England Seamount Chain Research Articles

Deep Mesoscale and Submesoscale Circulations Around the Atlantis II Seamount

AbstractThis study investigates the complex interactions between the North Atlantic Current (NAC) and the New England Seamount chain, focusing on the Atlantis II seamount. Employing a high‐resolution submesoscale permitting regional ocean circulation model nested within a basin‐wide simulation, it explores three distinct periods, each 2 weeks long, showcasing varied deep mesoscale and submesoscale circulations around the seamount. The analysis includes Eulerian statistics and Lagrangian particle tracing experiments to explore the transport and mixing impacts of the highly dynamic flow around the seamount. The results reveal significant density variations across the water column, attributed to the seamount's influence on local ocean currents. Specifically, mesoscale and submesoscale vortices, a seamount wake, and lee waves form during periods of increased near‐bottom current flow. These findings highlight the critical role of topographic features in modulating oceanic flows and ocean mixing, which have implications, among others, for nutrient distribution, acoustic propagation, and climate modeling. The variety and variability of the mesoscale and submesoscale circulations that arise in response to the variability in the NAC strength and position in relation to the New England Seamount Chain demonstrate the difficulty in extrapolating general behaviors from isolated observational campaigns.

Deep-water ambient sound on near-bottom receivers in the New England Seamount Chain

Ambient sound was recorded for about two months by three synchronized, single-hydrophone receivers that were moored at depths of 2573, 2994, and 4443 m on seamount flanks. Hydrophones were located within 5 m from the seafloor. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.5 to 4000 Hz. Statistical distribution of pressure amplitude exhibits much heavier tails than the expected Rayleigh distribution throughout the frequency band of observations. Spatial variability of the observed ambient sound is controlled by the seafloor properties, bathymetric shadowing, and nonuniform distribution of the noise sources on the sea surface due to the Gulf Stream and its meanders. Interferometry of the ambient sound recorded by the receivers with the horizontal separations of 7.0 km shows strong variations of the acoustic travel time between the receivers along a surface-reflected path due to the evolution of the Gulf Stream position. The magnitude of the variations of the passively measured acoustic travel time is consistent with the available contact measurements of the sound speed profiles. Environmental inferences derived from the ambient, shipping, and flow noise data will be discussed. [Work supported by the ONR TFO DRI.]

Impact of the New England Seamount Chain on Gulf Stream Pathway and Variability

Abstract The potential role of the New England seamount chain (NESC) on the Gulf Stream pathway and variability has been long recognized, and the series of numerical experiments presented in this paper further emphasize the importance of properly resolving the NESC when modeling the Gulf Stream. The NESC has a strong impact on the Gulf Stream pathway and variability, as demonstrated by comparison experiments with and without the NESC. With the NESC removed from the model bathymetry, the Gulf Stream remains a stable coherent jet much farther east than in the experiment with the NESC. The NESC is the leading factor destabilizing the Gulf Stream and, when it is not properly resolved by the model’s grid, its impact on the Gulf Stream’s pathway and variability is surprisingly large. A high-resolution bathymetry, which better resolves the New England seamounts (i.e., narrower and rising higher in the water column), leads to a tighter Gulf Stream mean path that better agrees with the observed path and a sea surface height variability distribution that is in excellent agreement with the observations.

Spatial Variability of Movement, Structure, and Formation of Warm Core Rings in the Northwest Atlantic Slope Sea

AbstractGulf Stream Warm Core Rings (WCRs) have important influences on the New England Shelf and marine ecosystems. A 10‐year (2011–2020) WCR dataset that tracks weekly WCR locations and surface areas is used here to identify the rings' path and characterize their movement between 55 and 75°W. The WCR dataset reveals a very narrow band between 66 and 71°W along which rings travel almost due west along ∼39°N across isobaths – the “Ring Corridor.” Then, west of the corridor, the mean path turns southwestward, paralleling the shelfbreak. The average ring translation speed along the mean path is 5.9 cm s−1. Long‐lived rings (lifespan >150 days) tend to occupy the region west of the New England Seamount Chain (NESC) whereas short‐lived rings (lifespan <150 days) tend to be more broadly distributed. WCR vertical structures, analyzed using available Argo float profiles indicate that rings that are formed to the west of the NESC have shallower thermoclines than those formed to the east. This tendency may be due to different WCR formation processes that are observed to occur along different sections of the Gulf Stream. WCRs formed to the east of the NESC tend to form from a pinch‐off mechanism incorporating cores of Sargasso Sea water and a perimeter of Gulf Stream water. WCRs that form to the west of the NESC, form from a process called an aneurysm. WCRs formed through aneurysms comprise water mostly from the northern half of the Gulf Stream and are smaller than the classic pinch‐off rings.

Environmental and Geomorphological Effects on the Distribution of Deep-Sea Canyon and Seamount Communities in the Northwest Atlantic

Deep sea canyons and seamounts are topographically complex features that are considered to be biological hotspots. Anthropogenic pressures related to climate change and human activities are placing the species that inhabit these features at risk. Though studies have examined species composition on seamounts and canyons, few have compared communities between them, and even fewer studies have examined how species’ abundances correlate with environmental conditions or geomorphology. Consequently, this study compares species composition, community structure, and environmental variables between Northwest Atlantic continental margin canyons and seamounts along the New England Seamount Chain. Geoforms were also related to the occurrence of phyla and biodiversity. Overall, there was a significant difference in species composition between canyons and seamounts with sponges, corals, sea urchins and seastars contributing heavily to observed differences. Environmental conditions of temperature and salinity and the seafloor property slope contributed significantly to communities observed on seamounts, while substrate, depth and salinity contributed significantly to canyon communities. Abundances were significantly higher in canyons, but taxonomic richness, evenness, and diversity were all greater on seamounts. In an era where climate change and human activity have the potential to alter environmental parameters in the deep sea, it is important to examine factors that influence the spatial distribution of deep-sea benthic communities.

Abundance and diversity of deep-sea crustaceans (Decapoda, Lophogastrida, and Euphausiacea) in the micronekton of Bear Seamount, New England Seamount Chain

Bear Seamount (39° 55′ N; 67° 30′ W) is the oldest and westernmost seamount in the New England Seamount Chain. It is located in the Marine National Monument on the continental slope south of Georges Bank and is influenced by unique ecological factors and currents. We enumerated and calculated the diversity, catch per unit effort (CPUE), and distribution of trawl-susceptible crustacean micronekton (2–20 cm) in the vicinity of Bear Seamount. Sixty-six species of pelagic Crustacea (Decapoda, Lophogastrida, and Euphausiacea) were taken with large, double-warp midwater trawls at 33 stations in the spring of 2003 and 2004 and fall of 2014. Oblique trawl sampling occurred from the surface to 2238 m, with most sampling concentrated between 600 and 1700 m depth. In 2003–2004, the assemblage on the south side of the seamount was the most diverse, while the summit assemblage had the greatest CPUE and species richness. In 2014, the summit assemblage was the most diverse and species rich while the assemblage on the west side had the greatest CPUE. Rarefaction curves did not reach an asymptote, suggesting increased sampling effort will yield more species at Bear Seamount. Eusergestes arcticus and Meganyctiphanes norvegica were numerically dominant. Two species Pasiphaea hoplocerca and P. merriami are new records for Northwest Atlantic waters.

The Gulf Stream's path and time-averaged velocity structure and transport at 68.5°W and 70.3°W

Full-ocean-depth observations of horizontal velocity, temperature and salinity along 68.5° W chiefly over the period October 2010–May 2014 are analyzed in conjunction with repeated shipboard acoustic Doppler current profiler (SADCP) upper-ocean velocity sections occupied upstream at 70.3° W and regional satellite-altimeter-based sea surface height (SSH) data to construct estimates of the time-averaged Gulf Stream velocity, property structures and transport. A stream-coordinate mean section is created from two moorings near 68.5° W where data are binned relative to distance from the Gulf Stream axis, rotated into along- and across-stream coordinates, and then averaged. Transport in the upper 600 m inferred from the moorings excluding times of large Stream axis curvature and Gulf Stream ring influences is 59.9 Sv (with 95% confidence bounds between 58.6 and 61.6 Sv). This is in good agreement with a mean constructed from the SADCP sections at 70.3° W. Relative to the mean field at 70.3° W, the velocity core of the time-averaged Stream at 68.5° W appears broader with weaker maximum speed, consistent with a companion analysis of the altimetric SSH data. The time-averaged full-ocean-depth transport inferred from the moorings is 102.1 Sv (with 95% confidence bounds between 99.1 and 106.3 Sv), which is stronger than the mean inferred from an ensemble of 10 full-depth lowered acoustic Doppler current profiler (LADCP) sections collected along the moored array. The 2010–2014 time-averaged Gulf Stream inferred from the moored observations is weaker by about 10% than the time-averaged full-ocean-depth transport reported for the late 1980s at the same location using similar procedures, with much of this difference arising from flows at depths greater than 1000 m. Satellite altimetry provides spatial and temporal context for these results and suggests that there are small-scale recirculation cells flanking the separated Gulf Stream west of the New England Seamount Chain. Gulf Stream transport, which includes throughput and recirculating components, appears to be more sensitive to changes in these recirculations at 68.5° W compared to 70.3° W.

Slow Down of the Gulf Stream during 1993\u20132016

The Gulf Stream, the main heat-carrier from low to high latitudes in the North Atlantic Ocean, influences the climate and weather in the northern hemisphere. In this study we determine and analyze the position, speed, and width of the Gulf Stream (GS) from 80°W–50°W using satellite altimeter sea surface height (SSH) measurements to examine the possible link between changes in the strength of the GS and coastal sea levels along the U.S. East Coast. During our 24-year study period (1993–2016), the GS experienced a southward shift east of 65°W after passing the New England Seamount chain. This southward shift was accompanied by a weakening of the GS, associated with an increase in SSH to the north of the GS. West of 70°W, however, we found no statistically significant trends in the GS properties, consistent with results based on in situ measurements. This lack of a trend to the west fails to support a direct link between a long-term slowdown of the GS west of 70°W and sea level rise acceleration along the U.S. East Coast, though a slowdown of the GS east of 65°W may contribute to sea level rise. It is also possible that heat carried to the region by the GS may be responsible for these observed sea level changes.

Ring-shaped morphological features and interpreted small seamounts between southern Quebec (Canada) and the New England seamounts (USA) and their possible association with the New England hotspot track

Enhancements of recently available high-resolution multibeam echosounder data from the western Gulf of Maine and Atlantic continental margin and light detection and ranging (LiDAR) and digital elevation model data from southeastern Quebec (Canada) and the northeastern United States have revealed numerous ring-shaped morphological features and interpreted small seamounts between the Monteregian Hills igneous province and the New England seamounts. The morphological features onshore are mainly ring-shaped depressions, several of which surround mapped igneous intrusions in the Monteregian Hills igneous province and White Mountain magma series. Most of the rings offshore are also depressions, although a few rings are curved ridges above the seafloor. The largest ring in the western Gulf of Maine is the 30-km-diameter Tillies ring that lies 20 km east of Cape Ann, MA. Several small (<3 km in diameter) round, flat-topped submerged hills that we interpret to be volcanic necks are also present beneath the western Gulf of Maine. The rings between Cape Cod and the continental slope are more subtle because of thicker sediments and poorer spatial resolution of the sonar data in this area. The southernmost ring-shaped features are located on the continental slope and upper continental rise and coincide with the northwestern end of the New England seamount chain. The concentration of these features between the Monteregian Hills igneous province and the New England seamounts suggests that they are igneous features that may be associated with the New England hotspot track.

Cephalopod biodiversity in the vicinity of Bear Seamount, western North Atlantic based on exploratory trawling from 2000 to 2014

Bear Seamount (BSM) is the most inshore seamount in the New England Seamount chain. It is located within the U.S. Exclusive Economic Zone and is contained within the recently established Northeast Canyons and Seamounts Marine National Monument. In 2000, the National Oceanic and Atmospheric Administration’s (NOAA) National Systematics Laboratory began an exploratory trawling program to document nekton diversity at BSM and its vicinity. Here, we summarize eight exploratory sampling cruises conducted between 2000 and 2014, and describe the cephalopod biodiversity and assemblage structure around BSM. Over the course of 174 deep–midwater and 56 bottom tows, 5088 cephalopods were identified, measured, and documented. In total, 77 species were collected at BSM; 75 species were collected from midwater tows and 28 from benthic tows. Rarefaction curves did not reach an asymptote, suggesting that additional sampling will collect more species. Seventeen species accounted for 75% of the total midwater and bottom catch, including: Illex illecebrosus (n = 605), Magnoteuthis magna (n = 568), Abraliopsis morisii (n = 518), Abralia redfieldi (n = 358), Mastigoteuthis agassizii (n = 336), Histioteuthis reversa (n = 273), Taonius pavo (n = 239), Haliphron atlanticus (n = 195), Brachioteuthis beanii (n = 160), Ornithoteuthis antillarum (n = 153), Pterygioteuthis gemmata (n = 141), Pyroteuthis margaritifera (n = 120), Vampyroteuthis infernalis (n = 101), Chiroteuthis veranyi (n = 33), Bolitaena pygmaea (n = 30), Graneledone verrucosa (n = 11), and Stauroteuthis syrtensis (n = 29). Non-metric multidimensional scaling (NMDS) of significant analysis of similarity (ANOSIM) results showed that the 2000 cruise was different from other years, meteorological winter was different from other seasons, and that day and night shallow samples were different from each other and all other depths. Based on seasonal size variation in the most abundant taxa, we propose hypotheses of year-round, winter, and spring spawning for future critical assessment. This extensive description of the offshore cephalopod assemblage may be used to assess vulnerability to future environmental changes and human activities.

Seasonal occurrence of sperm whales (Physeter macrocephalus) around Kelvin Seamount in the Sargasso Sea in relation to oceanographic processes

Sperm whales (Physeter macrocephalus) are widely distributed in all oceans, but they are clumped geographically, generally in areas associated with high primary and secondary productivity. The warm, clear waters of the Sargasso Sea are traditionally thought to be low in productivity, however recent surveys have found large numbers of sperm whales there. The New England Seamount Chain bisects the north-western portion of the Sargasso Sea, and might influence the mesoscale eddies associated with the Gulf Stream; creating areas of higher productivity within the Sargasso Sea. We investigated the seasonal occurrence of sperm whales over Kelvin Seamount (part of the New England Seamount Chain) and how it is influenced by oceanographic variables. An autonomous recording device was deployed over Kelvin Seamount from May to June 2006 and November 2006 to June 2007. A total of 6505 hourly two-minute recordings were examined for the presence of sperm whale echolocation clicks. Sperm whales were more prevalent around Kelvin in the spring (April to June: mean=51% of recordings contained clicks) compared to the winter (November to March: mean=16% of recordings contained clicks). Sperm whale prevalence at Kelvin was related to chlorophyll-a concentration four weeks previous, eddy kinetic energy and month. The mesoscale activity associated with the Gulf Stream and the Gulf Stream׳s interaction with the New England Seamount Chain likely play an important role in sperm whale occurrence in this area, by increasing productivity and perhaps concentration of cephalopod species.

A New Genus and Species of Bamboo Coral (Octocorallia: Isididae: Keratoisidinae) from the New England Seamounts

Sampling along the New England Seamount Chain has produced a large number of new octocoral species, especially of the bamboo corals in the subfamily Keratoisidinae. The new genus described here is unusual in that it branches from the internodes yet has sclerites at the base of the polyp body that are arranged obliquely, and in having tentacles that can be pulled completely into the top of the polyp over the retracted oral disk. The genus and species are characterized using both morphological and molecular data.

The mean flow and variability of the Gulf stream-slopewater system from MICOM

The slopewater region is influenced by surface wind-driven, deep buoyancy-driven and shelf currents, whose complex interactions can affect both the northward heat transport and southward return flows. The mean flow and variability of the Gulf Stream-slopewater system are studied using four year outputs of Miami Isopycnic Coordinate Ocean Model (MICOM) realistic high-resolution simulation of the North Atlantic circulation. Special attention is focused on the eastward Slopewater Jet (SJ), a surface current characterized by a mean path coinciding with the strong outcropping temperature front in the slopewater. The water mass, path and transport of the SJ in MICOM are found to be in reasonable agreement with the existing observations. The modeled SJ is associated in part with a GS’ secondary branch, induced by a Taylor column effect of the New England Seamount Chain (NESC) on the upper GS. This upper-ocean-topographic coupling results in a spatial GS bifurcation, and advection of GS waters into the slopewater region shortly downstream of the NESC. An EOF analysis of the pycnocline depth confirms this tendency, as the first mode displays a qualitative dependence of the GS fan-shape streamline dispersion on the strength and intersecting latitude of the incident GS. Additionally, the model displays a strong influence of the Deep Western Boundary Current (DWBC) on the path of the SJ, by acting as a potential vorticity barrier. Important interactions between the two currents are suggested by the statistical EOF of the slopewater column, as in observations. Downstream of the NESC, the SJ transport variability is seasonal in MICOM, due to the north-south annual oscillation of the GS path and mergings with anticyclonic eddies. However, the variability of the SJ velocity profile is dominated (49% eigenvalue) by lateral translations of the current, at a 9-month timescale characteristic of GS meander-intensity variability. South of the Grand Banks, the transport variations of both the SJ and Labrador Current (LC) are captured by the first mode of the upper-slope water (37% of the variability), with predominant timescales corresponding to the upstream variability of the GS, and seasonality of the LC. Both modes are also in reasonable agreement with observations.

Studies on western Atlantic Octocorallia (Coelenterata: Anthozoa). Part 6: The genera Primnoella Gray, 1858; Thouarella Gray, 1870; Dasystenella Versluys, 1906

ABSTRACT The seven western Atlantic species belonging to three genera, Primnoella, Thouarella, and Dasystenella, are described and illustrated. Three new species of Thouarella are described, which constitute the first records of this genus for the northwestern Atlantic. An annotated and revised list of the 26 species of Thouarella is given. Most of the species included occur primarily in the southwestern Atlantic at depths of 50–5000 m; however, one species, Thouarella grasshoffi, is amphi-North Atlantic in distribution. Whereas most of the species included herein are small and inconspicuous, T. grasshoffi is fairly large and occurs abundantly on seamounts of the New England Seamount Chain as well as on eastern Atlantic seamounts. Specimens of all three genera are still relatively rare in museum collections.

Reproductive biology of the deep-sea polychaete Gorgoniapolynoe caeciliae (Polynoidae), a commensal species associated with octocorals

Some aspects of the reproductive biology of the polychaete Gorgoniapolynoe caeciliae have been described for the first time. Gorgoniapolynoe caeciliae is a deep-sea commensal species associated with Candidella imbricata, an octocoral that populates the New England Seamount chain. Gorgoniapolynoe caeciliae is a dioecious species with an equal sex ratio and fertile segments throughout most of the adult body. The gonads of both sexes are associated with genital blood vessels emerging from the posterior surface of most intersegmental septa. In the female, oogenesis is intraovarian with oocytes being retained within the ovary until vitellogenesis is completed. The largest female examined contained over 3000 eggs with a maximum diameter of 80–90 μm. In the male, the testes are repeated in numerous segments and consist of small clusters of spermatogonia, spermatocytes, and early spermatids associated with the walls of the genital blood vessels. Early spermatids are shed into the coelom where they complete differentiation into mature ect-aquasperm with a spherical head (4 μm), a small cap-like acrosome, and a short mid-piece with four mitochondria. Indirect evidence suggests that this species is an annual breeder that releases its gametes into seawater and produces a planktotrophic larva following fertilization. The reproductive biology of G. caeciliae is consistent with that of most other polynoids including many shallow water species suggesting that phylogenetic history strongly shapes its biology.

Biodiversity of Bear Seamount, New England Seamount Chain: Results of Exploratory Trawling

Biodiversity of Bear Seamount, New England Seamount Chain: Results of Exploratory Trawling

Hydrodynamical dispersion of dredged materials sequestered on the abyssal seafloor

This paper addresses the potential dispersion by hydrodynamical processes of dredged material disposed on the abyssal seafloor off the east coast of the United States. The importance of the large-scale mean currents and of the eddy kinetic energy of the large-scale flow are discussed both for the selection of potential isolation sites and for determining the potential dispersion pattern from selected sites. These processes play a particularly significant role in the Atlantic off the east coast of the United States. There the potential isolation sites are close to a highly energetic region of the abyssal circulation, the North Atlantic Deep Western Boundary Current. The Naval Research Laboratory Layered Ocean Model was used to simulate large-scale ocean circulation in the North Atlantic Subtropical Gyre. Numerical experiments were conducted using monthly climatological forcing. High horizontal resolution, 1/16°, was required to achieve a realistic level of kinetic energy in the deep ocean and a realistic deep mean flow. The prognostic variables came to statistical equilibrium after 50 years of simulation. Then the mean currents and eddy kinetic energy were calculated based on 4 years of simulated data. The patterns and magnitudes of the currents and kinetic energy from the simulation agreed with the observations. The simulated pathways of the Deep Western Boundary Current including bifurcations and reconnections conformed to current concepts of its circulation. The recirculation gyres under the Gulf Stream were simulated, as were the eddies, meanders and high levels of eddy kinetic energy northeast of the New England Seamount chain. The decrease in eddy kinetic energy to the south over the Hatteras Abyssal Plain and the weak currents in the region of 25°N to 30°N along 70°W were also in agreement with observations and verified the results of the site selection model. Tracers were released into the 1° squares that were chosen by the NRL site selection model as the best candidates for isolation of dredged material. Modest quantities of the tracers were dispersed beyond the initial release areas and only trace amounts were mixed out of the bottom layer of the model. The results of this study indicate that if contaminated material was released into the water column from isolation sites in the region chosen by the site selection model it would probably be dispersed over a limited area. However, the dispersion of the tracers was anisotropic and varied spatially among the sites. This underscores the need to conduct both boundary layer-scale and gyre-scale numerical simulations and careful measurement programs at the sites chosen for initial evaluation of deep-ocean isolation of dredged material. The next step is to run mesoscale models that use the results from this study as boundary conditions and the results from plume models of contaminant release to determine the amount of contaminated material that might leak out of the boundary layer into the deep-ocean circulation system.

Monitoring of the Gulf Stream path using Geosat and Topex/Poseidon altimetric data assimilated into a model of ocean circulation

Abstract Two years of altimetric data from the Geosat satellite and one year from Topex/Poseidon were assimilated into a numerical ocean circulation model of the North Atlantic. The results of these simulations were used to compute statistics concerning the movements of the Gulf Stream from Cape Hatteras to 45 °W. This approach enabled us to obtain almost continuous spatio-temporal coverage of the phenomenon; it also allowed observations of the vertical structure of the current to be made. The mean path of the Gulf Stream is described. This trajectory is affected by the New England Seamount chain which gives rise to a south-easterly deflection and to an increase in positional variability which reaches a maximum near 61 °W. Downstream from this chain, the Gulf Stream reaches its highest latitude, and favoured positions for meanders were identified: two crests near 57 °W and 51 °W flanking a trough near 53 °W. In terms of the vertical structure of the current, a southerly shift of the Gulf Stream path as a function of depth was clearly seen. The Gulf Stream has a marked seasonal cycle with northerly maxima in August–September and southerly maxima in February–March. This cycle is not, however, in phase with that observed near to the North American continent. Secondary positional extrema, on the other hand, do not always occur at the same time of year. Spectra of the lateral displacements of the Gulf Stream are remarkably similar at all points. The spectra considered are red spectra with slopes of almost — 2, except to the east of the New England Seamount chain where the slope falls off for periods of more than 50 days. The statistics presented in this article are very much in agreement with earlier work on the subject and thus show the feasibility and value of such a joint model/data assimilation approach for monitoring major ocean currents.

Assimilation of TOPEX/POSEIDON altimeter data into a circulation model of the North Atlantic

Assimilation experiments were conducted using the first 12 months of TOPEX/POSEIDON (T/P) altimeter measurements in a multilayered quasi‐geostrophic model of the North Atlantic between 20°N and 60°N. These experiments demonstrate the feasibility of using T/P data to control a basin‐scale circulation model by means of an assimilation procedure. Moreover, they allow us to recreate the four‐dimensional behavior of the North Atlantic Ocean during the year October 1992–September 1993 and to improve our knowledge and understanding of such circulation patterns. For this study we used a four‐layer quasigeostrophic model of high horizontal resolution (1/6° in latitude and longitude). The assimilation procedure used is an along‐track, sequential, nudging technique. The evolution of the model general circulation is described and analyzed from a deterministic and statistical point of view, with special emphasis on the Gulf Stream area. The gross features of the North Atlantic circulation in terms of mean transport and circulation are reproduced, such as the path, penetration and recirculation of the Gulf Stream, and its meandering throughout the eastern basin. The North Atlantic Drift is, however, noticeably underestimated. A northern meander of the north wall of the Gulf Stream above the New England Seamount Chain is present for most of the year, while, just downstream, the southern part of the jet is subject to a 100‐km southeastward deflection. The Azores current is shown to remain stable and to shift southward with time from the beginning of December 1992 to the end of April 1993, the amplitude of the shift being about 2°. The computation of the mean latitude of the Gulf Stream as a function of time shows an abrupt shift from a northern position to a southern position in January, and a reverse shift, from a southern position to a northern position, in July. Finally, some issues are addressed concerning the comparison of assimilation experiments using T/P data and Geosat data. The first results show that the T/P simulations are more energetic than the Geosat simulations, especially east of the Mid‐Atlantic Ridge, for every wavelength from 50 km to 500 km. This property is also verified in the deep ocean. The predicted abyssal circulation is indeed more energetic in the T/P case, which is more in accordance with what we know of the real ocean. Moreover, the good T/P altimeter coverage near the coasts greatly improves the model eddy kinetic energy levels in these areas, especially east of 25°W.

Determining the mean Gulf Stream and its recirculations through combining hydrographic and altimetric data

The altimetric data from the first 2.5‐year Geosat Exact Repeat Mission were used to estimate the mean sea surface height (SSH) field in the region of the Gulf Stream and its recirculation gyres. Assuming the instantaneous surface velocity field is composed of an eastward flowing jet and two westward recirculating flows, we used the time‐varying surface data from the altimeter to determine the shape of the along‐track mean SSH profiles and the historical hydrographic data to constrain the net SSH difference across the Gulf Stream system. The two‐dimensional mean SSH field was determined by objectively mapping the mean height profiles along the ascending and descending tracks. The SSH jump across the mean Gulf Stream has a maximum of 1.15m around 65°W and drops to an almost constant 0.9m downstream of the New England Seamount Chain (NESC). While the SSH jump associated with the mean northern recirculating flow is mostly uniform, we found that the Gulf Stream's southern recirculation has two local gyres that are separated by the NESC. An attempt was then made to estimate the mean deep circulation in this region by comparing the mean SSH field derived from the altimetry data and the surface dynamic height field based on the historical hydrographic data. Despite the large uncertainties, the mean deep flow pattern thus estimated agrees favorably with the overall circulation pattern from the long‐term current meter observations. Like the well‐defined northern recirculation gyre, we found that a continuous southward flow exists along 57.5°W, which follows closely along the deep layer potential vorticity contours. To the south of the Gulf Stream, the deep circulation consists of two separated recirculation gyres; the recirculation gyre to the east of the NESC appears to be trapped around the Corner Rise.