Gulf Stream North Wall Research Articles

Does the mid‐Atlantic United States sea level acceleration hot spot reflect ocean dynamic variability?

To test a hypothesized faster‐than‐global sea level acceleration along the mid‐Atlantic United States, I construct a Gaussian process model that decomposes tide gauge data into short‐term variability and longer‐term trends, and into globally coherent, regionally coherent, and local components. While tide gauge records indicate a faster‐than‐global increase in the rate of mid‐Atlantic U.S. sea level rise beginning ∼1975, this acceleration could reflect either the start of a long‐term trend or ocean dynamic variability. The acceleration will need to continue for ∼2 decades before the rate of increase of the sea level difference between the mid‐Atlantic and southeastern U.S. can be judged as very likely unprecedented by 20th century standards. However, the difference is correlated with the Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and Gulf Stream North Wall indices, all of which are currently within the range of past variability.

Variability of hydrophysical characteristics in the Gulf Stream

We consider the interannual variability of the intensity of the Gulf Stream and interannual fluctuations of seawater parameters in the Gulf Stream and in the Labrador Current during intense climate warming. We show that this intensity has increased during this period. The scales of fluctuations and their contribution to variance in the initial time series was determined from wavelet analysis of the Gulf Stream north wall. We noted a considerable decrease in water density of the main branch of the Gulf Stream, caused by the increase in temperature due to global climate warming, and an absence of trends in water density of the main branch of the Labrador Current.

Eighteen Degree Water formation within the Gulf Stream during CLIMODE

Analysis of wintertime CLIMODE data for 2007 indicates that a substantial portion of new Eighteen Degree Water (EDW) is likely ventilated within the eastward flowing Gulf Stream (GS) between 67°W and 52°W longitudes, possibly exceeding that formed elsewhere in the northern Sargasso Sea. Use of some global air–sea interaction data sets applied to the study region for Feb/Mar of 2007 indicate that this winter may have been anomalously energetic in air–sea exchange compared to the mean of the prior 19yr. The largest heat and freshwater fluxes found directly over the meandering warm core of the Gulf Stream are capable of removing most of the subtropical heat anomaly of the GS, but cross-frontal fluxes of salinity are required to account for the observed regional salinity structure. An isopycnal diffusivity of ∼100m2s−1 is inferred from the salinity balance. This mixing would also account for the observation that EDW formed in the GS is slightly fresher than that formed in northern Sargasso Sea. The lateral flux of heat across the GS north wall also acts to cool the resulting EDW water, but the heat balance for EDW production is largely determined from GS advection and air–sea fluxes, in contrast to salinity. Based on oxygen saturation data, we estimate that 1.8–3.0Sv-yr of new EDW is formed in the GS for the winter of 2007. EDW originating from the GS is generated in a separate location from where it is accumulated in the northern Sargasso Sea. This manner of EDW formation will produce unique characteristics of EDW found in the northern Sargasso Sea: ones that differ in T/S properties from that formed south of the GS under the more traditional 1D, cooling-driven convection process.

Transport of the North Atlantic Deep Western Boundary Current about 39°N, 70°W: 2004–2008

Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.

Interannual variability of Gulf Stream warm-core rings in response to the North Atlantic Oscillation

Analysis of a quality-controlled database of Gulf Stream warm-core rings (WCRs) between 75° and 50°W during 1978–1999 demonstrates a significant correlation between WCR occurrences and variations in large-scale atmospheric forcing related to the state of the North Atlantic Oscillation (NAO). The mechanisms for linking the NAO with the rate of WCR occurrences are two-fold: (1) the influence of the NAO on Gulf Stream (GS) position, which could affect the interaction of the Gulf Stream with the New England Seamounts chain and thus allow for a higher/lower number of WCR occurrences; (2) the NAO-induced eddy kinetic energy (EKE) variability in the Gulf Stream region (GSR), which is indicative of the baroclinic instability processes necessary for WCR formation. Variability in GS movement is studied by analyzing annual mean positions of the Gulf Stream North Wall obtained from satellite-derived sea surface temperature (SST) frontal charts. Response of GSR EKE to fluctuations in the state of the NAO is examined with a numerical simulation of the North Atlantic basin from 1980–1999. The North Atlantic basin is simulated using a 1/6°-resolution eddy-resolving Regional Ocean Modeling System (ROMS) model that spins up with Southampton Oceanography Center (SOC) ocean-atmosphere atlas-derived atmospheric forcing fields. Model-derived EKE estimates are observed to be in good agreement with TOPEX/Poseidon altimeter-based EKE estimates as well as with results from other modeling studies for the North Atlantic basin. We suggest that lateral movement of the GS may not be the primary mechanism causing variation in the rate of WCR occurrences, because GS position is observed to respond at a lag of one year, whereas annual rates of WCR occurrences respond at 0-year lag to the NAO. Based on results from numerical simulations of the North Atlantic basin, adjustment to NAO-induced wind forcing is seen to impact the GSR EKE intensity and possibly the related baroclinic instability structure of the GS at 0-year lag. These results suggest that NAO-induced interannual variability in GSR EKE is the most likely mechanism affecting WCR occurrences. Numerical simulations show that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, possibly resulting in higher (lower) occurrences of WCRs.

Changes in DMS production and flux in relation to decadal shifts in ocean circulation

A fundamental question is are the biological processes regulating dimethylsulphide (DMS) production by the marine ecosystem interconnected and responding to atmospheric or ocean signals at decadal timescales? Related to this is a need to quantify how climate change affects these interconnections and understand the expected levels of natural variability on decadal timescales. To explore this we have used indicators of climate variability [the Gulf Stream North Wall (GSNW) and the North Atlantic Oscillation (NAO) indices] as probes to demonstrate that a marine ecosystem model, incorporating DMS production, can extract and amplify a climatic signal, which is spread across a variety of meteorological variables. The GSNW signal is imparted through the wind and cloud forcing, despite the fact there was not significant relationship observed between the GSNW index and the meteorological forcing data. The model simulations appear to reproduce observed decadal variability in phytoplankton community structure in the eastern North Atlantic and imply that DMS(P) biogeochemistry may vary on decadal timescales as a consequence of changes in community structure. The GSNW index is a potential indicator of such changes and there may have been a regime shift in DMSP production in the eastern North Atlantic coincident with that observed for plankton. Sensitivity analysis indicates that the impact of climate variability on DMS biogeochemistry may potentially be damped by the ability of microbial communities to adapt physiologically to the effects of changes in light and nutrients.

Jellyfish abundance and climatic variation: contrasting responses in oceanographically distinct regions of the North Sea, and possible implications for fisheries

Jellyfish medusae prey on zooplankton and may impact fish recruitment both directly (top-down control) and indirectly (through competition). Abundances of Aurelia aurita, Cyanea lamarckii and Cyanea capillata medusae (Scyphozoa) in the North Sea appear to be linked to large-scale inter-annual climatic change, as quantified by the North Atlantic Oscillation Index (NAOI), the Barents Sea-Ice Index (BSII) and changes in the latitude of the Gulf Stream North Wall (GSNW). Hydroclimatic forcing may thus be an important factor influencing the abundance of gelatinous zooplankton and may modulate the scale of any ecosystem impact of jellyfish. The population responses are probably also affected by local variability in the environment manifested in intra-annual changes in temperature, salinity, current strength/direction and prey abundance. Aurelia aurita and C. lamarckii in the north-west and south-east North Sea exhibited contrasting relationships to change in the NAOI and BSII: north of Scotland, where the North Sea borders the Atlantic, positive relationships were evident between the abundance of scyphomedusae (data from 1974 to 1986, except 1975) and the indices; whereas west of northern Denmark, a region much less affected by Atlantic inflow, negative relationships were found (data from 1973 to 1983, except 1974). Weaker negative relationships with the NAOI were also found in an intermediate region, east of Scotland, for the abundance of A. aurita and C. capillata medusae (1971 to 1982). East of Shetland, the abundance of jellyfish was not correlated directly with the NAOI but, in contrast to all other regions, the abundances of A. aurita and C. lamarckii (1971 to 1986, not 1984) were found to correlate negatively with changes in the GSNW, which itself was significantly positively correlated to the NAOI with a two year lag. On this evidence, we suggest that, for jellyfish, there exist three regions of the North Sea with distinct environmental processes governing species abundance: one north of Scotland, another east of Shetland, and a more southerly group (i.e. east of Scotland and west of northern Denmark). Impacts by jellyfish are likely to vary regionally, and ecosystem management may benefit from considering this spatial variability.

Seven copepod species considered as indicators of water-mass influence and changes: results from a Northumberland coastal station

Abstract Zooplankton has been sampled monthly since 1969 at Station Z off the Northumberland coast. Seven copepod species were chosen as potential indicators of specific water masses. Data have been analysed to provide information about seasonal and interannual changes in the zooplankton community with special reference to the indicator taxa and to the possible role of hydrographic and climate drivers, including variations in the position of the Gulf Stream North Wall position and the North Atlantic Oscillation. Results show that, at this Northumberland coastal station, some copepod species are likely to be good indicators of water-mass influence and changes.

Inverse relationship between cod recruitment in the North sea and young fish in the continuous plankton recorder survey

There is a significant (p<0.01) negative correlation between cod recruitment (age 1 year) and the numbers of young fish taken in the Continuous Plankton Recorder (CPR) Survey in the North Sea. This correlation is analysed with reference to hydrographic, climatic and biological variables in order to determine whether it results from opposite responses to physical forcing, trophic relationships or a combination of these. The results of correlation analyses are consistent with cod recruitment being determined mainly during the planktonic phase of development in the preceding year. Multiple regressions indicate that a combination of the North Atlantic Oscillation index in the preceding year and the abundance of CPR young fish is the pair of variables that provide the best fit to the data. The abundance of CPR young fish is best predicted by the Gulf Stream North Wall Index and the abundance of phytoplankton in the spring of the previous year. Opposite responses to climatic forcing and the regional environmental consequences are identified as of primary importance but interactions, possibly both predation and competition, occur between the planktonic stages of cod and both the planktonic stages of fish and the stocks from which they were derived.

Long-term, predation-based control of a central-west North Sea zooplankton community

Abstract Long-term monitoring of the zooplankton community at a station 5.5 miles from the English coast in the central-west North Sea has been performed since 1968. Analyses of these data have revealed an inverse relationship between annual total zooplankton abundance and the position of the Gulf Stream North Wall (GSNW). This long-term relationship is opposite to the long-term positive association observed between the GSNW and total zooplankton abundances throughout most of the oceanic NE Atlantic region and the northern and central North Sea using Continuous Plankton Recorder data. This study investigates the mechanism behind the inverse relationship with the GSNW, focussing on the importance of zooplankton predators in influencing long-term changes in the zooplankton community of the central-west North Sea. The results suggest that the dominant zooplankton predator Sagitta elegans plays a key role in mediating spring copepod population growth rates and thus their maximum and overall productivity during any one particular year. In turn, the abundance of Sagitta during the spring appears to be related to climatic factors. The implications of this on the zooplankton community are discussed.

An extraordinary breach of the Gulf Stream north wall by a cold water intrusion

A breach of the Gulf Stream (GS) North Wall by a parcel of cold Middle Atlantic Bight shelf water was observed from October 2 to 5, 2001 by a sequence of NOAA AVHRR sea surface temperature (SST) images. Unlike the warm/cold core rings generated by GS meanders, the path of cold water eventually cuts into the GS. Subsequently, the cold water remained intact and eventually penetrated and transversed the entire width of the GS. This is readily apparent in the 3‐day SST sequence. Analysis of wind data from a nearby CMAN station reveals a high (over 12.8 m/s) and persistent (3 days) alongshore wind event occurred prior to the breach. An 11‐year time series (1991–2001) of local wind measurements show that this type of strong and persistent wind situation is uncommon, which may explain why the large‐scale surface shelf water breach of the GS has not been previously reported.

Observation of hurricane-generated ocean swell refraction at the Gulf Stream north wall with the RADARSAT-1 synthetic aperture radar

We analyze the refraction of long oceanic waves at the Gulf Stream's north wall off the Florida coast as observed in imagery obtained from the RADARSAT-1 synthetic aperture radar (SAR) during the passage of Hurricane Bonnie on August 25, 1998. The wave spectra are derived from RADARSAT-1 SAR images from both inside and outside the Gulf Stream. From the image spectra, we can determine both the long wave's dominant wavelength and its propagation direction with 180/spl deg/ ambiguity. We find that the wavelength of hurricane-generated ocean waves can exceed 200 m. The calculated dominant wavelength from the SAR image spectra agree very well with in situ measurements made by National Oceanic and Atmospheric Administration National Data Buoy Center buoys. Since the waves mainly propagate toward the continental shelf from the open ocean, we can eliminate the wave propagation ambiguity. We also discuss the velocity-bunching mechanism. We find that in this very long wave case, the RADARSAT-1 SAR wave spectra should not be appreciably affected by the azimuth falloff, and we find that the ocean swell measurements can be considered reliable. We observe that the oceanic long waves change their propagation directions as they leave the Gulf Stream current. A wave-current interaction model is used to simulate the wave refraction at the Gulf Stream boundary. In addition, the wave shoaling effect is discussed. We find that wave refraction is the dominant mechanism at the Gulf Stream boundary for these very long ocean swells, while wave reflection is not a dominant factor. We extract 256-by-256 pixel full-resolution subimages from the SAR image on both sides of the Gulf Stream boundary, and then derive the wave spectra. The SAR-observed swell refraction angles at the Gulf Stream north wall agree reasonably well with those calculated by the wave-current interaction model.

Gulf Stream and Ring Feature Analyses for Forecast Model Validation*

Abstract A series of Gulf Stream forecast model test cases were developed for the Data Assimilation and Model Evaluation Experiment (DAMEE). The model initialization and verification procedure relies heavily on a series of accurate synoptic snapshots of the Gulf Stream north wall and ring locations. Satellite infrared imagery, Geosat altimetry, and numerous in situ temperature profiles were combined using a geographic information system to construct Gulf Stream and ring location analyses at approximately weekly intervals during a 6-week, data-rich time period. To improve the accuracy of the feature analyses, a new image compositing technique called patching was developed to decrease the spatial smearing experienced with standard warmest pixel composites. During the 6-week test period, three ring formations, two ring absorptions, and one ring merger event were observed. The average difference between the weekly north wall positions ranged from 20 to 34 km (average 27 km). When the DAMEE GSR north wall posi...

Frontal enhancement of dimethylsulfide concentrations across a Gulf Stream meander

Abstract Aqueous and atmospheric dimethyl sulfide (DMS) concentrations (0.4–4 nM, 5–350 ppt) were determined across a Gulf Stream meander in April 1989. Physical processes along and across the front were expected to result in biological changes also reflected in DMS levels. Highest concentrations of aqueous DMS (3–4 nM) were observed in surface waters coinciding with Slope waters at, or inshore of, the Gulf Stream north wall. This water also contained the highest chlorophyll levels (up to 2.6 μg chl/l) which co-varied with the sea surface DMS concentrations. Concentrations of atmospheric DMS were lower and more constant (21 ± 9 ppt) outside the frontal area, either in Slope or Stream waters. At the northern wall of the Gulf Stream, a strikingly sharp increase in atmospheric DMS (up to 350 ppt) was observed.

Maltese Front variability from satellite observations based on automated detection

Maltese Front location statistics are obtained from multichannel sea-surface temperature (MCSST) images, derived from Advanced Very High Resolution Radiometer (AVHRR) observations of the Mediterranean Sea during March 3 through September 27, 1993. The statistics are based on semiautomated determinations of the Front's sea-surface temperature surface expression. Expert analyses from the Naval Oceanographic Office give an accuracy check. Expert techniques are largely manual, labor-intensive, subjective, and skill-dependent; therefore, automation could be beneficial. A mathematical morphology-based method successfully delineates the Front. It finds the temperature gradients most likely to be the Front's and presents the corresponding segmentations to the operator. The method was developed to find stars in astronomical images, and has successfully analyzed solar magnetograms and satellite Gulf Stream images. Rings, fronts, and sunspots are not star-like, but simple preprocessing adapts the technique to these problem domains. This work constitutes another successful application. The success in "moving" the technique is encouraging. The Maltese Front's thermal gradients are 5-10 times weaker than the Gulf Stream North Wall's, yet the method produces useful results, and it may work in other regions.

An Acoustic Telemetry System Implemented for Real-Time Monitoring of the Gulf Stream with Inverted Echo Sounders

From August 1989 until August 1990, a simple acoustic telemetry system was used for obtaining real-time data from four inverted echo sounders (IESs) deployed in the Synoptic Ocean Prediction Experiment (SYNOP) inlet array in the Gulf Stream east of Cape Hatteras. The telemetry system is based on encoding data as a time-delayed broadcast acoustic signal: the delay of the time of broadcast of the signal, with respect to a reference time, is proportional to the data value. The changes in delay time, from one broadcast signal to the next, are recorded at a remote receiving station. Moored near the sea floor, IESs are designed to emit high-frequency (10 kHz) acoustic pings toward the sea surface and receive the reflected signal. In the Gulf Stream region, the round-trip travel time of the emitted signal is proportional to the depth of the main thermocline. As the Gulf Stream meanders back and forth over the instrument, its position can be tracked with the thermocline depth changes. Every 24 h, each AES calculated a representative travel time from a set of 48 bursts of measurements (τ), and telemetered that value to a listening station on Bermuda. From the received data, a daily time series of the depth of the 12°C isotherm (our proxy for main thermocline depth) at each IES was calculated. The position of the Gulf Stream north wall through the IES array was calculated on a daily basis from the thermocline depth information at each IES site. Three of the four IESs were recovered in August 1990. Although the IES at site B2 was not recovered, its telemetered data was received at Bermuda. The rms agreement between thermocline depths, as calculated from the data on tape from the recovered IESs and as calculated from the received telemetry data, is 20 m. This compares favorably with the 19-m uncertainty in calibrating the τ's as a measure of the thermocline depth. The rms agreement between the position of the Gulf Stream path through the IESs as calculated from the tape data and the telemetry data is 5 km. This telemetry system is not IES specific. It could be used with other appropriately modified oceanographic instruments, such as current meters and pressure sensors.

Variations of the Gulf Stream's north wall east of Cape Hatteras

Twelve years (1977–1988) of NOAA infrared imagery data were used to study the variations of the north wall displacements of the Gulf Stream east of Cape Hatteras (i.e., between 74 W and 65 W). The variations of the north wall displacements were compared with the variations in the Straits of Florida transport, which was estimated using a multifrequency model based on the sea-level differences between Siboney, Cuba and Key West, Florida for the period 1977–1988, and with transport measurements based on the submarine cable between Jupiter, Florida, and Settlement Point in the Bahamas for the period 1982–1988. The north wall reached a maximum northward position in 1985 when the cable data indicated that the Straits of Florida transport was a maximum. The north wall reached a maximum southward position in 1981. Cable data were not available before 1982, but the available data suggested that there may have been a minimum transport in 1981. A weak annual cycle of the north wall was found with a maximum northward position in the fall and a maximum southward position in the spring. No relationship was found between the annual cycle of the Gulf Stream's north wall and the annual cycle of the Straits of Florida transport.

Sperm whales associated with Gulf Stream features off the north‐eastern USA shelf

ABSTRACTMarine mammal surveys were conducted during sum Mer 1990 and 1991 in shelf‐edge and off‐shelf waters between Cape Hatteras and Georges Bank. Sperm whales were the most frequently sighted large whale in both years, constituting 27.9% and 12.7% of total sightings in 1990 and 1991, respectively. Sighting rates of sperm whales that were within a 9.3 km buffer (5 n.m.) of the Gulf Stream north wall were not significantly different (x2= 2.86, P &gt; 0.05) from those within the Gulf Stream during 1990. In 1991, however, significantly more (x2= 51.0, P &lt; 0.05) sightings were associated with shelf‐edge areas where warm core rings were located.

Mesoscale pigment fields in the Gulf Stream: Observations in a meander crest and trough

In September‐October 1988 and April 1989 a series of hydrographic transects were completed across the Gulf Stream front in a meander crest and meander trough, respectively. One of the main experimental objectives was to relate the spatial distribution of chlorophyll to the physical fields of Gulf Stream meanders. Chlorophyll distributions are derived from conductivity‐temperature‐depth (CTD)/fluorescence profiles calibrated with discrete pigment samples collected from bottles at several depths at various stations. Objective analysis (OA) maps and vertical sections, in stream coordinates, of chlorophyll on density surfaces show the chlorophyll distribution was strongly related to the structure of the Gulf Stream front. In particular, chlorophyll concentrations greater than 0.4 mg m−3 were at, or inshore of, the Gulf Stream north wall. Characteristic length scales of chlorophyll distribution determined from the horizontal and temporal correlation function are the same order (50–100 km) as length scales of the physical variables. The maximum chlorophyll concentrations in the vertical were from the surface to 50‐m depth on the western flank of the meander crest and deepened to 75‐ to 100‐m depth on the eastern flank. This coincides with the deepening of the 24.4–25.7 σθ surfaces from the western to the eastern flank of the meander crest. Although in the spring cruise the maximum chlorophyll concentrations were also found at depths between the surface and 100 m, there were no clear distinctions in pigment distributions between the western and eastern transects of a relatively weak trough. Maximum chlorophyll concentrations, 1.0–1.5 mg m−3, from the 1989 spring bloom in slope waters were about double those observed in the 1988 fall data. The primary physical mechanisms influencing the mesoscale pigment distribution in Gulf Stream meanders observed in this study are (1) outcropping of nutrient‐bearing strata in the spring, (2) meander‐induced upwelling of nutrients along sloping isopycnals, and (3) Gulf Stream‐ring interactions.

Contour Analysis: A New Approach for Melding Geophysical Fields

This paper introduces a new approach, contour analysis, for combining independent estimates of a geophysical field to produce a single realization incorporating data from all sources. Contour analysis divides the field estimates into contours and then calculates the average position of each contour: it is shown that this method avoids the smearing and weakening of important dynamical features which can result from the traditional method of simply averaging data values at fixed geographical coordinates. The paper begins with an overview of how contour points are determined and averaged; this is followed by an illustrative example showing the basic steps one must go through to automate the process on a computer. The remainder of the paper presents comparisons between the traditional approach and the contour analysis approach over a wide range of data types: combining numerical model fields, averaging one-dimensional curves, space-time interpolation of Gulf Stream north wall positions and an application to data assimilation in the Gulf Stream ring and meander region.