Baroclinic eddies in the Arctic Ocean

Deep-SeaResearch, 1974,3/ol.21, pp.

707 to

719. PergamonPreu. Printedin GreatBritain.

Baroclinic

e d d i e s in t h e A r c t i c O c e a n *

J. L. NEWTON,~ K. AAOAARD~ and L. K. COACHMAN~ (Received 25 September 1973; in revisedform 8 March 1974; accepted 12 March 1974) Abstract--High-speed currents within the Arctic Ocean pycnocline are associated with small baro-

clinic eddies, most of which appear to be anticyclonic. A typical eddy has a central core with a radius of about 6 km, which is in solid-body rotation. This core is surrounded by a region of large vorticity gradient. At radii greater than about 8 km the motion is nearly irrotational, although the eddy noticeably influences the velocity field out to about 15 km from the center. Such eddies are apparent in ½-~ of the available current records and hence represent a fairly common feature. T-S correlations in the eddies differ from those in the ambient waters, implying that the eddies are not locally generated. INTRODUCTION

CURRENT speeds in the Arctic Ocean are generally less than 10-15 cm s -1. The mean speed is greatest at the surface and decreases through the pycnocline. Occasionally, however, currents o f 30-50 cm s -1 have been observed within the pycnoeline for durations of 2-8 days. The relatively high speeds appear to be confined to a narrow depth range (50-300 m). Such events have been documented by GALT (1967) and COACHMAN and NEWTON (1972), and they are also evident in a series of 15-day current records from ice station North Pole 2 (NIKITIN, 1954--5). Three such events were observed during the 1972 Arctic Ice Dynamics Joint Experiment (AIDJEX) Pilot Study. Sufficient data were collected to identify the features as baroclinie eddies, although their exact origin is still undetermined. OBSERVATIONS

As a part of the spring 1972 A I D J E X Pilot Study, three manned camps were located in a 110-kin triangular array on the ice pack near the center of the Beaufort gyre in the Arctic Ocean (Fig. 1). Hydrography and currents were observed at each camp for 4 weeks. The hydrography consisted of twice-daily synoptic casts to 1000 m. The temperatures were determined from reversing thermometers, and the salinities were determined on station with portable salinometers. At each camp, four current meters (Aanderaa RCM-4 and Bralncon 316) were suspended beneath the ice at 30, 150, 500, and 850 m; the sampling interval was 10 rain for all meters. Current meter malfunctioning caused the loss of directional data at 500 and 850 m at Brass Monkey (BM) camp and of all current data below 30 m at Jump Suit (JS) camp. However, the JS data are augmented by twice-daily velocity profiles down to 170 m measured with a direct readout instrument using a Savonius rotor and vane (HUNr~NS, 1973). The camp positions and drifts were determined every 1-2 h by a satellite navigation system installed at each camp ( A I D J E X STAR, 1972). *Contribution 780, Department of Oceanography, University of Washington, Seattle, Washington 98195, U.S.A. "l'Naval Undersea Center, San Diego, California 92132, U.S.A. Y,.Department of ~ o g r a p h y , University of Washington, Seattle, Washington 98195, U.S.A.

707

708

J . L . NSWTON, K. AJ~G,~mD a n d L. K. COACHMAN

160"

150"

\

\

140°

80*

~30"

\/

120°

",,\

\\

-.

"" - - ~- ,,.

75*

~ . : "-/~":;~'t':.. . ~, .... I

y; [:: . •

0.40

\

x

\

!::~..'%.,

8O'

,-.-t3

I

I

~oo0,.

"'.Y 170'

u)

I

..0.50..

"

-I

.-- ..,

~ x,-.. ~

ao- ~97z

" ' ~ L \ `°°~5 \

\\

\

,

AIDJEX / /

~

" L: ~ ,

,.

/

/

I

~

'.~

I

I

',/

2 0°

//

;

~:¢~' '..I

//

.~.'.':'(:~fi: Xhz'°"

75'

,,, .,. :z;.

/

\ ,,,j~

[70 °

160 °

x, 70 °

0 8M

.*""

150"

SATELLITE CAMP BRASS

~N;~EY

8o BLuE ooG

', 140"

Fig. 1. T h e C a n a d i a n Basin o f the Arctic O c e a n a n d location o f the 1972 A I D J E X camps. Location o f t h e Beaufort gyre indicated by the l o n g - t e r m m e a n d y n a m i c t o p o g r a p h y 30/500 db (unit is d y n a m i c meter).

RESULTS

The pycnocline extends from 50 to 300 m and coincides with the strong haloeline (Fig. 2). Sigma-t values at 90, 150, and 210 m may be considered to represent the density near the top, center, and bottom of the pycnoeline (Fig. 3). The strong temporal density variations labelled l, 2, 3, in Fig. 3, were characterized by (1) Density surfaces that, in the upper half of the pycnocline (above 150 m), were always displaced upward by ~ 20 m and, in the lower half, downward by about the same amount. (2) A density variation or displacement that, in each case, was a solitary event rather than an oscillation about a mean value. (3) Displacements that occurred at only one camp at a time. During each density variation, there were relatively high-speed currents within the pycnocline, but only at the station where the density event was observed. The high speeds and associated displacements of isopycnal surfaces are thus apparently of a smaller horizontal scale than the 110-kin station spacing. Previous analyses of fast currents in the pycnocline (e.g. GABT, 1967) have considered the observations as Eulerian time series. We shall instead treat the station data as resulting from a quasi-synoptic survey, during which the ice camp moved rapidly (5-30 cm s-1) across features being adveeted slowly with the mean flow ( ~ 2 em s-X).* Successive measurements of current or hydrography will be considered as spatially (rather than temporally) separated samples, with the spacing between measurements determined from the average ice drift between stations. • T h e average d y n a m i c t o p o g r a p h y (e.g. Fig. 1) a n d the m e a n true c u r r e n t in the u p p e r 150 m at B D c a m p (where n o eddies were observed) b o t h indicate an average flow o f ~ 2 c m s -1.

Baroclinic eddies in the Arctic Ocean

o'f 94 T (~.J -2 S (eke) 30 0

25 31 J

~ -1 ~12 i

27

28 0 34 i

33

709

35 J

200

40C

BOO

I

800

1000

Fig. 2. Typical temperature (T), salinity (S), and at profiles from the Canadian Basin (approx. 76°N, 149°W; 22 March 1972).

26.251"

~"

N26.oo

1.,4..1~.r.1,8., . 2 ~ . .

~6

,~

~!

30

/

. 3AIpr " ~ . . .

"25

\/x,

2~7~I < 26251,

L,') r'~

O 27D0

26.75,

Fig. 3.

",

-20

Sil~3a-t versus time at 90, 150, and 210 m at the 3 A[DJ-E,X camps: ( . . . . ) Blue Dog,

(- - -) Brass Monkey, and ( ) Jump Suit. Sections of the records showing strong temporal density variations are labeled 1, 2, and 3. The vertical displacement scale is based on the mean vertical ~ distribution.

710

J . L . NEWTON, K. AAOAARD and L. K. COACHMAN

Our interpretation of event 2 (Fig. 3) is shown in Figs. 4 and 5. The general agreement between the measured and calculated current differences(Fig. 4) indicates that the interpretation of the data is internally consistent and that the geostrophic approximation is fairly good. For example, if the high-speed current core were not nearly stationary (in contrast to the rapidly moving ice station), the spatial scale of the current core would be in error, resulting in a consistent over- or underestimate of the calculated geostrophic currents. The suggestion is that the high-speed currents and density structure of event 2 were associated with an anticyclonic horizontal eddy contained within the pycnocline. The total radius of influence of the eddy was ~ 15 km, and the maximum speed ( ~ 35 cm s -x) was at 6-7 km from the center. Similar representations of events 1 and 3 are shown in Figs. 6 and 7. In neither of these cases did the ice camp completely traverse the eddy, although it is clear that the eddies were generally similar to that of event 2. During event 1, JS camp was initially in the southern portion of an anticyclonic eddy and subsequently moved away to the southwest. A current core, the magnitude of which agrees fairly well with the direct current measurements, is indicated by the geostrophic calculations (Fig. 6a). The current vector at 150 m (Fig. 6b) was initially directed west and then rotated to the northwest and decreased as the camp moved across the southwest quadrant of the eddy. The radius and tangential speed of the eddy appear to be similar to those of event 2.

ICE DRIFT (kin) i

3~)

,

2p

,

10

,

O't = 2 4 . 5

25.5 I00 26.0 o 26.5

22

-~00 27.0

27.5 --

BOO

27.9 --

Fig. 4. Vertical section through event 2 (Fig. 3), assuming stationary conditions. The vertical arrows locate the hydrographic stations. The isotachs of computed geostrophie currents are relative to 20 m (taken to represent the surface layer); the measured current differences (ia boxes) are between the surface layer (represented b y t h e measurement at 30 m), with current speeds less than 5 cm s -t, and the center of the pyenoeline (150 rn), containing the current maximum.

Baruclinic eddies in the Arctic Ocean

//

J

.. "

I

,4

711

" " - , ,,

2~4c~',

-

i

%

Fig. 5. Two-hour averages of true 150-m currents during event 2 (Fig. 3), 24-26 March 1972. Hydrographic stations taken at positions X. The dashed circles show the extent of the eddy's influence and approximate radius of maximum velocity.

ICE DRIFT(km) 10

O N

I00 m -I-

~27.0

~

200

~)

,

2,0cm

s e c "~

?km

9 I00

(a)

Co)

Fig. 6a. Fig. 6b.

Vertical section through event 1 (Fig. 3). See legend Fig. 4. Currents at 150 m from twice-daily current profiles during event 1, 13-19 March 1972. Dashed arc indicates approximate eddy circumference.

Event 3 (Fig. 7) also appears to be associated with an anticyclonic eddy with about the same characteristics as those of events 1 and 2, and the measured and calculated geostrophic shears are in reasonably good agreement. As JS camp moved across the southern part of the eddy, the current vector at 150 m increased to the north, then rotated counter-clockwise through west. This eddy was observed for 8 days and appears to have been displaced northward during that time at about 2 cm s -x. Vertical profiles of the currents within the eddies (Fig. 8) show both computed geostrophic and measured currents to be slow at the surface, to increase to a maximum

712

J.L. NEWTON,K. AJ.OA.~,D and L. K. COACHMAN

ICE DRIFT (kin} 1o 2.0

300 ~

2 7,5.....

Fig. 7a. Vertical section through event 3 (Fig. 3). Se¢ legend Fig. 4.

N

o

2.oc m

9

sec~

skin

%

\\

Fig. 7b. Currents at 150 m from twice-daily current profil¢~ during event 3, 29 March-7 April 1972. For clarity currents associated with dashed portion of the track (3-7 April) are shown above the main figure. Hcffivydashod lines indicate tramlatkm of eddy.

Baroclinic eddies in the Arctic Ocean

E 'EVE.T2' 0

I

10

713

I JUMPSUIT'EVENT 1}

20(NW)

0

10

20

~7

I

3~) (NW)

300

i 10 •

• ,~_ 3/'30 ~',I

J,,UMP,~JIT (EVENT 3) 10 20 30 (N)

20(N) 0 0

,,

¢~,1-~o

4/2

I 0

100 200"

\

300"

Fig. 8.

Vertical current profiles during events 1, 2, and 3: ( o • • e) measured currents, and ( ) computed geostrophic currents relative to 20 m.

('.') T-S ENEg'rsI,~&3

0,0

!

t~" T-S AL.LOTHERPOINTS

2

-1.,"

I

60m:

| .31

i

i

|

I &2

|

|

|

i

i

33

*

*

SALINITY- "/,.

Pig. 9. TemDerature~alinity correlations. The envelope includes all ( ~ 130) s:tations not associated with events 1, 2, and 3 ( ~ 40 stations). The latter are separately shown by the dashed lines. The hatched areas of the envelope indicate the T-$ correlations at 60 m (left) and 270 m (right), excluding those of events I, 2, and 3. The numbered depths refer to events, I, 2, and 3.

at about 150 m, near the center of the pycnocline, and then to decrease with depth. A current speed record (direction not available) at 500 and 850 m during event 2 indicated no motion at those depths other than that of the meter being pulled through the water by the ice movement. Therefore, the effects of the eddy apparently were not felt below 300-400 m. The temperature-salinity correlations (Fig. 9) show that the water characteristics

714

J . L . NEWTON, K. AAGAARD and L. K. COACHMAN

of the eddies differed considerably and, more significantly, were different from those at any of the other approximately 130 hydrographic stations occupied during the onemonth drift. Since a local displacement of the ,, surfaces would not move the T - S points of the eddy water outside the ambient water T - S envelope, the eddies must have their individual origins at some other location and subsequently have been advected into the region of observation. The T - S correlations within the eddies are less smooth than in the ambient water. This is particularly evident for events 2 and 3, so that the T - S structure suggests a series of vertically layered intrusive water parcels with relatively little mixing between them. RESULTS FROM OTHER OBSERVATIONS

Fast currents in the pycnocline are evident in three of the five 15-day current records from ice station North Pole 2 (NIKITIN, 1954-5). The ice drift track was determined by assuming no current at 1000 m and using the speed recorded at this depth and the opposite direction as an estimate of the ice movement. The resulting track agreed satisfactorily with that indicated by occasional celestial fixes. No hydrographic data coincident with the current records are available. The observations from ice island T-3 (GALT, 1967) included current profiles adequate to define the vertical extent of the high-speed current. The drift was determined from celestial navigation. Data from the 1970 AIDJEX pilot study included current observations at 150 m and hydrographic casts. The ice track was determined by Decca radionavigation. Table 1 summarizes the available data on fast pycnocline currents in the Arctic Ocean and the current characteristics.

/~1 AUG 50 O. NORTH POLE 2 ?~aOO'N 170eOO'W

!

31

b. NORTH POLE 2 80~I'N 163eOO'W

27

d

C. NORTH POLE 2 81*~O'N 17P 30'W

Z4 -23 MAR

: : :

7~'30'N

IcEDRIFT 150m CURRENTS

26 DIRECTION OF

~'~. © r ~

0

n

7-3 14~°100~W

~km

O 40¢m$1c"1 * * , , , |12hr w l ) OF

CENTER e. AIDJEX 1970 71*ZO'N 136"30'W

Fig. 10.

f. AIDJEX 1972 76eOO'N i49o00'W

Ice drift tracks, 150m currents, and probable eddy characteristics deduced from observations listed in Table 1. Event 2 is shown for eOmlmrbon.

79-00N 170-00W

80-30N 163-00W

81-30N 171-30W

75-30N 142-00W

71-20N 136-30W

76-00N 149-00W

75-00N 148-00W

75-00N 148-00W

North Pole 2 Series 4

N o r t h Pole 2 Series 5

T-3

A I D J E X 1970

AIDJEX 1972 Brass Monkey

A I D J E X 1972 Jump Suit

AIDJEX 1972 Jump Suit

Approx. location

North Pole 2 Series 2

Record

30

30

30

12

7

15

15

15

(days)

31

30

25

22

45

24

20

34

(cm s-~)

8

4

3

5

6

5

5

2"5

(days)

Location~characteristics Duration Record Max. of max. duration speed current Ice drift

90 °

C C W 90 °

CW

C W 180°

C C W 65 °

CCW 2150

Satellite Navigation System

Decca Radionavigation

Celestial fixes

CCW 50 ° "1 Assumed measured t current at 1000 m was directed opposite C W 185° and equal in magnitude to ice drift. Checked with occasional CCW 45 ° celestial fixes

Angular rotation

Profiles to 170 m (Jump Suit Camp); Recording current meters at 30 and 150 m (Brass Monkey)

twice daily to 1000 m

HtmrdNs (1973)

NEWTON a n d COACHMAN (1973)

CoAc~n~.~a~ Once per and NEWTON day to 55 m (1972)

Recording current meter at 150 m

GALT ( 1967)

Nigrrr~ (1954-5)

Source

4 stations

None

Hydrographic

Profiles through pycnocline

Every 2 h at 75, 150, and 1000 m

Currents

Data available

Table 1. Location and characteristics of relatively high-speed pycnocline currents and data available during observations.

-,d

O

O

716

J.L. N~eTON,K. AAGA.~'U:) and L. K. COACHMAN

We have interpreted these data in a manner similar to the 1972 AIDJEX observations, i.e. as spatially (rather than temporally) separated samples (Fig. 10). In each case, the high-speed currents could have been associated with an eddy within the pycnocline. The probable rotation was generally anticyclonic, with the exception of that shown in Fig. 10e (which, however, could have been anticyclonic with the center of the eddy northeast of the drift track). In cases where the sense of rotation was somewhat ambiguous (e.g. in Fig. 10b), that sense which involved the least eddy propagation required to fit the observed currents was chosen. SOME ASPECTS OF THE EDDY DYNAMICS During event 2 (Fig. 5), the oceanographic section appears to have bisected the eddy, so that further analysis will be focused on those observations. For anticyclonic circular flow, the equations of horizontal motion are v= --+fv r

av + a-t

1 ap . . . . par

0, and

l

A,

\drZ -t- r a r

(1)

v) r = = O,

(2)

where v is the tangential speed, r the radial distance from the center of the eddy, f t h e Coriolis parameter, p the density, p the pressure, and Ah the eddy viscosity. The individual terms in (1), shown in Fig. 11, indicate the meander flow approximation to be consistent with the observations of event 2. The solution to the radial component equation (2) has been discussed in a slightly different form by McEwL~ (1948). It is useful to consider the problem as one in which an eddy initially consists o f a central region in solid-body rotation (with v = klr, and vorticity ~ = 2kl) embedded in an irrotational flow (with v = k d r ) . Votticity 5 2

4

IJJ U n-

1

%

5

R(km)

10

15

Fig. 11. Radial distribution of forces for eddy of event 2. The Coriolis force (fv) and centrifugal force (vZ/r) were computed from the radial velocity distribution determined from Fig. 5. The pressure gradient force (p-10p/Or) was calculated from the hydrographic data and is relative to 20 n~ The sum (dashed fine) of the pressure gradient force and the centrffu~ force is within 20 % of the Coriolis force. For radii < 10 km, the centrifugal force is important; at 5 km it equals the pressure gradient force.

Baxoclinic eddies in the Arctic Ocean

717

subsequently diffuses radially outward, modifying the initial velocity distribution. The general form of the velocity distribution at a subsequent time during the decay of an idealized eddy is illustrated in Fig. 12. For event 2, the ratio v/r, which for solid-body rotation should be constant, actually decreases in magnitude with radial distance; there is, however, a region from about 2.5 to 5.5 km where the ratio does not differ much from -- 6.5 × 10-5 s -1 (Fig. 13). Similarly, the quantity vr (constant for irrotational motion) changes relatively little from -- 2.5 × 10v cm s s -1 in the region from 7 to 13 km (Fig. 13). Matching velocities for the initial rotational and irrotational fields gives a maximum tangential speed of 40 cm s -1 at a radius of 6.2 Ion. The observed velocity distribution and that constructed from the idealized initial representation (Fig. 12) agree well out to about 12 kin. While the values of kl and ks would change during decay, the observed radial distribution of tangential velocity closely matches the model of a central core in solidbody rotation surrounded by a region of irrotational motion (Figs. 12 and 13). Near the center of the eddy the magnitude of the relative vorticity, calculated from =

r

(3)

~r

(Fig. 14) is nearly as large as the planetary vorticity, and this condition may represent the limiting stable case (e.g. EAvY, 1951). The largest vorticity gradient occurs near 7 km from the eddy center, and beyond 8 km the vorticity is about one order of magnitude less than near the center. For certain dynamic analyses it is appropriate to identify the extent of the eddy with the region of solid-body rotation, rather than with the total sphere of influence of the eddy. For example, (1) can be scaled by NB

~ Ro,

(4)

3o

o

~2¢

/

xN

\

xx

,~u

K.~2

o

~.

V- R

xx

; R (krn)

Fi& 12. Radialdistributinnofeddyvelocityforevcnt2:(oo, ee)obscrvcd, and(

)

smoothed observed; (- - -) idealized initial velocity, and (xxxx) general form of velocity during decay [according to McEw~q (1948)].

718

J.L. N~wroN, K. AAOAARDand L. K. COACHMAN

°,VR -4

A o ~

~'-

•

A A ~

-2

-5

o

oo

•

• •

•

.

•

oe •

•

A

•

8 • o

0

•

0 0

o

m

•

i

J

I

5

"i0

•

•

A •

t

o

o

AI

8,

eA

AA

m

A

15

R(kin)

Fig. 13. Observedradial distribution of vr (© ©, • 0) and v/r (zx zx, AA) (event 2).

where the eddy number is lOAD N~

(Rf) ~ ,

(5)

and the Rossby number f2 R0 ..... f

(6)

For the eddy of event 2 (with the dynamic height difference across the eddy AD ~ 0.01 dm, the eddy radius R ~ 6 kin, and the angular velocity of the eddy f2 = kl ~ 6 × 10-s s-X), ArE ~ 0.14 and R0 ~ 0.43 which satisfy (6). If, on the other hand, the eddy had been sealed by the total extent of the bar•clinic structure (AD ~ 0-03 din, R ~ 15 km, and f~ = v/r ~ 3 × 10-~ s4), then ArE ~ 0.07 and R0 ~ 0.02. These values do not satisfy the scaling equation particularly well, but more importantly they illustrate the considerable effect on the estimate of the Rossby number if the characteristic radius is taken to extend beyond the region of approximately solid-body rotation. Likewise, the Coriolis force can be meaningfully scaled by f O R or f~R (of. SAUNDERS, 1973) only within the region of solid-body rotation where the Coriolis force increases linearly with radial distance (Fig. 11). If R is taken at the extreme range of influence of the eddy (e.g. 15 km for event 2), then such scaling will overestimate the Coriolis force by a full order of magnitude. Therefore, the selection of the appropriate length scale in a vortex analysis is critical. We wonder whether WILKINSON'S(1972) interesting result, that oceanic eddies vary little in either the eddy number or the Rossby number, might not have been influenced by the characteristic radius being taken too large, which would result in significant underestimates of the Rossby number.

Baroclinic eddies in the Arctic Ocean

719

-12 -8

•

.

.

X X 'Kq o ~ X ,Kq o R

-4

•

R

a • •

Q

_. . . . . . .

X .

I

,

5

| I0

"_'.,

X ,

K ! 111

R (kin)

Fig. 14. Radial distribution of vorticity (event 2).

We are unable to estimate the life span of the Arctic Ocean eddies, but the eddies off southern California desoribed by McEW~N (1948) were clearly identifiable for 100 days or so. McEWEN'S eddies extended to the surface and could reasonably be expected to decay faster than the Arctic eddies, which are confined to the pycnocline in what is probably an ocean of relatively low turbulence.

Acknowledgements--Support for this work came from the Office of Naval Research, Contract No. N00014-67-A-0103-0021. REFERENCES AIDJEX STAFF (1972) Station positions, azimuths, weather: 1972 AIDJEX pilot study preliminary data. AIDJEX Bulletin, 14, 63-71. COACHMANL. K. and J. L. NEWTON(1972) Water and ice motion in the Beaufort Sea, spring 1970. AIDJEX Bulletin, 12, 61-91. EADY E. T. (1951) The quantitative theory of cyclone development. In: Compendium of meteorology, American Meteorological Society, pp. 464-469. GALTJ. A. (1967) Current measurements in the Canadian Basin of the Arctic Ocean. Summer, 1965. University of Washington, Department of Oceanography Technical Report, 17 pp. HUNKINS K. (1973) Twice daily vortical current profiles at Jump Suit Camp, AIDJEX 72. AIDJEX data files. (Unpublished.) McEwEN G. F. (1948) The dynamics of largo horizontal eddies (axes vortical) in the ocean off Southern California. Research, London, 7, 188-216. NEWTON J. L. and L. K. C o A c n ~ (1973) 1972 AIDJEX interior flow field study preo liminary report and comparison with previous results. AIDJEX Bulletin, 19, 19-42. NnOT~ M. M. (1954--5) Observations of currents. (In Russian). In: Morskoi Transport, 1(3), 171--403. Translation: American Meteorological Society. SAUNDERS P. M. (1973) The instability of a baroclinic vortex. Journal of Physical Oceanography, 3, 61-65. WILKINSOND. L. (1972) An apparent similarity among ocean eddies. Deep-Sea Research, 19, 895--898.