Warm-core Ring 82B Research Articles

Euphausiid distribution, abundance and succession in North Atlantic warm-core ring 82B

Zooplankton collections were made with a multiple opening/closing net and environmental sensing system (MOCNESS) both day and night in warm-core ring 82B in the North Atlantic Ocean and at night in the Slope Water between March and August 1982. Species succession of euphausiids in 82B was presented during the lifespan of this warm-core ring, aiming at providing basic information on possible response of North Atlantic marine ecosystem to global warming. Species succession of euphausiids (32 species) in this long-lived warm-core ring was investigated. Major physical changes of 82B occurred in March to April by convective mixing and thermostad cooling, in April/May by stratification of the surface waters and in August by the interaction with Gulf Stream. Substantial changes in species composition were observed that corresponded to these physical changes. Four different patterns were found in temporal change in abundance of warm-water species. There were species that decreased in number from March to August, species that decreased from March to June, but increased in August, species that increased from Match to August and species that showed no systematic trend. These differences may be attributable to a species tolerance to the thermostad temperature decrease and their vertical distribution. There was also a large change from April to June with ascendance of the transition species, Thysanoessa gregaria. Cold-water species had variable patterns of abundance in 82B and occurred more abundantly in the Slope Water than in the ring. The monthly percentage decrease in the abundance of warm-water species in 82B was high compared with that of cold-water species in cold-core rings as a result of the more rapid changes in the physical structure and the shorter lifetimes of warm-core rings in the western North Atlantic.

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.

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.

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.

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.

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.

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.

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.

Meridional circulation and mixing associated with the decay of warm-core ring 82B

Observations of a warm-core ring are used to formulate a model for diagnosing the physics of a ring change over a 2-month period. About 30 hydrographic casts and acoustic doppler current measurements are used to generate estimates of an equivalent radially symmetric ring with radial contrasts of stratification, temperature, salinity, azimuthal velocity, angular momentum and potential vorticity. A series of related models is inverted for the ring circulation and mixing coefficients. The circulation is insensitive to the model details, is well resolved, and is a radial outflow and upwelling. Eddy coefficients are only partially resolved; determining the mixing with any degree of confidence appears to require a much more elaborate data set than the one available.

Moored observations of current and temperature on the shelf and upper slope near ring 82B

An array of current meters was deployed along the continental shelf and upper slope from April 15 to July 1, 1982, as part of the Warm Core Rings Experiment. The array consisted of four mooring sites along the 325‐m isobath making an “X” with another line of moorings extending from the 60‐m to the 900‐m isobath. The purpose of the array was to examine the current and temperature variability induced at the shelf break and upper slope by the formation and passing of ring 82B. The velocity records consisted of 2‐ to 10‐day fluctuations embedded on a long‐period signal with a period ranging from 15 to 26 days, increasing offshore. The temperature records contained three time scales: The 2‐ to 10‐day scale, longer (about 1–2 months) scales due to movements of the shelf‐slope front induced by the ring, and an increasing trend due to seasonal warming. The 2‐ to 10‐day fluctuations of velocity and temperature were not statistically coherent with each other or with the weak surface wind stress but appear to be visually correlated with small (sub‐ring) scale undulations along the shelf‐slope front as seen in the advanced very high resolution radiometer (AVHRR) sea surface temperature imagery on an event by event basis. The observed periods and across‐shore scales of the long‐period motions were used with barotropic topographic Rossby wave (TRW) theory to estimate alongshore wavelengths of the order of 1000 km for these motions, westward and slightly offshore phase velocities, and onshore group velocity. Friction and wave refraction will not allow barotropic TRWs of the observed period to penetrate the upper slope unless the source is located nearby on the continental rise, which strongly suggests Gulf Stream warm core ring 82B as the energy source for the waves. The observed wave energy was used with the radiation model of Louis and Smith (1982) to estimate the rate at which energy was being lost from the source of the waves. The decay rate calculated from the moorings and the model was 0.24×10−13 J d−1, averaged over 6–14 weeks after ring formation, compared with 0.81×10−13 J d−1 for the decay rate of ring 82B (Olson et al., 1985) based on observed changes in the ring's available potential energy during April‐June 1982.

Seasonal changes in the silicon cycle within a Gulf Stream warm-core ring

The silicon cycle within Gulf Stream warm-core ring 82-B was examined before and after spring stratification using silicon isotope tracers. In April, prior to stratification, the ring was isothermal and isohaline to 400 m. Ambient silicic acid concentrations were ca 3 μmol l −1 and biogenic silica concentrations ranged from 80 to 100 nmol l −1 throughout this depth range. Specific production rates tended to be greatest at the surface and were vertically uniform below 20 m. Net silica production occurred from the surface to 300 m on 25 April, about three times deeper than has been observed before in the sea. The zone of net production shoaled to <150 m by 4 May. These depths were 2 to >4 times the 1% light depth, indicating that the diatom assemblage was producing substantial amounts of silica at depths where little or no photosynthesis was taking place. By June, a seasonal pycnocline had developed and silicic acid concentrations were depleted to ⩽0.2 μmol l −1 in the upper 20–30 m. Vertically integrated particulate silica concentrations in the upper 80 m had increased four-fold compared to those for the upper 110 m in April/May. Siliceous biomass was concentrated in the upper 40 m, with a subsurface maximum of ca 1000 nmol Si l −1 within the seasonal pycnocline. The upper 80 m remained a zone of net silica production, but net dissolution was occurring at all depths >100 m. The average specific production rate of biogenic silica (0.0098 h −1) was five times less than observed during April/May (0.041 h −1), suggesting slower diatom division rates in the stratified water column. The decrease in specific production rates following stratification was nearly offset by the increased particulate silica levels such that the vertically integrated production rate of biogenic silica for the upper 80 m was essentially unchanged from that observed for the upper 110 m during April/May. The percentage of silica production that could be supported by in situ dissolution increased from ⩽30% for the upper 110 m during April/May to 68% for the upper 80 m in June. This transition from a “new production” cycle to a “regenerated production” cycle is the first observation of a seasonal trend in the silicon cycle in the surface ocean. Furthermore, while net silica production occurred throughout the upper 110–300 m in April, net silica production in June was confined to the relatively nutrient-rich waters below seasonal pycnocline, wih a near balance between silica production and dissolution in the oligotrophic watersabove (average production:dissolution = 1.03). These changes in the production:dissolution ratio and the increased reliance on new nutrients with depth imply that the cycling of particulate silica in June fit a two-layer model of stratified oceanic systems where nearly complete and rapid recycling of materials occurs within a nutrient-poor surface layer with a zone of net production below.

Enhanced near-surface nutrient availability and new production resulting from the frictional decay of a Gulf Stream warm-core ring

Warm-core eddies in at least three different current systems have been observed to evolve over a few weeks' time from local minima to local maxima in phytoplankton biomass and primary productivity. The causes of this biological enrichment have been uncertain. We report here a possible explanation of this phenomenon, based on events observed in Gulf Stream warmcore ring 82-B. During the time when a phytoplankton biomass maximum was present in this ring, nutrient concentration gradients beneath a nutrient-depleted surface layer were consistently both greater in magnitude and closer to the sea surface in the interior of the ring than in the surrounding Slope Water. In addition, surface-layer densities were greater inside than outside the ring, resulting in much smaller density differences between the surface and the nitracline (shallowest depth at which nitrate was detectable by conventional methods). The cumulative result of these two factors was that, compared to the surrounding Slope Water, less than one fifth as much wind energy would have been needed near the center of the ring to mix the water column from the nitracline to the surface. The altered nutrient and density profiles within the ring were consistent with physical and biological models of eddy dynamics, which predict that the frictional decay of the rotational flow field should result in shoaling of the main thermocline, and thus upwelling, in the interior of a warm-core eddy. There is reasonably good quantitative agreement between calculated rates of vertical nutrient transport associated with upwelling in the interior of the ring and the measured net uptake rates of nitrate and silicic acid by the phytoplankton.

Net phytoplankton in a Gulf Stream warm-core ring: species composition, relative abundance, and the chlorophyll maximum layer

Gulf Stream warm-core ring (WCR) 82B and surrounding waers (Sargasso Sea, Slope Water) were sampled during three cruises from April to August 1982, to assess changes in the species composition and relative abundance of the large, rare netphytoplankton. Analysis of net samples indicated that the major changes in dinoflagellate species composition in the ring occurred between the April and June cruises, while the major changes in diatom species composition occurred between the June and August cruises. Centric diatom species composition and relative abundance were stable over cruise-length time periods (3 weeks) in April and June but varied greatly in the 1.5 month interval between cruises. Between April and June, the dominant centric diatom species in the ring changed dramatically, although the overall species assemblages (presence or absence of diatom and dinoflagellate taxa) remained similar. The opposite occurred between the June and August cruises: the overall species assemblages showed large changes while the abundant centric diatoms did not. Based on the species composition and relative abundance of centric diatoms,the chlorophyll maximum layers exhibited a greater similarity to surface layers at the same station than to chlorophyll maximum layers at other locations. This is consistent with the hypothesis that the chlorophyll maximum forms as a result of cells sinking from the surface, rather than each layer being composed of distinct, separate species assemblages. The chlorophyll maximum in WCR 82B represented a phytoplankton numerical abundance maximum in 70% of the profiles examined.

Spatial distributions and variability of suspended particulate matter in warm-core ring 82B

Spatial distributions and variability of suspended particulate matter (SPM) were determined optically during CTD/transmissometer surveys of warm-core ring (WCR) 82B and surrounding waters in June 1982. SPM distributions within 30 km of ring center from the surface to the depth of the 27.6 kg m −3 potential density surface at 800 m were different from surrounding waters. Waters within this core region of the ring had higher SPM concentrations between potential densities of 26.0 and 26.75 kg m −3 and had lower concentrations between 26.8 and 27.5 kg m −3 when compared to sorrounding waters. This result suggests that processes within WCR 82B (as opposed to lateral exchange processes with surrounding waters) were most important in governing particle distributions near ring center. Such isolation was not true for entrainment zone waters 70–80 km from ring center. In this region, waters shallower than 100 m showed a strong influence of waters from the continental shelf. In the entrainment zone, SPM concentrations between the 26.8 and 27.5 kg m −3 potential density surfaces (between 100 and 380 m) were higher compared with waters of similar density both within the ring and in the surrounding Slope Water. In addition, higher SPM concentrations were found in the region of offshore flow behind the ring than in the onshore flow ahead of the ring. This produces a net offshore transport of SPM by WCR 82B for entrainment zone waters of this potential density interval of between 2900 and 15,800 g SPM s −1. These results suggest that WCR's may be very important in the dispersal of resuspended continental slope sediments 100–200 km away from the continental margin.

Utilization of nitrogen and phosphorus by primary producers in warm-core ring 82-B following deep convective mixing

Rates of utilization for NO 3 −, NH 4 +, urea, and PO 4 − by primary producers and the abundances of particulate C, N, and P were determined for the euphotic zone of warm-core ring 82-B in April and early May 1982. The ring had formed in late February, and our study took place between a period of deep convection and the formation of a stable seasonal thermocline. Deep convection enriched the surface waters with NO 3 − and PO 4 −, which remained in sufficient quantities to preclude nutrient limitation of primary production during the course of the study. Concentrations of HN 4 + and urea were always less than or equal to the limit of detection (0.03 μmol kg −1). The fraction of total nitrogen taken up as NO 3 − (ƒ ratio) was high (0.62–0.66) in the core region of the ring. During the course of this study, the concentration of NO 3 − decreased near the surface, allowing a NO 3 − gradient to develop, while the concentration of particulate N (PN) and the rate of N uptake increased. However, these processes were not in balance. Only about 16% of the NO 3 − based production remained in the euphotic zone. The PN:Chl a ratio was higher at the bottom of the euphotic zone than at the top. This indicates that detrital PN was being formed faster than it was exported by either sinking or mixing processes. The overall reduction in euphotic zone NO 3 − concentrations was about one third the measured consumption rate. Midway through the study a storm was apparently responsible for redistributing NO 3 − in the euphotic zone, and perhaps mixing NO 3 − up from greater depths. After that event, the mean NO 3 − uptake rate was about twice the rate of NO 3 − disappearance. Upward flux of NO 3 − associated with the relaxation of depressed density surfaces as the ring lost kinetic energy could have supplied NO 3 − at a rate equivalent to 40–110% the mean NO 3 − uptake rate for the period. The highest rates of NO 3 − uptake were observed at the end of the study, and were in excess of the calculated rates for upward flux. This may explain the increasing magnitude of NO 3 − gradients observed durinng the final few days of this study.

Nutrient distributions in warm-core ring 82-B April–August

The chemical, physical and biological distributions of a single warm-core ring, WCR 82-B, were examined in a multidisciplinary study of the ring from March to August 1982. WCR 82-B formed in late February 1982; in April the core water of the ring was isothermal and isohaline, with a homogeneous distribution of nutrients and oxygen from the surface to a depth of 350 m. Thermal stratification occured in early May; nutrients were depleted from the surface layer after stratification. In the core of the ring (radius < 20 km) nitrate lost from the surface layer after stratification was regenerated in the thermostad (a layer of isothermal and isohaline water extending from the base of the seasonal thermocline to the top of the main thermocline); silicate lost from the surface water was not regenerated in the thermostad, but was regenerated in the main thermocline. Between 2 July and 5 August the ring had a series of interactions with the Gulf Stream; the ring diameter was reduced from 110 to 70 km and the thickness of the 15–16°C thermostad was reduced from 350 to 200 m. Nutrient and oxygen distributions in the thermostad were not significantly changed by the Gulf Stream interactions.

Velocity Profiles in a Warm-Core Ring

A transect of velocity profiles across Gulf Stream warm-core ring 82-B shows that the thermostad contained virtually no vertical shear. Enhanced downward-propagating near-inertial wave shear was present at the base of the core, consistent with these waves encountering critical layers as they try to leave the region of negative vorticity. Wave amplification and a shrinking vertical wavelength at the critical layer should lead to instability and shear production of turbulence. High dissipation rates have been observed at the base of ring 81-D's core.

Upper-ocean velocity structure of Gulf Stream warm-core ring 82B : Joyce, T.M. and M.A. Kennelly, 1985. J. geophys. Res., 90(C5):8839–8844

Upper-ocean velocity structure of Gulf Stream warm-core ring 82B : Joyce, T.M. and M.A. Kennelly, 1985. J. geophys. Res., 90(C5):8839–8844

Spatial and temporal patterns in pigment biomass in Gulf Stream warm-core ring 82B and its environs : Smith, R.C. and K.S. Baker, 1985. J. geophys. Res., 90(C5):8859–8870

Spatial and temporal patterns in pigment biomass in Gulf Stream warm-core ring 82B and its environs : Smith, R.C. and K.S. Baker, 1985. J. geophys. Res., 90(C5):8859–8870

Chronology of warm-core ring 82B : Evans, R.H., K.S. Baker, O.B. Brown and R.C. Smith, 1985. J. geophys. Res., 90(C5):8803–8811

Chronology of warm-core ring 82B : Evans, R.H., K.S. Baker, O.B. Brown and R.C. Smith, 1985. J. geophys. Res., 90(C5):8803–8811