studies of the Beaufort Sea under the Outer Continental Shelf Assessment ..... Descrip- tions of the drifters and drifter experiments have been reported elsewhere.
Ocean Management, 6 (1981) 223--234 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Observations of Under-Ice Circulation in a Shallow Lagoon in the Alaskan Beaufort Sea
J. B. Matthews
Geophysical Institute, University of Alaska, Fairbanks 99701 (U.S.A.) (Accepted for publication September 2, 1980)
ABSTRACT Measurements are presented of temperature, salinity and current speed and direction under a relatively smooth ice sheet in water of 5 m average depth. Mean currents under the ice at 122 cm and 323 cm above the sea floor over a 50-day period in November and December were 1.91 cm/sec and 1.34 cm/sec, respectively, in a consistent direction away from the coastline. A semidiurnal component in both the current and sea-level data was recorded. Sea-level variations of 1.2 m, 10 times the semidiurnal tidal range, were recorded. The salinity of the water at the instruments increased at the rate of 0.04%o per day which is consistent with an ice growth rate of 0.8--1.0 cm/day for sea ice of salinity 6--13%o. A computed mean shorewards current of 5.9 cm/sec had peak currents in excess of 30 cm/sec resulting from tidal and surge pumping. The existence of the shorewards current under the ice sheet was confirmed by drifter experiments but further measurements are required for full verification of the current magnitude. The implications of the work for under-ice oil spill trajectory computations is discussed.
INTRODUCTION T h e c o a s t a l w a t e r s o f t h e B e a u f o r t S e a o f f s h o r e o f t h e P r u d h o e B a y oil f i e l d h a v e b e e n t h e site o f i n t e n s i v e r e s e a r c h o v e r t h e p a s t f e w y e a r s i n p r e p a r a t i o n f o r t h e l e t t i n g o f leases f o r f u r t h e r p e t r o l e u m e x p l o r a t i o n a n d
0302-184X/81/0000--0000/$ 02.50 © 1981 Elsevier Scientific Publishing Company
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development. Very little work had been done prior to the initiation of the studies of the Beaufort Sea under the Outer Continental Shelf Assessment Program (OCSEAP) of the National Oceanic and Atmospheric Administration (NOAA) and the Bureau of Land Management (BLM). The work reported here was carried o u t under the OCSEAP sponsorship. The initial summer-season work used a new approach for determining the potential impacts of oil and gas development on the coastal lagoons. The ecosystem process analysis, b y which fully integrated inter-disciplinary research studies were carried out, have been reported elsewhere (Matthews, 1980a) and will n o t be repeated here. Matthews (1980a) reported the movement of brackish (7°C) water masses of riverine origin moving along the coast in a generally westward direction during August 1977. Prevailing winds f r o m the east have been reported by Searby and Hunter (1971). Kozo (1979) has shown that a strong sea breeze regime is responsible for the increased persistence of easterly and northeasterly winds observed in August. Matthews (1980b) has shown that these easterly winds result in a generally westward flow of the nearshore waters at rates 3--4% of the windspeed. Summer storms can result in sea-level surges of 3 m or more along the Beaufort Sea coast (Schafer, 1966; Matthews, 1970) and account for the large coastal recession rate of 1.4 m per year. The studies referred to above have begun to fill data gaps for the 3-month open-water season. Studies have reported currents and water structure under the Arctic Ocean ice-pack in winter (Smith, 1974}. However, very little was known a b o u t winter conditions under the landfast and nearshore ice. Studies were initiated in the fall of 1978 of under-ice ecosystem processes under the landfast ice. This paper details the first observations of the under-ice physical oceanography which resulted from these studies.
LOCATION OF THE STUDIES The site chosen for the most intense study was in Stefansson Sound (Fig. 1) to the east Of Prudhoe Bay on the Beaufort Sea coast. The site was chosen because it supports a unique c o m m u n i t y of arctic kelp (Laminaria solidungla). Stefansson Sound is a typical lagoon of the Beaufort Sea coast protected by offshore barrier islands. Water depths in the Stefansson Sound average a b o u t 3 m and by early spring the waters are covered by about 2 m of ice. Thus, much of the Sound has b o t t o m f a s t ice by late winter. Offshore of the barrier islands, the water depths fall rapidly and pack-ice movement is observed there all winter. The ice cover within the Sound is landfast from late October and is generally smooth and level. It is probably because the waters in mid-Stefansson Sound are deeper than 2 m and protected from
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grounding ice ridges by barrier islands that the kelp community can survive there. We used Aanderaa current meters fitted with conductivity and temperature sensors and Aanderaa tide gauges for these studies. The equipment was deployed through a hole in the ice in an arrangement shown schematically in Fig. 2. The floats, polypropylene line, instruments and concrete anchor were all fitted with quick-connect bronze shackles to allow the pieces to be flown by helicopter in modular units and quickly assembled on the ice. The instrument arrays were deployed in the narrow time window between the times when the ice thickness was sufficient to support the helicopter (about 0.5 m) in early November and when the sun set for the winter in midNovember. The instrument array had an acoustic pinger which produces 27KHz signals at 1-sec intervals with a range of about 1 km. In March the instrument arrays were relocated by combined use of the helicopter's satellite navigation system and an acoustic underwater direction finder. Divers were able to remove and replace the instruments underwater so that a long record could be obtained from the site and so that the tidal datum could be preserved. Data were recovered from the instruments on serial digital magnetic tape. These data were transferred to a 9-track digital magnetic tape and analyses
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Fig. 2. Schematic diagram showing the instrument array under the arctic shorefast ice at Stefansson Sound. carried o u t on a Honeywell 66/20 computer system and a Tektronix 4051 graphic display system. The sea-going instruments were calibrated before and after each deployment. The computed engineering values of the recorded digital data are derived from means of the two sets of calibration data. Sealevel records were corrected for barometric pressure variation using data from Deadhorse Airport, approximately 10 km from the Stefansson Sound site. Water
