Observations of the Cromwell Current near the Galapagos Islands

Deep-Sea Research, 1971, Vol. 18, pp. 27 to 33. Pergamon Press, Printed in Great Britain.

Observations of the Cromwell Current near the Galapagos Islands NIELS CHRISTENSEN, J R . *

(Received 14 September 1969; in revisedform 17 April 1970; accepted27 April 1970) Abstract--Temperature cross-sections and a dense network of current stations down to 240 m have been recorded near the eastern terminus of the Pacific Equatorial Undercurrent. Much of these data axe presented in plotted form because observations from this area are sparse. The records show that the core of the undercurrent is not always centered on the equator, its depth is not constant, its width varies by a factor of two, and that volume transport is an order of magnitude less than in the Central Pacific. INTRODUCTION

THE Cromwell Current (also known as the Pacific Equatorial Undercurrent and herein referred to as the Undercurren0 flows eastward along the equator beneath the westward-flowing Pacific South Equatorial Current. In the central Pacific its propertics are well known (KNAUSS, 1960; 1966; MONTGOMERY and STROUP, 1962). There, it is approximately 200 m thick and 400 km wide with a peak velocity of 1.25 m/see at a depth of 75-100 m. It transports 30 x l0 e mS/see. After traversing most of the Pacific Ocean, the Undercurrent abandons its persistent course along the equator some distance from the coast of South America. Mechanisms involved in the diminution of its eastward flow are unclear. Before an understanding of them can be reached, the Undercurrent must be adequately described in the region of its terminus--near the Galapagos Islands. The Equator Expedition (Fig. l) o f the Naval Undersea Research and Development Center, (NUC), San Diego (February and March, 1966), was designed to study this region.

Cruise description

OBSERVATIONS

Data were taken (1) to make a detailed temperature study, (2) to measure the change in volume of water transported by the undercurrent, (3) to measure north-south velocities in the Undercurrent, and (4) to describe changes in flow east o f the Galapages Islands. The track (Fig. 1) consisted of four north-south sections more or less equally spaced between 89°W and 97°W, and one east-west section along the equator. Continuous temperature profiles and fifty-six current observations were made from U.S.S. Marysville along the track.

Temperature U.S.S. Marysville is equipped with a 270-m chain that has 34 equally-spaced thermistor beads and four current meters attached to it (CHRmTENSEN, 1966). This *Naval Undersea Research and Development Center, San Diego, California 92132. 27

28

NIELS CHRISTENSEN,JR.

4°N

3 FEB

10 MAR

/

X

2°N

OO

18 MAR

A

,~

3 FEB l

_.]

.dL

-' 8 FEB

4,13 7~/ MA

ISLANDS 16FEB

~// 6 MAR

7 FEB 96°W

94°W

O,

A~LA G p~G~S"Y| 15MAR

y

2°S

4°S

SECTIONIV

SECTIONIV 17 FEB

92°W

90°W

Fig. 1. Portions of the track of the Equator Expedition (February-March, 1966)where the NUC thermistor chain was towed and where temperature and current velocity data were taken. equipment is lowered into the sea and towed at a speed of 6 knots and at this speed it extends to approximately 240 m depth. The outputs of the thermistors (Fig. 2) are electronically scanned, interpolated, and plotted in whole-degree Celsius isotherms ever 12 sec (RIcI-IARDSON and HUBBARD, 1960). These data are smoothed and replotted with compressed distance scales (Fig. 3). The distance scales are not linear because of variations in ship's speed. One feature in particular stands out in the original data. Away from the Undercurrent (as determined by current measurements), the temperature structure (Fig. 2a) is typified by vertical variations in depth of isotherms as large as ten meters over a distance of one or two miles. At the boundary of the Undercurrent, where horizontal current shear is greatest, these oscillations increase in amplitude and are present from the surface to 240 m (Fig. 2b). The temperature structure inside the Undercurrent (Fig. 2c) is unusually ' s m o o t h '. This general pattern consistently repeats itself and is independent of horizontal direction. Smoothed and compressed temperature plots (Fig. 3) show many details of the large scale temperature structure. A weakening of vertical temperature gradients near the equator is known to be a signature of the Undercurrent (KNAUss, 1960; MONTGOMERYand STROUP, 1962). Spreading of isotherms is irregular and sometimes asymmetric with respect to the equator or depth. Spreading is sometimes deeper on

29

Observations of the Cromwell Current near the Galapagos Islands SURFACE

~ -,'-",'~

~ ~,

.,_.,

.

,

100

14oc

.

, ,;

I

!

6 HI ,~,' ,.%

,.~.~ .

- -

200

(a)

SURFACE

lOO

16°C

,,\-.,

, p

..,

U MI 200

1 HR

,-

:,.,.~

(b)

SURFACE .....

,,~...,..-.-~ ..,

16°C

.~. 100

14°C

.,,,

,,,.,

6 MI 200

,,d~,,~,,,~""~'~'~ -'~ ~.,,~,',~,~.~t.~t~.,,,,.,...~ 1 HR , . r~. * , ,.~.~..,~.,....,~,...,~ •

(c) Fig. 2.

Original thermistor chain temperature data from (a) 1°40'N on Section IL Co) 1°40!S on Section II, and (c) 0°15'S on Section II. Temperatures are in °C.

30

N]~.s CHRISTENSEN,..TR. T

I

l

r

[ [

i

i

i

SECTIONII AT 94045, 4°N

,,,o,"""

,,""

.........................

(',,'1

/,,1 //[

I

/ S

2°N

O

~" •

%"., %%., / I """';Z,

.eti'Rs'...............

,2

2%

7s .~e~

-,,~ \ ~25o

75 METERS

/,......../

UETERS

4°S

80

40

0

40

I

.

80

,o

o

(a) I

I

4ON

I

!,,,,,,

,

T

...............""'~T,CN

' SECTI--ONIg '94°W-B2°W ' '

'

8'o

[(b)

r

'

I

WEST EAST CURRENT[CM PERSECONDI

WEST EAST CURRENTICM PERSECOND}

'

;o

r

,v AT9sow

4°N

.,:............i2 .ETERS <,.J.. ""''"

~METERS

2ON

2°H b

[ 9,~,5 .ETERS

...............~

.(

'~¢";:' ....' .......... '"'.,4'"'-4 ,,~. u

"~'- z5o METERS .........."--..',4..,;' j

I ./"".,,._

2os

2~S

J

( , g

4oS

4°S l

00

I

40 WEST

1

l

0

I

I

40

EAST CURRENT(CM PERSECONO)

(c)

I

80

I

?

80

L

~

I

]

I

40 0 40 WEST EAST CURRENT(CM PER SECOND]

I

80

(d)

Fig. 5. East-wcst current profiles as a function of latitude for 12 m (dashed lines), 75 m (solid linm), and I 5 0 m (long ~ lin~) for (a) Section I, (1}) Section II, (c) Section III, (d) Section IV. Positive values are eastward and n¢gative values are westward. All currents are relative to 240m.

]L]N] HLd]f]

~

2

-

[SUlLgn] HLdga

c~

t

mm ,~o~

~o (Slili]nJ~dlQ

~~ g ~'~

Observations of the Cromwell Current near[the Galapagos Islands

!

1

I

SECTIONI AT flT°W

4°N

I

I

I

I

I

i

3]

i

,

I

SECTION IIAT 94°45"W

.?

2"N

i50 METERS

tL

75 Ii11EltS .2os

\:

'~

75 METERS

i5o m['r~s

'k

./

4"S !

I

10

I

I

I

I

I

I

I

8O

40 40 SOUTH NORTH CORIIENT[CM PER SECOND)

I

I

(a) I

I

I

I

I

l

t

40 ' O 40 SOUTH NORTH CURRENT(CM PERSECOND)

I

80

(b) I

i

I

SECTIONIN AT O4°W- 92°W

i

)

,

SECTIONIV AT 89°W

4ON

150 METERS ~'

ETERS

2ON

f

)

2%

4os

75 METERS I

/

I

I

4O

L

I

4O SOUTH NORTH CURRENTfCM PER SECONDi

(c)

I

I

80

80

I

I

50 METERS

1 %

~I

I

I

40 0 40 SOUTH NORTH CURRENT[CM PERSECONOI

I

80

(d)

Fig. 6. North-south curr-~aUprofiles at 75 m (solid lines), and 150 m (long dashed lines) for (a) Section I, Co) Section IL (c) Section HI, (d) Section IV. Positive values are northward and negative values are southward, All currents are relative to 240 m,

32

N~Ls CHRISTENSEN,JR,

one side of the Undercurrent (Fig. 3a) and sometimes it occurs off the equatorial line (Fig. 3b). Generally, weakening of the thermocline occurs at the equator and where the Undercurrent is observed. The thermocline rises from west to east (Fig. 3e) and no mixed layer occurs over much of this region (Fig. 3b-e).

Current measurements Four Marine Advisors Ducted Current Meters (Model B-7C) were attached to the thermistor chain to operate at depths of 12, 75, 150, and 240 m. It is assumed that the current at 240 m is negligible and that current speed never exceeds ship's speed (3 m/sec). Current observations (Figs. 4a and b) were made by recording flow through the current meters as the ship steamed in a square pattern. Accuracy in determination of the orthogonal current components was estimated to be ± 0.1 m/see, so that the method gives more meaningful results where current speeds are high. The Undercurrent is much narrower from 92°W to 97°W than it is in the central Pacific. Eastward flow at the depths of the Undercurrent is confined to a 160-km wide band near the equator (Figs. 5a-c). East of the Galapagos Islands the Undercurrent widens (Fig. 5d). At 97°W the largest observed eastward speed was 0.45 m/see. This occurs on the equator and at depths of 75 and 150 m (Fig. 5a). At 94°W it is 0.5 m/see, 80 km south of the equator, and at a depth of 75 m (Fig. 5b). Most of the land mass of the Galapagos Islands lies north of the equator but one of the islands lies directly on the equator (Fig. 1). Where the Undercurrent approaches these islands, the maximum eastward velocity of 1.05 m/see at a depth of 75 m is slightly north of the equator (Fig. 5c). Here, the surface current also has an eastward component (0.25 m/see). East of the islands the Undercurrent has speeds less than 0.30 m/see and is north of the equator (Fig. 5d). The north-south current components (Fig. 6) are small compared to east-west components, but in several instances features are prominent enough to be recognized. Horizontal divergence in the Undercurrent is seen as a meridional flow away from its center (Fig. 5a, c). This is contrary to earlier findings by KNAUSS(1966), but evidence for divergence can be found in a report by MONTGOlVmRYand STRouP (1962). Using the current profiles of Figs. 4a--d, volume transport of the Undercurrent was calculated (Table 1). A linear variation of speed with depth and zero current at 240 m were assumed. Agreement in transport for sections on both sides of the Galapagos Islands indicates that essentially all the water of the Undercurrent flows past the Galapagos Islands. All values are an order of magnitude smaller than those Table 1.

Values of volume transport of the Undercurrent from meridional sections (Fig. 1). Sections 97°W 94045'W 920W * 89°W *

Transport

(m3see-l) 2 3 3 3

× × × ×

106 106 10a 106

• F o r the sections at 89°W and 92°W only eta'rent m e a s ~ n t s north of the Galapagos Islands were used because currents s o u t h o f the islands were weak and irregular.

Observations of the Cromwell Current near the C_ralapagosIslands

33

given by KNAUSS (1960) and MONTGOMERYand STROm' (1962) for the Undercurrent in the central Pacific but are of the same order o f magnitude as, but smaller than, those reported by KNAUSS (1966) for the Galapagos Islands region. SUMMARY

The following conclusions can be made: I. Weakening of vertical temperature gradients is irregular near the equator in thisregion. 2. The depth of the core of the Undercurrent is not constant. 3. The core is sometimes displaced south (Fig. 5b) and sometimes north (Fig 5c, d) of the equator. 4. The width of the Undercurrent varies by at least a factor of two. 5. Large variationsin the depths of isotherms existat the horizontal boundaries of the Undercurrent and could be caused by turbulence. This suggests a possible mechanism for the transferof the Undercurrent's eastward momentum. 6. Isotherms are unusually smooth in the Undercurrent. 7. Downstream (east)of the Galapagns Islands the eastward flow is much weaker and spread over a larger meridional distance. 8. Volume transport is an order of magnitude smaller than it is in the central Pacific and varies littlebetween 97°W and 89°W. 9. East of 90°W, east-west speeds at the depths of the Undercurrent are irregular south of the equator. 10. There is no clear indication that water flows from the Undercurrent into westward-flowing currents, although this possibility is suggested by north-south components on two equator crossings. REFERENCES

Cimisxe~seN N., Jr. (1966) Determination of currents from instruments attached to the NEL thermistor chain. Tech. Memo. U.S.N. Undersea Warfare Center, 8 Aug. 1966, TM-974, (Unpublished manuscript). KNAuss J. A. (1960) Measurement of the Cromwell Current. Deep-Sea Res., 6, 265-285. KNAUSS J. A. (1966) Further measurements and observations on the Cromwell Current. J. mar. Res., 24, 205-240. M o ~ r c - o ~ Y R. B. and E. D. SxRom' (1962) Equatorial waters and currents at 150°W in July-August 1952. Johns Hopkins Oceanogr. Stud., 1, 68 pp. R I C ~ u s o N W. W. and C. J. HunnAm3 (1959-1960) The contouring temperature recorder. Deep-Sea Res., 6, 239-244.