Warm-core Eddy Research Articles

The effect of cold- and warm-core eddies on the distribution and stoichiometry of dissolved nutrients in the northeastern Mediterranean

The nutrient distribution and phytoplankton production in the Levantine Sea of the eastern Mediterranean are principally determined by the duration and the intensity of deep winter mixing in the quasi-permanent anticyclonic and cyclonic eddies. In the seasons of stratification, a nutrient-poor aphotic layer is formed between the euphotic zone and the nutricline; interestingly, it consistently extends down to depths of about 29.0–29.05 isopycnal surfaces, but nearly vanishes in the core of the cyclonic Rhodes Gyre (RG) due to the upwelling of the Levantine deep water (LDW) up to the base of the euphotic zone. Accordingly, the nutricline is much sharper and shallower in the cyclonic RG; nevertheless, it is consistently established between the density surfaces of 29.00–29.05 and 29.15 throughout the basin. In the severe winters of 1992 and 1993, the upper 1000 m of the cyclonic Rhodes Gyre was occupied by the LDW with its associated chemical properties and abnormally high nutrient concentrations (NO 3=3.8–4.7μM; PO 4=0.14–0.16μM and Si=7.3–7.8μM) were observed in the euphotic zone. However, the surface nutrient concentrations of the anticyclonic regions were raised merely from the summer–autumn values of <0.02 and nearly 0.2μM to about 0.03 and 0.8μM for phosphate and nitrate, respectively. The molar ratios of nitrate to phosphate in the water column range between 5 and 20 in the euphotic zone but exhibit well-defined peak values (as large as 40–120) at the top of the nutricline (corresponding to nearly the depths of the 29.05 isopycnal surfaces) for most of the year. Such prominent maxima are the result of the apparent shift between the onsets of the nitracline and phosphacline due to as yet undefined factors. Below the nutricline the N/P ratios decrease regularly and reach an almost constant deep value (=28) over the basin. The mean ratio, derived from linear regression of the pooled phosphate and nitrate data from March 1991 to March 1994 is about 23.6, substantially higher than deep ocean values.

The mean and variability of ocean circulation past northern New Zealand: Determining the representativeness of hydrographic climatologies

Eastward flow in the Tasman Sea, from the separated East Australia Current, reattaches to the shelf break near North Cape, New Zealand, and then continues alongshore to the southeast as the East Auckland Current. A series of three permanent warm core eddies occurs along the offshore side. The mean transport of the East Auckland Current is about 9 Sv, with an additional 10 Sv or more of circulation in the eddies. An extensive hydrographic data set, archived broad scale expendable bathythermograph (XBT) data, two repeating high‐resolution XBT transects, neutrally buoyant float trajectories, and TOPEX altimetric data are used to estimate the temperature and absolute flow fields and to characterize variability. The aim is to examine the usefulness of time series information and absolute velocity measurements in the interpretation of hydrographic snapshots and climatologies, as well as to describe a region having intrinsic oceanographic interest and complexity. Issues of representativeness of the hydrographic data, of the magnitude and scales of the underlying variability, of the existence of permanent fine‐scale features, and of the appropriateness of deep reference levels are addressed directly. The relatively well sampled hydrographic climatology is shown to contain the equivalent of as many as 10 independent realizations. Temperature errors, relative to the true mean, are typically a few tenths of a degree. Significant seasonal bias is identified in the surface layer, and interannual bias is seen in the position of an eddy near North Cape. The dynamic height field at 1000 dbar relative to 2000 dbar is similar to estimates based on float trajectories and the assumption of geostrophic dynamics. This study underlines the value of time series data in the interpretation of a hydrographic climatology, in quantifying the errors in the estimated mean field as well as determining the magnitude and nature of variability. It also highlights the fact that the mean circulation of the oceans contains significant mesoscale structure, unnoticed in coarsely smoothed climatologies.

Analytical Solutions for Circular Eddies of the Reduced-Gravity, Shallow-Water Equations

New analytical, circular eddy solutions of the nonlinear, reduced-gravity, shallow-water equations in a rotating system are presented. While previous analytical solutions were limited to the description of pulsons, which are oscillating, frontal, warm-core eddies with paraboloidic shape and linear velocity components, the new solutions describe more general radial structures of eddy shape and azimuthal velocity. In particular, the new solutions, which contain as a subset the circular pulson solution, also allow for the description of circular, frontal, warm-core eddies with small azimuthal velocities at their periphery and/or with motionless cores, which are frequently observed characteristics of warm-core eddies in the World Ocean.

A basin‐scale oceanic instability event in the Gulf of Alaska

A composite NOAA 14 satellite advanced very high resolution radiometer image of the Gulf of Alaska for early March 1995 reveals a sequence of six anticyclonic oceanic eddies spanning more than 1500 km of the continental margin of North America from central British Columbia to central Alaska. Although isolated mesoscale eddies have previously been documented for this eastern boundary region, the composite image provides the first evidence that trains of such eddies can develop “simultaneously” along the entire coast. The 80 km radius and 250 km spacing of these warm‐core eddies are consistent with a baroclinic instability event in the poleward coastal current that prevails along the northwest continental margin of North America in “winter” (roughly October through March). Wind data from offshore meteorological buoys, combined with partial cloud‐free thermal images for January and February, suggest that the event was triggered in mid January by an abrupt, coast‐wide reversal in the prevailing poleward wind. The instability event disrupted the alongshore transport of relatively warm, high‐salinity water and presumably resulted in a greatly enhanced cross‐shore flux of brackish, nutrient‐rich coastal water into the open ocean. On the basis of its basin‐scale extent and more than 2 month duration it is feasible that the event had a major impact on the early spring recruitment and survival of zooplankton and pelagic fish within the highly productive fishery zones of coastal British Columbia and Alaska. Less extensive eddy sequences found in several earlier satellite images for the region suggest that these instability events are fairly common. If so, this indicates that the poleward current that forms over the eastern Pacific continental slope in late fall (herein, the “Northeast Pacific Coastal Current”) is marginally stable and likely to deform into a series of mesoscale eddies following abrupt, basin‐scale reversals in the seasonally dominant wind. Provided these wind events occur early enough in the season, the coastal current could reform with the onset of poleward winter winds.

Application of a modified Arakawa ‘a’ grid ocean model having reduced numerical dispersion to the Gulf of Mexico circulation

We describe a simple, flux conserving robust advection scheme that reduces numerical dispersion in application to passive scalar advection. Our hydrostatic semi-collocated (modified Arakawa ‘a’ grid) ocean model performance is enhanced by this modification in combination with a modified strategy for incompressibility. The modified advection scheme also reduces overshooting errors resulting from numerical dispersion in the evolution of a sloshing front using a non-hydrostatic model. In Gulf of Mexico simulations, many of the realistic detailed features from our original model using 1/12° (about 9 km) resolution occur using the modified numerics with 20-km resolution. These are not seen with the original numerics and 20-km resolution. These detailed features include: strong fronts and frontal eddies; many cyclonic eddies, which spin off major warm core Loop Current eddies; and sharp eastward deflection of the Loop Current directly into the Florida Strait after major warm core eddy shedding.

Buildup of microbial biomass during deep winter mixing in a Mediterranean warm-core eddy

Buildup of microbial biomass during deep winter mixing in a Mediterranean warm-core eddy

Micro-algal carbon and nitrogen uptake in post-coccolithophore bloom conditions in the northeast Atlantic, July 1991

A study of carbon and nitrogen uptake by phytoplankton was conducted at two sites in the subpolar northeast Atlantic during July 1991. The main study site at 61°N, 20°W was located within a cyclonic warm core eddy, and the other site was at 62°30′N, 20°W. Based on evidence of coccolithophore sedimentation and the relatively slow movement of the eddy, this structure was probably within the bounds of a mesoscale coccolithophore bloom that was in decline 10–20 days before the present study. As little is known about the conditions leading to the onset or collapse of such a bloom, the data presented here are, to our knowledge, the first to quantify phytoplankton dynamics following such an event. In addition, data from the main site provide one of the first assessments of the carbon and nitrogen dynamics within a cyclonic eddy that is typically observed in the northeast Atlantic. The findings at both sites revealed relatively high rates of primary production and concomitant uptake of inorganic nitrogen, suggesting that production rates were not enhanced within the eddy relative to the surrounding waters. These rates were comparable with those noted in late spring in the northeast Atlantic at lower latitudes (47°N, 20°W; 0.7 g C m −2 day −1) and are slightly in excess of modelled estimates of production for the subpolar North Atlantic in midsummer (0.5–0.6 g C m −2 day −1), but less than estimates based on functional photosynthetically active radiation (PAR)-productivity relationships derived from the North Atlantic Bloom Experiment (NABE) (greater than 1.0 g C m −2 day −1). Such functional relationships, derived at lower latitudes, may not be applicable to the subpolar northeast Atlantic. The findings of this study concur with the previous suggestion that the subpolar Atlantic is indeed productive at this time and may deserve further attention from modellers. A comparison of physical, chemical and biological conditions during the coccolithophore bloom and post-bloom periods indicates little change in environment and suggests the role of some self-regulatory mechanism, such as cell self-shading, as the cause of the collapse of the bloom. Invoking such a non-environmental factor may also explain the observed high rates of post-bloom production.

South china sea warm pool in boreal spring

During the boreal spring of 1966, a warm-core eddy is identified in the upper South China Sea (SCS) west of the Philippines through an analysis of the U.S. Navy’s Master Oceanographic Observation Data Set. This eddy occurred before the development of the northern summer monsoon and disappeared afterward. We propose that this eddy is a result of the radiative warming during spring and the downwelling due to the anticyclonic forcing at the surface. Our hypothesis suggests an air-sea feedback scenario that may explain the development and withdrawal of the summer monsoon over the SCS. The development phase of the warm-core eddy in this hypothesis is tested by using the Princeton Ocean model.

Stability of a front separating water masses with different stratifications

The linear instability of a narrow current flowing eastward between two semi-infinite watr masses of different mean stratification is studied with a multi-mode quasi-geostrophic model. The mean stratification in each domain has an exponential profile. The current is symmetric about the vertical plane separating the two water masses and its vertical profile is determined by the difference between the mean stratification of the two domains, as imposed by the thermal-wind equation. The current so defined is unstable over the entire parameter range we studied. As the ratio between the asymptotic depth of the thermocline in the two domains tends to unity, the current weakens and becomes eventually stable, while the length of the fastest-growing wave increases toward a finite upper limit. Increasing the depth or gradient of the thermocline in both domains by the same ratio also stabilizes the jet. Decreasing the width of the jet destabilizes it, but the length of the most unstable wave is almost independent of current width. The stability results are applied to the Gulf Stream after it has separated from Cape Hatteras, using a realistic stratification. The most unstable wave has a length of 200 km, which is comparable to twice the diameter of warm-core rings. The maximum amplitude of the unstable waves in the cross-stream direction is shifted towards the northern domain. We interpret this shift as inducing formation of warm-core eddies largely to the north of the current axis, while cold-core eddies capture water originally lying almost as much north of the axis as south of it. Anticyclonic eddies in the model would thus tend to exhibit a weaker temperature contrast with the surrounding water mass than the cyclonic ones, as observed in the Gulf-Stream system. The effect of adding a barotropic component to the basic jet depends on the sign and magnitude of this component. For a small westward component or large components of either sign the jet is less stable, while an intermediate range of east- or westward barotropic components reduces the instability. Both the Gulf Stream and Kuroshio are in this latter range, near their respective separation points, so their meandering is mainly induced by baroclinic instability.

A high resolution numerical study of Gulf of Mexico fronts and eddies

The Gulf of Mexico (GOM) circulation is simulated using the DieCAST ocean model, with a horizontal resolution of 1/12° and 20 vertical layers. The results compare well with observations of both large and small scale features, including Loop Current frontal occlusions associated with frontal eddies. The simulation is carried out without any data assimilation. The frontal eddies tend to be spaced at about 90° intervals around the Loop Current, leading to a Loop Current head shaped like a square with rounded corners. The pattern rotates as the eddies circle the Loop, and frontal eddies elongate as they squeeze through the Florida Strait. Major warm core eddies separate regularly from the Loop Current and propagate to the western GOM. Old warm core eddies in the western Gulf dissipate through bottom drag effects, which also generate cyclonic parasitic eddies. Newly arrived warm core eddies merge with old ones in the western GOM. Recently separated elongated Loop Current eddies can rotate and reattach temporarily to the Loop Current. The barotropic flow component develops eddies between the main separated warm core eddy and the Loop Current due to eastward dispersion, as the main eddy itself propagates westward into the Gulf.

Biogeographical structure of the microphytoplankton assemblages in the region of the Subtropical Convergence and across a warm-core eddy during austral winter

The distribution of microphytopla nkton in the region of the Subtropical Convergence (STC) and across a warm-core eddy shed from the Agulhas Return Current was investigated along two transects in late austral winter (June-July) 1993, during the South African Antarctic Marine Eco- system Study (S AAMES) III cruise. Sampling was undertaken for the analysis of nutrients, and for the enumeration and identification of microphytoplankton species. Along both transects, chlorophyll con- centrations were highest at stations at the southern boundary of the STC and at the periphery of the warm-core eddy. Of the variance associated with chlorophyll concentration, temperature accounted for 65% of the total. Along both transects, a decrease in species richness from north to south was observed. The spatial distribution of the numerically dominant diatom species was similar in both tran- sects. The microphytoplankton assemblage was dominated by the subtropical diatom species Chactoccros constnctus north of the STC, and by Pseudoeunotia doliolus within the eddy and south of the STC. Using cluster and ordination analyses, three significantly different groupings of stations were identified along the combined transects. These coincided with stations located north and south of the STC and with the warm-core eddy proper, confirming that the STC represents a strong biogeograph- ical boundary. The predominance of the warm-water species P.doliolus and Planktoniella sol in and around the warm-core eddy south of the STC suggests that eddies are important in the transfer of micTophytoplankton across this strong biogeographical boundary.

Mesozooplankton community structure and grazing impact in the region of the Subtropical Convergence south of Africa

Mesozooplankton distribution and community structure in the region of the Subtropical Convergence (STC) south of Africa were investigated during the SAAMES III cruise in austral winter (June–July) 1993. Both the STC and an associated warm-core eddy (WCE) exhibited enhancements in zooplankton abundance, compared to the Subantarctic waters. Particularly, elevated zooplankton densities were found in the centre of the STC and in the region north of it as well as at the edge of the WCE. Copepods (mainly Pleuromamma abdominalis and Metridia lucens), euphausiids (Euphausia spinifera, E.similis and E.recurva), pteropods (Limacina spp.) and chaetognaths (Eukrohnia hamata and Sagitta spp.) dominated numerically and accounted for >60‰ of the mesozooplankton community. Both cluster and ordination analyses indicated the occurrence of two major plankton communities located north and south of the STC. These results confirm that the STC is one of the strongest regional zoogeographical barriers, but south of Africa, WCEs may represent the most important source of disturbance in the meso-/macroscale zonation of the zooplankton assemblages. The daily grazing impact of the epipelagic community was estimated to remove ∼1–18‰ of phytoplankton standing stock and 20–165‰ of daily primary production. The highest rates of phytoplankton consumption were found in the centre of the STC and within the WCE, suggesting that the zooplankton assemblage transferred by the WCE may enhance substantially the mesoscale carbon flux in the Subantarctic zone south of Africa.

Distribution of seabirds in the northern Gulf of Mexico in relation to mesoscale features: initial observations

The presence of seabirds was related to oVshore surface eddies and the freshwater plume of the Mississippi River in the northern Gulf of Mexico during autumn, winter, spring, and summer 1992‐1993. Skuas (Stercorarius spp.) were the most common bird seen in autumn, whereas skuas and gulls (Larusspp.) were the most common in winter. Few birds were seen in the spring, and terns (Sterna spp.) were the most common birds seen in late summer. During summer, terns were associated with the freshwater plume of the Mississippi River. In the winter, herring (Larus argentatus) and laughing gulls (L. artricilla) were associated with areas with steep thermoclines, while pomarine skuas (Stercorarius pomarinus) were found in areas of low productivity. Herring and laughing gulls in the winter, all birds in the spring, and terns in the summer were more common outside eddies. Only pomarine skuas in the winter were seen more commonly inside warm-core eddies. ? 1997 US Government

Assimilation of Sea Surface Height Data into an Isopycnic Ocean Model

Abstract Two different methods for assimilating sea surface height data into an isopycnic primitive equation model were developed and tested for the idealized case of an analytical Gaussian warm core (anticyclonic) eddy, then implemented using remotely sensed data from the Brazil–Malvinas confluence region. The first method makes a geostrophic assumption about the flow to relate sea surface height field gradients to the model velocities. The second method nudges the model sea surface height itself toward the observed values using a linear vertical influence function in the upper layers. The relationship between the surface height observations and the layer interface displacements is derived from observations of eddies in different regions of the Atlantic Ocean. Both assimilation methods were successful in transferring the dynamical influence of the sea surface height measurements deep into the water column, but a combination of both gave the best results. The application of both methods reproduced the det...

Variability of the oceanic boundary layer characteristics in the northern Bay of Bengal during MONTBLEX-90

Variability of the ocean surface boundary layer characteristics on daily time-scale is studied utilizing the 3-hourly hydrographic data collected at a stationary location (20°N, 89°E) in the Bay of Bengal during August (18th–31st) and September (9th–19th), 1990 under MONTBLEX-90 field programme. The daily variations of temperature, salinity, σ0, mixed layer thickness, stability, heat content and rate of change of heat content in the upper 100 m are discussed in relation to prevailing weather (depressions) and hydrographic conditions (influx of fresh water, presence of eddies). The mixed layer thickness is examined through temperatureand σ0-based criteria considering also the surface salinity in the latter. TheT-based mixed layer thickness is always higher than that of σ0-based thickness. The rate of change of heat content is also computed up to the depth of 20°C and 14°C isotherms which takes into account the vertical motion and hence divergence. With the development of a low into a deep depression close to the study area, intense upwelling of subsurface cold waters is noticed from 100 m to the bottom of the surface mixed layer (20m) from 18th to 20th August. The upwelling is weakened by 21st August when the depression moved away from the study location. This variation of upwelling is supported by the variation of surface mixed layer thickness, static stability at 30 m depth, heat content in the upper 100 m and the heat content up to the depth of 20°C isotherm from 18th to 21st August. The rate of change of heat content in the upper 100 m and up to the depths of 20°C and 14°C isotherms leads to net heat storage during August and to net heat depletion during September. This together with the net surface heat gain lead to an import (197Wm−2) and export (233 Wm−2) of heat during August and September respectively through horizontal advective processes. These advective processes are attributed to the presence and movement of a warm core eddy through the study location.

Structure and grazing of the microzooplankton communities of the Subtropical Convergence and a warm-core eddy in the Atlantic sector of the Southern Ocean

Structure and grazing of the microzooplankton communities of the Subtropical Convergence and a warm-core eddy in the Atlantic sector of the Southern Ocean

Gyres off the Somali coast and western boundary currents in the Bay of Bengal during the south-west monsoon

Abstract Satellite remotely-sensed sea surface temperature (SST) data were obtained during the initial phase of the onset of the south-west monsoon in the north-western Indian Ocean for the years 1987and 1988. Large wedge-like areas of upwelled water during 1988 were observed at 5°Nand 10°N after the Somali current spinup, indicative of a two-gyre circulation in the Somali current system. Satellite infrared observations of the Bay of Bengal during the early February 1990revealed the existenceof two bands of warm water that resembled a western boundary current (WBC) along the east coast of India. A warm core eddy, with a major axis of nearly 120km, appeared at 89°E and 19°N at the end of the axis of the current. The existence of the two gyre system ofT the Somali coast and the presence or absence of the WBC in the Bay of Bengal in response to the onset of the summer monsoon has been discussed in detail.

Agulhas Eddies: A Synoptic View Using Geosat ERM Data

Abstract Warm core rings formed in the, Agulhas Retroflection transfer water from the Indian Ocean to the South Atlantic. In an attempt to measure the strength of this exchange, a combination of satellite altimeter and hydrographic data are used to examine Agulhas eddy paths and decay rates in the South Atlantic. Because the surface dynamic height of a warm core eddy is higher than surrounding waters, the rings are visible in satellite altimeter measurements. Over 20 Agulhas eddies have been tracked from maps of anomalous sea surface height (SSH) derived from the Geosat Exact Repeat Mission (ERM) dataset. The correlation (r2) of dynamic height referenced to 2000 dbar and anomaly SSH for one coincidentally sampled area is 97% within an Agulhas eddy, dropping to a fraction of that outside of it, indicating that the SSH anomaly signal is a reliable measure for strong features like Agulhas eddies. The sizes and distribution of the Agulhas eddies in the ERM record compare favorably with those in recent hydrogr...

A global survey of the impact on acoustic propagation of deep water warm core ocean eddies

Warm core eddies have been reported throughout the oceans of the world, usually associated with warm boundary currents such as the Gulf Stream. The physical characteristics and impact on acoustic propagation conditions were compared for various eddies at locations ranging from the East Greenland Current to the Tasman Sea. The eddies were found to have some common features such as a conical shape and increased sound speed. On the other hand, the acoustic impact was significantly determined by the sound-speed profile in the adjacent water to the eddy and this was found to vary greatly from site to site. In addition, seasonal secondary sound channels were formed across the eddies at some locations. In general, the greatest acoustic impact of a warm core eddy was for a shallow source and receiver configuration.

Postbreeding Dispersal and Drift-Net Mortality of Endangered Japanese Murrelets

ABSTRACr.-The incidental catch of seabirds in high-seas drift nets was recorded in 19901991 by scientific observers on commercial squid and large-mesh fishery vessels operating in the North Pacific Transitional Zone. Twenty-six Synthliboramphus murrelet deaths were recorded in the months of August through December. All but one were from the Korean squid fishery in a small area bounded by 38?-44?N and 142?-157?E. Five specimens of the dead birds were collected and later identified as Japanese Murrelets (S. wumizusume). As fishing effort was widely distributed over a large area east of Japan, these data suggest that postbreeding Japanese Murrelets migrate north to winter in a relatively small area southeast of Hokkaido, where persistent eddies form at the confluence of the Oyashio and Kuroshio currents. Fronts between cold Oyashio water and Kuroshio warm-core eddies promote the aggregation of zooplankton and pelagic fishes, which in turn may attract murrelets during the nonbreeding season. The estimated total mortality of Japanese Murrelets in high-seas drift-net fisheries represents a significant proportion of the total world population of this rare and endangered species. Received 12 October 1992, accepted 4 April 1993.