third of NPP possibly supported by regenerated dissolved organic nitrogen (DON). Accounting for DON uptake decreased the average f‐ratio from 0.28 ± 0.26 to 0.22 ± 0.21, with the highest f‐ratio estimated for an eddy‐edge station (0.64) and the lowest for its center (0.00). While the low productivity metrics measured in the eddy‐center could imply that Agulhas leakage decreases carbon export potential in the CB, enhanced nutrient supply and consumption at eddy edges may (partly) offset this reduction.

Bluefin Larvae in Oligotrophic Ocean Foodwebs, investigations of nutrients to zooplankton: overview of the BLOOFINZ-Gulf of Mexico program

Abstract Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general introduction to the BLOOFINZ-GoM project (Bluefin tuna Larvae in Oligotrophic Ocean Foodwebs, Investigation of Nitrogen to Zooplankton) and highlight the finding, based on backtracking of experimental waters to their positions weeks earlier, that lateral transport from the continental slope region may be more of a key determinant of available habitat utilized by larvae than eddy edges per se.

Submesoscale processes-induced vertical heat transport modulated by oceanic mesoscale eddies

Oceanic submesoscale processes mainly arise from phenomena such as jets, density fronts, and mesoscale eddies. Previous studies suggest that the overall submesoscale processes transport heat vertically upgradient, i.e. from cold to warm. However, it is not clear whether the submesoscale processes-induced vertical heat transport (VHT) in mesoscale eddies remains upgradient. Therefore, the present study focuses on submesoscale-induced VHT modulated by mesoscale eddies. A high-resolution oceanic numerical model product is applied to examine the VHT induced by submesoscale processes associated with a pair of cyclonic and anticyclonic mesoscale eddies in the Kuroshio Extension (KE) region. Eddies at different time steps are collocated in terms of the eddy centers to reconstruct a time series of a new eddy dataset that presents eddy-modulated submesoscale processes. The frequency-wavenumber spectra, Rossby numbers, and strain rates in the pair of eddies reveal the existence of submesoscale motions surrounding mesoscale eddies. The variables are decomposed into monthly mean mesoscale and submesoscale components to calculate submesoscale VHT at low-frequency (LF) and high-frequency (HF). The results indicate that submesoscale VHT is modulated by mesoscale eddies mainly along the eddy boundaries. The LF submesoscale VHT (upward in both cyclonic and anticyclonic eddies) dominates the monthly-averaged submesoscale VHT. The magnitude of the hourly HF submesoscale VHT is larger than the LF component, while its monthly average becomes negligible because it is caused by periodical internal waves. The submesoscale VHT along the eddy edges is related to strong frontogenesis which develops the vertical flux.

Transport modes of inertial particles and their effects on flow structures and heat transfer in Rayleigh–Bénard convection

We investigate the flow characteristics and kinetic behaviors of particles in turbulent Rayleigh–Bénard convection particulate flows. Direct numerical simulations combined with a Lagrangian point-particle strategy were carried out in the range of Stokes numbers 2×10−4≤StL≤7.3×10−2 for Rayleigh numbers from 2×106 to 108 at the Prandtl number Pr=0.678. A two-way coupling model is employed in which the momentum exchange between the dispersed particles and the carrier fluid is taken fully into account. Based on various patterns of particle motion, we find three transport modes of inertial particles which are labeled as the circling transport (CCT) mode, the channel transport (CNT) mode, and the downpour transport (DPT) mode, respectively. These modes can switch to each other when Stokes numbers and Rayleigh numbers vary and exhibit different effects of particle motions on the flow field and heat transfer. For the CCT and DPT modes, compared with the CNT, a weakening alteration of flow structures and thermal plumes leads to no significant effect on the transport of momentum and heat. For the CNT mode, a pronounced effect of particles on enhancements of the turbulent momentum transport and heat transfer relates to the strong interaction between the particle clusters and the chaotic structures of eddies. What is more, the particles tend to homogeneously distribute for the CCT and DPT modes, although the particles exhibit different transport states. As for the CNT mode, under both preferential sweeping and centrifugal effects, particles accumulate into clusters that hover toward the region of high strain rate and the edges of eddies. We found that the averaged particle settling speeds are almost proportional to the Stokes number. The particle settling speeds are larger than the terminal velocity of Stokesian particles for the CCT and CNT modes as particles tend to settle in the downward fluid. In contrast, it becomes smaller than the terminal velocity for the DPT mode due to the drag of the upward fluid.