15
Deep-Sea Research, Vol. 32. No. 3, pp; 299 to 313. 1985. 0198-0149/85 $3.00 + 0.00 Printed in GreatBritain. ~ 1985Pergamon ~ Ltd. The subthermoeline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean MIZUKI TSUCHIYA* (Received 23 January 1984; in revised form 23 July 1984; accepted 31 August 1984) Abstract--Measurements of nutrient salts were made in two periods, November to December ! 98 I and February to March 1982, along nearly the same tracks in the far eastern Pacific betwcen the Galfipagos Islands and the coast of South America. The subthermocline circulation was deduced principally from the distribution of phosphate. The major flow patterns were found to be basically the same for the two periods. Along the 85°W meridian, the major observed zonal currents were the Equatorial Undercurrent within 2 ° of the equator, the westward return flows on both sides of the Undercurrent, the subsurface South Equatorial Countercurrent centered at about 6°S, and additional eastward flows at about 4 ° and 8°S. East of 85°W, the subsurface South Equatorial Countercurrent veered southeastward and crossed the 10.5°S parallel in the vicinity of 8 ! °W. Sig- nificant decreases in the nutrient concentrations occurred in the domain of the Equatorial Undercurrent from November to December 1981 to February to March 1982. These decreases can be explained by an increase in the eastward speed of the Undercurrent during February to March 1982. INTRODUCTION IN THE subthermocline layer of the eastern equatorial Pacific, the fields of salinity and geopotential (or acceleration potential) are so uniform that it is difficult to infer details of the circulation from these properties. However, non-conservative properties such as the con- centrations of dissolved oxygen and nutrient salts are useful in identifying the sources of water masses and deducing the flow patterns (TsuCHZYA, 1968, 1981). Measurements of nutrient salts were made in 1981 and 1982 to the east of the Galfipagos Islands on two cruises of EPOCS (Equatorial Pacific Ocean Climate Studies), a program sponsored by NOAA (National Oceanic and Atmospheric Administration), thereby supplementing the CTD/Oz data taken by NOAA investigators. The primary purpose of the cruises was to monitor temporal variations of the thermal and current fields between 5°N and 11 *S and between 85°W and the coast of South America. This domain ofthe eastern Pacific is known to be an area of high annual and interannual variabilities. The entire nutrient data set from the cruises has been analyzed. This report presents the results of the analysis, primarily on the basis of the phosphate data; however the silica and nitrate data give essentially the same results in deducing the subsurface circulation. There have been a considerable number of studies on the nutrient distribution and its rela- tion to the ocean circulation in the eastern equatorial Pacific (WOOSTER and CROMWELL, 1958; WOOSTER et al., 1965; WOOSTER, 1967; AUSTIN, 1960; BENNETT, 1963; LOVE, 1971, 1972; * Institute of Marine Resources, A-030, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, U.S.A. 299

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Page 1: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Deep-Sea Research, Vol. 32. No. 3, pp; 299 to 313. 1985. 0198-0149/85 $3.00 + 0.00 Printed in Great Britain. ~ 1985 Pergamon ~ Ltd.

T h e s u b t h e r m o e l i n e p h o s p h a t e dis tr ibut ion and c i rcu la t ion in the far

eas tern equator ia l Paci f ic O c e a n

MIZUKI TSUCHIYA*

(Received 23 January 1984; in revised form 23 July 1984; accepted 31 August 1984)

Abstract--Measurements of nutrient salts were made in two periods, November to December ! 98 I and February to March 1982, along nearly the same tracks in the far eastern Pacific betwcen the Galfipagos Islands and the coast of South America. The subthermocline circulation was deduced principally from the distribution of phosphate. The major flow patterns were found to be basically the same for the two periods. Along the 85°W meridian, the major observed zonal currents were the Equatorial Undercurrent within 2 ° of the equator, the westward return flows on both sides of the Undercurrent, the subsurface South Equatorial Countercurrent centered at about 6°S, and additional eastward flows at about 4 ° and 8°S. East of 85°W, the subsurface South Equatorial Countercurrent veered southeastward and crossed the 10.5 °S parallel in the vicinity of 8 ! °W. Sig- nificant decreases in the nutrient concentrations occurred in the domain of the Equatorial Undercurrent from November to December 1981 to February to March 1982. These decreases can be explained by an increase in the eastward speed of the Undercurrent during February to March 1982.

INTRODUCTION

IN THE subthermocline layer of the eastern equatorial Pacific, the fields of salinity and geopotential (or acceleration potential) are so uniform that it is difficult to infer details of the circulation from these properties. However, non-conservative properties such as the con- centrations of dissolved oxygen and nutrient salts are useful in identifying the sources of water masses and deducing the flow patterns (TsuCHZYA, 1968, 1981). Measurements of nutrient salts were made in 1981 and 1982 to the east of the Galfipagos Islands on two cruises of EPOCS (Equatorial Pacific Ocean Climate Studies), a program sponsored by NOAA (National Oceanic and Atmospheric Administration), thereby supplementing the CTD/Oz data taken by NOAA investigators. The primary purpose of the cruises was to monitor temporal variations of the thermal and current fields between 5°N and 11 *S and between 85°W and the coast of South America. This domain ofthe eastern Pacific is known to be an area of high annual and interannual variabilities.

The entire nutrient data set from the cruises has been analyzed. This report presents the results of the analysis, primarily on the basis of the phosphate data; however the silica and nitrate data give essentially the same results in deducing the subsurface circulation.

There have been a considerable number of studies on the nutrient distribution and its rela- tion to the ocean circulation in the eastern equatorial Pacific (WOOSTER and CROMWELL, 1958; WOOSTER et al., 1965; WOOSTER, 1967; AUSTIN, 1960; BENNETT, 1963; LOVE, 1971, 1972;

* Institute of Marine Resources, A-030, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, U.S.A.

299

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300 M. TSUCHIYA

LOVE and ALLEN, 1975; THOMAS, 1972, 1979; PATZERT, 1978; BARBER and HUYER, 1979; CODISPOT, and PACKARD, 1980). Pertinent background information about the circulation and the general distributions of phosphate, silica, and nitrate in the subthermocline layer in the vicinity of the study region, based on a composite of various data, can be found in TSUCHIYA (1981).

In the tropical and subtropical Pacific Ocean, the concentrations of phosphate, silica, and nitrate just below the thermocline are highest off the coast of South America near the study region and, north of the equator, in the corresponding region off central America. Although the locations of the highest concentrations are not the same for the three nutrients they all appear to occur slightly south of the .study region. Thus, the concentrations of the nutrients are generally high in the east and south and low in the west and north. Farther southwest, the concentrations decrease toward the center of the South Pacific subtropical gyre, but this is well outside the region of the present study (REID, 1962, Fig. 3; TSUCHIYA, 1981, Figs 9 to II).

There are three subsurface eastward zonal flows that approach the region from the west: the Equatorial Undercurrent along the equator and the subsurface Equatorial countercurrents along about 5°N and 5°S. The Equatorial Undercurrent appears to extend eastward past the Galapagos Islands, at least from time to time (KNAUSS, 1966; STEVENSON and TAFT, 1971; CHRISTENSEN, 1971; PAK and ZANEVELD, 1973, 1974; LUKAS, 1981; LEETMAA, 1982), and the subsurface countercurrents have been shown to extend at least as far east as 95°W in the north and 88°W in the south (TsuCmYA, 1975). The three subsurface eastward flows trans- port low-nutrient water from the west and, therefore, each is associated with tongues of low phosphate, silica, and nitrate extending eastward. Near the coast of South America, the eastward flows, in part, are believed to turn poleward, continuing as the eastern boundary undercurrents; however, the actual connections between the eastward flows and the poleward undercurrents have not been well established. Part of the eastward flows turn back to return westward along about 3°N and 3°S between the eastward flows. The westward return flows are associated with tongues of high nutrients extending westward on both sides of the equator. The Peru Current, which flows into the region from the south, tends to bring in water of high nutrients.

D A T A

The first cruise was in November to December 1981 on R.V. Researcher, and the nutrient measurements were made on the stations shown in Fig. I a. The second cruise took place in February to March 1982 on R.V. Discoverer, and essentially the same stations were occupied and a few stations were added along 5°S (Fig. lb). On both cruises, the stations were spaced at intervals of 55 km except between l °N and 1 °S and near the coast, where they were spaced more closely (,,,30 kin). On each station, water samples were collected from 12 depths (0, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800, and 1000 m). The samples were analyzed aboard ship with an automated chemical analyzer for phosphate, silica, nitrate, and nitrite. The processed data have been published in a data report (TsuCmYA, 1983). The preci- sion of the analyses has been estimated to be _+ 1 to 1.5% (HAOER et al., 1972; BAINBRIDGE, 1981).

The two cruises produced a nutrient data set with an along-track horizontal resolution finer than that in most of the existing data from this region but along widely separated sections. Also the limited numbers of sampling bottles resulted in a vertical resolution coarser than desirable for a detailed study of the distribution in the thermocline and sub-

Page 3: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Subthermocline phosphate distribution and circulation 301

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Page 4: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

302 M. TSUCHn'̂

thermocline layers. With this limitation of the data in mind, the results of the analysis are pre- sented and discussed in the following sections.

RESULTS

November to December 1981 cruise

Figure 2 shows the distribution of phosphate in the upper 600 m along 85°W in November to December 1981. The heavy dashed lines in all but one (Fig. 7) of the phosphate distribution figures (Figs 2 to 7) represent the isopleths ofthermosteric anomaly (isanosteres)for 1.6, 1.8, 1.9, and 2.0 cm a kg -I (l cm a kg -j = 102 tit -I) calculated from CTD data. There is a sharp increase in phosphate from the sea surface down to 50 m. Although the depth of the maximum gradient is not well resolved because of the coarse sample spacing, this sharp vertical gradient of phosphate is associated with the primary thermocline.* When the Equatorial Undercurrent is present at this longitude, its speed core is usually found below the center of the primary thermocline (KNAuSS, 1966; S'mVENSON and TAFT, 1971; LEETMAA, 1982).

As was mentioned in the introduction, the concentration of phosphate in the study region is generally high in the east and low in the west. Therefore, if a quasi-steady distribution can be

J0"S 5 ° O* 5"N Om J'"

6o(

Fig. 2.

0m

200

400

600 25 ~ 3 0 2 ~ . 3 4 ~ 37 3 e , 4 0 6 s 57'59 6 0 s 2 6 4 6 6 6 8 7 0 ~ 7(; e o e 2 e 4 0 6 e e e 9 9 1 ~

Phosphate, in umol dm -3, along 85°W, 25 November to 6 December 1981, Researcher. The vertical exaggeration is 2000.

* Within about 5 ° of the equator, a deeper secondary thermocline that marks the bottom of the Equatorial 13°C Water is found at about 300 m. This secondary thermocline coincides with a stronger vertical gradient of phosphate at about 300 m than immediately above and below (Fig. 2). The term 'subthermocline' used in this report refers to the primary thermocline.

Page 5: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Subthermocline phosphate distribution and circulation 303

assumed (a justification for this assumption is discussed in the next section), domains of meridionally low (high) phosphate can be identified as those of eastward (westward) zonal flows. By far the clearest signal of the zonal circulation in the phosphate distribution can be found in the subthermoeline layer near the equator. Within about 2 ° of the equator, meridionally low-phosphate water (1.6 to 1.8 gmoldm -3) is present at 50 to 200m immediately beneath the thermocline. Along the isanostere for 1.9 cm 3 kg -~, for example, the phosphate concentration is < l . 7 g m o l d m -3 between I°N and 2°S, while it is >1.7 ~tmol dm -3 farther from the equator. The presence of low-phosphate water at the equator on some subsurface isothermal (isanosteric) surfaces was first noted by WOOSTER and CROMWELL (1958), and its relation to the Equatorial Undercurrent was pointed out by MONTGOMERY (1962). It is seen in Fig. 2 that this water contains an isolated core of low phosphate (< 1.6 ~tmol dm -3 at 150 m accompanied by an overlying maximum (> 1.7 lamol dm -3) centered at 100 m near 0.5°S. The existence of a vertical minimum of phosphate in the subthermocline layer near the equator also has been shown by some earlier data (AUSTIN, 1960; ROTSCm et

al . , 1967) and indicates that the low concentrations near the equator are not a result of vertical mixing but due to eastward advection of low-phosphate water in the Equatorial Undercurrent (TstJcmVA, 1981). On the other hand, there is no clearly recognizable indication of the Equatorial Undercurrent in the field of thermosteric anomaly except a slight troughing of the isanosteres for 1.6 and 1.8 cm 3 kg -~ at the equator, which is associated with a weak eastward geostrophie flow just beneath the thermocline (see next section).

The distributions of silica and nitrate (not presented) also show meridionally low concentra- tions at 50 to 200 m within about 2 ° of the equator, and isolated cores of low silica and nitrate accompanied by overlying maxima are found in the same location as the isolated core of low phosphate.

Just north of the equator between 1.5 and 3°N, the subthermocline layer at 1.8 to 2.5 cm 3 kg -~ (50 to 200 m) shows a narrow band of high phosphate, as indicated b y an upward excursion of the isopleth for 1.8 grnol dm -3. This high-phosphate water must be associated with the westward return flow north of the equator. The westward return flow south of the equator is also indicated at 2 to 3°S by high phosphate (1.7 to 1.8 grnol dm -J) along the !.9 cm 3 kg -~ isanostere.

The equatorward deepening of the isanosteres for 1.6 and 1.8 em 3 kg -1 between 3 and 9°S suggests that the zonal component of the subthermocline geostrophic flow is generally eastward here, although the meridional component may not be small because the section is close to the South American coast. In these latitudes, the isopleths of phosphate are more nearly parallel to the isanosteres than near the equator. This distribution makes it difficult to deduce the direction of the zonal flow from the phosphate field, since the interpolated values of phosphate on the isanosteres are not sufficiently accurate to estimate the weak meridionai gradient. Small-scale irregularities in the isopleths of phosphate seen in Fig. 2 may be due to the linear interpolation of samples generally 50 m apart. (There is no vertical interpolation problem for thermosterie anomaly because it is based on CTD data.) Moreover, it is possible that some of the irregularities represent isolated m a s s e s of water associated with small-scale eddies. Herein, only features that are spatially coherent and of a magnitude >0.1 gmoi dm -3 in the concentration difference are interpreted as representing quasi-steady flows. In the thermosteric-anomaly range 1.6 to 1.8 cm 3 kg -l, an eastward flow is suggested by low ptlosphate at 3.5 to 4.50S. It shows a trend to shit~ equatorward with increasing depth. Another eastward flow is suggested at 5.5 to 70S. The latter eastward flow is identified as the subsurface South Equatorial Countercurrent; a previous study has shown that its axis is

Page 6: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

304 M. TSUCmYA

Fig. 3.

Om Om I ~ . i . I l I

~ ~ / . \ ~ . . •

~ " " I. ~ :~./. - "~ .... !.~ V

¢~00 ~ i i i ~ i i , i i i 600 2322 19 I]8 t I 15 14 12 II 9 8 6 S

Phosphate, in Ilmol dm -3, along I0.5°S, 2l to 25 November 1981,Researcher.

located near 6°S at 88°W (TsucalvA, 1975). A weak indication of an eastward flow is found also at 8°S.

Figure 3 shows the phosphate section along 10.5 °S in November 1981. The sharp vertical gradient of phosphate at about 50 m again coincides with the thermocline. As was noted before, the concentration of phosphate is generally higher to the south than to the north of the 10.5°S section, so that domains of zonally low (high) phosphate along isanosteres can be identified as those of southward (northward) meridional flow. In the subthermocline layer at 1.6 to 1.8 cm 3 kg- ' , a northward flow is indicated at, and east of, 80.5°W by high phosphate and by an eastward rise of the isanostere for 1.6 cm 3 kg -]. The northward flow contains an isolated core of high phosphate (>2.6 IJmol dm -3) at 150 to 250 m. Additional northward flows are found at 81 to 82°W and near 84°W, but neither is clear except at 1.6 cm 3 kg -I. On the other hand, a southward flow is indicated by low phosphate at 81 °W and the slope of the isanostere for !.6 cm 3 kg -i also suggests a strong southward flow.

February to March ! 982 cruise

The distribution of phosphate observed along 85°W in February to March 1982 is illustrated in Fig. 4. The presence of low-phosphate water (1.2 to 1.7 lamoi dm -3) associated with the Equatorial Undercurrent is clearly evident within about 2 ° of the equator at 50 to 200 m just beneath the primary thermocline. The phosphate concentration of this water, however, has decreased to 1.2 to 1.7 l~nol dm -3 from 1.6 to 1.8 I~mol dm -3 observed on the November to December 1981 cruise. At 50 to 150 m, the concentration is so much reduced that the subsurface maximum found at 100 m on the previous cruise is no longer present, and the underlying low-phosphate water, which formed an isolated core at 150 m, is only recog: nized as a lateral (meridional) minimum.

Page 7: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Subthermoeline phosphate distribution and circulation 305

IO*S 5 ° 0 ° 5°N

Om

7: 200

4 400

E G00 4140 3 9 3 8 3 7 3 6 3534333,? . 31 3021 20 1918 I? 16 r$ 14131Z 111098 r 6 5 4 3 2 I

F ig . 4. P h o s p h a t e , in ~ m o l drn -~, a l o n g 85 * W , 28 F e b r u a r y to 13 M a r c h 1982, Discoverer.

Coincident with the decrease in phosphate, an increase in thermosteric anomaly is observed in the same domain. As is evident from a comparison of Fig. 4 with Fig. 2, the isanosteres for 1.9 and 2.0 cm 3 kg -I have deepened by 60 to 80 m to form a trough between 2°N and 2°S. As a result, if the phosphate concentration is compared along isanosteres, the decrease is smaller than along horizontal surfaces but still significantly large along certain isanosteres (Table l), because the deepening of the isanosteres is exceeded by that of the phosphate isopleths. Clearly, the decrease in phosphate along the isanosteres could not be caused by a short-period downward motion of water associated with internal waves. The trough of the isanosteres at the equator can be taken as evidence of a stronger development of the Equatorial Undercurrent in February to March 1982 than in November to December 1981.

The distribution of silica and nitrate in February to March 1982 (not presented) also show meridionally low concentrations at 50 to 200 m in the subthermocline layer within 2 ° of the equator, and the silica and nitrate concentrations of this water are significantly lower than those in November to December 1981 (Table 1). The first sign of the well-publicized equatorial warming in 1982 to 1983 was not observed at 85°W until October 1982 (L~TMAA et al., 1983). Thus, the changes in the nutrient and steric fields described above are probably not associated with this warm event.

Northward from the equator, the subthermocline concentration of phosphate along the isanosteres increases gradually. This increase is consistent with the northward decrease irl the eastward speed of the Equatorial Undercurrent. The reversal of the slope of the isanosteres for 1.6 to 2.0 cm 3 kg -~ near 3°N suggests that the subthermocline flow is weakly westward north of 3°N, although it is not obvious from the phosphate distribution. Near 3°S the southern limit of the Equatorial Undercurrent is marked by an upward bulge of the 1.8 lamol dm -3 isopleth, which indicates a narrow band of westward flow.

Page 8: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

306 M. TsucmYA

Table 1. Concentrations ~ f nutrient salts within 1 o o f the equator

Concentration (JLmol drn -3)

5T(cm J kg -I) Nutrients Nov. to Dec. 1981 March 1982

2.0 Phosphate 1.5-1.7 1.3-1.5 Silica 16-17 15-16 Nitrate 22-23 20-21

Phosphate 1.6-1.7 1.6-1.7 1.9 Silica 17-18 ~18

Nitrate 22-24 ~24

Between 3 and 10°S, the phosphate distribution shows three bands of subthermocline eastward flows. One is found between 4 and 5 °S at 2.0 cm 3 kg -I and shifts cquatorward to 3 to 4°S at 1.6 cm 3 kg -I . Another is centered at 6°S and can be identified as the subsurface South Equatorial Countercurrent. The third eastward flow is indicated at 7.5 to 8.5°S just below the thermocline and becomes broader in deeper layers.

The phosphate section along 10.5°S made in March 1982 is shown in Fig. 5. In the sub- thermocline layer (1.6 to 1.8 cm 3 kg-I), two northward flows are suggested near 80 and 84°W by slightly higher phosphate than elsewhere. At 1.6 cm 3 kg -~, southward flows are seen at the eastern end of the section and at 80.5°W. The southward flow at 80.5°W is associated with an isolated core of low phosphate with concentrations below 2.4 pmol dm -s. As can be seen in Fig. 6, which illustrates the phosphate section along 5°S in the same period, the phosphate concentration at 1.6 cm 3 kg -I along 5 °S is everywhere higher than 2.4 pmol dm -3 (Fig. 7) so that low-phosphate water at 10.5°S, 80.5°W must have come from the eastward

Fig. 5.

Om

B 5 • I I 1 I I

• - ~ • " • ' i ' i

e O ' W ~ 0 .4 - - Om

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6oo , i i ~ ; ,~-~, ~ , ,I 6oo 41 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9 5 0 5 1 5 2 5 3 5 4

Phosphate, in pmol dm -~, along i 0.5 °S, 13 to 17 March 1982, Discoverer.

Page 9: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

Subthermocline phosphate distribution and circulation 307

Om

20O

85"W 81*

0.4 0m

200

Fig. 6.

400 400

. ~,32 600 600 T J i I I 1 i li 30 29 2:8 27 26 25 232422

Phosphate, in lamol dm -3, along 5°S, 7 to 10 March 1982, Discoverer.

90" ~° I [ I | I I I I l

5" November - December 1981

I0'

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Fig. 7. Phmpha te , in l~no l d m -J, on the surface where thennoster ic anoma ly equals 1.6 cm 3 k8-~: (a) 21 November to 6 December 1981, Researcher;, (b) 28 February to 17 March 1982, Discaoerer.

Arrows indicate inferred flow direction.

Page 10: The subthermocline phosphate distribution and circulation in the far eastern equatorial Pacific Ocean

308 M. Tsucm,,,^

flow (phosphate <2.4 I~mol dm -3) of the subsurface South Equatorial Countercurrent centered at 6°S along 85°W. Another indication of a southward flow is found at 1.8 cm 3 kg -I near 82°W.

D I S C U S S I O N

It has been demonstrated that the subthermocline distribution of phosphate can be interpreted qualitatively in terms of the circulation. As was pointed out in the preceding section, one of the basic assumptions behind this interpretation is that the distribution is quasi-steady. A comparison of the distributions observed on the two cruises three months apart indicates that, although there are some quantitatively significant changes, the major flow patterns are indeed much the same for the two cruises (Fig. 7). In this figure, the phosphate field is shown at 1.6 cm 3 kg -~ of thermosteric anomaly for the two periods. The sections are too far apart for mapping, but the isopleths have been drawn on the maps in an attempt to relate the distribution on the 85°W section to that on the 10.5°S section. Along 85°W, eastward flows are found to be centered at about the same latitudes near the equator, 4, 6, and 8°S on both cruises. The eastward flow at the equator is obviously the Equatorial Undercurrent. The eastward flow centered at about 6°S has been identified as the subsurface South Equatorial Countercurrent. Downstream of 85°W, it veers southeastward to reach 80.5 to 81 °W on the 10.5°S section.

The similarity in the phosphate pattern between the two cruises (Fig. 7)justifies deducing the flow field from that of phosphate as we have attempted and interpreting it as the mean- flow field. The deduced circulation pattern is generally, but not everywhere, in agreement with that of the geostrophic flow relative to 5 MPa (500 dbar). The choice of a shallow reference for the geostrophic flow and the weak gradient of acceleration potential may be responsible for some disagreement. It must be pointed out, however, that the observed mass field may include short-period ageostrophic fluctuations, which obscure the weak signal Of the geostro- phic flow in low latitudes, while the phosphate distribution on isanosteric surfaces is not affected by such fluctuations. Hence, the flow field deduced from the distribution of phosphate more probably represents the quasi-steady flow of present interest, and the disagreements do not necessarily invalidate the use of phosphate as an ocean circulation tracer. (The same comment also applies to comparison of the deduced flow and short-term direct current-measurement data such as those from current profilers.)

The present attempt to deduce the flow field from the distribution of phosphate is impeded by the inadequate station coverage and the coarse vertical spacing of samples, but the useful- ness of the concentration of phosphate as a tracer of the circulation in the eastern equatorial Pacific has been confirmed. Differences in local biological processes do not seem to have a strong effect on the pattern of the quasi-steady subthermocline distribution of phosphate in the region of the present study. Although no data are presented in this report, the concentra- tions of silica and nitrate also can be used for a circulation tracer, except in the area south of about 5 °S where denitrification occurs, as evidenced by the presence of a secondary maximum of nitrite in the subthermocline layer [see nitrite data in the data report (TsuCHn'A, 1983)]. The concentration of dissolved oxygen may also be used, but, because of the extremely low concentration in the subthermocline layer of the study region, accurate measurements of oxygen are difficult and laborious (CARRrrr and CAXPENrER, 1966; BROENKOW and CLINE, 1969).

A number of studies on the circulation and the nutrient distribution have been carried out

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Subthermocline phosphate distribution and circulation 309

in the region south of 10°S, especially in the coastal upwelling region near 15°S (RICHARDS, 1981). However, despite the fact that characteristics of the upwelled water near 15°S are determined to a large extent by water carried southward in the poleward undercurrent over the continental slope and shelf and despite the general belief that poleward undercurrent water is fed from the west by the Equatorial Undercurrent and the subsurface South Equatorial Countercurrent (FRmDERICH and CODISPOTI, 1981), few previous studies exist in the offshore region north of 10°S that can be compared with the present observations. WVRrKI (1963), whose station coverage is as coarse as that in the present study, used the distribution of geopotential to discuss the flow pattern in this region, although the small gradient of geopotential makes the deduced flow field unreliable. He noted the presence of a southward flow, best developed at 100 m, that flows almost due south from Punta Aguja (5°S) as far as 24°S and passes near 81°W at 10°S, where the present data also indicate a southward flow (Fig. 7). In contrast to Fig. 7, however, Wyrtki's maps do not show this southward flow to be the downstream extension of the subsurface South Equatorial Countereurrent, which turns sharply southward along 85°W and returns to the west south of 10°S. WHITE (1971) discussed both the acceleration-potential and oxygen fields between 89°W and the South American coast. He found evidence of an eastward subthermoeline flow between 7 and 11 °S at 89°W and interpreted it as the (subsurface) South Equatorial Countereurrent. At !.6 cm 3 kg -1, the eastward flow does not extend beyond 86°W but returns to the west on both sides of it; at 2.0 em 3 kg -m, however, it shifts north to 6 to 8°S and appears to extend nearly to the coast at about 10°S. The maps of oxygen and nutrients in the El Nifio Watch Atlas (PATZERT, 1978) do not show a clear-cut flow pattern in the subthermoeline layer, probably because the stations are too far apart, The maps of oxygen and phosphate at 1.6 cm 3 kg -~ prepared by TSUCHIYA (198 I) suggest that the tongues of high oxygen and low phosphate associated with the subsurface South Equatorial Countereurrent veers southeastward east of 90°W, but the indication is not strong.

Superimposed on the quasi-steady nutrient distribution associated with the major flow pattern are changes that occurred between the fwst and second cruises. The clearest of these changes of shorter time scales is observed in the Equatorial Undercurrent, e.g., the Equatorial Undercurrent is clearly evident on both cruises in the fields of phosphate and other nutrients but significant decreases in the concentrations of the nutrients are found in the domain of the Undercurrent from November to December 1981 to February to March 1982. The largest decreases occur at 2.0era 3 kg -1 of thermosterie anomaly (57.) near the core of the Undercurrent (Table 1) and can be shown to be associated with an inca'ease in the eastward speed of the Undercurrent.

Assume that the flow is zonal and occurs along isanosteric surfaces and that the zonal scale of the nutrient distribution is much larger than the meridional scale. Further assume that the fluctuation of the nutrient concentration about the quasi-steady state is independent of longitude. Then the nutrient balance equations are

i)N c32N c~2N u =A, ÷ + R (1)

and

~n ~H ~2n ~2n

--+ u ~ ~z ~ r, 8t ~x = A t + A , ~ + (2)

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310 M. TsocnPt^

where the capital letters U, N, and R are the zonal speed along an isanostere, the concentra- tion of a nutrient, and the time rate of nutrient regeneration in a quaff-steady state; the lower- case letters u, n, and r denote fluctuations of these quantifies about the quasi-steady state; A t and A, are the coefficients of lateral and vertical mixing, t is time, and the x-, y-, and z-axes are in the east-west, north-south, and vertical directions. Since we are discussing the distribu- tion along an isanostere, the effect of vertical advection does not explicitly appear in equations (l) and (2).

We can examine the relative importance of the processes involved in equation (2) by evaluating the time scale of each process. First, the decrease in the nutrients occurred on a time scale of 3 months or less. Second, the time scale of lateral mixing is given by L2A7 ~, where L is the meridional scale of the Equatorial Undercurrent. With L----- 100 km and A t = 5 x 105 m s s -~, L2A7 ~ ,,-2.2 x 10 s days. Similarly, with the vertical scale of the Undercurrent H = 1 5 0 m and A,=10-4m2s -~, the time scale of vertical mixing H2A; -I ~ 2.5 x 105 days. The values of the coefficients of lateral and vertical mixing used here are arbitrary, but they probably overestimate the intensity of mixing on this horizontal scale and at this depth (GaEc_,o, 1976; OsBoar~, 1980; AgMI and SrOMI~EL, 1983). Therefore, for the changes on the time scale of 3 months, the effect of mixing is not important. Finally, changes in the nutrient-regeneration rate are not likely to be important either. The fluctuation of the regeneration rate, r, is difficult to estimate, but that for the quasi-steady state, R, can be estimated from the observations of primary production in the overlying euphotie layer (Lovr~, 1972; Love and ALLEN, 1975) by assuming that new production is given by an empirical formula proposed by EpPLE~' and 1NrEasoN (1979) and that one-half the new production decomposes in the layer at 50 to 200 m to regenerate the nutrients. R turns out that the time scale of regeneration given by An x R-~, where An is the observed decreases in the nutrients, is in the range of 2 to 8 x 102 days. This implies that, even if the regeneration rate drops to zero (i.e., r = -R) , it is not possible to account for the observed decreases in the nutrients. Thus, although this estimate of the regeneration rate involves various assumptions, it is unlikely that the uncertainty in the estimate alters the present conclusion. Estimates obtained in the coastal upweUing area farther south (FRIEDRICH and CODISI'OTI, 1981) may lead to a shorter time scale of regeneration, but it cannot be compared with that in the Equatorial Undercurrent 500 km offshore.

The estimates of the time scale indicate that the dominant factors in the nutrient balance must be lateral advection and the local time rate of change in concentration. Thus, equation (2) reduces to

i~n 0N + u = 0 . ( 3 )

Ot Ox

Assuming that the speed of the Undercurrent increases by Au at a uniform rate over a time period of At and that the nutrient concentration increases by An over the same time period, we can rewrite equation (3) as

An Aui~N - - = ( 4 ) At 2 ~x"

in this equation, Au/2 represents the average speed increase over the time period At. The zonal gradient of a nutrient ~N/ax along the equator can be estimated from the maps of nutrients at 1.6 cm J kg -j of thermosterie anomaly presented by TSUCmYA (1981). The maps indicate that, in the eastern Pacific, ~N/~x,~l.! x 10-Tgmoldm-3m -~ for phosphate,

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Subthermoclin¢ phosphate distribution and circulation 31 1

--,6.4 x 10 -7 ttmol dm -3 m -~ for silica, and ,,,1.1 x 10 -6 ~tmol dm -3 m -~ for nitrate; these values are used here as representative of the subthermocline layer. With An = -0.2 Itmol dm -3 for phosphate, - l j tmoldm -3 for silica, -2 lamol dm -3 for nitrate (Table 1), and A t = 3 months ,--8 x 106 s, equation (4) yields speed increases (Au) of 0.45 ms -I (phosphate), 0.39 ms -I (silica), and 0.45 ms -t (nitrate).

To compare the estimates based on the nutrient distributions with another independent estimate, the geostrophic speed of the Equatorial Undercurrent has been calculated for the two cruises. The geostrophic speed was computed first for the two station pairs between i °N and 0 ° and between 0 ° and I °S and then averaged. The averaging procedure is equivalent to the use of JERLOV'S 0953) meridionally differentiated form of the geostrophic equation (MONTGOMERY and STROUP, 1962, p. 49). Since we are interested only in the difference between the two cruises, the 2.4 MPa (240 dbar) surface, below which no systematic changes in the nutrient distributions are found, was used as the reference. The eastward geostrophic speed thus estimated at 2.0 em 3 kg -~ is 0.12 ms- ' for the November to December 1981 and 0.51 ms -~ for the February to March 1982 cruises. Therefore, the increase in the geostrophic speed is 0.39 ms -I. This value is in agreement with the speed increase estimated from the nutrient distributions. Although estimates of the near-equatorial geostrophic speed from individual hydrographic observations are not always reliable (LuK^s and FINING, 1984), this agreement is encouraging. In fact, the intensified Equatorial Undercurrent in March is con- sistent with the seasonal variation of the Undercurrent east of the Gal~pagos Islands (LOKAS, 1981).

In equation (4) it was assumed that the speed of the Undercurrent increases linearly with time. More generally, however, the nutrient increase An is determined by the time integral of the speed u over the time period At, as equation (3) indicates. Hence, the speed maximum may have occurred between December 1981 and February 1982, or even if the speed maximum was reached in February to March 1982, the nutrient concentration may have con- tinued to decrease after the occurrence of the speed maximum. I f the changes are sinusoidal, there should be a phase difference of n/2 between the nutrient concentration and the Undercurrent speed. A time series of data are required to answer these questions. Nevertheless, the result of the present analysis indicates that the nutrient concentrations can be used as useful parameters for monitoring the seasonal variation of the Equatorial Undercurrent.

Acknowledgements~Tlf~ study was supported by the EPOCS program of the National Oceanic and Atmospheric Administration under Contract NA82RAC00002. I thank Ants L_,~_man and Don Hamam for their a~'~mce in the field work and for the use of their CTD data and R. W. Eppley for his help with the estimation of the nutrient- regeneration rate. The assistance of the Physical and Chermcal Oceanographic Data Facility at Scripps is gratefully acknowledged; I would particularly like to thank Doug Masten and David Boa for carryin8 out the nutrient analysis at sea and Kristin Sanborn for processing the data into the final form.

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