between 92 ø 00'W and the Galapagos Islands

Transcription

1 VOL. 78, NO. 15 JOURNAL OF GEOPHYSICAL RESEARCH MAY 20, 1973 Optical and Hydrographic Observations of the Cromwell Current between 92 ø 00'W and the Galapagos Islands RONALD V. ZANEVELD, HASONG PAX, AND WILLIAM 8. PLANK School o] Oceanography, Oregon State University, Corvallis, Oregon Optical and hydrographic observations were made at two meridional sections across the Cromwell Current at 90 ' and 91ø40'W during February The distribution of hydrographic and optical properties near the Galapagos Islands is described. Beneath the thermocline the Cromwell Current is characterized by deepening of the light-scattering isolines. Above the thermocline, shallowing of the light-scattering isolines results in a surface minimum in light scattering directly above the core of the current. Splitting of the core of the current into northern and a less clearly indicated southern branch is inferred from the horizontal distributions of temperature, oxygen content, and light scattering just beneath the depth of maximurn horizontm velocity. The water surrounding the Galapagos Islands tent. The scattering of light with a wavelength are of considerable oceanographic interest. The of 546 nm was determined by means of a Bricewestern boundary of the island group straddles Phoenix light-scattering photometer, and parthe equator at approximately 91ø30'W. In this ticle concentration was measured with a Model area a major ocean current, the Pacific equa- A Coulter Counter equipped with a 100-t m torial undercurrent, often called'the Cromwell aperture that is capable of counting particles Current, undergoes major modifications in direc- between 2.2 t m and 40 t m. Particles in this tion, speed, and depth over a relatively short range are thought to be responsible for most of distance. One of the objectives of Yaloc-69, a the scattered light at 45 ø [Jerlov, 1968]. The cruise to the Panama Basin by the Department use of these two instruments in optical oceanof Oceanography of Oregon State University, ographic applications has been discussed by was to apply the methods of optical ocean- Pak [1970] and Carder [1970]. ography to the study of the waters near the Galapagos Islands. By determining the distribu- HYDROGRAPHIC OBSERVATIONS tion of optical and hydrographic properties im- The presence of the Cromwell Current in the mediately to the west of the islands it was felt region of study may be inferred from the mothat some insight could be obtained into the tion of a parachute drogue released at a 100- processes which modify the Cromwell Current meter depth on the equator at 91ø40'W. The and that some information could be obtained drogue was traced for 12 hours using radar concerning the final destination of the large observations of Isla Isabela as a reference. The volumes of water transported by the current. observed velocity was approximately 50 cm/sec to the northeast. Velocity measurements of 50 EXPERIMENTAL PRdGRAlVf and 15 cm/sec were made by Knauss [1960, During February 1969 the Cromwell Current 1966] at a 100-meter depth on the equator at was traversed by two north-south fines of 90 ' and 92ø16'W, respectively, and Christenhydrographic stations at 91ø40 ' and 92ø00Wg. sen [1971] observed velocities of 50 cm/sec at Water samples collected to a depth of 94ø45'W and 105 cm/sec at 92ø00'W. Our meameters at each station were analyzed for fight surementhus falls in the range already obscattering at 45 ø from the forward direct}on served by others. and for concentration of particulate matter as Our hydrographic observations (Figures 1 well as temperature, salinity, and oxygen con- and 2) demonstrate a feature previously observed further to the west by several investi- Copyright 1973 by the American Geophysical Union. gators [Wooster and Jennings, 1955; Wooster 2708

2 ZANEVELD ET AL.' BRIEF REPORT 2709 STAT NS 0 20{ _ 0 30O TEMPERATURE (*C) [,,,--t ' ', ß I,,,,ß.,. ß I... I ß 4 ø.3 ø 2 ø I o 0 ß I ß LATITUDE STATK S _ o,,. ß 0 ß., j4. 4_ 35 I 35.0 SALINITY,,!...!,, ß!...!-- 4 2o I o 0 ß I o LATITUDE STATK)NS OXYGEN (ML/L) t,,, '., ' '!,,,!, ',!... [ 4 ø.3o 2 o I ø 0 ø 10 LATITUDE Fig. 1. Temperature, salinity, and oxygen content at 92ø00'W. Dots indicate bottle spacing.

3 2710 ZANEVELD ET AL.' BRIEF REPORT U ø ß ß,. -o. =o- -,- =,. -,.--.,. ß ; : : : : :. : ' :.._. 1.o 4' $* 2' I* 0"' I e LATITUDE: ß IOO ß. ß ß $ $./ ' 3.0./ 0 i 4ooL,.. I,,_..T; 1 4' 3' 2' I' I' ( UTH) LATITUDE ( TH) 0.,m m m. m. _.m =.o =',, : :' ' ::... - ioo ß ß OXYGEN ( ML! L ) 40C --,4 ø 3 o I.,,!... I,,, 2 ø I o 0 o I o L AT I TUDE Fig. 2. Temperature, salinity, and oxygen content at 91ø40'W.

4 , ZANEVELD ET AL.: BRIEF REPORT and Cromwell, 1958; Knauss, 1960, 1966; Bennett, 1963], that is, the spreading of isolines of temperature, salinity, and oxygen content. The shallowing of these isolines at depths less than 100 meters and the corresponding deepening between 100 and 300 meters results in reduced vertical gradients of these properties between about 1 øn and 2øS. OPTICAL OBSERVATIONS The distribution of light scattering at 92ø00 ' and 91ø40'W (Figure 3) is similar to the distribution of hydrographic properties in that spreading of the isolines is once again detected between about 2øS and IøN. It has been noted by Jerlov [1959] that high light-scattering gradients are normally present at the level of the 2711 thermocline, and this feature can be seen in our cross-sections south of 2øS and north of iøn. The gradient is greatly reduced near the equator, where the thermocline is also weaker. A related feature is a meridional surface mini- mum in light scattering above the Cromwell Current, which results from the intersection of rising isolines with the surface. During the Swedish Deep Sea expedition, Jerlov [1953] observed a meridional particle concentration maximum between 2øN and 2øS, presumably a result of biological actvity stimulated by upwelling. This data would seem to conflict with the light scattering minimum we observed; however, it is believed that the surface minimum we observed is embedded within this maximum and must be attributed to mixing or o o. "' 2OO 500 (45) (M-STER) - xlo -4 [.,,, [ [ 4' 3' 2' (SDUTH) LATITUDE I* O0... i:, '.:., o!- ø,oo;. '.'..<_;-,oo - ß.. E " ' 'k - ß Fig. 3. oor. (45) (M-$TœR) - xlo. -4. ß. 4* 3' 2* I* O* I* L AT I TU DE ( NOR TH ) Light scattering at 45* from the forward direction at 92ø00'W (top) and 91ø40'W (bottom).

5 271'2 ZANEVELD ET AL.' BRIEr REPORT upwelling of deeper and clearer water. Once this east of the Galapagos Islands [Knauss, 1966; water reaches the surface and moves toward White, 1969; Christensen, 1971;Stevenson and higher latitudes, the nutrients contained within Ta[t, 1971]. Also, a deep eastward flowing curit stimulate organic production and the result rent has been shown to exist to the north of is the larger scale particle maximum observed the islands by Knauss [1966] and Christensen by Jerlov. [1971]. These investigators found no evidence Due to the lower precision of particle counts of a current to the south; however, White relative to the other parameters discussed in [1969] describes a southern branch which apthis paper, we do not expect to see the same pears to extend to the South American contidetail in the cross-sections. However, Figure 4 nent. If the current splits, it would seem that shows that the particle concentration isolines the distribution of optical and hydrographic deepen between 0ø30'N and 1ø00'S at depths properties immediately west of the islands greater than 50 meters. would contain some evidence of this. Montgomery [1962] indicates that the high SrL TT G or T E CROMWELL CURRENT velocity core of the Cromwell Current coincides The Cromwell Current, although greatly with the isotherm which is most nearly horiweakened, has been shown to exist to the zontal in the north-south direction. In our case, -i' 'i- IOO ø o 20 i o 0 o i o ß 1... ß ;ARTICLE ' ' ' CONT ENT " ' ' ' oo, (p$/ml) :,,,!,,,.N,,,!,.'! LATITUDE 0 --i -/ooo. :-.-" /!- '!-!' -!''i' 'i''i-'!''i' -!''!''i' o ß ß -- ''-_... ' -_'_ ß ß ß ß!. ß ß PARTICLE CONTENT. ( ß (p3/ml)... ' ' --.,..., ' I, '- 4oo 4 o o 2o i o OO io LATITUDE Fig. 4. Total number of particles per mi. with diameters greater than 2.2 m at 92ø00'W (top) and 91ø40'W (bottom). :

6 92'W 91'W 90'W ZANEVELD ET AL.: BRIEF REPORT 92'W 91'W 90'W 'W 91'W 90'W I I ß ten I-' ') 1 Fig. 5. Temperature oxygen content, and light scattering at a 100-meter depth. this is the 20øC isotherm at approximately 80 meters. Since we have seen from the cross- sections that the isolines of most properties deepen below the core of the current, the axis of the current should be visible as a meridional maximum in these properties in the horizontal plane at depths greater than 80 meters. Plots of temperature, oxygen content, and light scattering at 100 meters (Figure 5) show tongues of water with high values of these properties extending to the northwest and southwest of Isla Isabela. These tongues are suggestive of a splitting of the current, with the northern branch being the more dearly defined. CONCLUSIONS A study of optical and hydrographic properties has demonstrated the existence of the Crom- well Current immediately west of the Galapagos Islands. A previously unobserved surface minimum in light scattering was noted along the axis of the current. Evidence for the splitting of the current into a northern and southern branch is presented. However, it is believed that a detailed survey over a larger area in the vicinity of the islands is needed to accurately delineate the path of the current. Acknowledgment. This research was supported by the Office of Naval Research through contract N A-03694)007 under project NR REFERENCES Bennet, E. B., An oceanographic atlas of the eastern tropical Pacific ocean, based on data from the EASTOPAC Expedition, October-December 1955, Inter-Amer. Trop. Tuna Comm. Bull., 8(2), 165, Carder, K. L., Particles in the eastern Pacific Ocean' Their distribution and effect upon optical parameters, 140 pp., Ph.D. thesis, Oregon State Univ., Corvallis, Christensen, N., Jr., Observations of the Cromwell Current near the Galapagos Islands, Deep Sea Res., 18, 27-33, Jerlov, N. G., Particle distribution in the ocean, Rep. $wed. Deep Sea Exped., 3, 73-97, Jerlov, N. G., Maxima in the vertical distribution of particles in the sea, Deep Sea Res., 5, , Jefiov, N. G., Optical Oceanography, 194 pp., Elsevier, Amsterdam, Knauss, J. A., Measurements of the Cromwell Current, Deep Sea Res., 6, , Knauss, J. A., Further measurements and observations on the Cromwell Current, J. Mar. Res., 26, , Montgomery, R. B., Equatorial undercurrent observations in review, J. Oceanogr. $oc. Jap., 20th Anniv. Vol., , Pak, H., The Columbia River as a source of marine light scattering particles, Ph.D. thesis, 110 pp., Oregon State Univ., Corvallis, Stevenson, M. R., and B. A. Taft, New evidence of the equatorial undercurrent east o.f the Galapagos Islands, J. Mar. Res., 29, , White, W. B., The equatorial undercurrent, the South equatorial countercurrent, and their extensions in the South Pacific Ocean east of the Galapagos Islands during February-March 1967, 74 pp., Texas A&M Univ., College Station, 1969.

7 2714 ZANEVELD ET AL..' BRIEF REPORT Wooster, W. S., and F. Jennings, Exploratory graphic description of the eastern tropical Paoceanographic observation in the eastern trop- cific, / ull. Scripps I st. Oceano 7r., 7(3), 169- ical Pacific, January to March 1953, Cal ;]. F sh 182, Ga ne,,1, 79-90, (Received January 8, 1973; Wooster, W. S., and T. Cromwell, An oceano- revised January 30, 1973.)