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California Undercurrent Off Northern

Baja California'

W arren S. Wooster and James H. J ones

Scripps I nstitution of Oceanography University of California La J olla, California 92037

ABSTRACT

T he Cali fornia Undercurrent is a subthermocli ne poleward fl ow of warm and salty low-oxygen water along the coasts of California and Baja California. T he salinity maximum is centered on the isanosteric surface of thermosteri c anomaly 150 cl/t, and the region influ-enced by this circulation is shown by the distribution of salinity on that surface (at depths of 250-300 m). Direct measurements in August 1966 at about 31°N showed that the Under-current li es close to the continental slope. The Undercurrent had a width of about 20 km, a thickness of about 300 m, and an average speed of about 30 cm/sec, corresponding to a transport of about 2 x 106 cm3 sec- 1 • Numerous temperature and salinit y inversions were as-sociated with the high-salinity regime. Within these laminae, the verti cal density gradient was reduced, and, even with the low values of average velocity shear observed, the Richard-son number was close to the cr itical value for dynamic stabilit y.

Introduction. Observations on eastern boundary currents have shown that, beneath the prevailin g equatorward fl ow at the surface, a poleward undercur-rent is commonly found close to the boundary (Wooster and Reid 1963). A theoretical basis for this circulation has been provided by Yoshida ( 1967 ). Such undercurrents are evidenced by a coastward deepening of subsurface isopycnals (associated with the poleward geostrophic fl ow) and by the presence of water with properti es characteristic of tropical subsurface conditions.

The California Undercurrent is present along the western coast of N orth America and extends from southern Baja Californi a to north of Cape M en-docino (Reid et al. 1958). During the EXJIBIA Expedition, aboard R. V. THO-MAS WASHINGTON in August 1966, we had an opportunity to study this Undercurrent off Punta Colnett, Baja Cali fo rnia (at about 31°N). The

1. Contri bution from the Scripps Inst itution of Oceanography. This work was supported by the Office of Naval Research and by the M arine Lif e Research Program, a component of the Cali fo rnia Cooperati ve Oceanic Fisheri es Invest igations. The current meters were loaned by the USN Pacifi c M issil e Range, Point Mugu, Califo rnia.

Accepted fo r publication and submitted to press 14 February 1970.

2 35

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Journal of Marine Research

' ' ----

120"

1.04

110"

CAL COFI

40"

20"

110"

Figure r. M ean August geostrophic flo w at the surface, in dynamic meters, relati ve to 500 db; 1950-1964 (from Wyllie 1966). Contour interval 0.04 dynamic meters. Current direction indicated by arrows.

temperature and salinit y were measured across and along the stream with an in situ profiling device (the Bissett-Berman 9006 STD system) and the vertical distribution of horizontal velocity was measured with Richardson-type current meters.

The California Undercurrent. The most complete description of circulation in the California Current system is that of W yllie (1966), whose atlas contains

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Wooster and 'Jones: California Undercurrent

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Figure 2. Mean August geostrophic flow at 200 db, in dynamic meters, relative to 500 db; 1950-1964 (from Wyllie 1966). Contour interval 0.04 dynamic meters. Current direction indicated by arrows. Dotted lin e shows approximate 200-m bottom contour.

monthly charts of the relative geostrophic flow at the surface and at 200 m. The average surface charts (Fig. 1) show a slow meandering drift toward the southeast. Seasonal variations in the intensity of the surface current are related to the wind strength, and the period of strongest equatorward flow differs with latitude; off southern California, such flow occurs during late spring and summer.

One noteworthy feature of the .surface circulation is pertinent to this study.

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J ournal of M arine Research [28,2

South of Point Conception (about 35°N) there is a large semipermanent cyclonic eddy, such that the inshore circulati on off southern _California is co_m-monly to the north. When the north winds a~e weak, as _is common dun_ng the winter, this northward fl ow extends to Point Conception, beyond which it is known as the Davidson Current. The southern limit of the cyclonic

TEMPERATURE, °C G· a· 10· 12· 14• 33_0.--,--.----'i--.--'C'-r-..----'i' ........ .--.~7

'

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' ' ' ' ..... '\ .. ·: .. ' . .. :· ' . . . .. · :· : ' ....

",.... -,, '

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3 5 . 0 L__L_L__,__J....>.._....J..._J._---L.;.__.,____._~

Figure 3. Temperature-salinity relationship fo r selected stati ons in the Cali fornia Undercurrent and farther offshore.

eddy li es at about 32°N, so that surface fl ow off the northernmost part of Baja California, the locus of this study, is often directed on-shore.

The subsurface circulation is evident in W ylli e's charts of mean relative geostrophic fl ow at 200 m (Fig. 2). At this depth, poleward fl ow is often present near the coast and is particularly well devel-oped during the summer. Whereas the surface California Current at-tains geostrophic speeds as high as 2 5 emf sec, the mean speed of the Undercurrent seldom exceeds 8 cm/sec.

Direct measurements of the subsurface currents off northern Baja California were made by

Reid (1963) in December 1962. Parachute drogues at a depth of 250 m showed moti on toward the northwest only within 50 km of the coast, where the maximum speed observed was 8 cm/sec. There was relatively good agree-ment with geostrophic fl ow. Observati ons farther offshore suggested that the poleward fl ow inshore may have been part of an eddy rather than a simple current fl owing parall el to the coastlin e.

Water transported by the Undercurrent comes from below the surface in the tropical region and is characteri zed by high temperature and salinit y and by low dissolved-oxygen content (Reid 195 8 ). Examination of appropriate station data on a temperature-salinit y plot (Fig. 3) shows the Undercurrent water as a high-salinit y bulge centered on the isanosteri c surface where ther-mosteri c anomaly equals 150 cl/t (ae equals 26.54), with the maximum salinit y values (> 34.3°/oo) occurring at temperatures between about 8° and I 1°C.

The average characteri sti cs of the sli ghtly deeper isopycnal surface of at 26.60 (thermosteric anomaly, 145 cl/t) have been charted by Barkley (1968; fi gs. 65-67). Off the coasts of Cali fornia and Baja Cali fo rnia in the summer, the surface lies at 250- 300 m; the isopleths of salinit y and oxygen bend sharply northward. Salinit y distribution on the isanosteric surface 150 cl/t is

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Wooster and Jones: California Undercurrent

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CCOFI CRU I S E 6507

15 JUNE - II AUGUS T 1965

POINT CO NCEPT ION

SAN DIEGO

115°

2 39

110•

35•

30•

20·

110"

Figure 4. Salinity (0/ 00) on the isanosteric surface 150 cl/t, CCOFI cruise 6507, summer of 1965.

shown in Fig. 4, based on CCOFI Cruise 6507 in the summer of 1965 (University of California I 967 ). The highest salinities were observed off southern Baja California. At about 30°N, the region of high salinity sud-denly narrowed, and, to the north extending beyond San Diego, salinities greater than 34.3 °/oo were present only in a narrow band just offshore from the 200-m isobath.

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'Journal of Marine Research

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CCO F I CRUISE 6607

8-29JULY 1966

POINT CONCEPTION

115°

[28,2

110 •

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35•

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20•

110°

Figure 5. Salinity (0/ 00) on the isanosteric surface I 50 cl/ t , CCOFI cruise 6607, summer of 1966.

Similar data for the years 1954-1966 were examined to see if year-to-year differences in the northward penetration of the high-salinity water could be detected. The very narrowness of this fl ow made this analysis difficult, since failure to make an inshore station on any line could make it impossible to detect the feature. The northward extent of the 34.3 °/oo isopleth appeared to vary from year to year, reaching only to 30°N in 1958, 1964, and 1966, and nearly to 34°N in 1960 and 1965.

40"

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Wooster and Jones: California Undercurrent

120·

.4 2

125° 120°

PO INT CO NCEPTION

115°

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115°

40°

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30"

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Figure 6. Geostrophic Row at 200 m, relative to 500 db, CCOFI cruise 6607, 8-29 July 1966 (University of California 1968). Contour interval 0.02 dynamic meters. Current direction indicated by arrows. Dotted line shows approximate 200-m bottom contour.

The 1966 summer survey of the CCOFI Program (8-29 July 1966; University of California 1968) took place about one month before the ExJIBIA observations. Coverage of the 1966 summer survey is shown in charts of the salinity distribution on the isanosteric surface 150 cl/t (Fig. 5) and of the relative geostrophic flow at 200 m (Fig. 6 ). As noted above, the northward

]ournal of Marine Research [28,2

Figure 7. Locations of reconnaissance and current stations, ExJIBIA Expedition, 2-10 August 1966. The STD stations are identified by circles, the current stations, by triangles.

penetration of high-salinity water was not remarkable, although the geostro-phic chart shows northward flow along the coast nearly to San Francisco.

M easurements on ExJIBIA. The location of observations is shown in Figs. 7 and 8. Profiles of the temperature and salinity were obtained with the STD at intervals of r 0-20 km along a reconnaissance line (Sts. 270- 284). The manufacturer's estimate of the precision of the instrument is ± o.02°C, ± 0.03°/oo, and ± 0.25°/o in depth; several Nansen-bottle stations along the line confirmed these estimates.

Following completion of the reconnaissance section, taut-wire moorings were placed for navigational control during a fine-scale survey and current-measuring program. The fin e-scale survey consisted of STD measurements at 1.5-km intervals across and parallel to the Undercurrent. The position of the ship was established by radar fixes on the reference buoys; repeated bathy-metri c surveys established that these buoys remained in position throughout the measurements.

At four positions (Sts. I-IV), profiles of the horizontal velocity were measured, using three Richardson-type current meters (manufactured by the

Wooster and Jones: California Undercurrent 243

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Figure 8. Locations of fine-scale and current stations, ExJIBIA Expedition, 2-ro August 1966. The arrows connecting the triangles indicate the net ship drift during the current station. St. I is not shown.

Geodyne Corp.). On these meters, speed is sensed by a Savonius rotor; the manufacturer claims a starting speed of less than 2.6 cm/sec and a precision of 2.6 cm/sec at speeds below 5 1 cm/sec. Direction is recorded as a vane direc-tion relative to the instrument case, and a case orientation; directional un-certainty below 20 m is estimated to be 6°. The three sensors are recorded internally on photographic film every 2.5 seconds.

On current stations, one meter was maintained at about 20 m. The other two meters, separated by 5 m of wire, were successively lowered to five depths, at each of which they were maintained for about 30 minutes. The depth of each recording was estimated from the wire angle (held relatively constant by use of the bow-thruster) and the length of wire out. Reversing thermometers at JOO m and 250 m above the current meters during the deepest measure-ments confirmed that the depth error was not greater than 3 m. The velocity at 20 m was determined from the ship motion (relative to the reference buoy) and from the 20-m measurements; at greater depths, readings of the two meters were averaged.

Distributions of the temperature and salinity on the reconnaissance sec-tions (Figs. 9, 1 o) illustrate characteristic features of the Undercurrent. Off-shore, the thermocline and deeper isotherms are relatively level. At a distance of less than 25 km from the edge of the continental shelf, isotherms above 10° rise toward the coast while deeper isotherms (below 150 m) descend abruptly. The shallow salinity minimum of the California Current (Reid et al. 1964) extends across the section in the lower part of the thermocline.

-(/) 0:: w I-w ::E -I I-a.. w a

244 Journal of Marine Research [28,2

0

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200

300

400 /' ----

70 / \

'- ~-- / \

"- / \

---- / '-- ' 500 60 ,,,.--------.__ 1\

200 150 100 50 0 Distance from 150-meter contour(km)

Figure 9. Distribution of temperature (0 C) on the reconnaissance section. See Fig. 7.

Below 200 m, near the coast, an intrusion of high-salinity water (greater than 34.2°/oo) is present in a very narrow zone next to the continental slope. Geostrophic computati ons from the reconnaissance line indicate an inshore northward fl ow of about IO cm/sec (relative to 500 db) at depths of 200 m and 300 m.

The abrupt nature of the transition between waters of the Undercurrent and of the California Current offshore can be seen in data from the fine-scale crossing (Fig. 1 I). The transition ( near St. 330) was marked by temperature and salin ity inversions below I 50 m; inshore, the salinity in the core of the Undercurrent exceeded 34.3 °/oo- The fin e-scale section parallel to the shelf (Fig. I 2) illu strates the intricate temperature and salinity distributions as-sociated with the flow . (Variati ons in the salinit y maximum suggest that this

Wooster and 'Jones: California Undercurrent 2 45 270 275 280 284

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400

>34.2

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Figure 10. Distribution of salinit y (0/ 00) on the reconnaissance section. See Fig. 7.

section was not precisely along the axis of Row.) Comparison of the tempera-ture and salinity distributions at Sts. 327 and 333 (about 9 km apart) shows that the Undercurrent water below I oo m was not only warmer and more saline than that offshore but was characterized by numerous temperature and salinity inversions above 400 m (Fig. 1 3).

The average velocities measured at various depths are shown in Fig. 14. St. I was far offshore; Row rotated with increasing depth-from eastward at 20 m toward the northwest at I 02 m, then changed abruptly toward the east and southeast at greater depths. St. IV was just west of the transition boundary; the Row at all depths was weak and generally toward the north and northeast. St. II was approximately on the transition boundary; speeds were higher (about 20 cm/sec) than at St. IV and toward the northeast. St. III was ap-proximately in the core of the Undercurrent; here speeds between I oo and 300 m were high (to nearly 40 cm/sec) and the direction was uniformly

246 'Journal of Marine Research [28,2

DISTANC E (k m) DI STA NCE ( km)

0 5 10 15 0 5 10 15

0 ---20•~

0

=---=-= ::::-- 15•~

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toward the north (more exactl y, about 350°). The Undercurrent appeared to weaken below 300 m, but at 400 m the northward component was still nearly 20 cm/sec. Unfortunately, measurements were not obtained at greater depths.

These measurements indicate that, in August 1966, the California Under-current off Punta Colnett was about 20 km wide, was at least 300 m thick, and had an average speed of about 30 cm/sec. The volume transport was about 2 x 106 m3/sec, or about one-fifth that of the California Current (W ooster and Reid 1963).

Discussion. The existence of the California Undercurrent has long been known. Alth ough few direct current measurements have been made, the Undercurrent is evident in the computed relative geostrophic circulati on and by the subsurface distributions of temperature, salinit y, and dissolved oxygen. Because of the narrowness of the feature, as shown by the ExJIBIA measure-ments, conventional station spacing is too coarse fo r an adequate description of the fl ow.

Direct measurements in the core of the Undercurrent showed speeds as high as 40 cm/sec. The fl ow was swift, narrow, and pressed against the con-tinental slope. It is of interest to examine the fin e-scale distribution of mass to see if a comparable geostrophic fl ow was present. A t this latitude, the observed maximum speed corresponds to a isobaric slope of 3 cm in 1 0 km. O n the assumption of no moti on at 500 m, data from the fin e-scale section across the current are not inconsistent with such a slope. The topography of

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Wooster and 'Jones: California Undercurrent

DISTANCE (km) 0 5 10 15

= 20• ,--::

- 15• ----------------------::~::------ - - ---- :::.::o c:;,::::::

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DISTANCE (km) 0 5 10

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®"

2 47

15

Figure n . Distributions of temperature and salin ity on the fine-scale section along the Undercur-rent. See Fig. 8.

the surface 200/ 500 db varies irregularly by 2 cm over a distance of 5-10 km. However, it is difficult to determine the exact geostrophic speed for a number of reasons, including the following: (i) Uncertainties in the measured values of temperature, salinity, and pressure lead to uncertainties of at least ± 1

dynamic cm in geopotential anomaly (i.e., about the observed range). (ii) Time variations due to vertical oscillations contribute an additional and unknown error. (iii) Suitability of 500 m as the reference level is unknown (flow was still 20 cm/sec at 400 m at St. III).

The abruptness of the western edge of the Undercurrent is surprising. Within a few kilometers, as one approaches the coast, the isotherms deepen steeply, the salinities become high, and the profiles of temperature become extremely irregular. The relationship between the observed density and velocity structure and the mixing can be examined in terms of the Vaisala frequency, N, and the Richardson number, Ri (Jones 1969).

The Vaisala frequency is defined as

where g is the acceleration of gravity, e is the density, ae/az is the vertical density gradient (z increasing downward), and c is the speed of sound. Values of Nin the ocean typically range from 10-2 sec-, in the thermocline to 10-4

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TEM PERATURE, °C

Figure 13. Profiles of temperature and salinity at Sts. 327 and 333.

sec' in the deep ocean (Phillips 1966); when N 2 < o , the water is statically unstable.

The Richardson number is defined as

where o u/oz is the vertical gradient of horizontal velocity. Low values of Ri ( < 1 /4) are beli eved to indicate dynamic instability (Phillips 1966) while higher values imply increasing dynamic stability.

The average density gradient in the high-salinity water at St. 327 was 6.4 x 10-8g cm-4, corresponding to N equal to about 4 x 10-3 sec'. The average vertical shear (between 100 and 400 m) at St. III (Fig. 15) was no greater than 10-3 sec '- less than one fifth that recently observed beneath the core of the Equatorial Undercurrent (Jones 1969). The resulting Richard-son number of about 16 is high, indicating a high degree of dynamic stability.

With in the layers of warm salty water, the density gradient can be con-siderably reduced. A value of o.6 x 10-3 sec' for N was arbitrarily selected as indicative of reduced static stability. At Sts. 326-329, within the Under-current, a third to a half of the layer between 150 and 450 m consisted of

Wooster and Jones: California Undercurrent

.. 0

0

180

249

.. 0

Figure 14. Average velociti es measured at ExJIB!A Sts. I-IV; number alongside vector indicates depth in meters.

numerous laminae 5-25 m thick, with N less than the selected value (in a number of cases, N 2 < o in these laminae). In contrast, at Sts. 333-336, beyond the Undercurrent, less than a fifth of this layer consisted of such laminae.

If one uses a value of o.6 x I o-3 sec I for N and an average shear of 10-3 sec', the resulting Richardson number is 0.36---close to the critical value for dynamic instability. If the shear were locally larger, Ri would be further reduced and the vertical mixing might be vigorous.

Several desirable experiments are suggested by these observations. It would be interesting to use a profiling device, such as the STD, close inshore to map out the northward extent of the high-salinity water. Such a survey should reveal appropriate locations where subsurface salinity measurements could

Journal of Marine Research

monitor variati ons in the Undercurrent. Littl e is known about the time varia-tions in this fl ow-from those of high frequency that could be studied by methods described in this paper to those of lower frequency requiring long-term monitoring. The relationships between the intensities of the Under-current, the surface current, and the surface winds are poorly understood. The effects of changes in the bottom topography are not known. Finally, detailed velocity profiles in the Undercurrent should facilitate interpretation of the mixi ng processes related to the flow and to the thin layer of high salinity.

REFERENCES BARKLEY, R. A .

1968. Oceanographic atlas of the Pacific Ocean. University of Hawaii Press, Honolulu. 20 pp + 1 56 plates.

JONES, J . H . 1969. Verti cal mixi ng in the Equatorial Undercurrent. Ph.D . dissertation, Univ. of Calif.,

La Jolla; 68 pp.

PHILLIPS, 0. M. 1966. The dynamics of the upper ocean. University Press, Cambridge. 261 pp.

REID JR., J . L . 1963. Measurements of the California Countercurrent off Baja California. J . geophys.

Res., 68 : 4819-4822.

REID JR., J . L., G. I. RoDEN, and J. G. WYL LI E 1958. Studies of the California current system. Prog. Rep. Calif. Coop. Oceanic Fish.

Invest., 1 July 1956-1 January 1958; pp. 27-56.

REID JR., J. L. , C. G. WORRALL, and E. H. COUGHRAN 1964. D etail ed measurements of a shallow salinity minimum in the thermocline. J . geo-

phys. Res., 69: 4767-477 r.

UNIVERSITY OF CALIFORNIA 1967. Physical and chemical data. Scripps Inst. Oceanogr. Ref. 67-17; 119 pp.

1968. Physical and chemical data. Scripps Inst. Oceanogr. Ref. 68- 21; 75 pp.

WOOSTER, W. S., and J . L. REID JR. 1963. Eastern boundary currents, In The Sea, vol. 2, pp. 253-280. M. N. Hill , Ed.

lnterscience, N.Y . 554 PP·

WYLLIE, J. G. 1966. Geostrophic fl ow of the California Current at the surface and at 200 meters. State

of California Marine Research Committee, Calif . Coop. Oceanic Fish. Invest. Atlas No. 4 ; pp. xiii + 288 charts.

YosHIDA, Kozo 1967. Circulation in the eastern tropical oceans with special references to upwelling and

undercurrents. Jap. J . Geophys., 4: 1-75.