ACCESS NUMBER: 30599

STUDY TITLE: A Physical Oceanographic Field Program Offshore North Carolina 

REPORT TITLE: A Physical Oceanographic Field Program Offshore North Carolina, 

Final Synthesis Report and Instrument Performance and Data Quality Control Analyses 

for the Physical Oceanography Field Program Offshore North Carolina Science 

Applications International Corporation 

CONTRACT NUMBER: 14-35-0001-30599 

SPONSORING OCS REGION: Atlantic 

APPLICABLE PLANNING AREAS: South Atlantic; Mid-Atlantic 

FISCAL YEARS OF PROJECT FUNDING: 1991; 1992; 1993; 1994; 1995 

COMPLETION DATE OF REPORT: March 1995 

COSTS: FY 1991: $419,693; FY 1992: $897,172; FY 1993: $989,731; FY 1994: 

$662,121; FY 1995: $44,988 

CUMULATIVE PROJECT COST: $3,013,710 

PROGRAM MANAGER: Thomas J. Berger 

AFFILIATION: Science Applications International Corporation 

ADDRESS: 615 Oberlin Road, Suite 300, Raleigh, North Carolina 27605 

PRINCIPAL INVESTIGATORS*: Jackson O. Blanton, William C. Boicourt, James H. 

Churchill, Peter Hamilton, D. Randolph Watts, Robert J. Wayland 

KEY WORDS: Physical Oceanography, Cape Hatteras, Middle Atlantic Bight, South 

Atlantic Bight, Gulf Stream, Warm Core Rings, Continental Shelf, Slope Sea, Virginia 

Coastal Water, Deep Western Boundary Current, Shelf Water Export, Nearshore Zone 

BACKGROUND: The present study is part of a sequence of programs designed to 

provide MMS with a basis for evaluating the potential environmental impacts of oil and 

gas exploration in Cape Hatteras region. Mobile proposed in the late 1980's to drill for 

gas in Manteo Block 467 and conducted engineering studies at the proposed drill site. 

These studies were determined by a review panel of physical oceanographers to be 

insufficient to serve the purposes of a comprehensive impact statement for the region. 

In keeping with the finding and in accordance with Congressional requirements, two 

studies were identified as necessary (1) a thorough literature and data review (OCS 

Study MMS 93/0031); and (2) this program, a detailed field measurement study. 
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OBJECTIVES: (1) To develop an adequate understanding of the physical 

oceanography of the Cape Hatteras region in a study area which extended from 

34°30'N to 37°00' and from the coast eastward to 73°00'W (but excluding Chesapeake 

Bay); (2) the extent to which contaminants released over the continental slope can 

penetrate onto the shelf and thus shoreward; and (3) to address processes associated 

with flow between the slope area and the shelf break, flow across the shelf break, 

alongshore flow on the shelf, and cross-isobath flow towards the beach in the inner 

shelf zone. 

DESCRIPTION: The study encompassed two years of field work from February 15, 

1992, through February 22, 1994. Nine cruises were competed to perform mooring 

placement, servicing and recovery, and hydrographic measurements. Field 

measurement techniques included (1) 15 current meter moorings on the shelf and in 

deep water; (2) quarterly hydrographic surveys concurrent with current meter servicing 

cruises; (3) three special event surveys, which consisted of detailed hydrographic and 

shipboard acoustic doppler current profiler (ADCP) surveys of offshore flow or Virginia 

coastal water intrusions into Raleigh Bay, and two nearshore experiments with global 

positioning system (GPS) drifters during June and September 1993; (4) seven sets of 

langrangian drifter deployments designed to evaluate the potential for materials 

discharged near the shelf break to reach the nearshore region; and (5) collection of 

satellite infrared imagery of sea surface temperature, and meteorological and water 

level data along the coast within 500 km of Cape Hatteras, to support analysis and 

synthesis of the primary data. 

SIGNIFICANT CONCLUSIONS: Data from this program confirm many notions about 

the nature of the complex circulation of the Cape Hatteras region, including the 

dominant role of the Gulf Stream and its meanders on Raleigh Bay and the shelf break 

just north of Diamond Shoals. Export of water from the Middle Atlantic Bight shelf 

occurs here in a band whose location can shift depending on the location of the Gulf 

Stream. Material released on the shelf outside a nearshore frontal zone, approximately 

2-4 km from the beach, appears likely to be eventually entrained along the Gulf Stream. 

STUDY RESULTS: The dominant oceanographic feature of the region is the Gulf 

Stream and it associated meanders and frontal eddies; while Slope Sea influences 

appear to be limited in the study area. Gulf Stream rings also appear to have little affect 

on regional circulation patterns except indirectly during interactions near Cape Hatteras 

between cold core rings and the Gulf Stream. Cold core ring interactions in April, June 

and December 1992 reduced Gulf Stream currents to practically zero at the slope 

moorings: C4, during all three events; B4, in April and December and A5, only in 

December. There were few current reversals above 300 m at the three moorings; the 

strongest reversal was at B4 during the December cold core ring interaction. The Gulf 

Stream path has its least variability, after leaving the confines of the Straits of Florida, in 

the area just southeast of Cape Hatteras. No seasonal patterns in the Gulf Stream path 

were discernible but there were clear one- to four-month trends and a possible 10 km 

annual shift in position. Gulf Stream currents were observable on the shelf consistently 

at moorings C3 and B3. Gulf Stream water was identifiable at mooring A3, the 
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northernmost shelf and generates marked contrasts in the temperature-salinity 

characteristics of the shelf region. Hydrographic data show possible interannual 

variability resulting from changes in Gulf Stream position, spring runoff, and winds. 

Wind-forced currents on the shelf in winter present a classic Ekman response. Currents 

at all levels north of Diamond Shoals and at C1 and C2 surface were highly coherent 

with winds and sea level. Large wind-forced current fluctuations on the inner shelf north 

and south of Cape Hatteras had important contributions from pressure gradients caused 

by sea level differences around Cape Hatteras. Currents at lower levels in Raleigh Bay 

were less coherent with the winds and more coherent with the Gulf Stream. Gulf 

Stream forcing on the shelf is almost absent in winter because of the insulating affect of 

a strong shelf break front and stratification and stronger wind forcing. During summer 

the wind stress and sea level fluctuations are about half the magnitude of those in 

winter. Winds in summer 1993 were weaker than in 1992. In the summers of 1992 and 

1993 there was evidence that Gulf Stream fluctuations were as far as the coast in 

northern Raleigh Bay (Line C). 

Seasonal progression of stratification in the Middle Atlantic Bight, primarily through 

vernal warming, was observed; this process was augmented in 1993 by an anomalous 

high and early buoyancy flux from Chesapeake Bay. Stratification decreases spatially 

from north to south as a punctuated change concentrated at cross shelf front separating 

Middle Atlantic Bight and South Atlantic Bight shelf waters in the vicinity of Diamond 

Shoals. Stratification in Raleigh Bay is, in general, weak and may be driven by either 

temperature or salinity fluctuations. 

Export of water from the Middle Atlantic Bight Shelf occurs in the region just north of 

Diamond Shoals, generally between moorings B3 and D2 when the Gulf Stream was 

away from the shelf but between D2 and D1 when the Gulf Stream was closer. This 

shifting region is where direct export to the Gulf Stream takes place. Some Middle 

Atlantic Bight water may also be exported to the Gulf Stream from Raleigh Bay after first 

instruding into Raleigh Bay over Diamond Shoals. A second mechanism for shelf water 

export is past the northern boundary of Gulf Stream north of Diamond Shoals is 

estimated to be about the same magnitude as the volume transport along the 

continental shelf north of Cape Hatteras. Raleigh Bay shelf water also appears to be 

exported by entrainment along the Gulf Stream front just south of Diamond Shoals. 

There often are converging shelf flows in the region of Diamond Shoals from the 

southward flowing Middle Atlantic Bight and northward flowing Raleigh Bay water. 

A series of drogued drifters were recovered ashore in the Cape Hatteras region; while 

seven were found beached in England (1), France (3), the Canary Islands (1), and the 

Bahamas (2) 14-35 months after deployment. Estimates of residence time on the shelf 

ranged from less than eight to more than 27 days. In general the drifters deployed in 

Raleigh Bay had the shortest residence time. 

Drifters tracks during the nearshore experiments reveal vigorous and highly variable 

coastal currents near the sea surface, which are clearly influenced by the wind to a 
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large degree. Comparison of the drifter velocities with the wind records from the Corps 

of Engineers facility at Duck, North Carolina show occasional departures from a 

classical nearshore wind response. Buoyancy-driven and surface wave-induced 

currents may also affect drifter motions. With regard to the former, cross-shore motion 

of the drifters appears to often have been influenced by the nearshore front common to 

the study area. Drifter convergence to the front is indicated by the tracks of two drifter 

deployments. On two other occasions, the front appears to have been the outer 

boundary of a zone of surface transport onto the beach. This transport is attributed to 

driving by the onshore wind component with likely assistance from surface waveinduced currents. On two occasions during these experiments ARGOS tracked drifters 

were eventually entrained in the Gulf Stream. 

STUDY PRODUCT: Berger, T.J., J.O. Blanton, W.C. Boicourt, J.H. Churchill, P. 

Hamilton, D.R. Watts, and R.J. Wayland. 1994. A Physical Oceanographic Field 

Program Offshore North Carolina, Final Synthesis Report. OCS Study MMS 94-0047. 

U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, Louisiana. 375 pp. 

Berger, T.J., J.O. Blanton, W.C. Boicourt, J.H. Churchill, P. Hamilton, D.R. Watts, and 

R.J. Wayland. 1996. Instrument Performance and Data Quality Control Analyses for 

the Physical Oceanography Field Program Offshore North Carolina Science 

Applications International Corporation. OCS Study MMS 96-0004. U.S. Department of 

the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

Louisiana. 172 pp. 

*P.I.'s affiliation may be different than that listed for Program Managers.