Between Florida and Cape Hatteras, North Carolina, the Gulf Stream carries warm, salty waters poleward along the continental slope. This strong current abuts the edge of the South Atlantic Bight (SAB) continental shelf and is thought to influence exchange of waters between the open ocean and the shelf. Observations from a pair of instruments deployed for 19 months in the northern SAB are used here to examine the processes by which the Gulf Stream can impact this exchange. The instrument deployed on the SAB shelf edge shows that the time‐averaged along‐slope flow is surface‐intensified with only few flow reversals at low frequencies (>40‐day period). Time‐averaged cross‐slope flow is onto the SAB shelf in a lower layer and off‐shelf above. Consistent with Ekman dynamics, the magnitude of lower‐layer on‐shelf flow is correlated with the along‐slope velocity, which is in turn controlled by the position and/or transport of the Gulf Stream that flows poleward along the SAB continental slope. In the frequency band associated with downstream‐propagating wave‐like meanders of the Gulf Stream jet (2‐15 day period), currents at the shelf‐edge are characterized by surface‐intensified flow in the along‐ and cross‐slope directions. Estimates of maximum upwelling velocities associated with cyclonic frontal eddies between meander crests occasionally reach 100 m/day.
In situ observations from a 19‐month deployment of current‐ and pressure‐sensor equipped inverted echo sounders (CPIESs) along and across the Gulf Stream near Cape Hatteras capture spatial and temporal variability where this western boundary current separates from the continental margin. Regional hydrographic casts and two temperature cross‐sections spanning the Gulf Stream southeast of Cape Hatteras are used with the CPIESs' records of acoustic travel time to infer changes in thermocline depth
- NSF-PAR ID:
- 10450097
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 126
- Issue:
- 9
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The convergence of different water masses on the shelf and along the shelfbreak, and cross‐isobath shelf‐open ocean exchanges contribute to the complex circulation near Cape Hatteras. We examine the mean and variability of these circulations using data from nine bottom‐mounted acoustic Doppler current profilers, deployed over the mid‐ to outer‐continental shelf north and south of Cape Hatteras as part of the Processes driving Exchange At Cape Hatteras program. The 18‐month‐mean depth‐averaged shelf flows are mostly aligned with isobaths and oriented toward Cape Hatteras. At two sites just north of Cape Hatteras, mean flows have a strong cross‐shelf component. Two dominant spatial patterns in the velocity field are obtained from an empirical orthogonal function analysis. The two leading modes contain 61% of the total variance. The spatial variation of Mode 1 exhibits an along‐shelf flow pattern, while that of Mode 2 shows a convergent flow pattern. The principal component (PC) series of Mode 1 is significantly correlated with the local wind stress, confirming that the along‐shelf flow is wind‐driven as expected. The PC of Mode 2 is highly correlated with the Gulf Stream lateral position as inferred from the current‐ and pressure‐sensor‐equipped inverted echo sounders over the slope south of Cape Hatteras, which indicates that Gulf Stream movement drives time‐varying shelf flow convergence. Conditionally averaged sea‐surface temperature and high‐frequency radar‐measured surface currents based on PC1 and PC2 confirm these relationships and further illustrate how the wind and Gulf Stream forcing work together to influence the flow regime in this region.
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Abstract The Gulf Stream affects global climate by transporting water and heat poleward. The current’s volume transport increases markedly along the U.S. East Coast. An extensive observing program using autonomous underwater gliders provides finescale, subsurface observations of hydrography and velocity spanning more than 15° of latitude along the path of the Gulf Stream, thereby filling a 1500-km-long gap between long-term transport measurements in the Florida Strait and downstream of Cape Hatteras. Here, the glider-based observations are combined with shipboard measurements along Line W near 68°W to provide a detailed picture of the along-stream transport increase. To account for the influences of Gulf Stream curvature and adjacent circulation (e.g., corotating eddies) on transport estimates, upper- and lower-bound transports are constructed for each cross–Gulf Stream transect. The upper-bound estimate for time-averaged volume transport above 1000 m is 32.9 ± 1.2 Sv (1 Sv ≡ 106 m3 s−1) in the Florida Strait, 57.3 ± 1.9 Sv at Cape Hatteras, and 75.6 ± 4.7 Sv at Line W. Corresponding lower-bound estimates are 32.3 ± 1.1 Sv in the Florida Strait, 54.5 ± 1.7 Sv at Cape Hatteras, and 69.9 ± 4.2 Sv at Line W. Using the temperature and salinity observations from gliders and Line W, waters are divided into seven classes to investigate the properties of waters that are transported by and entrained into the Gulf Stream. Most of the increase in overall Gulf Stream volume transport above 1000 m stems from the entrainment of subthermocline waters, including upper Labrador Sea Water and Eighteen Degree Water.more » « less
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Abstract The relationship between Gulf Stream (GS) transport and coastal sea level is investigated using monthly GS transport between 1993 and 2019 at Florida Straits and 10 altimeter tracks. The results show that GS transport decorrelates quickly along its path, indicating it is misleading to assume that transport at a particular location represents strength of the GS as a whole. GS transport south of Cape Hatteras is significantly correlated with coastal sea level in South Atlantic Bight from both altimetry and tide gauges. North of Cape Hatteras, sea level changes associated with GS transport decay rapidly away from GS on the onshore side and become negligible approximately 300 km northwest of GS axis. In this region, the correlations between GS transport and sea level are primarily in the deep ocean and rarely on the shelf, indicating that coastal sea level is unlikely to be driven by geostrophic adjustment to changes in GS transport.
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This dataset includes multiple fields: (i) files for monthly and annual fields for the max curl line and the zero curl line at 0.1 degree longitudinal resolutions; (ii) files for monthly and annual GS path obtained from Altimetry and originally processed by Andres (2016) at 0.1 degree longitudinal resolution. The maximum curl line (MCL) and the zero curl line (ZCL) calculations are briefly described here and are based on the original wind data (at 1.25 x 1.25 degree) provided by the Japanese reanalysis (JRA-55; Kobayashi et al., 2015) and available at https://zenodo.org/record/8200832 (Gifford et al. 2023). For details see Gifford, 2023.
The wind stress curl (WSC) fields used for the MCL and ZCL calculations extend from 80W to 45W and 30N to 45N at the 1.25 by 1.25-degree resolution. The MCL is defined as the maximum WSC values greater than zero within the domain per 1.25 degree longitude. As such, it is a function of longitude and is not a constant WSC value unlike the zero contour. High wind stress curl values that occurred near the coast were not included within this calculation. After MCL at the 1.25 resolution was obtained the line was smoothed with a gaussian smoothing and interpolated on to a 0.1 longitudinal resolution. The smoothed MCL lines at 0.1 degree resolution are provided in separate files for monthly and annual averages (2 files). Similarly, 2 other files (monthly and annual) are provided for the ZCL.
Like the MCL, the ZCL is a line derived from 1.25 degree longitude throughout the domain under the condition that it's the line of zero WSC. The ZCL is constant at 0 and does not vary spatially like the MCL. If there are more than one location of zero curl for a given longitude the first location south of the MCL is selected. Similar to the MCL, the ZCL was smoothed with a gaussian smoothing and interpolated on to a 0.1 longitudinal resolution.
The above files span the years from 1980 through 2019. So, the monthly files have 480 months starting January 1980, and the annual files have 40 years of data. The files are organized with each row being a new time step and each column being a different longitude. Therefore, the monthly MCL and ZCL files are each 480 x 351 for the 0.1 resolution data. Similarly, the annual files are 40 x 351 for the 0.1 degree resolution data.
Note that the monthly MCLs and ZCLs are obtained from the monthly wind-stress curl fields. The annual MCLs and ZCLs are obtained from the annual wind-stress curl fields.
Since the monthly curl fields preserves more atmospheric mesoscales than the annual curl fields, the 12-month average of the monthly MCLs and ZCLs will not match with the annual MCLs and ZCLs derived from the annual curl field. The annual MCLs and ZCLs provided here are obtained from the annual curl fields and representative metrics of the wind forcing on an annual time-scale.
Furthermore, the monthly Gulf Stream axis path (25 cm isoheight from Altimeter, reprocessed by Andres (2016) technique) from 1993 through 2019 have been made available here. A total of 324 monthly paths of the Gulf Stream are tabulated. In addition, the annual GS paths for these 27 years (1993-2019) of altimetry era have been put together for ease of use. The monthly Gulf Stream paths have been resampled and reprocessed for uniqueness at every 0.1 degree longitude from 75W to 50W and smoothed with a 100 km (10 point) running average via matlab. The uniqueness has been achieved by using Consolidator algorithm (D’Errico, 2023).
Each monthly or annual GS path has 251 points between 75W to 50W at 0.1 degree resolution.
Please contact igifford@earth.miami.edu for any queries. {"references": ["Andres, M., 2016. On the recent destabilization of the Gulf Stream path downstream of Cape Hatteras. Geophysical Research Letters, 43(18), 9836-9842.", "D'Errico, J., 2023. Consolidator (https://www.mathworks.com/matlabcentral/fileexchange/ 8354-consolidator), MATLAB Central File Exchange. Retrieved June 17, 2023.", "Gifford, Ian. H., 2023. The Synchronicity of the Gulf Stream Free Jet and the Wind Induced Cyclonic Vorticity Pool. MS Thesis, University of Massachusetts Dartmouth. 75pp.", "Gifford, Ian, H., Avijit Gangopadhyay, Magdalena Andres, Glen Gawarkiewicz, Hilde Oliver, Adrienne Silver, 2023. Wind Stress, Wind Stress Curl, and Upwelling Velocities in the Northwest Atlantic (80-45W, 30-45N) during 1980-2019, https://zenodo.org/record/8200832.", "Kobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., Onogi, K., Kamahori, H., Kobayashi, C., Endo, H. and Miyaoka, K., 2015. The JRA-55 reanalysis: General specifications and basic characteristics.\u202fJournal of the Meteorological Society of Japan. Ser. II,\u202f93(1), pp.5-48. Kobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., Onogi, K., Kamahori, H., Kobayashi, C., Endo, H. and Miyaoka, K., 2015. The JRA-55 reanalysis: General specifications and basic characteristics.\u202fJournal of the Meteorological Society of Japan. Ser. II,\u202f93(1), pp.5-48."]}
