Mesoscale Eddies

In this study, uncoupled and coupled ocean–atmosphere simulations are carried out for the California Upwelling System to assess the dynamic ocean–atmosphere interactions, namely, the ocean surface current feedback to the atmosphere. The authors show the current feedback, by modulating the energy transfer from the atmosphere to the ocean, controls the oceanic eddy kinetic energy (EKE). For the first time, it is demonstrated that the current feedback has an effect on the surface stress and a counteracting effect on the wind itself. The current feedback acts as an oceanic eddy killer, reducing by half the surface EKE, and by 27% the depth-integrated EKE. On one hand, it reduces the coastal generation of eddies by weakening the surface stress and hence the nearshore supply of positive wind work (i.e., the work done by the wind on the ocean). On the other hand, by inducing a surface stress curl opposite to the current vorticity, it deflects energy from the geostrophic current into the atmosphere and dampens eddies. The wind response counteracts the surface stress response. It partly reenergizes the ocean in the coastal region and decreases the offshore return of energy to the atmosphere. Eddy statistics confirm the current feedback dampens the eddies and reduces their lifetime, improving the realism of the simulation. Finally, the authors propose an additional energy element in the Lorenz diagram of energy conversion: namely, the current-induced transfer of energy from the ocean to the atmosphere at the eddy scale.

Macrourids are among the most abundant and diverse demersal fishes in all deep oceans, including the Southwestern Brazilian continental slope. Although not targeted by Brazilian fisheries, they suffer impact similar than the target species, being among the most discarded fishes by deep bottom trawling.
Trophic Ecology: Data from research surveys and commercial fishing were used to analyze the trophic ecology of four species inhabiting the upper slope of southern Brazil: Coelorinchus marinii, Malacocephalus occidentalis, M. laevis and Lucigadus ori. For the two abundant ones, ontogenetic changes, seasonal variations, intra- and interspecific dietary overlap, parasite fauna and aspects of functional morphology are also described. C.marinii had an extremely diverse diet, preying infauna, epifauna, plankton, necton and carcasses.
M.occidentalis fed on larger and nektonic prey, but also included crabs and carcasses in the diet. Both species showed ontogenetic shifts and seasonal variations in diet composition, both leading to changes in intra- and interspecific diet overlap patterns. Species showed quite distinct feeding anatomy and proportions of body with mouth size, reflecting on feeding strategies. There was little interspecific food overlap. In most cases when the diet was more similar there was a spatial segregation. The coexistence of these species appears to be facilitated by the development of different functional morphologies and feeding strategies. A considerable portion of the diet of these species is due to the consumption of carcasses of pelagic and mesopelagic organisms, and even insects, bypassing the benthic trophic web. Conservative (minimum) estimates of the mean weight
of carcasses in diet ranged from 3 to 20%, increasing with the size of the predators and towards deeper waters. C.marinii showed a lower consumption of carcasses and a high proportion of mesopelagic fishes and cephalopods, however, the analysis of the feeding morphology and prey size leads to believe that most of these two groups of prey were consumed as carcasses. This source of food bypass the detritus food chains and connect the concentrations of macrourids to fluctuations in the abundance of epi and mesopelagic organisms and to oceanographic processes that increase their concentration and mortality (e.g. mesoscale anticyclonic eddies).

Distribution, Biomass and Oceanography: Data from two seasonal bottom trawl surveys were used to provide information on distribution, abundances, densities, sizecomposition and biomass estimates for seven species: Coelorinchus marinii, Malacocephalus occidentalis, M. laevis, Lucigadus ori, Hymenocephalus billsam, Ventrifossa macropogon and V. mucocephalus. The total biomass was estimated in 5.5 and 8.3 kt respectively in winter-spring and summer-autumn. C.marinii and M.occidentalis comprised 98% of the biomass. For these two abundant species, surface maps were made with spawning areas, feeding index, sex and immature/mature ratios, and were related to
oceanographic processes, providing insights on strategies and important processes regulating distribution and abundance patterns. Both species showed a marked seasonal variation in the extent and location of spawning areas. Most C.marinii females were mature (90%), suggesting an early maturation during pelagic phase and acquiring demersal habit just prior the onset of maturation, while M.occidentalis showed few matures females and settle to bottom well before maturity. Temperature rather than depth seems to be the main factor regulating the batimetric distribution of both species. We describe three processes responsible for distribution and abundance patterns found in these species. Different patterns of spatial segregation were found in both species, related with depth, sex and maturity. It is suggested that areas with high biomass Macrouridae (scavengers) are induced by zones of occurrence semi-permanent mesoscale processes (e.g. eddies). These processes increase productivity and enable large biomass of short-lived organisms found in the upper layers, and also increase the concentration, mortality and availability of carcasses, favoring scavenger predators. These processes may be responsible for inconsistencies in biomass of megafauna and macrofauna found in some studies, where biomass of megafauna was of the same order of magnitude or larger than macrofauna, contradicting the Eltonian principle. It is suggested that future studies attempt to relate mesoscale processes with the biomass of potential short-lived prey in surface waters and higher biomass of scavengers. This work highlights the importance of the study of ocean dynamics, combining biological and oceanographic observations, trying to understand the role of mesoscale physical processes on the distribution and abundance patterns of species.

1] The South China Sea (SCS) summer circulation often has a dipole structure associated with an eastward jet, appearing off central Vietnam. The dipole has an anticyclonic eddy (AE) south of the jet and a cyclonic eddy (CE) north of it. The life cycle of the dipole structure is analyzed using satellite altimetry data and a reduced gravity model. On average the dipole structure begins in June, peaks in strength in August or September, and disappears in October. The dipole evolution lags behind the basin scale wind by about 40 days, and 40 days are exactly what it takes for baroclinic planetary waves to cross the southern SCS. Our results show that the vorticity transports from the nonlinear effect of the western boundary currents are crucial for the generation of the dipole structure. In addition, the strength and direction of the offshore wind jet also play a significant role in determining the magnitudes and the core positions of the two concomitant eddies.

Abstract Using two dimensional continuous wavelet transforms, a novel method for identification of mesoscale eddies is presented to facilitate extraction of characteristics for area, amplitude, type, and location from maps of sea level anomalies. In comparison with the previously established growing method for eddy identification, it is found that the wavelet method identifies more than twice the number of eddies and is particularly better at resolving small eddies down to the 0.25 degree resolution of the data. Such research into eddy identification and tracking is significant to the assessment of eddies with potential to impact on coastlines of small islands. The method is applied to the identification of eddies on tracks towards islands of the Eastern Caribbean over 23 years. Spatial and temporal variation in rate of occurrence and magnitude is established. For Barbados there is an average of 9 anticyclonic incidents a year with maximum amplitude of typically 0.22 m in the dry seasons and 0.16 m in the wet seasons. Seasonal variation is reversed for the other islands with twice the number of anticyclonic incidents having maximum amplitudes of about 0.20 m annually.

Anticyclonic mesoscale eddies (ACME) have been proposed as a mechanism by which new nutrients are episodically delivered into the euphotic zone, thereby enhancing new production as well as shifting phy-toplankton community structure. In this paper, we report on a 34-month sediment trap experiment at the Cape Verde Ocean Observatory (CVOO; ca. 18°N, 24°E; December 2009–October 2012), occasionally influenced by ACME passages. The typically oligotrophic, weakly seasonal particle flux pattern at the CVOO is strongly modified by the appearance of a highly productive and low oxygen ACME. Out of four recorded diatom flux maxima at CVOO, three were associated with the passage of ACMEs. The recorded diatom maxima events support the view that local ACME dynamics promotes upward nutrient supply into the euphotic zone leading to a rapid response of diatoms. This response is clearly reflected by the flux seasonality: between 40% and 60% of the total annual diatom flux at the CVOO site was intercepted in a relatively short time interval (<60 days). A highly diverse diatom community characterized the dia-tom fluxes throughout. Along with the ACME passages, small species of the genus Nitzschia, and Thalassionema nitzschioides var. parva dominated and delivered a major portion of the opal and organic carbon into deeper waters at site CVOO. Several pelagic, warm-water background species became dominant during intervals with low nutrient availability in the euphotic zone. Results of our interannual time-series suggest that ACMEs impact on total diatom production and the species-specific composition of the assemblage north of the Cave Verde Islands, and can strengthen the biological pump in open-ocean, olig-otrophic subtropical regions of the world ocean. Our observations are useful for testing biogeochemical ocean models and will also help in improving the knowledge of processes and mechanisms behind inter-annual time-series of bulk components and microorganisms in pelagic and hemipelagic ocean areas.

Particle fluxes at the Cape Verde Ocean Observa- tory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time-series data collected at 1290 and 3439 m water depth. The typically oligotrophic par- ticle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen (mini- mum concentration below 2 μmol kg−1 at 40 m depth) anti- cyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes of up to 151 mg m−2 d−1, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks of ∼ 15 and 13.3 mg m−2 d−1, respectively, were ob- served in February–March 2010 when the eddy approached the CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated with that of organic car- bon, in particular in the deep trap samples, suggesting a tight coupling. The lithogenic ballasting obviously resulted in high particle settling rates and, thus, a fast transfer of epi-/meso- pelagic signatures to the bathypelagic traps. We suspect that the two- to three-fold increase in particle fluxes with depth as well as the tight coupling of mineral dust and organic car- bon in the deep trap samples might be explained by parti- cle focusing processes within the deeper part of the eddy. Molar C : N ratios of organic matter during the ACME pas- sage were around 18 and 25 for the upper and lower trap
June 2016
samples, respectively. This suggests that some productivity under nutrient (nitrate) limitation occurred in the euphotic zone of the eddy in the beginning of 2010 or that a local nitrogen recycling took place. The δ15N record showed a de- crease from 5.21 to 3.11 ‰ from January to March 2010, while the organic carbon and nitrogen fluxes increased. The causes of enhanced sedimentation from the eddy in Febru- ary/March 2010 remain elusive, but nutrient depletion and/or an increased availability of dust as a ballast mineral for organic-rich aggregates might have contributed. Rapid rem- ineralisation of sinking organic-rich particles could have con- tributed to oxygen depletion at shallow depth. Although the eddy formed in the West African coastal area in summer 2009, no indications of coastal flux signatures (e.g. from di- atoms) were found in the sediment trap samples, confirming the assumption that the suboxia developed within the eddy en route. However, we could not detect biomarkers indica- tive of the presence of anammox (anaerobic ammonia ox- idation) bacteria or green sulfur bacteria thriving in photic zone suboxia/hypoxia, i.e. ladderane fatty acids and isore- nieratene derivatives, respectively. This could indicate that suboxic conditions in the eddy had recently developed and/or the respective bacterial stocks had not yet reached detec- tion thresholds. Another explanation is that the fast-sinking organic-rich particles produced in the surface layer did not interact with bacteria from the suboxic zone below.

As the mesoscale eddies in oceans and semi-enclosed seas are significant in horizontal dispersion of pollutants, we investigate the seasonal variations of these eddies in the Persian Gulf (PG) that are usually generated due to seasonal winds and baroclinic instability. The sea surface height (SSH) data from 2010 to 2014 of AVISO are used to identify and track eddies, using the SSH-based method. Then seasonal horizontal dispersion coefficients are estimated for the PG, using the properties of eddies. The results show an annual mean of 78 eddies with a minimum lifetime of one week. Most of the eddies are predominantly cyclonic (59.1%) and have longer lifetimes and higher diffusion coefficients than the anti-cyclonic eddies. The eddy activity is higher in warm seasons, compared to that of cold seasons. As locations with high eddy diffusion coefficients are high-risk areas by using maps of horizontal eddy diffusion coefficients, perilous times and locations of the release of pollutants are specified to be within the longitude from 51.38 • E to 55.28 • E. The mentioned areas are located from the Strait of Hormuz towards the northeast of the PG, closer to Iranian coast. Moreover, July can be considered as the most dangerous time of pollution release.

Particle fluxes at the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time-series data collected at 1290 and 3439 m water depth. The typically oligotrophic particle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen (minimum concentration below 2 µmol kg&lt;sup&gt;−1&lt;/sup&gt; at 40 m depth) anticyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes of up to 151 mg m&lt;sup&gt;−2&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks of ∼ 15 and 13.3 mg m&lt;sup&gt;−2&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;, respectively, were observed in February–March 2010 when the eddy approached the CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated wi...

Generation of mesoscale eddies in the eastern South China Sea (SCS) in winters during August 1999 to July 2002 is studied with a reduced-gravity model. It is found that the orographic wind jets associated with the northeast winter monsoon and the gaps in the mountainous island chain along the eastern boundary of the SCS can spin up cyclonic and anticyclonic eddies over the SCS. Results suggest that direct wind forcing could be an important generation mechanism for the rich eddy activity in the SCS, and that to simulate this mechanism the resolution of the wind forcing has to be high enough to resolve the local wind jets induced by orographic effects.

Bathypelagic particle flux signatures from a suboxic eddy ate fluxes dropped from ∼ 52 to 21.4 mg m −2 d −1 from January to February 2010, respectively, mainly due to reduced contribution of shallow-dwelling planktonic foraminifera and pteropods. The deep-dwelling foraminifera Globorotalia menardii, however, showed a major flux peak in February 2010, most probably due to the suboxia/hypoxia. The low oxygen conditions forced at least some zooplankton to reduce diel vertical migration. Reduced "flux feeding" by zooplankton in the epipelagic could have contributed to the enhanced fluxes of organic materials to the bathypelagic traps during the eddy passage. Further studies are required on eddy-induced particle production and preservation processes and particle focusing.

Manuscript revised for Global Biogeochemical Cycles Key points 1. An inter-model comparison reveals that oxygen deoxygenation is sensitive to the representation of lateral mixing. 2. Stronger lateral mixing results in greater deoxygenation, but largely in zones which have high oxygen in the base state. 3. Cross-model differences in oxygen fields are dominated by biological drawdown, but are driven by ventilation rather than consumption.

1] The impact of mesoscale eddies on the production of radiolarian species Rhizoplegma boreale in the central subarctic Pacific was investigated through analysis of sinking particle fluxes collected using sediment-trap and altimetry data from satellite observations. Altimetry observations provided the locations of mesoscale eddies in time and space and indicated that mesoscale eddies propagating around sediment trap Station SA were closely related to the high R. boreale fluxes at the station. The mesoscale eddies likely provided the deep-sea region of the central subarctic Pacific with coastal nutrient-rich waters derived from the region around the Aleutian Islands, and influenced the productivity of microzooplankton groups at Station SA.

A more than two-decadal sediment trap record from the Eastern Boundary Upwelling Ecosystem (EBUE) off Cape Blanc, Mauritania, is analysed with respect to deep ocean mass fluxes, flux components and their variability on seasonal to decadal timescales. The total mass flux revealed interannual fluctuations which were superimposed by fluctu- ations on decadal timescales. High winter fluxes of biogenic silica (BSi), used as a measure of marine production (mostly by diatoms) largely correspond to a positive North Atlantic Oscillation (NAO) index (December–March). However, this relationship is weak. The highest positive BSi anomaly was in winter 2004–2005 when the NAO was in a neutral state. More episodic BSi sedimentation events occurred in several summer seasons between 2001 and 2005, when the previous winter NAO was neutral or even negative. We suggest that distinct dust outbreaks and deposition in the surface ocean in winter and occasionally in summer/autumn enhanced par- ticle sedimentation and carbon export on short timescales via the ballasting effect. Episodic perturbations of the ma- rine carbon cycle by dust outbreaks (e.g. in 2005) might have weakened the relationships between fluxes and large- scale climatic oscillations. As phytoplankton biomass is high throughout the year, any dry (in winter) or wet (in sum- mer) deposition of fine-grained dust particles is assumed to enhance the efficiency of the biological pump by incorpo- rating dust into dense and fast settling organic-rich aggre- gates. A good correspondence between BSi and dust fluxes was observed for the dusty year 2005, following a period of rather dry conditions in the Sahara/Sahel region. Large changes of all bulk fluxes occurred during the strongest ElNiño-Southern Oscillation (ENSO) in 1997–1999 where low fluxes were obtained for almost 1 year during the warm El Niño and high fluxes in the following cold La Niña phase. For decadal timescales, Bakun (1990) suggested an intensifi- cation of coastal upwelling due to increased winds (“Bakun upwelling intensification hypothesis”; Cropper et al., 2014) and global climate change. We did not observe an increase of any flux component off Cape Blanc during the past 2 and a half decades which might support this. Furthermore, fluxes of mineral dust did not show any positive or negative trends over time which might suggest enhanced desertification or “Saharan greening” during the last few decades.

Using a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 1448 and 1548E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual vari-abilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

The South China Sea (SCS) summer circulation often has a dipole structure associated with an eastward jet, appearing off central Vietnam. The dipole has an anticyclonic eddy (AE) south of the jet and a cyclonic eddy (CE) north of it. The life cycle of the dipole structure is analyzed using satellite altimetry data and a reduced gravity model. On average the dipole structure begins in June, peaks in strength in August or September, and disappears in October. The dipole evolution lags behind the basin scale wind by about 40 days, and 40 days are exactly what it takes for baroclinic planetary waves to cross the southern SCS. Our results show that the vorticity transports from the nonlinear effect of the western boundary currents are crucial for the generation of the dipole structure. In addition, the strength and direction of the offshore wind jet also play a significant role in determining the magnitudes and the core positions of the two concomitant eddies.

Particle fluxes at the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time-series data collected at 1290 and 3439 m water depth. The typically oligotrophic particle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen (minimum concentration below 2 µmol kg&lt;sup&gt;−1&lt;/sup&gt; at 40 m depth) anticyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes of up to 151 mg m&lt;sup&gt;−2&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks of ∼ 15 and 13.3 mg m&lt;sup&gt;−2&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;, respectively, were observed in February–March 2010 when the eddy approached the CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated wi...

Macrourids are among the most abundant and diverse demersal fishes in all deep oceans, including the Southwestern Brazilian continental slope. Although not targeted by Brazilian fisheries, they suffer impact similar than the target species, being among the most discarded fishes by deep bottom trawling. Trophic Ecology: Data from research surveys and commercial fishing were used to analyze the trophic ecology of four species inhabiting the upper slope of southern Brazil: Coelorinchus marinii, Malacocephalus occidentalis, M. laevis and Lucigadus ori. For the two abundant ones, ontogenetic changes, seasonal variations, intra- and interspecific dietary overlap, parasite fauna and aspects of functional morphology are also described. C.marinii had an extremely diverse diet, preying infauna, epifauna, plankton, necton and carcasses. M.occidentalis fed on larger and nektonic prey, but also included crabs and carcasses in the diet. Both species showed ontogenetic shifts and seasonal variatio...

The results of simulating the hydrophysical fields of the Black Sea with a resolution of 1.64 × 1.64 km for January–September 2006 with the use of real atmospheric forcing are analyzed. Both vertical turbulent momentum exchange and vertical turbulent heat and salt diffusions are parameterized using the Mellor-Yamada level 2.5 scheme. The results of this numerical experiment are compared with similar data obtained with a horizontal resolution of 5 km. The features of the meso- and submesoscale dynamics of waters in individual sea regions are given. Possible physical mechanisms of forming meso- and submesoscale vortices are studied on the basis of energy analysis. It is shown that, in the absence of significant wind forcing, the main contribution to kinetic energy is made by the buoyancy force and wind-field inhomogeneities result in significant variations in both total vertical viscosity and total vertical diffusion.