Gulf Stream Warm-core Ring Research Articles

Shelf water entrainment by Gulf Stream warm-core rings between 75°W and 50°W during 1978–1999

Previous work concerning Gulf Stream warm-core rings (WCRs) and their associated shelf water entrainments have been based upon single surveys or time series from individual WCRs. To date, estimates of annual shelf water volume entrained into the Slope Sea by WCRs and its interannual variability have not been made. Using a long time series of satellite-derived sea surface temperature (SST) observations of Slope Sea WCRs, we have completed an analysis of 22 years of WCR data (1978–1999) between 75°W and 50°W to understand the interannual variability of WCRs and their role in entraining shelf water. Satellite-derived SST data digitized at Bedford Institute of Oceanography are analyzed using an ellipse-fitting feature model to determine key WCR characteristics including WCR center position, radius and orientation. Key characteristics are then used to compute WCR swirl velocity by finite-differencing WCR orientations ( θ) obtained from the feature model time series. Global mean WCR-edge swirl velocity calculated from all observations is 105.72±10.7 km day −1 (122.36±12.4 cm s −1), and global mean WCR radius is 64.8±6.2 km. Primary and derived WCR data are incorporated into a two-dimensional ring entrainment model (RM) using the quasi-geostrophic approximation of the potential vorticity equation. The RM defines ambient water as entrained by a WCR only if the gradient of relative vorticity term (horizontal shear) dominates the potential vorticity. Proximity of a WCR to the position of the shelf-slope front (SSF) is then used to determine whether the ambient water is entrained from the outer continental shelf. WCR-induced shelf entrainment derived from the RM displays considerable spatial variability, with maximum entrainment occurring offshore of Georges Bank, advecting a mean total annual shelf water volume of 7500 km 3 year −1 from the region. Estimates of shelf water fluxes display significant interannual variability, which may be in part due to the observed covariance between WCR occurrences and the state of the North Atlantic Oscillation (NAO). Increased (decreased) occurrences of WCRs are evidenced during positive (negative) phases of the NAO. The total mean annual shelf-wide WCR-induced shelf water transport is estimated to be 23,700 km 3 year −1 (0.75 Sv), accounting for nearly 25% of the total transport in the Slope Sea region neighboring the outer continental shelf.

Mapping Gulf Stream warm core rings from shipboard ADCP transects of the Oleander Project

The flow patterns of the Gulf Stream warm core rings and surrounding shelf break and slope water are constructed from shipboard acoustic Doppler current profiler (ADCP) transects of the Oleander Project. For each warm ring, consecutive ADCP transects are colocated to a common ring center and are mapped into a stream function. Methods to locate ring centers from the ADCP transect and advanced very high resolution radiometer image are developed and tested. Two warm rings in 1999, which have relatively complete data coverage, are examined to study the ring‐induced warm and cold streamers. For cold streamers, the estimated volume flux, based on more than 10 independent ADCP transects, is at least 1 × 106 m3 s−1. This result agrees well with the previous estimate in the Warm Core Ring Experiment. For warm streamers, the result is new. On the basis of a large number of ADCP transects, the estimated volume flux is about 2.5 × 106 m3 s−1. By pulling large transports associated with the cold/warm streamers, the warm rings likely play a fundamental role in the water exchange in the Slope Sea.

Chaotic advection and nonlinear resonances in an oceanic flow above submerged obstacle

The effect of an isolated submarine obstacle on the motion of fluid particles in a periodic external flow is studied within the framework of the barotropic, quasi-geostrophic approximation on f -plane. The concept of background currents advanced by Kozlov [1995. Background currents in geophysical hydrodynamics. Izvestia, Atmos. Oceanic Phys. 31 (2), 245–250] is used to construct a dynamically consistent stream function satisfying the potential vorticity conservation law. It is shown that a system of two topographic vortices revolving about a rotation center can form in a circular external flow. Unsteady periodic perturbations, associated with either variations in the background current or deviations of the external flow from circulation, are analyzed. Unsteadiness in the external flow essentially complicates the pattern of the motion of fluid particles. Vortex-type quasi-periodic structures, identified with nonlinear resonances that form in Lagrangian equations of fluid particle advection, are examined. They either surround the stationary configuration by a vortex chain—a ringlet-like structure [ Kennelly, M.A., Evans, R.H., Joyce, T.M., 1985. Small-scale cyclones on the periphery of Gulf Stream warm-core rings. J. Geophys. Res. 90(5), 8845–8857], or they form a complex-structure multivortex domain. Asymptotic estimates and numerical modeling are used to study the distribution and widths of the nonlinear resonance domains that appear under unsteady perturbations of different types. The onset of chaotic regimes owing to the overlapping of nonlinear resonance domains is analyzed. Transport fluxes determined by chaotic advection and barriers for transport (KAM-tori) and the conditions of their existence are studied. The relation of the rotation frequency of fluid particles on their initial position (when the dependence is calculated in the undisturbed system) is shown to completely determine the main features of the pattern of Lagrangian trajectories and chaotization effects. Because of nonlinear effects, the domain involved in quasi-periodic and chaotic motions can be much larger than the domain occupied by steady topographic vortices. The results of study by Sokolovskiy et al. [1988. On the influence of an isolated submerged obstacle on a barotropic tidal flow. Geophys. Astrophys. Fluid Dyn. 88(1), 1–30] concerning the due regard on the irrotational background component as the necessary factor for the transportation of fluid particles from the vortex domain to infinity are confirmed.

Satellite Observations of Seasonal and Interannual Changes of Sea Level and Currents over the Scotian Slope

Abstract Seasonal and interannual sea level and current variations over the Scotian slope are examined using 10 years of Ocean Topography Experiment (TOPEX)/Poseidon (T/P) satellite altimeter data. Geostrophic surface current anomalies normal to ground tracks are derived from the along-track gradients of sea level anomalies. The altimetric current anomalies are combined with a climatological mean circulation field of a finite-element model to construct nominal absolute currents. The seasonal mean results indicate that the sea level is highest in late summer and lowest in late winter and that the surface slope circulation is strong in winter/autumn and weaker in summer/spring. The total transport associated with the westward shelf-edge current and with the eastward slope current, calculated by combining the T/P data with a climatological seasonal mean density field, reveals a substantial seasonal change dominated by the barotropic component. The present analysis reveals prominent interannual changes of the sea level and current anomalies for the study period. The sea level was lowest in 1996/97, when the Gulf Stream was in its most southern position. The mean winter circulation over the Scotian slope was strongest (up to 30 cm s−1 in both the southwestward shelf-edge current and northeastward slope current) in 1998 and weakest (weaker and broader shelf-edge current) in 1996, which may be related to the fluctuation of the equatorward Labrador Current strength and of the Gulf Stream north–south position. The study also suggests that the root-mean-square current magnitude is positively correlated with the occurrence of the Gulf Stream warm-core rings (WCRs) on the interannual scale, while WCR yearly mean kinematic properties seem to have small variations.

Spatial and temporal distributions of surface temperature and chlorophyll in the Gulf of Maine during 1998 using SeaWiFS and AVHRR imagery

The temporal and spatial distributions of SST and surface chlorophyll in northeastern coastal areas (46.0°N, 74.0°W to 40.0°N, 62.0°W) were examined using a year-long time series of composite SST and chlorophyll images during 1998. Five study areas were identified in these images: Gulf of Maine, Nantucket shoals/Georges Bank, Scotian shelf waters, slope water and coastal waters (depth < 100 m) from Cape Cod to Nova Scotia. The mean SST and surface chlorophyll for each region was compared to the mean for the entire area. The coldest water (2.5 °C) was observed on the Scotian shelf in late March (YD 89). Throughout the year, coastal waters were cooler than the mean for the entire area. In the Gulf of Maine during a single day in summer (YD 208) SST ranged from 10 °C south of Nova Scotia to 20 °C in the central Gulf with a variance of over 8 °C. The Maine Coastal Current and its offshore extension were responsible for both these observations. Slope water was consistently warmer than the mean for the entire area. Several warm core Gulf Stream rings were observed in the SST images during 1998. These rings were partially responsible for the warm SST and high variances in slope water. Surface chlorophyll levels were higher than the mean in the Gulf of Maine, Nantucket shoals/Georges Bank and coastal waters; levels were below the mean on the Scotian shelf and in slope water. Using the SST and chlorophyll composite images a transect line along 67.6°W was extracted and surface contour plots for the entire year were made. The SST contour plot showed a strong surface manifestation of the Maine Coastal Current first appearing in late spring (YD 150) and continuing until the end of October (YD 300). High chlorophyll on Georges Bank throughout the year was the dominant feature in the surface contour plot of chlorophyll.

TOPEX/Poseidon-Jason Comparison and Combination off Nova Scotia

Sea level observations from the tandem TOPEX/Poseidon (T/P) and Jason-1 altimetry missions (2002–2003) are used to study characteristics of sea level and surface currents over the Scotian Shelf and Slope off Nova Scotia. The consistency and error characteristics of T/P and Jason-1 measurements are examined not only in terms of sea level and cross-track current anomalies but also with respect to current anomalies at crossovers, kinematic properties associated with Gulf Stream warm core rings (WCR), and the shelf-edge current transport. Nominal absolute currents are constructed by adding the altimetric geostrophic current anomalies to an annual-mean model circulation field. The concurrent frontal analysis data are analyzed for occurrence of the WCRs and associated kinematic properties are derived from altimetric current anomalies. The comparison of the sea level and cross-track current anomalies from January to July 2002 shows overall good agreement between T/P and Jason, with correlation coefficients different from zero at the 5% significance level at essentially all locations for sea level and at most locations for currents. The cross-track geostrophic current anomalies from January to July 2002 and from September 2002 to December 2003 are further used to calculate the root-mean-square (rms) current magnitude, and the normalized relative vorticity associated with WCRs. The altimetric currents are consistent with each other and complementary to frontal analysis data in deriving the properties of the WCRs. The rms current magnitude is ∼55 cm/s and the normalized relative vorticity is ∼0.15. The model-altimetry combined absolute currents are used to estimate near-surface transport associated with the shelf-edge current, showing good correlation between T/P and Jason estimates and strong seasonal changes. The current anomalies derived from altimetry and moored measurements are significantly (at the 5% significance level) correlated and comparable in the rms magnitude.

Scotian Slope circulation and eddy variability from TOPEX/Poseidon and frontal analysis data

Sea level observations from TOPEX/Poseidon (T/P) altimetry, in conjunction with concurrent frontal analysis data from infrared imagery, are used to study temporal and spatial variability of sea‐surface currents over the Scotian Slope. Geostrophic surface current anomalies normal to ground tracks are derived from the sea level anomalies relative to local means for the study period. The altimetric results reveal prominent spatial variability of cross‐track currents with an overall intensification toward the west and south, typically 20–30 cm/s over the upper and lower slope, 20–50 cm over the continental rise, and up to 70–75 cm/s near the Gulf Stream northern wall. A rapid westward intensification on the western slope contrasts with nearly uniform distribution on the eastern slope. The intensification seems to be associated with the high occurrence of Gulf Stream warm core rings (WCR) and with the close proximity to the Gulf Stream. Crossover analyses of altimetric current anomalies indicate that the total current variability can be estimated by a factor of 1.5 from the cross‐track current variability and the variance is isotropic to within 14%. The rotational speed of the Gulf Stream rings can reach 1–2 m/s over the Scotian Slope, with the root‐mean‐square variability of ∼60 cm/s. The relative vorticity of the interior core is estimated to be ∼1.5 ×10−5 1/s. When combined with CTD data, T/P observations can produce the vertical profile of the total geostrophic current that is in approximate agreement with ADCP measurements.

Stability‐induced modification of sea surface winds over Gulf Stream rings

Satellite‐borne scatterometer and infrared data collected over Gulf Stream warm and cold core rings are used to study the effect of the sea‐air temperature difference on the wind speed over rings. The observed acceleration of the wind over warm core rings and deceleration over cold core rings is found to be consistent with that predicted by the planetary boundary layer model of Brown and Foster [1994]. In addition it is shown that the distance over which the winds respond to an ocean surface temperature step is short (≤25km) while the distance over which the marine boundary layer responds to a surface temperature step is long (≥175km).

Influence of a Gulf Stream warm-core ring on water mass and chlorophyll distributions along the southern flank of Georges Bank

During late spring 1997, a Gulf Stream warm-core ring (WCR) strongly influenced water mass and chlorophyll distributions along the southern flank of Georges Bank. Entrainment of Georges Bank shelf water by the WCR, centered at the central southern flank, persisted through May. By late May, shelf water encircled nearly the entire WCR, and horizontal convergence toward the central southern flank was evident. Scotian Shelf water (SSW) extended across Northeast Channel, onto Georges Bank near the northeast peak, and along the southern flank between the 60-m isobath and the shelfbreak front. SSW extended furthest southwest along the southern flank shelfbreak. Satellite imagery from the ocean color and temperature sensor (OCTS) showed that by late May, in addition to the high chlorophyll concentrations within the entrained shelf water encircling the WCR, pigment rich bands of chlorophyll developed along the 60- and 100-m isobaths. Where this biological enhancement developed along >100 km of the shelfbreak (100-m isobath), shelf waters extended furthest seaward due to entrainment by the WCR. The enhanced shelfbreak chlorophyll was sampled in situ along two transects separated by ≈40 km along shelf. Along both transects, the enhanced chlorophyll coincided with divergent cross-shelf flow, maximum along-shelf flow and minimum surface temperature. Coincidence of the highest surface chlorophyll and lowest surface temperature with divergent cross-shelf flow is consistent with upwelling. Coincidence with along-shelf jets and their associated vertical shear is consistent with turbulent vertical mixing. The importance of turbulent vertical mixing was supported. Maximum velocity at the shelfbreak coincided with the subsurface temperature minimum of SSW near 30-m depth. Estimated gradient Richardson numbers (Ri) above the SSW were below critical (<0.25) within the shelfbreak chlorophyll maximum along both transects, and surface chlorophyll was significantly, inversely correlated with Ri along the southern flank. WCR entrainment of shelf water continued into June as the WCR propagated southwestward. On June 11, high chlorophyll concentrations along the shelfbreak north of the WCR were observed by the OCTS. Satellite sea-surface temperature showed that in addition to WCR entrainment, breaking waves ( λ≈30 km) of the shelf-slope front coincided directly with the locally enhanced chlorophyll. The waves propagated west at ≈12 km day −1 . Vertical mixing due to these breaking frontal waves may have contributed to local nutrient enrichment of near-surface waters at the shelfbreak.

High-resolution satellite-derived sea-surface temperature variability over the Gulf of Maine and Georges Bank region, 1993–1996

Empirical orthogonal function (EOF) analyses were completed using 3 yr (fall 1993 through late-summer 1996) of 1.4-km resolution, satellite-derived sea-surface temperature (SST) imagery from a large domain that includes the Gulf of Maine, Georges Bank, southern New England and western Scotian shelves, and the adjacent slope water region located seaward of the 200-m isobath. Prior to analysis, raw satellite-derived SST images were carefully cloud masked using automated and manual cloud detection techniques and then interpolated to a 7-km resolution grid using optimal interpolation (OI). OI resulted in 218 cloud-free, five-day-averaged SST fields, produced at five-day intervals for the study period. After removal of the 3 yr temporal mean SST at each gridpoint, a covariance EOF analysis of OI SST shows three significant modes, accounting for a total of 96.5% of observed SST variance. A gradient EOF analysis also shows three significant modes, accounting for a total of 92.7% of observed SST variance after removal of the spatial mean SST from each OI SST field. Covariance EOF modes 1 and 3 represent 92.2 and 1.3% of observed SST variance, and depict rich spatial structure in the seasonal heating cycle and additional spatial variation in the seasonal heating cycle over the study domain, respectively. SST variability noted for covariance EOF mode 1 is largely related to local differences in the seasonal mixed-layer depth, with deeper mixed layers resulting from strong tidal mixing over offshore banks and shoals contrasting with more- shallow-mixed layers and seasonal density stratification caused by weaker tidal mixing over regions of greater bottom depth. Additional spatial differences in seasonal SST variability and noted in covariance EOF mode 3 are related to regional differences in seasonal thermohaline and density characteristics of surface waters across the Gulf of Maine. Regional surface water differences across the Gulf of Maine are caused by different phasing between annual cycles of vertical heat flux and inputs of low-salinity Scotian Shelf water in eastern Gulf of Maine during late winter and local freshwater runoff from rivers in western Gulf of Maine during late spring. Covariance EOF mode 2 represents 3.0% of observed SST variance and describes interannual SST variability manifested by a cooling trend within the slope water region caused by a decrease in Gulf Stream warm-core ring activity during the study period. The gradient EOF analysis shows results that support and supplement the covariance EOF analysis by depicting the year-round nature of the SST gradient associated with the shelf break front (mode 1, 86.4% of SST variance) and SST gradients arising from spatial variation in the seasonal heating cycle (modes 2 and 3, 4.3% and 2.0% of SST variance, respectively) and some degree of interannual variability as described for covariance EOF mode 2 (mode 3).

Strange encounters of eddies with walls

The impingement of a westward drifting lens-like anticyclonic ring on a meridional boundary is examined analytically and numerically using an (inviscid) layer-and-a-half model. A nonlinear analytical solution is constructed with the aid of a slowly varying perturbation expansion and numerical simulations are performed with the Bleck and Boudra model. It is found that, in contrast to conventional wisdom which implies that an anticyclonic ring encountering a wall moves poleward due to the image effect, the ring stays roughly in a e xed latitude. As it slowly moves into the wall due to b , it gradually leaks e uid toward the equator until it loses all of its mass. As expected, different eddies lose their mass in different rates. The (nondimensional) central thickness of an intense zero potential vorticity ring decays as 1/[1 1 b (2g8Hi) 1/2 t/18f ] 2 where Hi is the initial central thickness, t is time, g8 the ‘ ‘ reduced gravity,’ ’f the Coriolis parameter, andb is the familiar variation of the Coriolis parameter with latitude. Due to the gradually decreasing depth, the ring’ s westward drift into the wall also gradually slows down; i.e., the ring advances toward the wall and loses mass like a gradually peeling onion. The ‘ ‘ peeling’ ’ rate is 2 b Rd /9 [where Rd is the instantaneous Rossby radius based on the instantaneous maximum thickness] implying that the eddy migrates toward the wall at a rate that is one-third of the free open ocean rate. Numerical simulations are in excellent agreement with the above solution.As expected, they show that frictional forces decrease the intensity of the leakage and, hence, slow down the draining process. Unfortunately, no analytical solution can be found for weak or moderately strong rings because, for such lenses, there is no known analytical solution even for the nonmigrating state (i.e., rings on an f-plane). Numerical simulations show, however, that such weak or moderately strong rings display (qualitatively) similar characteristics to those of intense zero potential vorticity rings. It is argued that rings such as warm-core Gulf Stream rings, Loop Current rings,Agulhas rings, as well as other eddies (e.g., Meddies) may all experience similar encounters. As the walls that they encounter are subsurface (e.g., the continental rise, the Walvis Ridge and the mid-Atlantic Ridge), the leakages are not expected to be easily detected.

A Model for Vortex-Trapped Internal Waves

Regions of negative vorticity are observed to trap and amplify near-inertial internal waves, which are sources of turbulent mixing 10–100 times higher than typically found in the stratified ocean interior. Because these regions are of finite lateral extent, trapped waves will not form a continuum but be quantized in modes. A model for the radial structure of near-inertial azimuthal modes in an axisymmetric vortex is described in order to explain intense near-inertial motions observed in the cores of a Gulf Stream warm-core ring and a vortex cap above Fieberling Seamount. Observed signals exhibit little variability of the rectilinear phase ϕ = arctan(υ/u) in the core and evanesce rapidly outside the swirl velocity maximum, where u is the zonal velocity and υ the meridional velocity. The authors focus on azimuthal mode n = −1 (propagating clockwise around the vortex) and the gravest radial mode (no zero crossing) that appears to dominate observations. Model solutions resemble Bessel functions inside the velocity maximum and modified Bessel function decay outside, consistent with observations and solutions previously found by Kunze et al. using a less complete model. The improved model supports their conclusions concerning radial wavelengths, vertical group velocities, and energy fluxes for trapped waves.

A Numerical Simulation of the Evolution and Propagation of Gulf Stream Warm Core Rings

The evolution and propagation of Gulf Stream warm core rings in a flat-bottom, β-plane ocean are studied using a three-dimensional primitive equation model. Rings are produced by a heat source that is turned on and off slowly in the upper 750 m of the water column. Besides an anticyclone in the upper ocean, a deep cyclone is generated below the surface eddy. In the first 30 days, the surface anticyclone moves slowly southwestward because of β dispersion and vorticity advection. In waters 4000 m deep, both the anticyclone and the cyclone intensify, and a barotropic vortex pair is formed. The vortex pair moves rapidly southeastward. Its propagation becomes steady and eastward after the cyclone sheds an eddy. The cyclone in the vortex pair moves away from the ring at the end of 6 months, and both vortices begin to propagate westward separately. Fluid to a depth of 3000 m, much deeper than that of forcing, is transported by the ring. The formation of a strong vortex pair is associated with the generation of relative vorticity in both vortices by unstable waves of the second azimuthal mode. In strong rings, the increase in vorticity could produce rapid propagation. Eastward propagation is a result of change in planetary vorticity and loss of relative vorticity during cyclone splitting. In waters shallower than 4000 m, the vortex pair is less stable and more vorticity is lost by cyclone splitting. There is still a rapid movement toward the south but the eastward propagation is weak. Rings in waters shallower than 4000 m are likely to remain on the continental slope off the U.S. East Coast and induce large amounts of momentum and mass transfer over the continental margin.

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.

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.

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.

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.

Coarse-scale horizontal patchiness and vertical migration of zooplankton in Gulf Stream warm-core ring 82-H

A 1-m 2 MOCNESS with 20 nets was used to make a series of tows in Gulf Stream meander/ring 82-H (September/October 1982) including two 0–100 m undulating “towyos”. One towyo, made at dusk in the core of 82-H (of Sargasso Sea/Gulf Stream origin) permitted study of the effect of diel migration on the spatial variability of copepod and euphausiid species abundance, and species composition in a region of low physical variability. The other towyo taken across a front on the outer edge of 82-H (a mixture of Gulf Stream, Shelf and Slope Water), allowed comparison of spatial variability of the same biological properties in a region of strong physical variability. A sharp transition in euphausiid species composition occurred in the ring core after sunset as dielly migrating euphausiids moved into the surface waters. A similar, but less extreme change took place in copepod species composition because a smaller proportion of these migrated. All copepod migrants also entered surface waters after sunset with species living deeper in the water column during the day arriving in the surface waters later than those living shallower. Enright's (1977, Limnology and Oceanography, 22, 856–872) hypothesis for the metabolic advantages available through diel vertical migration does not account for the observed behavior of the migrating copepods and euphausiids at this time and place. Estimated swimming speeds (typically 50–200 m h −1) of migrating copepods and euphausiids were similar in spite of large differences in body size between the two groups. Variations in species composition were substantially larger at the edge of the ring where species proportions changed radically in concert with changes in water-mass properties. There were also large differences in species composition between the samples from the ring core and the front which equaled those which occurred across the front. Hydrographic differences were stronger than diel changes due to migration for copepods but not for euphausiids. Streamers of surface water which originated within the frontal region and spiraled into the ring core could provide colonizers of many species not present at the time of ring formation.

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.