Cold-core Cyclonic Eddy Research Articles

Influence of Mozambique Channel eddies on larval loss of two shallow-water commercial shrimp species

The shallow-water shrimp fishery is an important social and economic sector in Mozambique. However, catches of shrimps have been declining over the last decades, presumably due to poor recruitment. Climate change has been proposed as a potential cause of increased mesoscale eddy activity over the Mozambique Channel, which may have played a role in poor shrimp recruitment. Indeed, stronger eddies could transport matter, including increased numbers of shrimp larvae, from the coast to unfavourable offshore areas. In this study we used a biophysical model to investigate the influence of eddies on the dispersal of larvae of two commercial shrimp species (Penaeus indicus and Metapenaeus monoceros) from their spawning areas on the Sofala Bank. We found some 5 large events of offshore or southward transport loss (>10% loss) from the Sofala Bank caused by eddies, occurring sporadically. The northern and central Sofala Bank areas were the most affected by larvae lost offshore. Simulations revealed that temperature-induced larval mortality, associated with cold-core cyclonic eddies, could also play a role in larval loss of up to 40%, which is greater than the larval transport loss associated with the eddy circulation. However, when they survived, larvae transported offshore could travel long distances (600–1600 km in 15 days), potentially promoting connectivity with other stocks in the region.

Abundant Cold Anticyclonic Eddies and Warm Cyclonic Eddies in the Global Ocean

AbstractMesoscale eddies are ubiquitous features of the global ocean circulation and play a key role in transporting ocean properties and modulating air–sea exchanges. Anticyclonic and cyclonic eddies are traditionally thought to be associated with anomalous warm and cold surface waters, respectively. Using satellite altimeter and microwave data, here we show that surface cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs) are surprisingly abundant in the global ocean—about 20% of the eddies inferred from altimeter data are CAEs and WCEs. Composite analysis using Argo float profiles reveals that the cold cores of CAEs and warm cores of WCEs are generally confined in the upper 50 m. Interestingly, CAEs and WCEs alter air–sea momentum and heat fluxes and modulate mixed layer depth and surface chlorophyll concentration in a way markedly different from the traditional warm-core anticyclonic and cold-core cyclonic eddies. Given their abundance, CAEs and WCEs need to be properly accounted for when assessing and parameterizing the role of ocean eddies in Earth’s climate system.

Effect of phytoplankton community size structure on remote-sensing reflectance and chlorophyll a products

Abstract Remotely-sensed ocean colour is the main tool for estimating chlorophyll a (Chl-a) concentration and primary productivity on the global scale. In order to investigate the source of errors in remotely-sensed Chl-a concentration we obtained in situ bio-optical properties, in situ reflectances, satellite-derived reflectances and the Chl-a concentration satellite products of the Ocean and Land Colour Imager (OLCI) Instrument on board Sentinel-3 A in waters off eastern Australia. The mesoscale eddies of these oligotrophic waters provide contrasting phytoplankton communities that allowed us to focus on the effect of phytoplankton size as a source of errors. In these waters, cold-core cyclonic eddies (CE) are dominated by large phytoplankton cells, while small cells dominate warm-core anticyclonic eddies (ACE). The chlorophyll-specific absorption and backscattering from contrasting sites show significant difference due to the differing package effect of phytoplankton size distributions. After normalising the absorption and backscattering spectra to Chl-a associated with just small phytoplankton, the spectra of optical properties become much more similar, showing that small-sized phytoplankton dominate IOPs even when large cells contain the greater fraction of Chl-a concentration of the phytoplankton community. Measured in situ reflectances agreed with reflectances calculated using a simple optical model based on measured IOPs. Furthermore, the in situ measured reflectances agreed well with the OLCI reflectance (mean normalised bias (MNB) of 7% for wavelengths

Gulf Stream evolution through the Straits of Florida: the role of eddies and upwelling near Cuba

The meandering of the Gulf Stream through the Straits of Florida is associated with eddy activity to the north (along the Florida Keys) and the south (along the Cuban coast). This study focuses on recently identified processes along the Cuban coast, namely anticyclonic eddies (Cuban ANticyclones: CubANs) and cyclonic activity characterized by cold-core eddies (cyclones) and coastal upwelling that enhances them. It is shown that these processes are an important factor for the evolution of the Loop Current/Florida Current (LC/FC) system. In particular, the Gulf Stream meandering inside the Straits (that is manifested as the position of the FC branch) is strongly related to CubANs either inside the retracted (closer to the Cuban coast) LC branch (CubAN “A”) or outside the LC as independent eddies (CubAN “B”). There are also mixed cases that both types of CubANs can be present, sharing a common place of origin, but evolving differently. These anticyclones are found to be strongly related to cyclonic activity and resulting temperature gradients along Cuba, characterized by cold-core cyclonic eddies and additional pools of cold waters that have upwelled under wind influence. These processes are shown to influence Gulf Stream meandering and also largely control the offshore export of upwelled, generally cooler and more productive, coastal waters. High-resolution simulations, in tandem with a variety of observational data, are used during a period of 8 years (2010–2017) to describe the evolution of CubANs and their contribution to Gulf Stream variability and associated coastal to offshore interactions. These findings are important for connectivity pathways between US and Cuban marine protected areas and between US coasts and the main Cuban area of oil exploration.

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea (SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic (warm-core anticyclonic) eddies tend to cool (warm) the overlying atmosphere and cause surface winds to decelerate (accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce (enhance) the vertical momentum transport over the cold (warm) eddy, resulting in the decrease (increase) of sea surface wind. Diagnoses of the model’s momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.

In situ measurement of the biogeochemical properties of Southern Ocean mesoscale eddies in the Southwest Indian Ocean, April 2014

Abstract. Several open-ocean mesoscale features – a "young" warm-core (anti-cyclonic) eddy at 52° S, an "older" warm-core eddy at 57.5° S and an adjacent cold-core (cyclonic) eddy at 56° S – were surveyed during a R/V S.A. Agulhas II cruise in April 2014. The main aim of the survey was to obtain hydrographical and biogeochemical profile data for contrasting open-ocean eddies in the Southern Ocean, which will be suitable for comparative study and modelling of their heat, salt and nutrient characteristics, and the changes that occur in these properties as warm-core eddies migrate from the polar front southwards. The major result is that the older warm-core eddy at 57.5° S is, at its core, 2.7 °C colder than a younger eddy at 52° S, while its dissolved silicate levels are almost 500 % higher and accompanied by chlorophyll a levels that are more than 200 % higher than that in the younger eddy. A total of 18 CTD stations were occupied in a sector south of the Southwest Indian Ridge, along three transects crossing several mesoscale features identified from satellite altimetry data prior to the cruise. The CTD data, as well as chlorophyll a and dissolved nutrient data (for NO3−, NO2−, PO43− and SiO2), have been processed, quality controlled and made available via the PANGAEA Data Archiving and Publication database at doi:10.1594/PANGAEA.848875.

Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Niña conditions: relationship between distribution and hydrological conditions

Abstract. The spatial distribution of heterotrophic prokaryotes was investigated during the Tokyo–Palau cruise in the western part of the North Pacific subtropical gyre (NPSG) along a north–south transect between 33.60 and 13.25° N. The cruise was conducted in three different hydrological areas identified as the Kuroshio region, the subtropical gyre area and the transition zone. Two eddies were crossed along the transect: one cold-core cyclonic eddy and one warm-core anticyclonic eddy and distributions of the heterotrophic prokaryotes were recorded. By using analytical flow cytometry and a nucleic acid staining protocol, heterotrophic prokaryotes were discriminated into three subgroups depending on their nucleic acid content (low, high and very high nucleic acid contents labelled LNA, HNA and VHNA, respectively). Statistical analyses performed on the data set showed that LNA, mainly associated with low temperature and low salinity, were dominant in all the hydrological regions. In contrast, HNA distribution seemed to be associated with temperature, salinity, Chl a and silicic acid. A latitudinal increase in the HNA / LNA ratio was observed along the north–south transect and was related to higher phosphate and nitrate concentrations. However, the opposite relationship observed for the VHNA / HNA ratio suggested that the link between nucleic acid content and oligotrophic conditions is not linear, underlying the complexity of the biodiversity in the VHNA, HNA and LNA subgroups. In the Kuroshio Current, it is suggested that the high concentration of heterotrophic prokaryotes observed at station 4 was linked to the path of the cold cyclonic eddy core. In contrast, it is thought that low concentrations of heterotrophic prokaryotes in the warm core of the anticyclonic gyre (Sta. 9) are related to the low nutrient concentrations measured in the seawater column. Our results showed that the high variability between the various heterotrophic prokaryote cluster abundances depend both on the mesoscale structures and the oligotrophic gradient.

English

Eddy pumping is a process by which mesoscale eddies induce isopycnal displacements that lift nutrient-replete waters into the euphotic zone, driving a set of biogeochemical processes in the ocean. To investigate the potential effect of physical processes on the microbial community functions, the microbial community metaproteomic profiles were determined in a cold-core cyclonic eddy in the South China Sea (SCS). The significant differences of metaproteomic patterns accurately predicted their distinct functional responses to environmental changes caused by the mesoscale cyclonic eddy perturbations. From the protein gel profiles, 11 proteins were successfully identified by MALDI TOF MS/MS, including nitrogen regulatory proteins, ribosomal proteins and substrate transport proteins, etc, which implied the metabolic responses of the microbial populations to the nutrients change within the cyclonic eddies. These results highlight the potential of metaproteomics for studying complex microbial consortium in the ocean and contributed to our understanding of the biological consequences of a mesoscale cyclonic eddy in the SCS.

Phylogenetic diversity of bacterial communities in South China Sea mesoscale cyclonic eddy perturbations

Eddy pumping drives a set of biogeochemical processes by lifting deep waters into the euphotic zone. To address the potential effect of such physical processes upon the bacterial community, phylogenetic diversity was determined in two cold-core cyclonic eddies in the South China Sea. 16S rDNA terminal restriction fragment length polymorphism analysis of the microbial communities through the whole water column showed a wider depth range for the intermediate transition water mass at sites inside the eddies than for those outside. This water mass contained a relatively more complex community than the euphotic and deep-water zones. Stratification of prokaryotic populations between the surface and chlorophyll maximum layer of eddy-related sites versus homogeneity of communities in the euphotic zone of the reference site, revealed by statistical analysis of 16S rDNA libraries, is most likely a reflection of isopycnal displacement induced by differing water movement inside and outside eddies. Phylogenetic analysis revealed that eddy center sites were characterized by deep-water group Alteromonadales-affiliated clones, the psychrophilic genus Octadecabacter cluster and the nitrogen-fixing phototrophic Rhodospirillaceae cluster, while Paracoccus, an important functional group, abundantly existed at the reference site outside eddies. Our analysis revealed that bacterial community structure was significantly influenced by cyclonic eddy perturbations.

Role of mesoscale cyclonic eddies in the distribution and activity of Archaea and Bacteria in the South China Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 56:65-79 (2009) - DOI: https://doi.org/10.3354/ame01324 Role of mesoscale cyclonic eddies in the distribution and activity of Archaea and Bacteria in the South China Sea Yao Zhang1, Eva Sintes2, Jianing Chen1, Yong Zhang1, Minhan Dai1, Nianzhi Jiao1, Gerhard J. Herndl2,3,* 1State Key Laboratory for Marine Environmental Sciences, Xiamen University, 422 Siming South Road, Xiamen 361005, China 2Department of Biological Oceanography, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, The Netherlands 3University of Vienna, Ecology Center, Department of Marine Biology, Althanstr. 14, 1090 Vienna, Austria *Corresponding author. Email: gerhard.herndl@univie.ac.at ABSTRACT: We examined the effect of 2 cold-core cyclonic eddies on the activity of the prokaryotic plankton community in the South China Sea. The abundance of bulk prokaryotes and major prokaryotic groups (Bacteria, marine Crenarchaeota Group I and marine Euryarchaeota Group II) and the number of cells taking up D- vs. L-aspartic acid (Asp) were determined using microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). At all sites, the bulk D-Asp:L-Asp uptake ratio by the prokaryotic community increased with depth. Concurrently, the contribution of marine Crenarchaeota Group I to total prokaryotic abundance and total active cells also increased with depth, while an opposite pattern was observed for Bacteria. Marine Euryarchaeota Group II were generally more dominant in near-surface than in deep waters. Significant differences were observed between sites inside and outside the cyclonic eddies in terms of distribution and activity of prokaryotic communities and the concentration of fluorescent dissolved organic matter (FDOM, an important refractory fraction of DOM). Generally, higher bulk D-Asp:L-Asp uptake ratios by the prokaryotic community and a greater crenarchaeotal contribution were found in the upper mesopelagic water column inside the cold-core eddies as compared to the outside sites. Taken together, the MICRO-CARD-FISH data and the pattern of FDOM indicate that the higher contribution of refractory DOM induced by upwelled water in the cyclonic eddy may have led to a more prominent role of Crenarchaeota in the organic carbon cycling in the mesopelagic realm of the cold-core eddy than outside the eddy in the South China Sea. KEY WORDS: Cyclonic eddy · Archaea · Bacteria · Enantiomeric amino acid uptake · DOM fluorescence Full text in pdf format PreviousNextCite this article as: Zhang Y, Sintes E, Chen J, Zhang Y, Dai M, Jiao N, Herndl GJ (2009) Role of mesoscale cyclonic eddies in the distribution and activity of Archaea and Bacteria in the South China Sea. Aquat Microb Ecol 56:65-79. https://doi.org/10.3354/ame01324 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 56, No. 1. Online publication date: July 09, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

Diatoms in the desert: Plankton community response to a mesoscale eddy in the subtropical North Pacific

Abstract As part of the E-Flux project, we documented spatial variability and temporal changes in plankton community structure in a cold-core cyclonic eddy in the lee of the Hawaiian Islands. Cyclone Opal spanned 200 km in diameter, with sharply uplifted isopycnals (80–100 m relative to surrounding waters) and a strongly expressed deep chlorophyll a maximum (DCM) in its central core region of 40 km diameter. Microscopic and flow cytometric analyses of samples from across the eddy revealed dramatic transitions in phytoplankton community structure, reflecting Opal's well-developed physical structure. Upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by picophytoplankton. In contrast, the DCM was composed of large chain-forming diatoms dominated by Chaetoceros and Rhizosolenia spp. Diatoms attained unprecedented levels of biomass (nearly 90 μg C l−1) in the center of the eddy, accounting for 85% of photosynthetic biomass. Protozoan grazers displayed two- to three-fold higher biomass levels in the eddy center as well. We also found a distinct and persistent layer of senescent diatom cells overlying healthy populations, often separated by less than 10 m, indicating that we were sampling a bloom in a state of decline. Time-series sampling over 8 days showed a successional shift in community structure within the central diatom bloom, from the unexpected large chain-forming species to smaller forms more typical of the subtropical North Pacific. The diatom bloom of Cyclone Opal was a unique, and possibly extreme, example of biological response to physical forcing in the North Pacific subtropical gyre, and its detailed study may therefore help to improve our predictive understanding of environmental controls on plankton community structure.

Physical dynamics and biological implications of Cyclone Noah in the lee of Hawai’i during E-Flux I

Abstract Quasi-synoptic observations of the horizontal and vertical structure of a cold-core cyclonic mesoscale eddy feature (Cyclone Noah) were conducted in the lee of Hawai’i from November 4–22, 2004 as part of the E-Flux interdisciplinary collaborative research program. Cyclone Noah appears to have spun up to the southwest of the ‘Alenuihaha Channel (between Maui and Hawai’i) as a result of strong and persistent northeasterly trade winds through the channel. Shipboard hydrographic surveys 2.5 months later suggest that Noah weakened and was in a hypothesized spin-down phase of its life cycle. Although the initial surface expression of Noah was limited in scale to ∼40 km in diameter and, as evidenced by surface temperatures, 2–3 °C cooler than the surrounding waters, depth profiles revealed a fully developed semi-elliptical shallow feature (∼200 m), ∼144 km long and ∼90 km wide (based on sigma-t=23 kg m−3) with tangential speeds of 40–80 cm s−1, and substantial isopycnal doming. Potential vorticity distribution of Noah suggests that radial horizontal flow of the core water was inhibited from the surface to depths of ∼75 m, with high vorticity confined above the sigma-t=23.5 kg m−3 isopycnal surface. Upward displacements of isopycnal surfaces in the eddy's center (∼50 m) were congruent with enhanced pigment concentrations (0.50 mg m−3). Comparisons of the results obtained for E-Flux I (Noah) and E-Flux III (Opal) suggest that translation characteristics of cyclonic Hawaiian lee eddies may be important in establishing the biogeochemical and biological responses of the oligotrophic ocean to cyclonic eddies.

Cobalt, manganese, and iron near the Hawaiian Islands: A potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals

Abstract The vertical distributions of cobalt, iron, and manganese in the water column were studied during the E-Flux Program (E-Flux II and III), which focused on the biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were sampled for cobalt ( n =147), all of which demonstrated nutrient-like depletion in surface waters. During E-Flux III, two depth profiles collected from within a mesoscale cold-core eddy, Cyclone Opal , revealed small distinct maxima in cobalt at ∼100 m depth and a larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt concentrating effect within the eddy, where upwelled cobalt was subsequently associated with sinking particulate organic carbon (POC) via biological activity and was released at a depth coincident with nearly complete POC remineralization [Benitez-Nelson, C., Bidigare, R.R., Dickey, T.D., Landry, M.R., Leonard, C.L., Brown, S.L., Nencioli, F., Rii, Y.M., Maiti, K., Becker, J.W., Bibby, T.S., Black, W., Cai, W.J., Carlson, C.A., Chen, F., Kuwahara, V.S., Mahaffey, C., McAndrew, P.M., Quay, P.D., Rappe, M.S., Selph, K.E., Simmons, M.P., Yang, E.J., 2007. Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean. Science 316, 1017–1020]. There is also evidence for the formation of a correlation between cobalt and soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we suggest is due to increased productivity, implying a minimum threshold of primary production below which cobalt–phosphate coupling does not occur. Dissolved iron was measured in E-Flux II and found in somewhat elevated concentrations (∼0.5 nM) in surface waters relative to the iron depleted waters of the surrounding Pacific [Fitzwater, S.E., Coale, K.H., Gordon, M.R., Johnson, K.S., Ondrusek, M.E., 1996. Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Research II 43 (4–6), 995–1015], possibly due to island effects associated with the iron-rich volcanic soil from the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total dissolved cobalt were observed at 400–600 m depth, suggestive of the release of metals from the shelf area of comparable depth that surrounds these islands.

Particle export within cyclonic Hawaiian lee eddies derived from 210Pb–210Po disequilibrium

Abstract Particle export from the upper waters of the oligotrophic ocean may play a crucial role in the global carbon cycle. Mesoscale eddies have been hypothesized to inject new nutrients into oligotrophic surface waters, thereby increasing new production and particle export in otherwise nutrient deficient regimes. The E-Flux Program was a large multidisciplinary project designed to investigate the physical, biological and biogeochemical characteristics of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. There, we investigated particle dynamics using 210Pb–210Po disequilibrium. Seawater samples for 210Pb and 210Po were collected both within (IN) and outside (OUT) of two cyclones, Noah and Opal, at different stages of their evolution as well as from the eddy generation region. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO2) export fluxes were determined using water-column PC, PN, and bSiO2 inventories and the residence times of 210Po. PC and PN fluxes at 150 m ranged from 1.58±0.10 to 1.71±0.16 mmol C m−2 d−1 and 0.22±0.02 to 0.30±0.02 mmol N m−2 d−1 within Cyclones Opal and Noah. PC and PN fluxes at OUT stations sampled during both cruises were of similar magnitudes, 1.69±0.16 to 1.67±0.16 mmol C m−2 d−1 and 0.30±0.03 to 0.26±0.03 mmol N m−2 d−1. The bSiO2 fluxes within Cyclone Opal were 0.157±0.010 mmol Si m−2 d−1 versus 0.025±0.002 mmol Si m−2 d−1 at OUT stations. These results of minimal PC and PN export, but significant eddy-induced bSiO2 fluxes, agree very well with other studies that used a variety of direct and indirect methods. Thus, our results suggest that using elemental inventories and residence times of 210Po is another independent and robust method for determining particle export and should be investigated more fully.

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

Ichthyoplankton assemblage structure in two meso-scale Leeuwin Current eddies, eastern Indian Ocean

Larval fish assemblages within two meso-scale eddies, located off the coast of south-western Australia, were investigated as part of an interdisciplinary study examining the biophysical nature of the eddies. Larval fish were sampled using oblique and depth-stratified plankton tows, taken within the centre, body and perimeter of both a cold-core cyclonic eddy and a warm-core anti-cyclonic eddy. Larvae from the oceanic families Myctophidae, Phosichthyidae and Gonostomatidae were the most abundant within the larval fish assemblage, with larvae from the families Sternoptychidae, Paralepididae and Stomiidae also common. Larval fish assemblages from the warm-core eddy were significantly different to those from the cold-core eddy. Significant differences were found between assemblages from the centre and body, and the centre and perimeter of the warm-core eddy, but there were no significant differences across zones in the cold-core eddy. Assemblages were more strongly structured with depth in the cold-core eddy than in the warm-core eddy. The correlation of environmental data to larval fish assemblages showed that sea-surface temperature was the variable best correlated with larval fish assemblage structure between eddies. Depth of the mixed-layer was best correlated with assemblages in the warm-core eddy only, but no significant environmental correlations existed in the cold-core eddy assemblages. We hypothesise that this was a result of the differing oceanographic structure between the two eddies.

Seasonal variability of upper-layer geostrophic transport in the tropical Indian Ocean during 1992–1996 along TOGA-I XBT tracklines

Upper layer (0–400 m) geostrophic volume transport associated with the major current systems of the tropical Indian Ocean along the near-meridional TOGA-India XBT tracklines from Mumbai to Mauritius during 1992–1996 is discussed. The transport of the eastward flowing Indian Monsoon Current (IMC) varies between 9 and 14 Sv (1 Sv=10 6 m 3 s −1) during southwest monsoon. The westward flowing North Equatorial Current (NEC) transports about 8 Sv during boreal winter. The westward flowing south equatorial current (SEC) has its lowest transport (10–12 Sv) during austral fall and spring and highest transport (18–21 Sv) during austral winter and summer. The northern boundary of the SEC extends equatorward as far north as for 4–5°S during southwest monsoon from its usual location of 8–10°S. The eastward flowing South Equatorial Counter Current (SECC) exhibits highest transport (37 Sv) during peak austral summer (January) and between 5 and 20 Sv during the rest of the year. In March and May, eastward undercurrents are noticed near the equator. In June and July, the undercurrents are present south of Sri Lanka away from the equator. The equatorial flow in the zonal belts 1–2°N and 1–2°S exhibits westward transport of 6–20 Sv during January–February. This broad westward flow and the SECC constitute an anticlockwise equatorial gyre during boreal winter. In addition to the above current systems, signatures of a warm-core anticyclonic eddy centered at 8°N, 72.5°E and a cold-core cyclonic eddy centered at 10.5°N, 72.5°E are also noticed in February and October, respectively, off the southwest coast of India. The volume transport associated with the warm-core eddy is about 10 Sv directed equatorward and that with the cold-core eddy is 2.5 Sv directed poleward.

Eddies and a mesoscale deflection of the slope current in the Faroe–Shetland Channel

The mesoscale dynamics of the Scottish side of the Faroe–Shetland Channel have been investigated using synoptic in situ and remote sensing observations. A cold core cyclonic eddy, identified from an AVHRR image, had a diameter of about 50 km and surface current speeds of up to 50 cm s -1; it appeared to be attached to the 800 m isobath as it moved north-eastward along the edge of the channel at about 8 cm s -1. Speeds in the slope current were about 50 cm s -1 but increased to 70 cm s -1 where the current was compressed by the eddy. Offshore, over the 1000 m isobath in the cooler water, speeds in the current were slower (ca. 20 cm s -1). North-west of the Shetlands the offshore edge of the slope current was deflected across the channel for a distance of about 70 km from the shelf edge. The speed of drifters in the slope current increased to over 60 cm s -1 as they moved anti-cyclonically around this deflection. CTD profiles suggest that the movement of the surface waters was mirrored in the deep water of the channel. The deflection carried a very large quantity of North Atlantic Water into the central part of the channel; its cause and ultimate fate are not known, although it is likely to have had a significant impact on the dynamics of the channel.

Localization of Deep Ocean Convection by a Mesoscale Eddy

Abstract Observations of open-ocean deep convection indicate that it is a highly localized phenomenon, occurring over areas of tens of kilometers in diameter. The cause of this localization has been ascribed to “preconditioning”—the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface-intensified cyclonic circulation. However, most numerical and laboratory studies of localized convection have prescribed the localization artificially, by confining the surface buoyancy loss to a circular disk. In contrast, in the numerical simulations described here, deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings. The preconditioned region is associated with a cold-core cyclonic eddy in geostrophic and cyclostrophic balance. As in previous studies of disk-shaped cooling, the localized convection region undergoes baroclinic instability at late times, causing the br...

Brazil-Malvinas Confluence–1984

The Brazil-Malvinas (Falkland) Confluence is marked by a complex array of strongly contrasting water types. In October 1984 two large poleward meanders of South Atlantic thermocline water, separated by a cold-core cyclonic eddy near 40°S and 50°W, were observed west of 46°W. The western feature extended further south, to 43°S, with a geostrophic volume transport for the upper 1500 decibars (db) relative to 1500 db of 22,5 × 10 6 m 3 s −1. The branch to the east has a somewhat weaker baroclinic structure, though it is associated with warmer and more saline surface water. The warmer temperature reflects less atmospheric cooling than experienced within the western branch, which is exposed directly to the cold prevailing westerlies. The western branch is capped by a low salinity layer, the origin of which appears to be a mixture of continental shelf water with surface water from the Brazil Current. Near 39–40°S the surface water at the Confluence front is of very low salinity, yet too warm to be subantarctic water carried in with the Malvinas Current. It is likely that it represents warm river plume water from the Rio de la Plata, that has “folded-over” the northern tip of the cyclonic trough formed by the Malvinas Current and its return to the south. At depth the most concentrated North Atlantic Deep Water is associated with the western branch. This feature continues flowing south, below the Malvinas Current, indicating that the western boundary separation at depth need not be coherent with the thermocline separation position. South of the western branch are two warm-core eddies; the northern eddy is in the process of separation while the southern eddy has been separated for a period of 2–3 months and has undergone thermohaline modification by the late winter atmosphere. These modified warm-core eddies may not re-coalesce with the main thermocline and hence represent a significant salt flux into the subantarctic zone of the South Atlantic.