Gulf Stream Warm-core Ring Research Articles

Distribution of microbial biomass and secondary production in a warm-core Gulf Stream ring

Microbial biomass, bacterial abundance and secondary production were examined in relation to the changing physiochemical conditions within warm-core Gulf Stream ring 81-F. Highest densities of bacterioplankton (6.14 × 10 8 cells 1 −1) occured at the ring margin in continental shelf and slope water entrained by the ring. Likewise, both total bacterial secondary production (measured as incorporation of [ 3H]thymidine into DNA) and per-cell bacterial secondary production were highest at the ring margin, averaging 291.1 pmoles 1 −1 d −1 and 4.95 × 10 −7 pmoles cell −1 d −1, respectively. In contrast, high concentrations of ATP (mean, 1.45 μg l −1) and chlorophyll α (mean, 2.29 μg l −1) were found both at the periphery and near the center of the ring. A similar distribution pattern was found for the incorporation of [ 3H]adenine into microbial RNA. The elevated values for microbial biomass and activity at the ring margin may reflect the response of microbial populations to new conditions resulting from the juxtaposition of water masses comprising and surrounding the ring. Our results indicate that the distribution of microbial populations and the rates of microbial processes occuring within warm-core ring 81-F were influenced by the age of the ring and the extent of its interaction with continental shelf water, surrounding slope water and Gulf Stream meanders.

Temporal and spatial changes in epipelagic microzooplankton and mesozooplankton biomass in warm-core Gulf Stream ring 82-B

The vertical distributio of > 64 μ m zooplankton biomass in warm-core Gulf Stream ring (WRC) 82-B was studied when the ring was approximately 2,4 and 6 months old. Variability of zooplankton biomass estimates in 82-B was not significantly different on diel, 5 to 12 day, and seasonal (April, June, August) time scales as the ring evolved from its Sargasso Sea origin. Zooplankton biomass in 82-B was similar to concentrations found in the Slope Water. The average ratio of integrated (0 to 200 m) zooplankton ( > 64 μ m ) biomass in the surrounding Slope Water to that in 82-B was 1.6 at 2 months old and 0.7 at 4 months. Small zooplankton (64 to 333 μm), dominated by copepod nauplii and copepodite stages, increased from 376 mg C m −2 in April (26% of total zooplankton biomass ) to 427 mg C m −2 in June (35% of total biomass), suggesting that the zooplankton were producing more juveniles. Increased zooplankton biomass in WCR 82-B between April and June was likely a result of both in situ growth and the lateral advection of Slope Water zooplankton into the ring. Many zooplankton species which increased in WCR 82-B were of Slope Water origin. These apportunistic species of calanoid and cyclopoid copepods appear capable of exploiting the often high phytoplankton production and standing crops of WCRs, and result in a rapid evolution of the WCR epipelagic zooplankton community from its Sargasso Sea origin.

Wintertime Convection in a Gulf stream Warm Core Ring

Abstract Wintertime convection creates deep mixed layers which determine the water mass characteristics and potential vorticity for waters in the main pycnocline of the world ocean. Diabatic cooling is most intense in the Northern Hemisphere near the northwestern boundary of subtropical gyres where outbreaks of cold, dry continental air over warm surfaces waters produce large sensible and latent heat losses from the ocean. Gulf Stream warm-core rings containing warm salty Sargasso Sea Water are cast off to the north of the Gulf Stream in close proximity to the northeastern coast of North America. We have observed the convective cooling of one these rings, 821, during January 1982 from the USNS Bartlett. During one cold air outbreak, we calculate a net surface hear loss from the ring to the atmosphere of 800 W m−2 and a deepening of the surface mixed layer form 50 to 190 m. The heat budget for the ring required a lateral heat exchange with the surrounding Slope Water, in order to achieve a balanced heat bu...

The Nantucket Shoals Flux Experiment (NSFE79). Part I: A Basic Description of the Current and Temperature Variability

The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability. In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed over the upper slope. The wind stress during summer was weak and only moderately correlated with the subtidal current fluctuations. In the winter period (October–March), when the seasonal thermocline was destroyed and the shelf water locally homogenized by increased surface cooling and wind mixing, the mean currents observed over the shelf were also primarily alongshelf towards the west at speeds comparable to those measured in summer. However, the low-frequency current fluctuations over the shelf were much more energetic in winter. These subtidal current fluctuations were highly coherent horizontally and vertically over the shelf and with surface wind-stress fluctuations (which increased in magnitude by a factor of 5 over the summer period). The most energetic subtidal current events observed over the shelf also tended to extend into the upper slope region. The subtidal currents observed over the upper slope in summer were dominated by three bursts of large eastward currents which correspond to the passage of anticyclonic Gulf Stream warm-core rings near or through the moored army. The effect of these rings on the current field does not appear to penetrate shoreward of the shelf break. In winter only two rings passed near the array and their influence on the observed upper slope currents was unclear owing, in part, to the increased subtidal current variability caused by the stronger synoptic wind forcing in winter. Multiple regression analysis was used to identify possible annual variations in the NSFE79 moored current and temperature data. Significant annual variations were found in the temperature field over the shelf and upper slope and in the low-frequency current variability over the shelf. No significant annual variation was observed in the alongshelf current over the shelf, however, suggesting that there is, at least on time scales of one month and more, a continuous flow of shelf water into the Middle Atlantic Bight from the Georges Bank/Gulf of Maine region. The mean westward volume flux between the 40 and 120 m isobaths observed in NSFE79 was 38.3 ± 6.9 × 104 m3 s−1.

Tritiated thymidine incorporation and the growth of heterotrophic bacteria in warm core rings1

The time‐course of the incorporation rate of [methyl‐3H]thymidine ([3H]TdR) was established during 6–12‐h incubations of natural bacterial populations sampled from the surface layers of warm core Gulf Stream rings. Parallel estimates of changes in cell numbers were made in order to examine the relationships between TdR incorporation and population growth for oceanic bacterial populations. Our results indicate that a conversion factor of 4 × 1018 cells produced per mole of [3H]TdR incorporated yielded estimates of bacterial production which were within a factor of 2 or 3 of production estimates derived from changes in cell numbers in seawater cultures. We observed a significant, direct relationship between the initial rates of TdR incorporation per cell and specific growth rates and conclude that initial short term (15–45 min) assays of TdR incorporation are a valuable tool for studying bacterial production in oceanic waters.In most incubations, the rate of TdR incorporation increased more rapidly than did cell numbers. Analyses of the time‐courses of TdR incorporation rates yielded estimates of specific growth rates for the actively dividing bacteria ranging from 0.08 to 0.51 h−1. Very large conversion factor values were derived from these data. The discrepancy between “growth” determined from TdR incorporation rates and total bacterial numbers in seawater cultures has not been observed in previous studies of coastal, estuarine, or lacustrine bacteria, but was a consistent feature of our studies on oceanic populations. We suggest that this may reflect the opportunistic character of oligotrophic bacteria. Rapid enhancement of TdR incorporation may indicate the capability of oligotrophs to grow rapidly when favorable conditions arise.

The growth of heterotrophic bacteria in the surface waters of warm core rings1

The “seawater culture” approach is evaluated as a means of observing changes in cell numbers over time to estimate specific growth rates and production of heterotrophic bacterioplankton in oligotrophic open ocean waters. We use models of exponential growth limited by grazers, and of logistic growth limited by substrate availability, to estimate the specific growth rates of natural bacterial populations in warm core Gulf Stream rings. The two models yielded somewhat different rate estimates, but in general the results suggest that in situ specific growth rates for bacterial populations in oligotrophic ocean waters are as fast as rates in rich coastal waters and lakes (1–2 d−1). Estimates of hourly bacterial production averaged 10% of hourly primary production. Since bacteria grew at night, up to 40% of the daytime primary production was estimated to pass through the bacteria every 24 h. Estimates of removal rates by grazers are about the same as bacterial growth rates. However we estimate that only 3–7% of primary production was converted to protozoan biomass via ingestion of bacteria.Our data suggest that simple dilution of natural samples with cell‐free water is a useful, valid method for estimation of bacterial specific growth rates.

The concentrating of organisms at fronts: A cold-water fish and a warm-core Gulf Stream ring

The concentrating of organisms at fronts: A cold-water fish and a warm-core Gulf Stream ring

Gulf Stream warm core rings: phytoplankton in two fall rings of different ages

Warm core ring (WCR) 82-H was sampled in September–October (1982) as a Gulf Stream meander pinched off and became a ring. It is compared with the 3-month-old WCR 81-D, visited September–October (1981). Although the rings have different histories, their phytoplankton assemblages share some characteristics. Using cluster analyses based on quantitative group counts, a station from one ring occasionally clusters most closely with a station from the other ring, showing a similar balance of organisms. The younger ring at the time of sampling, WCR 82-H, had lower diversity, fewer shelf species, and greater consistency between stations, except for a high level of Oscillatoria in the meander before the ring pinched off. Interaction with slope water was seen principally at the ring margin. WCR 81-D, on the other hand, showed a great deal of structure, and immediate dilutions with slope water and the Gulf Stream were apparent, with higher diversity before and a week after such interactions. The upper water column of warm core rings, although showing evidence of physical mixing, can exhibit stratification of species, even after a storm.

The concentrating of organisms at fronts: a cold-water fish and a warm-core Gulf Stream ring : Olson, D.B. and R.H. Backus, 1985 J. mar. Res., 43(1):113–137

The concentrating of organisms at fronts: a cold-water fish and a warm-core Gulf Stream ring : Olson, D.B. and R.H. Backus, 1985 J. mar. Res., 43(1):113–137

On the ellipticity of isolated anticyclonic eddies

A simplified two-layer model is considered in order to describe the behavior of anticyclonic lens-like eddies embedded in a horizontally sheared flow. Attention is focused on light eddies (such as warm-core gulf stream rings) embedded in an infinitely deep fluid whose speed varies linearly in the horizontal direction. Non-linear solutions are constructed analytically using a power series expansion. It is found that a lens-like eddy embedded in a horizontally sheared flow, on an f plane, migrates at the average speed of the mean flow. As a result of the shear, the ring's shape is an ellipsoid whose orientation depends on the environmental flow. When the environmental shear is cyclonic, the ellipse is oriented perpendicular to the mean flow, and when the environmental shear is anticyclonic, the ellipse orientation is parallel to the mean flow. Although the ring migrates at a speed which differs from the speed of most of the surrounding fluid, the elliptical shape is fixed so that the whole eddy does not rotate due to the shear. A possible application of this theory to the warm-core rings found north of the gulf stream is discussed. DOI: 10.1111/j.1600-0870.1985.tb00271.x

Center of Mass Estimation in Closed Vortices: A Verification in Principle and Practice

The problem of tracking closed mesoscale vortices using center of mass estimation techniques is studied. Three estimators are evaluated using data from a warm core Gulf Stream ring. The comparisons show that a method based on the intersection of perpendicular bisectors and one using a least-squares fit of a conic section perform comparably. The perpendicular bisector algorithm is used in conjunction with a Gaussian ring model and a star-shaped survey pattern to produce an expected error curve as a function of vortex translation, survey speed and vortex size. For typical ring parameters, center estimation is usually possible to within + or - 5 km. The feasibility of using differing data sets to construct a history of ring motion based on a coordinate system moving with the ring is also investigated. In this way, the validity of using satellite-derived data and drifter trajectories to estimate the center of mass of a mesoscale feature is assessed. The results of the analysis demonstrate that the location of the deeper structure of the ring and the surface expression are sufficiently well correlated to permit dynamically relevant calculations based on surface measurements. It is shown that satellite-derived data can be used to approximate the center of mass trajectory to within the error in the individual center estimates for the period analyzed. The Lagrangian-drifter-derived centers are offset from the center of mass trajectory in a manner consistent with kinematic arguments.

The synoptic sound-speed field of a warm-core Gulf Stream ring

A synoptic expendable bathythermograph survey of the warm-core Gulf Stream ring 81-D, conducted in September 1981 when the ring was located 550 km south of Halifax, Nova Scotia, provided the basis for a detailed study of the ring’s sound-speed field. Salinities used for computing sound speed were derived from nonsynoptic conductivity–temperature–depth data collected during the survey. Analysis of the sound-speed field, using vertical and horizontal sections and satellite imagery, revealed the ring’s typical high sound-speed core, but also showed increased horizontal and vertical gradients along the ring’s western margin as compared to its eastern side. This asymmetry appears to be related to a surface streamer of warm, higher sound-speed Gulf Stream water overlying colder, lower sound-speed slope water, resulting in a subsurface sound duct centered 100 m below the surface along the ring’s western margin. The subsurface duct and increased vertical gradient resulted in 30–50 dB greater loss at depths of 100 and 200 m in the ring’s eastern margin as compared to its western side and center. Strong ray channeling and refraction of a 2000-Hz source at a depth of 100 m was also noted. Based on these observations and predictions, warm-core rings may possess regions of increased acoustic range dependence due to the interactions of waters along their margins.

Velocity and Hydrographic Structure of a Gulf Stream Warm-Core Ring

Abstract The hydrographic and velocity structure of Gulf Stream warm-core ring 81D are investigated using a deep CTD–O2 section through the ring together with horizontal currents at 100 m depth obtained with an acoustic-Doppler system. The pycnocline within the ring is depressed 500 m below that of the surrounding Slope Water and maximum currents at 100 m approach 2 m s−1 at a radius of 70–80 km from the ring center. Water-mass analysis reveals that the waters in the central core of the ring are similar to the Sargasso Sea and distinct from the Slope Water to depths of 1300 m. Near the surface, the central core is rotating as a solid body approximately once every 3.6 days. The azimuthal currents at 100 m were combined with the gradient current relationship to obtain the absolute dynamic topography of the ring. The sea surface in the ring center is 70 dynamic centimeters above that in the Slope Water and ring currents decrease quickly with increasing depth to undetectable levels below 2000 m. The ring is l...

Rapid evolution of a Gulf Stream warm-core ring

Gulf Stream warm-core rings form in the Slope Water between the North American continental shelf and the Gulf Stream by the separation of a north-extending meander1. The initial physical, chemical and biological properties of the core of these 100–200-km eddies are often similar to those of its parent water mass, the Sargasso Sea. A clockwise rotating remnant of the Gulf Stream circulates around the core with surface current speeds of 50–200 cm s−1. Although warm-core rings can slowly change over periods of months through interactions with the surrounding Slope Water, Gulf Stream meanders, continental slope, other rings, and the atmosphere, we have now discovered that major alterations to ring structure can occur during very short periods (2–5 days) when an interaction with the Gulf Stream is particularly intense. Such short-period interactions between a ring and the Gulf Stream are a major factor governing a ring's evolution.

An upper ocean current jet and internal waves in a Gulf Stream warm core ring

On June 22, 1982, the R/V Endeavor, while participating in a multi‐ship study of a warm core ring 82B, encountered a strong front in the core of the ring. The vessel was headed on a radial section outward from ring center while a CTD was repeatedly raised and lowered between 10 and 300 m. Current profiles in the upper 100 m were obtained continuously with a Doppler acoustic profiling system. Above the shallow 45 m seasonal thermocline, a current jet of 4 km width was encountered having a central core of relatively light water and a maximum current of 1.1 m/s. This jet was both highly nonlinear and totally unexpected. A high frequency packet of directional internal waves was acoustically observed in the seasonal thermocline at the outer edge of the jet. Vertical velocities were large enough (6 cm/s) as to be directly observable in the Doppler returns. The waves were propagating from the northeast, parallel to the ship track, and orthogonal to the jet toward the center of the warm core ring. While a nonlinear, centrifugal term was required for the force balance of the jet, the high‐frequency internal wave packet could be explained with linear, gravest‐mode wave dynamics.

Airborne dual laser excitation and mapping of phytoplankton photopigments in a Gulf Stream Warm Core Ring

Utilization of a two-color airborne lidar system in the systematic study of a major oceanographic feature is reported here for the first time. An excimer pumped dye laser was optically and electronically integrated into the NASA Airborne Oceanographic Lidar for simultaneous use with a frequency doubled Nd:YAG laser. The output beams exit the laser system along parallel paths after being produced on an alternating pulse basis at a combined rate of 12.5 pps. Results are presented for missions flown over a Gulf Stream Warm Core Ring (WCR) as well as over shelf, slope, Gulf Stream, and Sargasso Sea waters. From the airborne data a high coherence is shown between the two-color chlorophyll a data and between the Nd:YAG chlorophyll a and phycoerythrin responses within each of these water masses. However, distinct differences in the response patterns of these photopigments are shown to exist between the differing water masses. At certain of the boundaries separating the water masses a sharp transition is seen to occur, while at others a wider transition zone was observed in which the correlation between the photopigments appears to degrade.

Lagrangian Current Measurements within the Eastern Margin of a Warm-Core Gulf Stream Ring

Two satellite-tracked buoys drogued at 10 m depth were deployed within shelf water, close to the shelf/slope front in late summer 1980. Position and sea-surface temperature data from the two buoys were combined with surfaco-water mass analysis based an high resolution infrared satellite imagery. Buoy trajectories and surface-water mass analysis suggest that both buoys traversed a region of shelf water while moving to the southwest at less than 25 cm s−1. Stronger currents up to 54 cm s−1 from warm-core ring 80-A were encountered near 38°N latitude. These currents caused the buoys to move anticyclonically along the ring's eastern margin within a well defined shelf-water entrainment. Buoy sea-surface temperatures and positions showed good correlation with frontal boundaries based on satellite imagery. This was shown for both the ring shelf-water entrainment and another shelf-water feature located along the northern edge of the Gulf Stream. Volume transport of the ring entrainment was estimated to be 1.5 × 105 m3 s−1 (4700 km3 year−1). An estimated 40% of this yearly volume (1900 km3 year−1) could be shelf water, according to salt- and mass-balance considerations. The yearly estimate of 1900 km3 year−1, however, is too large according to estimates by other investigators, and because the entrainments are short-lived features. A proposed model uses three warm-core rings per year located in the Middle Atlantic Bight for 90 days each. If the rings entrain shelf water at the measured rate for 50% of the time, this amounts to 703 km3 year−1 or 30% of the total yearly shelf-water transport estimated by others.

Lagrangian current measurements within the eastern margin of a warm-core Gulf Stream ring

Lagrangian current measurements within the eastern margin of a warm-core Gulf Stream ring