Cyclonic Eddies Research Articles

The generation mechanism of cold eddies and the related heat flux exchanges in the upper ocean during two sequential tropical cyclones

The impacts of two sequential tropical cyclones (TCs), Kyarr and Maha, [from October 24 to November 06, 2019, over the Arabian Sea (AS)] on upper ocean environments were investigated using multiple satellite observations, Argo float profiles and numerical model outputs. To obtain a realistic TC strength, the Weather Research and Forecasting (WRF) model was used to reproduce Kyarr and Maha. During Kyarr and Maha, three distinct cold patches were observed at the sea surface with a maximum sea surface cooling of approximately 5°C. The comparison between WRF model simulation results and ERA5 wind field showed that the WRF model simulation indicated high simulation accuracy with respect to the SST decrease in the AS under the influence of Kyarr and Maha’s wind stress curls. Meanwhile, concentration of chlorophyll a (chl-a) and positive relative vorticity of sea surface also appeared in the three cold patch areas. Through the use of eddy detection algorithms, three mesoscale cold cyclonic eddies were identified along the track of TC Kyarr, and the locations of these cold eddies were highly correlated with three obvious negative sea surface height anomalies (SSHAs). The radii of the three cold eddies were 69 km, 50 km, and 41 km. With a focus on the thermodynamic responses of the three cold eddy fields to Kyarr and Maha, the central regions of the three cold eddies were explored. The central regions of the three cold eddies exhibited relatively shallow mixed-layer depths (MLDs) and low mixed-layer temperatures (MLTs). The depth integrated heat (DIH) content was also calculated to explore the heat flux exchanges occurring in different layers in the upper 200 m of the centre of each eddy. The results showed that DIH in each eddy centre varied by one order of magnitude, accounting for between 127.3 MJ m-2 and 1220.0 MJ m-2 of heat loss. This study suggests that the effect of long forcing time on intense positive wind stress curls can produce upwelling caused by Ekman response, which is the main influencing factor of the three cold eddies generation mechanism. At the same time, the positive relative vorticity injected into the sea surface also has some contribution. TC-induced vertical mixing and upwelling (strengthened by unstable structures inside the cold eddies) cause substantial redistribution of the DIH, and related heat flux exchanges at different layers occur in the eddy fields.

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.

Submesoscale eddies in eastern Guangdong identified using high-frequency radar observations

The statistical characteristics and mechanism underlying the generation of submesoscale eddies in eastern Guangdong were examined using high-frequency radar (HFR) observations of surface currents during 2019_2021. By applying the vector geometry-based (VG) eddy detection algorithm to HFR datasets, 313 submesoscale eddies with 229 cyclonic and 84 anticyclonic eddies were identified. Cyclonic eddies were significantly more common than the anticyclonic eddies, but they had similar distributions in terms of eddy lifespan and radius. The trajectories showed that the submesoscale eddies within the study area tended to move northward and westward. Submesoscale eddies revealed annual variations with decreasing cyclonic eddies and increasing anticyclonic eddies during the observation period. Submesoscale eddies displayed a remarkable spatial distribution. Most of cyclonic (anticyclonic) eddies occurred at the eastern and southeastern regions of the Huilai Headland (south of the Nanao Island). Eddy evolution was linked to the production of vorticity in a narrow layer along the headland, the injection of vorticity into the interior at the point of flow separation, and vorticity damping by bottom friction.

Seasonal variation in eddy activity and associated heat/salt transport in the Bay of Bengal based on satellite, Argo, and 3D reprocessed data

Abstract. Based on satellite altimetry data spanning over 26 years in combination with Argo profile data or three-dimensional (3D) reprocessed thermohaline fields, the eddy synthesis method was used to construct vertical temperature and salinity structures of eddies in the Bay of Bengal, and the seasonal thermohaline properties of eddies and the heat and salt transport by eddies were analyzed. Analysis revealed that mesoscale eddy activities and the vertical thermohaline structures in the Bay of Bengal have evident seasonal variation. Temperature anomalies caused by eddies are usually between ±1 and ±3 ∘C (positive for anticyclonic eddies (AEs) and negative for cyclonic eddies (CEs)), and the magnitude varies seasonally. Salinity anomalies caused by eddies are small and disturbance signals in the southern bay due to the small vertical gradient of salinity there; salinity anomalies in the northern bay are generally between ±0.2 and ±0.3 psu, negative for AEs and positive for CEs. Owing to seasonal changes in both the eddy activity and the vertical thermohaline structure in the Bay of Bengal, the eddy-induced heat and salt transport in different seasons also changes substantially. Generally, high heat and salt transport is concentrated in eddy-rich regions, e.g., the western, northwestern, and eastern parts of the bay, the seas to the east of Sri Lanka, and the region to the southeast outside of the bay. The southern part of the bay shows weak salt transport owing to the inconsistent salinity signal within eddies. The result of the divergence of eddy heat transport illustrates that the 10–20 W m−2 value of the eddy-induced heat flux is comparable in magnitude with the annual mean air–sea net heat flux in the Bay of Bengal. Compared with the large-scale net heat flux and freshwater flux at the surface, the eddy-induced heat/freshwater transport can contribute substantially to regional and basin-scale heat/freshwater variability. This work provides data that could support further research on the heat and salt balance of the entire Bay of Bengal.

Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic

Abstract. Mesoscale eddies modulate the ocean's physical, chemical, and biological properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophy and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To fill this gap, we investigated the horizontal and vertical variability in auto- and heterotrophic microbial activity (biomass production and respiration) within a CE that formed off Mauritania and along the ∼ 900 km zonal corridor between Mauritania and the Cape Verde islands in the Eastern Tropical North Atlantic (ETNA). Our results show how the physical disturbances caused by the CE affected the biomass distribution of phyto- and bacterioplankton and their metabolic activities. The injection of nutrients into the sunlit surface resulted in enhanced autotrophic pico- and nanoplankton abundance and generally increased autotrophic activity as indicated by chlorophyll a (Chl a) concentration, primary production (PP), and extracellular release rates. However, the detailed eddy survey also revealed an uneven distribution of these variables with, for example, the highest Chl a concentrations and PP rates occurring near and just beyond the CE's periphery. The heterotrophic bacterial activity was similarly variable. Optode-based community respiration (CR), bacterial respiration (BR) estimates, and bacterial biomass production (BP) largely followed the trends of PP and Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial abundance and activities occurred within the CE that was closely related to variability in autotrophic production. Consistent with this, we found a significant positive correlation between concentrations of semi-labile dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable amino acids and dissolved combined carbohydrates) and BR estimates. Extracellular release of carbon as indicated by primary production of dissolved organic carbon (PPDOC) was variable with depth and laterally and not always sufficient to compensate the bacterial carbon demand (BCD: BR + BP), with PPDOC accounting for between 28 % and 110 % of the BCD. Bacterial growth efficiency (BGE: BP / BCD) ranged between 1.7 % and 18.2 %. We estimated the metabolic state to establish whether the CE was a source or a sink of organic carbon. We showed that the CE carried a strong autotrophic signal in the core (PP / CR > 1). Our results suggest that submesoscale (0–10 km) processes lead to highly variable metabolic activities in both photoautotrophic and heterotrophic microorganisms. Overall, we revealed that the CEs not only trap and transport coastal nutrients and organic carbon to the open ocean but also stimulate phytoplankton growth, generating freshly produced organic matter during westward propagation. This drives heterotrophic processes and may contribute to the previously observed net heterotrophy in open Atlantic surface waters.

The Three-Dimensional Structure of the Mesoscale Eddy in the Kuroshio Extension Region Obtained from Three Datasets

The high-resolution observation data from June 2019, Argo data from 1997 to 2019, and the multi-observation ARMOR 3D dataset from 1993 to 2019 were used to study the three-dimensional (3D) structural characteristics of the mesoscale cyclonic eddy (CE) in the Kuroshio Extension region (KER). The observed eddy has a typical 3D structure of the KER CE, which was a longer lifespan eddy in this KER. The maximum anomalies of temperature and salinity were −7.69 °C and −0.71 PSU, which were located at the 350 m depth. In the vertical, the observed and composite eddy had a dipole structure, while ARMOR 3D had a monopole. The study of the velocity fields indicate that ARMOR 3D underestimates the velocity below 500 m. The 3D structures of the CE composite eddy of Argo were comparable to the observations, whereas the temperature and salinity anomalies were weaker than the observation. The surface of the Argo composite eddy shows a positive temperature anomaly within 50 m, which used to be opposite to the observation. This phenomenon was due to the limited Argo data of the composite eddy, and most of them were the observed profiles of winter CE in the weak years of EKE in the KER. We tried using ARMOR 3D to explore the reliability of ARMOR 3D composite eddy and compared the seasonal variations of temperature/salinity anomalies of the cyclonic and anticyclonic eddy. The anomalies of temperature and salinity caused by CE have seasonal variations: the anomalies have been strong in summer and weak in winter. This is consistent with the variant of eddy kinetic energy (EKE), but AE has no seasonal variation.

Investigating the Effects of Super Typhoon HAGIBIS in the Northwest Pacific Ocean Using Multiple Observational Data

Various multi-source observational platforms have enabled the exploration of ocean dynamics in the Northwest Pacific Ocean (NPO). This study investigated daily oceanic variables in response to the combined effect of the 2019 super typhoon HAGIBIS and the Kuroshio current meander (KCM), which has caused economic, ecological, and climatic changes in the NPO since August 2017. During HAGIBIS, the six-hourly wind speed data estimated a wind stress power (Pw) which strengthened around the right and left semicircles of the typhoon, and an Ekman pumping velocity (EPV) which intensified at the center of the typhoon track. As a result, firstly, the sea temperature (ST) decreased along a boundary with a high EPV and a strong cyclonic eddy area, and the mixed layer depth (MLD) was shallow. Secondly, a low sea salinity (SS) concentration showed another area where heavy rain fell on the left side of the typhoon track. Phytoplankton bloom (PB) occurred with a large concentration of chlorophyll a (0.641 mg/m3) over a wide extent (56,615 km2; above 0.5 mg/m3) after one day of HAGIBIS. An analysis of a favorable environment of the PB’s growth determined the cause of the PB, and a shift of the subsurface chlorophyll maximum layer (SCML; above 0.7 mg/m3) was estimated by comprehensive impact analysis. This study may contribute to understanding different individually-estimated physical and biological mechanisms and predicting the recurrence of ocean anomalies.

The Impact of a Southern Ocean Cyclonic Eddy on Mesopelagic Micronekton

AbstractMesoscale eddies shape the foraging ecology of predators such as marine mammals and seabirds. A growing number of animal tracking studies show that predators alter their swimming, diving, and foraging behavior within mesoscale eddies. However, little is known about how Southern Ocean eddies influence the distribution of mesopelagic micronekton (fish, squid, and crustaceans), which are major prey items of megafauna. Studies in other oceanic regions have found that eddies can influence the abundance and community composition of micronekton. Here, we analyze acoustic observations from a 14‐day survey of a cyclonic mesoscale eddy, its surrounding waters, and the Polar Frontal Zone (PFZ) waters where the eddy formed. We report and interpret spatial patterns of acoustic backscatter at 18 and 75 kHz, proxies indicating combined changes in species, size, and abundance of micronekton. We find that the vertical distribution of acoustic backscatter matched the underwater light conditions characteristic of the eddy core, periphery, and surrounding waters, at scales smaller than 10 km. The median water‐column integrated acoustic backscatter values in the eddy core were only half of those measured in the Sub‐Antarctic Zone waters surrounding the eddy, but similar to those measured in the PFZ, where the eddy originated 27 days prior. These results suggest that, as for physical and chemical tracers, the eddy maintained its biological characteristics from its source waters creating a unique habitat compared to its surroundings.

Global Mapping of Mesoscale Eddy Vertical Tilt

AbstractVertical motion induced by oceanic mesoscale eddies can strengthen ocean ventilation, and regulate vertical distribution of heat, dissolved CO2 and other tracers on a global scale. Eddy‐induced vertical motion is closely related to eddy vertical structure. Here, based on available satellite observations and Argo profiles, we find that mesoscale eddies in global oceans are prevalently vertical tilted. Anticyclonic eddies are frequently tilted westward and poleward from 300 dbar up to sea surface, while cyclonic eddies are frequently tilted westward and equatorward. The horizontal tilting distance of eddies is significantly correlated (r ≥ 0.84) with the Burger number (the ratio of vertical stratification to planetary rotation and eddy size), indicating a strong constraint of Earth's rotation and ocean stratification on eddy vertical tilt. On average, the maximum vertical velocity in tilted circular eddies is at least one order of magnitude larger than that in non‐tilted circular eddies. This finding would improve estimates of eddy‐induced vertical motion, ventilation, so as to heat/salt and biogeochemical responses.

The southwestern Atlantic Ocean mesoscale eddies: A review of their role in the air-sea interaction processes

This study presents an overview of oceanic mesoscale eddies and their influence on the overlying atmosphere and surrounding waters, with a focus on the southwestern Atlantic Ocean. The high values of eddy kinetic energy (EKE) in this region are related to the mesoscale eddies observed, which transport dynamic and thermodynamic specific properties away from their origin point, during their life cycle. This transport capacity interferes with several atmospheric and oceanic processes, such as the local marine atmospheric boundary layer (MABL) stability, heat fluxes, and primary production. In the Southern Hemisphere, cyclonic (anticyclonic) eddies are related to clockwise (counterclockwise) circulation and negative (positive) anomalies of the sea surface temperature (SST) and the latent and sensible heat fluxes. It has been observed that over the clockwise and cold (counterclockwise and warm) eddies, there is a decrease (increase) in the near-surface wind, as well as stable (unstable) conditions on the MABL, vertical downward (upward) movements in the atmosphere, and decreased (increased) precipitation. SST anomalies influence the curl, divergence, and magnitude of the surface wind and wind stress, while mesoscale surface oceanic currents mainly affect the stress and wind curls. The atmospheric response to SST anomalies can be explained using two main mechanisms: hydrostatic adjustment and vertical mixing. However, as will be shown, the feedback among the SST anomalies, wind, and oceanic currents is far more complex. This work compiles and provides a theoretical basis for future work concerning air-sea interactions of mesoscale oceanic structures.

Mesoscale eddies in the southwestern tropical Atlantic

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. However, origin, occurrence and characteristics of mesoscale eddies were still not described in the region. Using satellite altimetry data from 1993 to 2018 off Northeast Brazil (37-25°W; 13-1°S), we reconstruct eddy trajectories and analyse the main eddy surface characteristics (e.g., size, amplitude, polarity) and their spatiotemporal variations. The study reveals two distinct dynamic regions before quantifying mesoscale eddies characteristics. Approximately 2000 mesoscale eddies crossed the region during the study period, among which 76% were generated inside the region, with amplitudes and radii ranging between 1 and 2 cm and 25 and 205 km, respectively. Eddies are preferentially formed between August and September and propagate westward. In the region around the FN Archipelago (36-26°W; 6-1°S), the formation of cyclonic eddies is likely favoured by barotropic instabilities of surface currents and the wind stress curl. On the other hand, in the south of the region (36-26°W; 12-8°S), eddies formation is likely associated with the barotropic instabilities, wind stress curl and the meandering of surface currents. Based on vertical temperature and salinity profiles from Argo floats’ data, we determined that in average, the core of cyclonic eddies is centred at ~ 130 m (140 m) in the northern (southern) region while the core of anticyclonic eddies is centred at ~90 m (125 m) in the northern (southern) region. Moreover, mesoscale eddies formed in the tropical Atlantic do not connect the eastern tropical Atlantic and northeast Brazil.

Cyclonic and Anticyclonic Asymmetry of Reef and Atoll Wakes in the Xisha Archipelago

A high-resolution (∼500 m) numerical model was used to study the reef and atoll wakes in the Xisha Archipelago (XA) during 2009. Statistical analyses of simulation data indicated strong cyclonic dominance in the mixing layer (above ∼35 m) and weak anticyclonic dominance in the subsurface layer (35∼160 m) for both eddies and filaments in the XA. The intrinsic dynamical properties of the flow, such as the vertical stratification and velocity magnitude, and the terrain of reefs and atolls had a significant effect on the asymmetry. Without considering the existence of reefs and atolls, the “background cyclonic dominance” generated under local planetary rotation (f≈4.1×10−5 s−1) and vertical stratification (with mean Brunt–Väisälä frequency N = 0.02 s−1 at 75 m) was stronger for filaments than eddies in the upper layer from 0∼200 m, and the larger vorticity amplitude in the cyclonic filaments could greatly enhance the cyclonic wake eddies. Furthermore, inertial–centrifugal instability induced selective destabilization of anticyclonic wake eddies in different water layers. As the Rossby number (Ro) and core vorticity (Burger number, Bu) decreased (increased) with the water depth, a more stable state was achieved for the anticyclonic wake eddies in the deeper layer. The stratification and slipping reefs and atolls also led to vertical decoupled shedding, which intensified the asymmetry.

Insignificant effects of eddies and typhoons on the biogeochemistry of the tropical northwest Pacific Ocean

We investigated the effects of eddies and typhoons on the biogeochemistry of the tropical northwest Pacific by examining the distribution of nutrients, dissolved oxygen (DO), chlorophyll-a (Chl-a), gross primary production (GPP), dissolved organic carbon (DOC), and fluorescent dissolved organic matter (FDOM). Water samples were collected from anticyclonic and cyclonic eddies in September 2019 and 2020, and before and after the passage of a Category 2 typhoon in 2019. The study region was characterized by a deep nitracline (~150 m), which was deeper than both the pycnocline (~50 m) and the FDOM-depleted layer (~75 m). A subsurface chlorophyll maximum layer was observed at 100–150 m depth. No clear differences in Chl-a, DO, GPP, DOC, and FDOM were observed for the anticyclonic and cyclonic eddies, indicating that the eddies did not have a significant influence on biological production. Similarly, there were no discernable changes in Chl-a concentrations or other biogeochemical parameters after the passage of the typhoon, which induced water mixing to a depth of ~60 m. We conjecture that the nutrient-depleted layer was too deep for any eddy- or typhoon-induced vertical mixing to cause upwelling of nutrients to the euphotic zone. Our results imply that the disturbances caused by mesoscale processes in the upper layer of the highly oligotrophic northwest Pacific may have a smaller effect than in oceans in other parts of the world.

Submesoscale variability of subsurface chlorophyll-a across eddy-driven fronts by glider observations

Reflecting the local dynamic state of the pelagic system, subsurface chlorophyll maxima (SCM) are ubiquitous in the global ocean. Here, a high-resolution glider survey was conducted in the South China Sea to study the physical and biogeochemical change across the periphery of a mesoscale cyclonic eddy. We found intense vertical dispersion of SCM associated with enhanced diapycnal mixing at the frontal zones on the edge of the eddy. Submesoscale processes were found to play important roles within the eddy-driven fronts despite the general impact of the mesoscale Ekman upwelling and downwelling. Our results also suggested that wind-driven overturns at the frontal zone could result in a stronger gravitational instability at the northern edge of the eddy leading to an enhanced vertical mixing of subsurface Chl-a. However, turbulent mixing induced by symmetric instability with energy extracted from the lateral buoyancy gradient at the northeastern edge of the eddy had caused a divergence of the subsurface chlorophyll-a at the frontal zone. Our results provide direct evidence of the interaction between mesoscale and submesoscale processes at the oceanic frontal zone. These findings also call for tight cooperation between multidisciplinary observational and modeling approaches for a better understanding of the SCM variability and associated biogeochemical transformations at transient oceanic fronts.

Efficient Underwater Sensor Data Recovery Method for Real-Time Communication Subsurface Mooring System

Marine submerged buoys can effectively obtain various parameters of seawater, which plays an important role in the research of marine physical phenomena, marine environmental changes, and climate change. However, traditional self-contained submerged buoys usually work underwater at a depth of about 100 m, and the observation data cannot be obtained before their recovery, which cannot satisfy the needs of real-time data acquisition for marine scientific research. To solve this problem, this paper proposes a real-time communication subsurface mooring system that consists of a satellite communication buoy (SCB), conductivity–temperature–depth sensors (CTD), and an inductive coupling mooring cable. The underwater inductive coupling link collects the data from the underwater sensors and transmit it to the SCB. Then, the data will be transmitted to the station receiver via satellite communication module integrated into the SCB. In order to ensure a high success rate of data recovery, the stress analysis and hydrodynamic simulation of the SCB were carried out in this paper. The results show that the SCB maintained a relatively stable attitude in the 3–4 sea state. The attitude data obtained from the subsequent sea trial was consistent with the simulation results, and the success rate of satellite communication during this period was more than 95%. In this paper, a modular embedded hardware circuit was designed to meet the functional requirements of the subsurface mooring system. An efficient data recovery strategy was also developed, which ensured that the average power consumption of the system was low and the success rate of data recovery is not less than 90% when operating in the severe sea state for a long time. The system underwent sea trials in the South China Sea for more than 3 months from the end of 2021 to the beginning of 2022. It transmitted more than 2034 sets of seawater profile temperature, salinity, and depth data in real-time, with a success rate of over 91% of the total sample data. The CTD data returned in real-time from our system is consistent with the data of the HYCOM and World Ocean Atlas (WOA), and a cyclonic mesoscale eddy was detected in the operation area.

Evolution of oceanic near-inertial waves induced by typhoon Sarika (2016) in the South China Sea

Oceanic near-inertial waves (NIWs) are typically generated by wind. Utilizing moored observations and hybrid coordinate ocean model (HYCOM) analysis, we investigated the NIWs induced by typhoon Sarika (2016) in the South China Sea. Sarika-induced NIWs exhibited an approximately 10% blue-shift to the local Coriolis frequency, which was largely due to a cyclonic eddy (CE) formed around the mooring. The development, decay and modal content of Sarika-induced NIWs displayed spatial differences. High near-inertial kinetic energy (NIKE) was concentrated to a small region on the right side of Sarika's track and exhibited slow damping. In contrast, the NIKE in the CE damped quickly, because the downward propagation of NIKE was prevented by the strong stratification in the CE. Additionally, modal decomposition revealed that the mode-2 and mode-3 NIKE induced by Sarika was significant but exhibited different evolution characteristics.

Microbial metabolic activity in two basins of the Gulf of Mexico influenced by mesoscale structures

Information on microbial metabolic activity is essential for quantifying carbon and energy flows through marine food webs. We quantified community (Rcom) and prokaryotic (Rpro) respiration rates, bacterial production (BP), bacterial abundance (BA), and bacterial growth efficiencies (BGE) in the Perdido and Coatzacoalcos basins of the Gulf of Mexico (GOM) during summer and winter conditions in 2016. Our results showed seasonal, regional, and mesoscale eddy influences on those metabolic variables. Rpro accounted for >60% of total respiration in both regions, being three times higher in stations influenced by a cyclonic eddy (CE) in September (24.1 μM O2 d−1) than in stations affected by an anticyclonic eddy in March (7.2 μM O2 d−1) within the Coatzacoalcos basin where the eddy-trapping mechanism advected biomass-enriched waters from the Bay of Campeche. The eddy-stirring mechanism produced horizontal and vertical dipole patterns of metabolic variables increasing up to one order of magnitude Rcom and Rpro while decreasing BGE to 25-fold from the southeastern to the northwestern edges in CEs. This finding indicates that dissolved organic matter is more actively taken up to build bacterial biomass on the eastern edge of CEs in the GOM, while Rpro and Rcom increase on the western edges. Satellite integrated primary production was coupled with surface Rpro and Rcom at CEs and no eddies. BP was mainly regulated by CEs and was about 50% higher in the Coatzacoalcos basin (~0.03–0.14 μmol C L−1 d−1). BP increased in zones with high Rpro and Rcom, suggesting that Rcom is associated with heterotrophic prokaryote activity in both basins. BGE was lower than 25% within the upper 500 m during both cruises, but the highest values were quantified in the euphotic zone and during the September cruise. Metabolic variables integrated over the water column showed that 40–80% of the microbial activity occurred between the base of the euphotic zone and 150 m depth. Our findings contribute to a better understanding of the metabolic activity of the microbial communities in two regions of the GOM influenced by mesoscale eddies.

Eddy Induced Cross-Shelf Exchanges in the Black Sea

Cross-shelf exchanges in the Black Sea were investigated using remote sensing data and an ocean circulation model to which an eddy-tracking algorithm and Lagrangian particle tracking model was applied. An anticyclonic eddy in 1998 and a cyclonic eddy in 2000 were investigated in detail. Eddy-induced cross-shelf transport of low salinity and high Chl-a waters reached a maximum in the presence of filaments associated with these eddies. The daily mean volume transport by the eddies was comparable with the previously documented transport by eddies of similar size in the north-western shelf region. Lagrangian particle tracking results showed that 59% of particles initially released over the shelf were transported offshore within 30 days by the 1998 anticyclone and 27% by the 2000 cyclone. The net volume transport across the Black Sea shelf-break reached the maxima in winter, coinciding with the increase in wind stress curl and mean kinetic energy that is a measure of the intensity of the boundary current. Ekman transport directly influences the cross-shelf exchanges in the surface layer. The south-eastern Black Sea is presented as an important area for cross-shelf transport. The total cross-shelf transport can be divided into its “large-scale” and “eddy-induced” components. Eddy-induced transport was 34% and 37% of the total cross-shelf transport (1998–2014) in the Black Sea in the off-shelf and on-shelf directions, respectively, but these values ranged between 25% and 65% depending on the eddy activity over time.

Surface chlorophyll anomalies induced by mesoscale eddy-wind interactions in the northern Norwegian Sea

The substantial productivity of the northern Norwegian Sea is closely related to its strong mesoscale eddy activity, but how eddies affect phytoplankton biomass levels in the upper ocean through horizontal and vertical transport-mixing has not been well quantified. To assess mesoscale eddy induced ocean surface chlorophyll-a concentration (CHL) anomalies and modulation of eddy-wind interactions in the region, we constructed composite averaged CHL and wind anomalies from 3,841 snapshots of anticyclonic eddies (ACEs) and 2,727 snapshots of cyclonic eddies (CEs) over the period 2000-2020 using satellite altimetry, scatterometry, and ocean color products. Results indicate that eddy pumping induces negative (positive) CHL anomalies within ACEs (CEs), while Ekman pumping caused by wind-eddy interactions induces positive (negative) CHL anomalies within ACEs (CEs). Eddy-induced Ekman upwelling plays a key role in the unusual positive CHL anomalies within the ACEs and results in the vertical transport of nutrients that stimulates phytoplankton growth and elevated productivity of the region. Seasonal shoaling of the mixed layer depth (MLD) results in greater irradiance levels available for phytoplankton growth, thereby promoting spring blooms, which in combination with strong eddy activity leads to large CHL anomalies in May and June. The combined processes of wind-eddy interactions and seasonal shallowing of MLD play a key role in generating surface CHL anomalies and is a major factor in the regulation of phytoplankton biomass in the northern Norwegian Sea.

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.