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 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 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 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. made three other stations nearer the middle Apparently there have been numerous cur- of the channel (Fig. 1). At the station rent measurements in Bering Strait by Soviet approximately 24 km WNW of Ratmanov investigators in the past 25 years, including Island and 16.5 km SSE of Cape Peek, the a buoy station occupied from 20 June to flow was northerly at 30 cm/set; it was also 15 October 1958, but none of the data are northerly at a station 15 km NW of Ratmanov available. Fedorova and Yankina ( 1964) Island, while farther west the flow was have summarized the results in the form of southerly at the surface and northerly at mean monthly transport calculations. Values depth ( 25 and 35 or 40 m ) . The observa- for August, which generally appears to be tions were interpreted (Ratmanov 1937a, the month of maximum transport, range Fig. 12, p. 54) as representing southerly from 1.1 to 2.3 X 10” m3/sec, while values flow near Cape Dezhneva, the flow curv- for March (minimum transport) range from ing eastward and joining a northerly flow 0.45 to 0.7 x 10” m”/sec. The net trans- south and east of mid-channel. port was to the north during all months, Because of insufficient data, no transport and there is no mention of a southerly flow calculations were made for the western of even a sporadic nature occurring. Esti- channel. The single station in the eastern mates of the ratio of total transport to west- channel was used to calculate a northerly ern channel transport range from 2.16 to transport of 0.68 x 10” m3/sec through the 2.87 (Fedorova and Yankina 1964). castern half of the strait. American-Canadian observations were, Barnes and Thompson ( 1938) reported until 1964, confined to the eastern channel. one 21-hr anchor station taken approximately Lesser and Pickard ( 1950) reported current 15 km south of the center of the western observations made from the Cedarwood in channel in August 1934 ( Fig. 1) . The flow August 1949. Without exception, they was northerly, and the speed varied from 15 found northerly flow in the eastern channel to 38 cm/set; that is, the flow was similar to, both at the surface (8 stations) and at in- but only about one-half as strong as that tcrmediate and near-bottom depths (4 sta- observed by Ratmanov in 1932. The data tions), The wind was from southern quad- from this station have been extrapolated to rants during all but two (of 84) surface estimate the transport through the whole current measurements. They computed the strait as 0.3 X 10” m3/sec (see for example, northerly transport through Bering Strait as Vowinckel and Orvig 1961; Fcdolrova and 1.28 x lo6 m”/sec, but the computation for Yankina 1964 ) . tho western channel was based on measure- Maksimov (1945) made some obscrva- ments at the single station occupied by tions in July and August of 1935 through Barnes and Thompson (1938) in 1934. 1938 and in the winters of I938 and 1939, Lesser and Pickard found no obvious tidal but the data apparently were not published component in the surface current, although and no judgment can be made regarding during 18-20 August there was some evi- details of flow. Also using Ratmanov’s re- dence for a IO cm/set oscillation of ap- WATER EXCHANGE THROUGH BERING STRAIT 47

l NORTHWIND, 1964 + BARNES a THOMPSON, 1934 o RATMANOV, 1932 0 RATMANOV, 1933 A BROWN BEAR, 1959 A BROWN &CAR, 1960 A-O, TRANSPORT SECTIONS

30’ 1700 30’ l69O 30’ 16E” 30’

FIG. 1. Location of current measurement stations in Bering Strait, including Northwind stations of 5-7 August 1964, and transport sections. proximately 12-11r period. But since the 940 m long that was calibrated on 1 August tide, presumably at St. Lawrcncc Island, 1954 by comparison with a 14-hr series of :had a diurnal period during that time, the current measurements from seven simulta- authors questioned the validity of the obser- neous anchor stations. vation, If the tidal reference point was St. In 1956-1958, the observational program ILawrence Island, then the assumption that was expanded using a system consisting of the tide in Bering Strait was also diurnal three electrodes extending to 9.3 km west dots not seem warranted. In fact, Bloom of the beach (Bloom 1964). Calibration ( 1956, p. 18) found “the tides off Wales . . . was primarily by a four-month series of normally characterized by a scmidiurnal acoustic current measurements made 3.7 km variation.” It is also possible that the 18-20 west of the beach. Supplemental calibra- August observations represent inertial oscil- tion was accomplished by an ll-hr series lations, that at the latitude of Bering Strait of biplane surface current measurements would have a period of 13.2 hr-nearly between Wales and the in semidiumal. mid-August 1957. A comprehensive program to measure Based on the 1954 calibrations, the 1953- electromagnetically water transport through 1955 potential measurements were in tcr- the eastern channel on a year-round basis pretcd as rcprcsenting transport through was initiated in 1949 by the US. Navy the eastern channel. Large fluctuations in Electronics Laboratory (Bloom 1954, 1956, magnitude and direction were reported; for 1964 ) . The basic instrumentation consisted example, mean montl1Iy changes from 1.1 x elf electrode systems laid on the sea floor 10G m3/sec northerly to 1.4 x lo6 m8/sec from the N.E.L. field station just north of so,utherly in November-December 1953; rYales village west toward . from 0.5 x 10” m3/scc northerly to 3.8 x lo6 Tl1e 1953-1955 system was an electrode pair m3/scc southerly in April-May 1954; and 48 L. K. COACIIMAN AND K. AAGAARD

from 0.5 X lo6 ms/sec southerly to 3.0 x 10” hourly water velocities for the period Sep- mR/scc northerly in July-August 1955. As tember 1956 through August 1.957 (Bloom these transports appear to us unrealistic, in 1964, Fig. 5). Values ranged between about view of previous knowledge, examination 510 cm/see northerly ( 15 June 1957) to 410 was made of the calibration techniques. cm/set southerly ( 1 November 1956). At the seven simultaneous anchor stations Large changes in velocity were reported, of 1 August 1954, surface and subsurface for example, from 300 cm/set northerly to currents were observed using biplanes 230 cm/set southerly, 27-28 October 1956; (Pritchard and Burt 1951). Independent 200 cm/set southerly to 350 cm/set north- surface current measurements were also crly, 34 October 1956; and from about 210 made using biplane type drogues. The sur- cm/see northerly to 260 cm/set southerly, face drogue measurements did not agree 20-21 July 1957. As these results do not with the near-surface biplane measurements; appear to be credible, examination was the speeds measured by drogue were nearly made of the calibration data, always lower by 5 to 30 cm/set. The trans- The acoustic current meter located 3.7 port calculations, and hence the electrode km west of the field station was operated system calibration, were based on the bi- daily over the four-month period August- plane measurements. In view of the calibra- November 1956. Measurements obtained tion discrepancies and of the use, without from this meter were plotted against elec- independent verification, of biplanes at trode potentials to derive a calibration curve higher speeds and greater depths than those ( Bloom 1964, Fig. 4). The current mea- encountered by Pritchard and Burt (who sured by the acoustic meter did not exceed measured speeds less than 50 cm/set at 175 cm/set either to the north or the south depths of less than 15 m ) , we believe that during the four-month period, while the the magnitude of the transports obtained average hourly speeds according to the po- from the 1953-1955 electrode system are tential measurements (Bloom 1964, Fig. 5a- unreliable. c) on 39 of the 91 days exceeded 200 cm/ We also consider the directions of flow set, with extremes of 360 cm/set northerly reported for the period 1953-1955 to be ( 11 September) and 420 cm/set southerly unreliable. No southerly flow was observed ( 1 November ) , and on three days ( 7 Sep- at the time of electrode calibration, but tember, 26 October, and 2 November) were reversals of electrode potential at other times never less than 200 cm/set. This apparent were interpreted as representing southerly discrepancy was not discussed by Bloom. flow in the eastern channel. The electrodes The surface biplane measurements dur- were located in the lee (to northerly flow) ing 16-17 August 1957, were made at se- of Cape Prince of Wales: the electrode lected locations NW from Wales out to 42.6 system extended only 0.9 km west from km and were repeated on the return trip. the shore adjacent to the N.E.L. field sta- At the LB- and 5.6-km distances, the current tion, while Cape Prince of Wales proper, increased 18 and 20 cm/set in 10.5 and 8.5 hr about 1 km to the south of the electrode and had mean values of 55 and 87 cm/set, system, protrudes more than 1 km west respectively. On the other hand, at the 9.3- from the field station shore (Bloom 1964, km position, in the proximity of the western- Fig, 2). During northward flow, eddies most electrode, the current decreased 55 might well be formed north of Cape Prince cm/set in 7.75 hr, and had a mean value of of Wales; hence there is not necessarily a 74 cm/set. Over the same period (16-17 direct relationship between the sign of the August) the potential measurements (Bloom electrode potential in the lee of the cape 1964, Fig. 51) indicated an increase in the and the direction of flow in the eastern mean current from about 120 to 160 cm/set. channel of the strait. This discrepancy was not discussed by The observations from the 1956-1958 elcc- Bloom. trode system were reported as average We conclude that some unaccounted-for WATER EXCHANGE THROUGII BERING STRAIT 49 effects influenced the electrode potential nel (Fig, 1); the flow was essentially north- values and that the currents reported are erly with speeds ranging from 18 to 43 cm/ unreliable. The direction of flow also de- sec. The swiftest current was measured at pcnds to some extent on the magnitude as the midstream station, but this may have well as the sign of the recorded potential, been because the eastern station was less because the apparent rcvcrsal of current than 3 km from the beach. dots not coincide with the point at which The 1960 program included extended ob- the potential reverses sign (Bloom 1964, servations in the northern part of the Bering Fig. 4). Further, the major current reversals Sea, where the influence of northerly winds indicated by Bloom’s data involved speed was noted. Not only can northerly winds changes of 400 cm/set or more in less than retard the surface flow in Bering Strait, re- two days; if the flow is controlled primarily sulting in maximum current velocities at by the sea surface slope (see Theoretical mid-depth (normally at about 20 m ) , but considerations ) , the required changes in sea they may cause an accumulation of surface lcvcl appear to us unrealistic. water south of the strait. When the north- We are left with the evidence for south- erly winds diminish, the surface water may erly flow obtained from the acoustic current be released, resulting in large accelerations measurements in the period August-Novcm- of water in the eastern channel (Fleming ber 1956. These data would obtain in the and Heggarty, in press ) . area of the eastern channel 3.7 km west Among other pertinent points mentioned of Wales, but the data were not presented by Fleming and Heggarty were these: in a form which allows evaluation of dates 1) Currents in the Bering Strait area are and periods of the southerly flow. associated with the barotropic rather than Oceanographic surveys of the southeast- the baroclinic mode. Numerous investiga- ern Chukchi and northern Bering seas, in- tors (for example, Ratmanov 1937a; Barnes cluding current observations in the eastern and Thompson 1938; Goodman et al. 1942) channel of Bering Strait, were conducted have computed dynamic topographies rela- by the University of Washington from the tive to the bottom water layer, and the RV Brown Bear in August and early Sep- flows calculated from them assuming geo- tember of 1959 and 1960 ( Ozturgut 1960; strophic equilibrium are typically only one- Fleming and staff 1961; Fleming and Heg- garty, in press), tenth as swift as those observed. However, the cause of a continual water-surface slope In 1959, three 4-hr stations were occupied between Cape Prince of Wales and Little down to the north, which must be the pri- Diomede Island ( Fig. 1). The wind in- mary driving mechanism of the observed creased from calm to 10 m/see from the NE northerly flow, is unknown. during the observational period, The general 2) The flow through Bering Strait is direction of flow was northerly, with little somewhat erratic. This may be associated change in direction during the period, al- with eddies propagating through the sys- though there were irregular speed fluctua- tem; little or no periodic fluctuation such as tions of as much as 15 cm/set. The speeds would be associated with tides appears to were least at the station near Little Diomedc have been observed. However, the Brown Island, varying from about 30 cm/set at the Bear observations showed that tidal effects surface to 15 cm/set near the bottom. The can be pronounced to the north (Kotzebue maximum speed, about 60 cm/set at a Sound) and possibly to the south (near St, Lawrence Island and Nome). depth of 20-25 m, was observed at the sta- tion near Cape Prince of Wales, The trans- Shtokman ( 1957) has published the only port through the eastern channel calculated theoretical work on the flow through Bering from these data was 0.66 x 106 ms/sec. Strait. He assumed a model with unacceler- Similar results were obtained in 1960 ated flow in which the pressure gradient in from three stations across the eastern chan- the direction of flow, as given by the sur- 50 L. K. COACHMAN AND K. AAGAARD face slope, balanced the horizontal frictional 3) Observations should be extended north stresses. He also assumed a turbulent vis- and south of the strait, to permit calculation cosity coefficient decreasing monotonically of meridional variations in velocity and with depth. The integrated equation of water properties. motion (with an assumed wind field) yielded 4) Simultaneously, the level of the sea a velocity pattern that resembled the one surface should be monitored both in the observed by Ratmanov. Shtokman calcu- strait and in the Bering and Chukchi seas, lated a meridional surface slope down to the north of 1.4 x 10w7. He believed that METHODS the cause of the surface slope and of the The 1964 Northwind survey was con- apparent seasonal fluctuations in flow was ducted 5-7 August by the University of meridional differences in water density, and Washington and the U.S. Naval Occano- their seasonal changes, between the Pacific graphic Office, The measurements were and Arctic oceans. Shtokman’s work is dis- planned with points l-3 (above) in mind; cussed further under theoretical considera- because of ice-breaking committments, only tions. 1 and 2 were accomplished, and these not in the detail envisioned. NORTHWIND SURVEY 19 64 The location of stations (Fig. 1) was An observational program prerequisite to restricted by the requirement of remaining assessing the flow pattern through Bering outside USSR territorial waters; thus, the Strait, calculating the transport of water, observations across the western channel salt, and heat, and formulating an adequate were taken from the bay south of Cape model of the flow and its variations, must Dezhneva toward a point south of the have the following characteristics: Diomede Islands. To the north and west o,f 1) Observations of currents and water station 9, that is, in the bay south of Cape propcrties must be made in considerable Dezhneva, the bottom lies at depths less detail in both horizontal and vertical extent. than one-half those in the western channel A rather wide range of speeds can be an- proper. WC believe that by locating the sta- ticipated across the strait, and during sum- tions as shown, only a small part of the flow mer a strong velocity shear may bc associated through the strait was missed. with the pronounced pycnocline (Lesser At each station the ship was anchored. and Pickard 1950). At stations 2-7 and 9-16, hydrographic casts 2) Observations must be repeated to were made for temperature, salinity, and measure accelerations and changes in water dissolved oxygen, and analyses were made properties, and these repeated observations by standard methods. Direct current mca- must be arranged in time and space in surements to within 5 m of the bottom were such a manner that the significant fluctua- made at all stations. A buoy with .three tions, whether periodic or aperiodic, can be Roberts type meters suspended at depths defined unambiguously. This may require of 2, 20, and 38 m was located at station 8, special care to distinguish between oscilla- and the signals were monitored aboard the tions of similar period, such as tidal oscilla- Northtciind Rcpcated stations were ob- tions ( approx 12 hr ) and inertial oscillations tained at 1, 4, and 2, 16. The observations ( 13.2 hr at the latitude of Bering Strait). are reported in Aagaard and Coachman (in Ultimately, the observational program must press ) . extend throughout the year. It may prove Currents were measured by two direct- polssible to determine the flow regime in reading Kelvin-IIughes current meters low- sufficient detail to enable adequate monitor- ered with the hydrographic wire with heavy ing of the entire regime by certain selected weights attached. No roll of the vessel measurements, but the detailed knowledge occurred during the measurements, and as necessary to employ such a simplified pro- well as could be ascertained by visual ob- cedure is not presently available. servations and cheeks of the ship’s heading, WATER IZXCHANGE THROUGH BERING STRAIT 51

CAPE

L. DIOMEDE CAPE PRINCE L. DIOMEDE CAPE PRINCE

WATER SPEED, Cm %C-’

30

FOG. 2. Cross sections of salinity ( a), temperature (b), sigma-t (c), and water speed (d) from Northwin.d data of 5-6 August 1964. Distances between stations 7-14 and g-Cape Dczhneva have been foreshortened. the icebreaker rode to anchor without yaw- beyond Fairway Rock, with warm (6I0.K) ing. To compensate for magnetic effects of and low salinity (29.8-31.3%0) water over- the ship’s hull, current directions measured lying colder ( 14C) and more saline ( 31.3- in the upper 10 m were realigned to agree 32.7%0) water. Salinities greater than 32.7%0 with visual surface observations related to and temperatures less than 4.5C were char- the ship’s gyrocompass. If visual obscrva- acteristics of all the water flowing through tions were unavailable, the current direc- the western channel. tions in the upper layer have been extrapo- There were also horizontal density gradi- lated from deeper measurements. ents across the strait. A relatively weak The meters were calibrated at the Uni- gradient south of the Diomede Islands versity of Washington; speed values re- marked the transition from the low-density ported are reliable to -C 5 cm/set and direc- surface water of the eastern channel to the ltions probably to -k 10”. However, fluctua- denser surface water of the western chan- tions of this amount were frequently noted nel. Another weak horizontal density gradi- in the instantaneous currents, and so the ent occurred within the deep water of the meters were held at each depth for a few castern channel; low-salinity (< 32.5%0)deep minutes and averages recorded. water was found near the Cape Prince of Wales shore, and water of more uniform RESUI,TS AND DISCUSSION and higher salinity was found farther off- Cross sections of salinity, temperature, short to the west of station 6. Curiously, density ( sigma-t), and water speed are the coldest water (1C) was found in the shown in Fig. 2. A marked vertical density eastern channel at station 7. However, in- gradient was observed in the caster-n chan- terpreting these data requires caution, bc- nel. The pycnocline lay bctwecn 10 and 15 cause they are not truly synoptic, rn westward from Cape Prince of Wales to The water propertics and their distribu- 52 L. K. COACHMAN AND K. MGAARD tions resemble those observed previously in the month of August, although none of the previous investigations provided as much de- O-IOm I tail as the Northwind survey (cf. Ratmanov I’ LAT. = IOcm BBC-’ 1937a; Barnes and Thompson 1938; Good- man et al. 1942; Saur, Tully, and LaFond 1954; Ozturgut 1960; Aagaard 1964; Flem- ing and Heggarty, in press). The water masses are characteristic of the northern Bering Sea in summer, For example, the warm, least saline water corresponds to the Alaskan Coastal Water of Saur et al. (1954), while the cold, more saline water is their Modified Shelf Water. Thus, it appears that the oceanographic conditions observed in August 1964 were quite typical of Bering Strait during the summer, and since the dis- tribution of properties in the area is prob- ably determined primarily by advection (Aagaard 1964; Fleming and Heggarty, in press ) , it may also be expected that the ob- served currents wcrc typical of Bering Strait during the summer. A zone of marked velocity shear (Fig. 2d) was associated with the pycnocline in the region extending approximately 10-12 km west from Cape Prince of Wales; the low- density surface water was moving at 80-95 cm/see and the underlying layer at 30-55 cmjscc. A subsurface core of high speed (95 cm/see), associated with marked hori- zontal and vertical speed gradients, was ob- served at station 5 between 15-25 m depth. Over the remainder of the strait, the ve- I’ LAT. = IOcm set locities ranged bctwccn 20-70 cm/set, but showed little correlation with the water den- sity. In general, the velocities were greatest near the surface and least near the bottom and greatest in and near the eastern channel and least near the Asiatic shore. A representation of the current velocities and their ranges and variabilities is given in Fig. 3, showing plan views of the mean cur- rents between various depths. The arrows represent the mean current vector of each FIG. 3. Plan views of mean ‘velocities between 1aycr. The arced segments in the figures depth levels O-10 m (a), 15-30 m (b), and 30 m- mark the extremes of direction encountered bottom (c) from Northwind data 5-7 August 1964. within the layer over which the mean was Arrow represents vector of mean speed and direc- taken, No variations in direction were ob- tion. Arced segments reprcscnt range of directions observed within layer except at station 8 where arc tained in the O-10-m layer because of the rcprcscnts time variation of direction. See Fig. 1 method employed in data reduction. Direc- for station locations. WATER EXCHANGE THROUGI-I BERING STRAIT 53 tional variability with time, rather than ?,TAT,ON NUMBERS AND TIME OF OCCUPATION with depth, is represented at station 8, the anchored buoy station monitored for 27 hr (see below ) . The currents at all stations and levels were directed essentially north and west of north. Relatively little variation in direction was observed within the water column at any station (at most * 200). However, there was a marked tendency for the velocity vec- tor to rotate to the west (left) with increas- ing depth in the column (cf. Ozturgut 1960), FIG. 4. Water speeds observed at anchored an effect to be expected in a shallow viscous buoy station 8. The data have been smoothccl by fluid moving on a rotating earth. S-point running means. The currents in the western channel had a larger westerly velocity co,mponent than those in the eastern channel. However, this from a minimum at station 9 to a maximum was probably a result of the placing of the at station 13, followed by a decrease toward station line in the western channel approxi- station 15. The speed at station 15 should mately 24 km south of the narrowest section have been nearly the same as that at station lof the channel-between Ratmanov Island 12, assuming that the flow at stations 12 and and Cape Dezhneva. Proceeding north 15 was otherwise identical. An examination toward the Ratmanov Island-Cape Dczh- of Figs, 3a, b, and c appears to confirm these :neva constriction, the water might be ex- predictions, thus implying that an oscilla- pected to accelerate and the current vectors tion of about 12 hr period occurred through- to rotate eastward and thus be more nearly out the western channel. Hence, the actual &gned with the channel axis. Such effects speed gradients would be somewhat less occur, as is demonstrated by the observa- than depicted in the cross section (Fig. 2d). tions at station 8 which was located about The subsurface measurements at 20 and I1 km closer to the line Cape Dezhneva- 38 m at station 8 showed little variability in :Ratmanov Island. current direction; the flow was quite stead- Two distinct types of speed fluctuations ily to N 20” I?,. IIowever, the current at 2 m were observed during 5-7 August, First, fluctuated widely both to the cast and west periodic speed changes of 20-25 cm/set of north. It is not certain to what extent (about 50% of th e mean speed) were mea- thcsc fluctuations are attributable to erratic sured in the western channel at the anchored behavior of the directional sensor of the buoy station (station 8). In Fig. 4, the speeds meter, to effects of a slight sea on the buoy, are plotted vs. time, the values having been to turbulent motion, and to local wind fluc- smoothed by taking 5-point running means, tuations. The speed fluctuations occurred at all three The second observed type of speed change depths, although they were apparently some- was an apparently aperiodic increase in what damped near the bottom. The oscilla- northerly flow through both channels. The tion period appears to be about 12 hr and acceleration in the western channel can be thus may represent semidiurnal tidal influ- estimated from the measurements at the ence. The record is too short, however, to buoy station. From Fig. 4, it appears that distinguish between a tidal and an inertial the current speed increased at all levels by oscillation. Figs. 3 and 4 indicate that an approximately 10 cm/set in 26 hr, equiva- oscillation similar to that at station 8 also lent to an acceleration of 1 X lo+ cm/sec2. occurred at stations 9-15. If such an oscil- A minimum value of the acceleration in lation occurred, then, according to Fig. 4, the eastern channel can be estimated by com- there should have been an increase in speed paring stations 1 and 4 and 2 and 16. At sta- 54 L. K. COACEIMAN AND K. AAGAARD tions 1 and 4, the increase in mean speed TABLE 1. Volume transport, 5-6 August 1964 was about 20 cm/set over 14 hr or less, or Section at least 4 X lo-” cm/scc2, and at stations 2 A B C D and 16 the increase was about 20 cm/set ---- over 42 hr or less, or at least 1.3 x 10-a cm/ scc2. These accelerations probably are not associated with a semidiurnal oscillation. Transport The 14-hr interval between observations at lO’m”/sec stations 1 and 4 is approximately one semi- diurnal period, and hence a large semi- diurnal oscillatory contribution to the ob- Transport calculations served acceleration would not be cxpectcd. Volume transport north through Bering It therefore appears that mean accelerations Strait was calculated using stations 2-7, 9- of at least 1 X IO--* cm/sec2 were observed 15, and 15-7. The cross section of the strait in the entire Bering Strait during 5-7 August. through these stations was divided into sub- It appears that the accelerations were as- sections (A-D in Fig. 1 ), and the transport sociated with a major change in the wind calculation results are reported in Table 1. regime in the general area, For at least a The estimated total transport of 1.4 x 10” week prior to 6 August, northerly winds m”/sec does not include the flow between were observed aboard Northwind while station 9 and the Asiatic shore, but includ- transiting the , During the oc- ing such a flow would probably not ma- cupation of stations 2 through 7 (through terially affect the magnitude of the total 5 August), the northerly winds averaged transport, both because of the shallow nearly 10 m/set, but they may have been depths to the north and west of station 9 even stronger previously. During 6 August, and because of the relatively lo,w speeds ob- the wind decreased to nearly calm. The ef- served at stations 9 and 10. Another un- fects of the removal of wind stress included certainty in the transport estimate is associ- not only acceleration of the water, but also ated with the accelerations discussed above, changes in water properties, For example, since the data are not synoptic. However, it at stations 2 and 16, the temperature in- did not appear feasible to adjust the current creased nearly 5C at the surface and 2.X observations to represent measuremmts at at 27 m, while the salinity decreased 1.7%0at an instant of time. The transport calcula- the surface and 0.8%0 at 27 m. It is likely tion was based on current observations made over 33 hr, and to some extent it therefore that the sustained northerly winds over the represents mean transport. Judging from the Chukchi Sea had previously restricted the current measurements at station 8, which warm, low-salinity water observed at station included speed fluctuations of 50%, rela- 16 to the area southeast of Cape Prince of tively large short-term transport fluctuations Wales. This conclusion is in agreement must be expected. with that of Fleming and Heggarty (in press) By considering that the flow through the based on the 1960 Brown Bear observations. eastern channel was made up of the flow An interesting feature of the acceleration through subsection D and about one-fourth near Cape Prince of Wales is seen in Fig. of subsection C, the ratio of the total trans- 3a, in which mean current vectors of the port to the western channel transport was O-10 m layer for stations 1 and 4 and 2 and cstimatcd at about 2. This value is in gen- 16 may be compared. The swiftest currents, eral agreement with recent Russian esti- that is, those at stations 4 and 16, are di- mates ( Fedorova and Yankina 1964). rected about N 20” W, while the slower flows set very nearly north. This dif Eerence Theoretical considerations in current direction is probably an inertial The observations required for an ade- effect. quate description of the flow through Bering WATER EXCHANGE THROUGH BERING STRAIT 55

Strait are not yet available. It is possible, : x however, to formulate an equation incorpo- A rating estimates of the magnitudes of the parameters that control the flow through the strait. Such a formulation contributes to our understanding of the primary forces h governing the flow and thus may be instruc- tive in planning future observational pro- ,grams. In a left-hand coordinate system with x in the direction of flo’w and x positive FIG. 5. Axes and depth relationships used in the dolwnward, the x-component of the equa- mathematical analysis. tion of motion is sdu 1 dTllJm =---+220sin@+--1 dP d~~,/dy, and so they have been neglected. ‘dt P dx P &II Then (1) may be written

+-- 1 hzy + --1 h%3 1 dp 1 dT$fi (1) ~+u~+w~-~+-- (3) P dY p ax ’ P ax p dx * j’n which u and u are respectively the x- and If h is the water depth and - { is the ele- y- components of velocity, p is density, p is vation of the sea surface above the undis- pressure, 20) sin 4 is the Coriolis parameter, turbed sea level x = 0 ( Fig. 5)) integration iIIl d ~czrn,ray, and 7$X arc the tensor compo- of (3) with rcspcct to x over the water nents of stress in the x-direction. column gives The individual acceleration is composed of the local change in velocity and the non- linear field acceleration terms: du du -&‘iit+u~+w$+w$ (2)

The large local change in velocity observed from the NotihwincZ in 1964 requires that the term au/at be retained. Northwind sur- vey data are not adequate to evaluate u ( au/ in which g is the acceleration due to gravity. Jx); ho,wever, from Brown Bear data (Flem- Denoting the mean density of the water ing and staff 1961), it appears that u( du/dx) column by p’, 7zx at the free surface x = -t rnight bc of the same order of magnitude as by TP, and TEz at the bottom x = h by TJL, (4) dlu/dt and hence should be included. The can be written acceleration w ( du/dy ) can be made negligi- blc by orienting the x-axis in the direction p’ “u2 dx cd flow, so that u e 0. The magnitude of -ii- Se W( &J/&Z) is mlknown, but since the com- IL dw bincd integrated nonlinear terms can be u-dx - - 776 -To) evaluated, w( du/dx) does not have to be S-5 ay pl( 1 evaluated independently. IL x *%dl; dz-ph--. The term in ( 1) involving the Coriolis -5 ax parameter vanishes with the selected oricn- -Isrs-5 1 tation of axes. It is possible that &,,/a~ and The accelerations, boundary stresses, and arazl/ay are not negligible compared with baroclinic contribution to the pressure gra- dr,,/dx. However, available data are insuf- dient can all be computed from repeated ficient to adequately evaluate d~~,/dx and observations of the fields of velocity, den- 56 L. K. COACHMAN AND K. AAGAARD sity, and wind stress. The sea surface slope gradient, and the bottom stress. (The bot- d~‘dx can then be evaluated using equation tom stress was estimated to be four times (5)) thus permitting the determination of the mean wind stress in the period 5-7 the x-component of the total pressure gra- August.) It is therefore not surprising that dient Shtokman computed a sea surface slope of -1.4 X 10e7, an order of magnitude less than the slope -2.6 x lo+ that we have com- puted. The low surface-slope value ob- The stress profile is determined by (3). The tained by Shtokman has been criticized by vertical turbulent viscosity coefficient can Gudkovich ( 1962). However, the surface then bc evaluated from slope of about -2.6 x lo+ computed above appears reasonable in comparison with the (7) slopes computed for other straits (cf. Defant 1961, p. 526). The available observations do not permit The computed surface slope is the com- meaningful computation of the stress dis- ponent in the direction of flow. To the cx- tribution. However, an estimate of the sea tent that geostrophic equilibrium is ap- surface slope was made by the following proached there will also be a transverse procedure: The 1964 Northwind data were slope of both the soa surface and the iso- used to evaluate the local accelerations at pycnals. stations 1 and 4 and 2 and 16. The field ac- celerations and the density distribution were SUMMARY AND CONCLUSIONS estimated by reference to the 1959 and 1960 1. The observations made in Bering Strait Brown Bear observations (Fleming and staff from the USCGC Northwind in August 1964 1961). This estimate of density distribution indicated that the oceanographic conditions was in reasonable agreement with one ob- at that time were typical summer conditions. tained by projecting the position of the There was a pronounced pycnocline in the water observed at various depths at station eastern part of the strait between 10 and 15 16 backwards in time. The wind stress was m separating a surface layer of warm (6- estimated from the 1964 Northwind data, IOC), low salinity (30-31%0) water from assuming it to be proportional to the square colder ( 1-4C ) , more saline ( 31-32.7%0) of the wind speed and the drag coefficient water. Relatively uniform, cold ( 1-4C) and to be 1.2 X lOa (see Deacon and Webb saline ( 32.7-33%0) water occurred in the 1962). The bottom stress was estimated by western part of the strait. extrapolating the observed 1964 Northwind The current was everywhere northerly. velocity profile to within 1 m of the bottom, The pycnocline was also, a region of velocity assuming a profile of the form u [x = (h - l)] shear, the surface layer flowing at 80-95 =u(x=-[)[l/h]‘h (Hansen 1950). The cm/set and the deeper layer at 30-55 cm/ bottom stress was assumed to be propor- sec. In the western part of the strait, the tional to the square of u [x = ( h - 1) 1. Fol- speeds ranged between 20-70 cm/set, with lowing Sternbcrg ( 1965), whose stress mea- a tendency for the values to be less toward surements were made over a similar type of the bottom and toward the Asiatic shore. bottom, the drag coefficient was assumed to The computed northerly transport of be 3.3 x 10e3. These estimates of the param- 1.4 x loo m3/sec, about one-half occurring etcrs of (5) are compared in Table 2. through each channel, was typical for Au- It appears that in general all parameters gust. of ( 5) must be retained to portray ade- 2. At least three types of speed fluctua- quately the flow through at least the eastern tions may be observed in Bering Strait: 1) channel of Bering Strait. The analysis of Short-term irregular speed fluctuations as Shtokman ( 1957) ignores the accelerations, large as IO-15 cm/set that are most pro- the baroclinic contribution to the pressure nounced in the upper water strata and are WATER EXCHANGE THROUGH BERING STRAIT 57

TABLE 2. Parameters of equation (5)

Parameter A B C D E 17

cm2sec-2 cll12sec-2 cm2 ~ec-~ cm2sec-2 cm2 sex+ cn~2scc-2 !Station 1 and 4 1.7 0.7 3.8 4.4 2.2 -12.8 Station 2 and 16 0.4 0.7 2.7 3.5 1.1 -8.4

h-dxau S-5 at

13

probably of a turbulent nature; 2) regular near Cape Dczhneva, it probably turns north speed fluctuations of as much as 50% of the before penetrating the strait. mean speed that have a period of about 12- Measurements from the eastern channel 1.3 hr and that are observable at all depths have shown northerly flow in summer, with (although they may be frictionally somewhat the exception of a set of acoustic current damped near the bottom) and arc probably measurements near Cape Prince of Wales, tidal or inertial oscillations resulting from August-November 1956 showing occasional major disturbances (for example, the movc- southerly flow. ment of atmospheric pressure disturbances 4. The mean sea level of the Chukchi Sea over the Bering or Chukchi seas); 3) long- is lower than that of the Bering Sea. In term speed fluctuations of perhaps as much August 1964, this difference was manifested as 100% that occur over 12 hr or more, ap- by a sea surface slope in the eastern channel plearing to be greatest in the eastern channel of Bering Strait of about 2.6 X lo-“. This and in the upper water strata and probably slope was sufficient to provide northerly in association with major changes in the flow at all depths. However, the baroclinic wind regime, the atmospheric pressure dis- contribution to the pressure gradient cannot tribution over the Bering or Chukchi seas, be considered negligible; for example, at 50 or both. m the pressure gradient is reduced to about In addition, seasonal transport fluctua- one-half its value at the surface. Undoubt- tions have been reported ( Maksimov 1945)) edly, variations in the slope occur with time the transport in winter being only about and position. For example, a reduced slope one-fourth that in summer, However, these some distance away from the strait may be seasonal fluctuations are insufficiently docu- expected. mentcd to warrant their acceptance at this The cause of the sea level diffcrcnce be- time. tween the Bering and Chukchi seas is un- 3. There appears to bc no substantiating known. Jacobs ( 1951) sought the cause in evidence for a southerly flow through the the meridional distribution of evaporation western channel of Bering Strait into the and precipitation in the North Pacific northern Bering Sea during summer (cf. Ocean. Shtokman (1957) attributed the sea M’eilakh 1958); all observations that have level difference to a meridional mean tem- been made south of a line between Cape perature gradient. These explanations wcrc Peek and Ratmanov Island have indicated discounted by Gudkovich ( 1962), who be- northerly flow. If southerly flow occasion- lieved the difference could be explained by ally occurs in the southwestern Chukchi Sea, the field of wind stress over the Arctic 58 L. K. COACHMAN AND K. AAGAARD

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