On the Water Exchange Through Bering Strait1
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ON THE WATER EXCHANGE THROUGH BERING STRAIT1 L. K. Coachman and K. Aagawd Department of Oceanography, University of Washington, Seattle 98105 ABSTRACT A critique of current observations from Bering Strait through 1960 elucidates the gross features of the flow. There is no substantiating evidence for a net southerly flow ever occurring through Bering Strait into the Bering Sea in summer, although the current may on occasion be southerly near Cape Dezhneva and Cape Prince of Wales. During 5-7 August 1964, the most intensive survey to date of the oceanographic condi- tions and currents was made from the USCGC Northwind. Hydrographic conditions and currents in 1964 were typical of Bering Strait in summer. In the eastern channel of the strait, there was a pycnocline at lo-15 m, which was also a region of velocity shear; the surface water layer speeds were typically 50-100 cm/set, ancl the lower layer speeds less than 50 cm/set. While speeds in the western channel were more uniform, they varied widely with time (20-70 cm/set). The northward transport calculated from the Northwind data was 1.4 x 10’ m’/scc, with about one-half flowing through each channel. At least three types of speed fluctuations may be observed in Bering Strait: 1) short-term irregular fluctuations of lo-15 cm/set, probably related to turbulence; 2) regular fluctua- tions of as much as 50% of the mean speed, occurring at all depths and having a period of 12-13 hr (tidal or inertial); and 3) long-term fluctuations of as much as 1000/o, probably associated with major changes in the wind regime, the atmospheric pressure distribution over the Bering or Chukchi seas, or both. The accelerations would give rise to corresponding fluctuations in transport. In addition, a seasonal variation in transport has been reported, showing the flow in winter to be approxi- mately one-fourth that in summer, but this effect is poorly documented. A mathematical analysis of the flow includes a balance among the pressure gradients in the direction of flow, frictional stresses in the horizontal plane, and the local and nonlinear accelerations. The calculated sea surface slope is about 2.6 X 10-O down to the north. This surface slope is the primary driving force of the northerly flow and can probably be modi- fied only by major variations in atmospheric conditions. INTRODUGTION CRITIQUE OF OBSERVATIONS TIIROUGII 1960 The Bering and Chukchi seas connect The first observations of the flow through through Bering Strait, which is about 85 Bering Strait were made by Captain Vitus km wide and 45 m deep. The general im- Bering in August 1728 (Bergh 1823 cited in portance to the hydrography of the Arctic Dal1 1891). He found a current through Ocean of the flow through Bering Strait has and north of the western channel (Asiatic been described by Coachman and Barnes side) that displaced the ship about 16 km ( 1961) and by Gushchenkov ( 1964). NNW during a day’s sailing. In the summer of 1779, Captain Charles Clerke experienced * Contribution No. 364 from the Department of a northward displacement of about 37 km Oceanography, University of Washington, Scattlc. in passing through the same part of the The assistance in the Iicld of W. Gsell and R. strait (King 1784). According to Clerke’s Magan of the US. Naval Oceanographic Office, and A. S. Frisch, J. A. Galt, and R. S. Winter of lieutenant, James King, Captain James Cook the University of Washington, is gratefully ac- had experienced a similar displacement the knowledged. The officers and men of the USCGC previous summer ( King 1784). However, Northwind were most cooperative. Miss D. Heg- garty assisted in the search of the historical litera- in 1778, lying north of Cape Prince of ture. Dr. D. V. Hansen contributed valuable ad- Wales and well in the lee to northerly flow, vice on the mathematical analysis and in reading Cook “found little or no current; nor could the manuscript. Financial assistance was provided by the Arctic Institute of North America and the perceive that the water either rose or Office of Naval Research. fell:’ ( Cook 1784, p. 453). 44 WATER EXCHANGE THROUGH BERING STRAIT 45 From the time of Cook’s expeditions until Systematic investigations of the flow in the advent of systematic current observa- Bering Strait were begun by Ratmanov tions in the 1930’s, many expeditions have ( 1937a, b ) in 1932 and 1933. His arc the worked in and around the strait, and a num- most extensive published data for the west- ber of them made observations of the flow. cm channel, and a number of transport These were made largely by recording the calculations, as well as interpretations Of a set and drift of the vessel; hcncc, the speeds southerly flow, have been based on these reported are probably accurate only to observations. within 50 cm/set, although the general di- In 1932, four anchor stations, two in the rection of flow was probably determined eastern and two in the western channel, satisfactorily. These estimates of the sur- were occupied between 3-15 August, for face flow made during the navigable season periods up to 25 hr each. Observations (June-October ) provide information on the were made generally at 0, 15, and 40 m gross features of the flow and its variability. depths. One set of observations (0, 15, 40 The results can be summarized as follows : m) was made in the middle of the western channel (Fig. 1). 1) There is normally a strong northerly The observations showed a northerly cur- flow through both the eastern and western rent at all stations; the directional variations channels of the strait, as noted by von from the axis of the strait (approx 030- Kotzcbue in 1816-1817 (von Kotzebue 1821), 210”T) being at most 2 45” at tho surface Shishmarcv in 1820-1821 (Lazarev 1950), and less at mid-depths and near-bottom. Beechey in 1826-1827 (Beechey 1831), (The term near-bottom is used to report ob- aboard the Thomas Corwin in 1880-1885 servations made within 5-10 m of the bot- (Muir 1917), and by Simpson in 1889 tom.) The speeds at the surface and mid- (Simpson 1890). Although the flow gcn- depths were similar in both channels, the erally stems to be stronger in the eastern values ranging in a rather erratic manner ‘channel (American side), two observers (von bctwcen 16 and 76 cm/set. The wind was Kotzebue and Simpson) reported the cur- from the northerly quadrants throughout rent stronger in the western channel (Asiatic the observations, At the station located 11 side) at the time of their observations. km NW of Cape Prince of Wales, the sur- 2 ) On relatively rare occasions, southerly face observations showed a regular current :Elow may occur north of and into the western variation between 20 and 70 cm/set with a ehanncl, as noted by Kellet on the Herald period of 12 to 13 hr, but this periodicity ( Secmann 1853 ) , Nordenskiold ( 1886) on was not observed at subsurface levels. the Vega, and Schmidt on the Chelyuskin Average values were calculated from { Baievski 1935 ) . these data ( Ratmanov 1937b). The surface 3) The speed of the flow may vary speed at each station was X1-20% lower widely, from less than 50 to as much as 150 than at mid-depth, while near-bottom values cm/set. Bcechcy reported 50 cm/see to be were only slightly less than those at the normal. Collinson ( 1855) made measure- surface. These average values were used to ments of 40 and 60 cm/set. Simpson re- calculate a net water transport to the north ported 100 cm/set to bc normal. Dal1 of 1.3 X lo6 m3/scc and a ratio of total (1882) reported a range of 75 to 150 cm/ transport to transport through the western set and believed that the flow was pri- channel of 1.8. rnarily of tidal origin, The situation during 6-9 August 1933 was 4) The flow, at least near the surface, markedly different. That year Ratmanov stems to be influenced greatly by the wind occupied only two 25-hr stations, one ap- regime, setting strongly to the north under proximately 15 km NW of Cape Prince of the influence of southerly winds and di- Wales and one about 3-3.5 km off the minishing, or even reversing, under northerly beach NE of Cape Peck (Fig, 1). The gales (see Beechey 1831; Simpson 1890). flow at the eastern channel station deviated 46 L. K. COACHMAN AND K. AAGAARD only * 30” from N but was about two times suits, Maksimov stated that summer flows swifter than in 1932, with speeds LIP to 200 were northerly, with speeds of 30 to 80 cm/ cm/set. SW, averaging 53 cm/set, in the eastern At the 25-hr station near Cape Peek, the channel and 20-60 cm/set, averaging 41 flow at 0, 15, and 35 m was northerly at 15 cm/set, in the western channel, These to 50 cm/set for the first 5 hr; the flow then values gave a total transport of I.6 x 106 reversed over the course of 2 hr and was m”/sec and a ratio of total transport to southerly for the remainder of the observa- western channel transport of 2.4. Mak- tional period, with speeds up to 115 cm/set. simov’s measurements in winter indicated To check the extent of southerly flow in the flow to be northerly, but the transports the western channel, Ratmanov, ten days were only about one-fourth those of the later, twice reoccupied his 25hr station and summer season.