Warm-core Ring Research Articles

Photosynthesis-irradiance relationships and production associated with a warm-core ring shed from the Agulhas Retroflection south of Africa

Photosynthesis-irradiance relationships and production associated with a warm-core ring shed from the Agulhas Retroflection south of Africa

Separation of Warm-Core Rings in the Gulf of Mexico

Abstract The separation of anticyclonic rings is studied using a 12-level primitive equation numerical model of the western North Atlantic. The “Gulf Stream Formation Region” model is based on the Bryan-Cox-Semtner code, and uses ¼ degree horizontal resolution. The eastern boundary of the model, near the mid-Atlantic Ridge, is forced by a “pumps and baffles” region to have the appropriate temperature and salinity structure, vertical shear, and total transport. The model is closed by a solid northern wall at 36°N and is forced by steady winds. In the results presented here, large rings separate from the Loop Current in the Gulf of Mexico at periods near 30 weeks. The separation of a single typical ring is shown in detail. The most striking feature is that the separation is not a single spectacular event but a long, gradual process involving recirculation between the ring and the main flow for many weeks after the time at which one would, on the basis of standard observational evidence, normally believe the...

Generation of ringlets

A mechanism for the generation of ringlets (i.e., small cyclonic eddies (20–40 km) which have recently been observed on the periphery of warm-core rings) is proposed. The suggested process is examined analytically using a reduced gravity one-and-a-half-layer and two-and-a-half-layer model. The underlying hypothesis is that the ringlets are formed by the expulsion of fluid from the outer rim of the warm ring and that this expulsion is the result of an absorption of foreign water into the core of the ring. This nonlinear process is examined using the not-so-frequently used integrated angular momentum constraint as well as the familiar conservation of potential vorticity and mass. These constraints show that when the ring interacts with other bodies of water such as shelf water or the Gulf Stream, the shelf water is sucked into the ring in such a manner that the entire ring is capped. To conserve angular momentum, some other fluid must then be pushed out and it is argued that via instability of the ring's edge, this expulsion forms ringlets.

Hydrography and spatial variability in the size distribution of phytoplankton along the Kurile Islands in the western subarctic Pacific Ocean

ABSTRACTSpatial variability in the size distribution of the phytoplankton community along the Kurile Islands, in the western subarctic Pacific Ocean, was investigated in terms of chlorophyll a concentrations in August and September, 1990. Analysis of dynamic height relative to 1,500 db and temperature–salinity diagrams at 50 m depth revealed five major oceanographic regions in this study area. The characteristic hydrography was clearly related to the surface distribution of chlorophyll a concentrations. As a warm core ring originating from the Kuroshio water was cooled by the surrounding water and meteorological conditions, its physical, chemical, and biological characteristics changed. Dominant picoplankton were replaced by a 10‐2 μm size fraction in the warm core ring. In the Oyashio region, physical structures were less variable but concentrations of chlorophyll a were most variable and cells larger than 10 μn dominated. An anticlockwise eddy found in the Oyashio water showed the highest concentration of chlorophyll a. In the Okhotsk region, the chlorophyll standing stock was small with a low percentage of cells larger than 10 μm in spite of high nitrate concentrations. Along the Kurile Islands and Hokkaido Island, the surface mixed layer was not well defined; surface nitrate concentrations were relatively high, and the chlorophyll standing crop was also high with low percentages of picoplankton. The spatial heterogeneity in the dominance of cells larger than 10 μm in phytoplankton assemblages seems to be clearly associated with the effect of nitrate availability determined by water structure.

Note on the isolated vortices over the topographic-β

The behavior of the isolated vortices over the topographic-βis considered with the quasi-geostrophic two-layer model, in the limit of very shallow upper layer, in the absence of planetary-β. The results are compared with the observed Kurshio warm-core rings. When a ring drifts southward (northward) relative to the meridional mean current, it radiates (does not radiate) Rossby waves in the lower layer. Even when the lower layer is radiating, the decaying is very slow as long as the ring drifts closely to a background current.

Evolution of a Kuroshio warm-core ring—variability of the hydrographic structure

The life history and structural variations of warm-core ring 86B, which detached from the Kuroshio Extension and moved northward interacting with neighboring currents over a life span of more than 3 years, were described by combining NOAA-infrared images and hydrographic data from 1986 to 1988. In the present study, interactions with neighboring currents and the atmosphere were found to strongly influence the evolution of 86B and to play a significant role in the northward transport of warm water. After the ring had detached from the main axis of Kuroshio meander, 86B was intensified by coalescence with a warm tongue extending from the Kuroshio Extension. Just before the detachment of 86B, another warm-core ring, 86A, was pushed northward as the Kuroshio meander developed. During the formation of 86B, warm water was carried northward as warm streamers through the warm tongue, 86B and 86A. Ring 86B moved intermittently northward. These northward movements were associated with the interaction with neighboring currents; they began as intrusions of warm streamers from the Kuroshio frontal disturbances or other warm-core rings and by coincidental deformation to an elliptical shape with a meridional major axis. Then 86B moved northeastward, rotating clockwise. The northward movement stopped when the supply of warm water ended, 86B then moved westward, returning to a circular shape. During the winter, when distinct warm-streamer intrusions were not observed, westward movements were dominant. Drastic structural variations after formation were brought about by wintertime convection and ring-ring interaction. During winter, not only the core temperature but also the salinity decreased and the diameter was reduced, suggesting an inward flow and mixing with Oyashio low-salinity water through wintertime convection. It was also observed that the negative relative vorticity increased, associated with the reduction of the horizontal scale and the deepening of the homogeneous layer. The ring-ring interaction occurred between 86B and the larger, westward-moving warm-core ring (OR) from offshore. A large amount of water from ring OR was transported northward and encircled 86B, conserving its vertical structure. As a result, 86B expanded and the volume of core water more than doubled, while OR decreased in size. The two rings had similar homogeneous waters and densities, and the interaction occurred between a stronger and smaller vortex and a weaker and larger one. Other developments of warm streamers were frequently observed through ring-ring or ring-Kuroshio interactions; these were related to the deserved near-surface structural changes and ring movements.

Differences in particulate nitrogen concentration and isotopic composition for samples collected by bottles and large-volume pumps in Gulf Stream warm-core rings and the Sargasso Sea

Nitrogen content and isotopic composition for particles collected by standard water-sampling bottles and in situ large-volume pumps are compared. Data are presented for stations in warm-core ring 82-B in April and June 1982, newly formed warm-core ring 82-H in September/ October 1982, and the OFP site off Bermuda. For warm-core rings, relatively large differences in γ 15N and PN concentration were observed between the two techniques. For the OFP site, except for PN concentration in the upper 200 m, good agreement was observed. Consideration of the effects of different retention sizes for the filter types used by the two techniques and whether or not large particles were included in the samples brought the OFP results into even closer agreement. However, only a fraction of the discrepancies in the warm-core ring results could be accounted for. Possible explanations include artifacts associated with differences in the details of the sample collection and handling techniques or differences in the size distributions of submicron particles and their γ 15N values in rings. The much better agreement in γ 15N between pumps at the OFP site, bottles at the OFP site, and pumps in ring 82-H suggests that minimal sample handling is desired. In addition, choice of filter material appears to be critical since significant quantities of PN can pass through commonly used varieties. These difficulties underscore the need to evaluate carefully the limitations of any particle-sampling technique regarding the specific measurements to be made.

Observations of offshore shelf-water transport induced by a warm-core ring

Gulf Stream warm-core rings often are observed to entrain water from the continental shelf and transport filaments of shelf water out over the continental slope. In June 1982 the R. V. Endeavor made a transect through one such shelf-water “streamer” that was seen from satellite thermal imagery to the east of warm-core ring 82B. Acoustic doppler current profiles together with repeated CTD stations were made through the streamer. The CTD system was also equipped with a dissolved oxygen sensor and transmissometer. We were able to make an unprecedented section of velocity and water mass structure of the shelf-water filament. The offshore velocity structure observed was complicated with velocities ranging from 10 to 50 cm s −1, temperatures, in a sub-surface temperature minimum, of less than 8°C, surface salinities less than 33‰, with high levels of suspended particulate matter and manganese, and with shelf-water properties as deep as 100 m in the center of the streamer. Transports calculated in different temperature and salinity classes showed a total volume transport of waters in the filament of 0.8–0.9 × 10 6 m 3 s −1 for water with a salinity <35‰. Thermal imagery shows that the streamer was not “wrapped around” the ring but was advected by the ring and deposited at the inshore edge of the Gulf Stream. Using adjacent stations not in the streamer, but in the Slope Water, as a reference for water that presumably replaced the above amount of shelf water, estimates have been made of the net exchange of heat, salt, particulates, dissolved oxygen and Mn due to the offshore transport of shelf water by 82B. These estimates indicate that warm-core rings are capable of inducing significant exchange across the Shelf/Slope Water front off the northeast coast of the U.S.A.

The spatial distribution of silicoflagellates in the region of the Gulf Stream warm-core ring 82B: application to water mass tracer studies

To delineate potential water mass affinities, we investigated silicoflagellates from the region of Gulf Stream warm-core ring (WCR) 82B in the northwestern Atlantic. Silicoflagellates from 202 samples from N-S and an E-W transects across WCR 82B during late April were analysed. Shelf to Sargasso Sea transects, one completed in early May and the other in June 1982 were also examined. Eight to 11 vertical profiles to 200 m comprised each of the transects. Six taxa of silicoflagellates were found in the samples studied and a total of more than 8000 specimens were encountered. Three major taxa dominated standing stocks: Distephanus speculum, Dictyocha messanensis (intermediate-sized form) and D. mandrai. D. speculum, considered a cold-water taxon in the literature, showed a higher standing stock in the cooler high-velocity region (HVR) of the warm-core ring, continental shelf (SH) and slope (SL) waters. Fewer were present in the wanner ring center (RC), Gulf Stream (GS) and Sargasso Sea (SS). D. mandrai showed a similar distribution to that of D. speculum, but its preference for slightly warmer waters (>~10°C) was noted. In contrast, Dictyocha messanensis (intermediate-sized) and Distephanus pulchra, known to be warm-water taxa, were relatively abundant in the warm ring center. In contrast to standing stock data, ratios between cold- and warm-water taxa correlate well with temperature and salinity in the warm-core ring. Since these ratios are not effected by convective loss, they are excellent water mass tracers in this system. Distribution of the silicoflagellate taxa suggests that WCR82B April had a higher affinity with the Gulf Stream than the Sargasso Sea. Scores derived from factor analysis indicate that silicoflagellate species distributions are highly correlative with water masses. This was evident from correlations with temperature, salinity and with distance from ring center. Nutrients were generally not correlated with species data. This may be due to deep vertical mixing in the ring center of 82B in April that resulted in loss of plankton cells below the euphotic zone which was responsible for low silicoflagellate standing stock. Silicoflagellate productivity, measured as % double skeletons of D. speculum, was approximately the same in all water masses. This result is consistent with the hypothesized plankton population loss due to the convection.

Warm-core rings, streamers and their role on the fishing ground formation around Japan

Eddy-shedding areas of the Kuroshio, the Oyashio and the Tsushima-Tsugaru Warm Currents are localized near the ridges of the stationary anticyclonic loop path of these ocean currents. Warm or cold streamers are entrained in the periphery of the warm-core ring, which form excellent fishing grounds for pelagic fish, such as skipjack, mackerel, flying squid and saury. Appearance and movement of the fishing grounds associated with the warm-core rings and streamers are discussed, using satellite thermal images and fish catch data.

Primary productivity and photosynthetic products around the Kuroshio warm-core ring

13C uptake experiments were carried out in the Kuroshio warm-core ring and adjacent waters during the cruise of R.V. Hakuho-Maru. The depth-integrated primary productivity at the warm-core center ranged from 840 to 1200 mgC m −2 day −1. These rates were higher than those obtained in the surrounding water and the warm streamer existing in the southern part of the warm-core ring, and lower than that in the cold Oyashio water. Production rates of 13 amino acids and eight monosaccharides at six stations were determined after hydrolysis of particulate matter by the 13C−GC−MS method. Protein and free amino acid (sum of 13 amino acids) carbon production rate accounted for 20–39% of organic carbon production at 10 m, whereas carbohydrate (sum of eight monosaccharides) was responsible for 11–21%. Composition of photosynthetic products and the spectrum of specific production rate (SPR) of organic compounds indicate that phytoplankton population in and around the warm-core ring was in a generally healthy state.

A description of the time-dependent hydrographic structure of the warm streamer around the Kuroshio warm-core ring 86B

The Kuroshio warm-core ring 86B entrained a large amount of warm water in late August 1987, which accompanied a partial coalescence with the offshore neighboring warm-core ring. Observations from mid-August to mid-September caught the hydrographic features before, during and just after the event. The formation process and inward spiraling of the streamer, as well as their effects on the temperature and salinity in the warm-core ring, are described based on the hydrographic data. Satellite thermal images of the warm streamers entrained by WCR 86B indicate that streamers occur about every 20 days or every 40–50 days, with a mean width of about 30 km; they complete one revolution in a week, after which they collapse.

Vertical distribution and species composition of midwater fishes in warm-core Gulf Stream meander/ring 82-H

The integrated abundance and vertical distribution of midwater fishes in the upper 1000 m of warm-core Gulf Stream ring 82-H show that the fauna of the ring was very similar to that of the northern Sargasso Sea. The data, indicate that warm-core rings have a large impact on the fauna of the Slope Water even though only a small fraction of the volume of rings is mixed into the Slope Water and the fishes (mostly non-migratory) are located in the “unreactive” parts of the ring.

Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring

Chcmiluminescent nitrate analysis was used in conjuction with 15N-labeled B = − B 0 e − 2 π a 0 x k to assess the rates of B = − B 0 e − 2 π a 0 x k uptake by phytoplankton in warm-core ring 82B. The relatively high precision of this method compared to conventional B = − B 0 e − 2 π a 0 x k analyses permits reliable estimates of B = − B 0 e − 2 π a 0 x k uptake in oligotrophic waters. Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of B = − B 0 e − 2 π a 0 x k turnover times ranging from about 4 h near the surface to 150 h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of 93 nmol kg −1 for B = − B 0 e − 2 π a 0 x k uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a B = − B 0 e − 2 π a 0 x k threshold concentration of about 16 nmol kg −1 was required for the initiation of uptake. These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of B = − B 0 e − 2 π a 0 x k . Maximum near-surface and deep-euphotic-zone eddy diffusivity values ( K z)were 3 × 10 −3 and 5 × 10 −4 m 2 s −1, respectively, prior toamajor storm. Following the storm K z values were substantially greater.

Detailed structure and water-type distribution of the warm-core ring 86B, September 1987

The cross-sectional structure and the water type-distribution across warm-core ring WCR 86B to the east of Honshu, Japan, in September 1987 indicate that the ring consists of two parts: the central part which contains original homogeneous water, and the outer part which contains water added from the ring-ring interaction about 1 month earlier. In the outer part, the various water types are distributed in a very complicated manner. The abnormally saline water found near the 170 cl/t surface in the outer part originated from the offshore ring which had interacted with WCR 86B. Considerable structure is also found in the salinity minimum layer beneath WCR 86B. Such a detailed water-mass analysis is useful to determine past events and to understand the evolution of oceanic structures.

Springtime distributions and variability of biogenic particulate matter in Gulf Stream warm-core ring 82B and surrounding N.W. Atlantic waters

Distributions of particulate matter, chlorophyll and other physical/bio-optical properties were investigated during April–May 1982 in warm-core ring (WCR) 82B and surrounding Shelf, Slope, Gulf Stream and Sargasso Sea waters. Biogenic particles present in the core waters of WCR 82B were strongly influenced by the day-to-day variation in air-sea heat flux. Particles grew/accumulated in surface waters during periods of transient stratification/quiescence and were removed from the euphotic zone as deep as 400 m during periods of strong heat loss and deep convective mixing. This process enriched ring core waters below the euphotic zone compared to all other waters sampled. Outside of ring-core waters, stratification was already permanently established and surface concentrations of particles/chlorophyll increased radially outward to maximum values in entrainment zone waters at the ring periphery. Slightly lower concentrations were observed in the Slope Water and Shelf Water. Comparisons of chlorophyll and beam attenuation coefficient data suggested that the beam attenuation coefficient reflects bulk distributions of particulate mass and not pigment biomass, consistent with earlier studies. Profiles of SPM and chlorophyll at a Gulf Stream station showed subduction of high (650 μg kg −1) concentrations of biogenic particles produced at the surface near the Gulf Stream/Slope Water front. Such subduction events may have significant impact on the oxygen budget of the N. Atlantic thermocline.

15N and 13C natural abundance in suspended particulate organic matter from a Kuroshio warm-core ring

Vertical profiles of the natural abundances of 15N and 13C in suspended particulate organic materials (POM) were determined together with PON and POC concentrations to the depth of ca 4000 m at five stations in and out of a warm-core ring (WCR). The 15N and 13C profiles showed generally, with one exception for a 15N profile at the northern edge of the ring, the same trend: an increasing abundance of 15N and 13C in the subsurface layer, followed by a decrease with depth. At the northern station, the 15N abundance of PON in the euphotic zone was unexpectedly high (∼20.4‰). PON with high 15N abundance was also observed at 30, 75 and 150 m at the southern periphery of the ring. This was not seen for the 13C of POC. T-S analysis indicated that POM with high 15N content was transported from the northern periphery by the ‘cold streamer’ of the WCR. In the depth zone of 100–500 m, where thermostad appeared in the ring, the 15N of PON increased with depth while the 13C of POC remained constant in the ring; conversely, the 13C of POC decreased with depth while 15N of PON remained constant out of the ring. Temporal variation was noted for the 15N abundance of PON in the thermostad of the WCR, but little variation was observed for the 13C of POC. The spatial and temporal differences between the 15N and the 13C profiles were explained assuming an intensified vertical mixing of the water column in the thermostad of the WCR, and an order of magnitude difference between the abundance of refractory fractions in POC and PON enriched with both 15N and 13C. It is suggested that the POM 15N and 13C isotopie ratios provide complementary information to study particle dynamics in the ocean.

Natural abundance of 15N in particulate nitrogen and zooplankton in the Gulf Stream region and warm-core ring 86A

The natural abundance of 15N (δ 15N) was measured in samples of suspended particles (PN) and macrozooplankton collected in the Slope Water and warm-core ring 86A in late May 1986. The vertical distribution of 15N in PN (δ 15PN) was characterized by a maximum below the mixed layer, and frequently by a minimum above the thermocline. The range of δ 15N values in PN, as well as the mean δ 15N for PN in the upper 100 m of the water column were comparable to literature values of these quantities. The δ 15N and concentration of PN were inversely correlated, a pattern that could be generated by isotopie fractionation during the remineralization of sinking PN. The fractionation factor estimated for this process (1.003–1.006) was similar to fractionation factors for the bacterially mediated decomposition of phytoplankton in laboratory studies. The δ 15N of the zooplankton sampled showed a general increase with trophic level: herbivorous species ( Salpa aspera and copepods) were characterized by δ 15NN values of ca 4‰, omnivorous species spanned a range of δ 15N values between 5.5. and 6.5‰, and carnivorous species had a δ 15N of 8–9‰. Despite this trophic level effect on δ 15N among the zooplankton, the range of δ 15N values measured for the animals was not distinctly different from the range of values of δ 15PN. The absence of a difference in δ 15N between the PN and the zooplankton in this ecosystem may be a consequence of the normal variation in δ 15N through the seasonal production cycle and the different turnover times for nitrogen in PN and the higher trophic levels of the food web. Representatives of five zooplankton species were maintained in filtered sea water for up to 74 h. Three of the species ( Euphausia krohnii, Paralhemisto gaudichaudii and Vibilia sp.) showed no significant changes in the δ 15N of body nitrogen with time. The data for the other two species ( Meganyctiphanes norvegica and Euphausia mulica) suggest that the δ 15N of body nitrogen increased with time of starvation. The fecal pellets released by zooplankton used in the starvation experiment were typically similar to or higher in δ 15N than the animals proper. In contrast, the (δ 15N of the nitrogen in molted exoskeletons was much lower than the whole-body δ 15N of the animals which produced them. The possible implications of these data are discussed in the context of the distribution of 15N in plankton in the upper ocean.

Physical structure and temporal evolution of Gulf Stream warm-core ring 82B

The hydrographic properties of warm-core ring 82B are shown from three cruises; April, June and August 1982. The velocity field for each cruise is formed from the gradient-current relationship, the observed hydrography, and the near surface now as measured by a ship-mounted Acoustic Doppler Current Profiler. The data for each cruise were sorted into radial bins 15-km apart, and into vertical bins based on potential density. The changes in salinity on isopycnals from April to June 1982 were modelled as being due to radial advection, and both diapycnal and isopycnal mixing processes. The diffusivities in the pycnocline were found from a least-squares fit to the data. Most of the observed changes in salinity were explained with a constant vertical diffusivity (0.53 × 10 −4 m 2 s −1), while the required lateral diffusivity was only 26.5 m 2 s −1 at ring center, decaying to zero at a radius of 40 km.

Origin of waters in warm-core ring 82-B

Warm-core ring 82-B (WCR 82-B) formed from a Gulf Stream meander in the winter of 1982. The T/S properties of the thermostad of the ring in April were different than the surrounding Slope Water and the possible source water, the Gulf Stream or the Sargasso Sea. A convective mixing model was applied to the formation of the thermostad of WCR 82-B. The T/S relationships derived using Gulf Stream source water were more consistent with the field observations in March and April than those obtained using the Sargasso Sea as the source water. Chemical data provided further support for distinguishing Gulf Stream rather than Sargasso Sea as the source. Contrary to earlier notions the source water for WCR 82-B was the Gulf Stream. The thermostad was not modified 18-degree water from the Sargasso Sea.