The termination of the Equatorial Undercurrent in the eastern Pacific

Prog. Oceanog. Vol. 16, pp. 63-90, 1986.

0079-6611/86 $0.00 + .50 Copyright ~ 1986 Pergamon Press Ltd.

Printed in Great Britain. All rights reserved.

The Termination of the Equatorial Undercurrent in the Eastern Pacific* ROGER LUKAS

Joint Institute for Marine and Atmospheric Research. University of Hawaii. Honolulu, HI 96822, U.S.A. (Submitted 12 February 1985; Received in final form 15 October 1985) Abstract--The termination of the Equatorial Undercurrent (EUC) in the eastern tropical Pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5°N, 10°S, 80°W and 100~W. Three distinctive hydrographic features associated with the EUC are used, along with dynamic topography, to trace the mean path of the EUC and to investigate aspects of its seasonal variation. These features are (1) the 13°C thermostad, (2) the high-salinity core, and (3) the high dissolved oxygen concentration tongue. The thermostad is thickest just south of the equator to the west of the Galapagos Islands, with a decreasing maximum extending southeast to the coast of South America near 7°S. The thermostad is poorly developed in the mean near the equator east of the Galapagos Islands. This pattern appears to be produced by convergence of eastward flow in the EUC and in the Southern Subsurface Countercurrent, as the EUC flows southeastward from the Galapagos. This interpretation is supported by the geostrophic flow calculated from the mean dynamic topography. The isolated high-salinity core is found to the south of the equator and is traced all the way to the coast of South America. The high oxygen tongue associated with the EUC splits at the Galapagos, with the southward projecting branch disappearing rapidly as an identifiable feature. The other branch of the tongue continues around the islands to the north and along the equator to the coast of Ecuador. Some evidence of recirculation of EUC water in the South Equatorial Current is seen to the northwest of the Galapagos. A strong seasonal pulsation of the EUC affects these features in a consistent way, with the pulse occurring first west of the Galapagos, and later along the coast. The thermostad first reaches maximum development, and then a pronounced downwelling of the thermocline occurs rapidly, leading to minimum thermostad extent. Associated with this downwelling is an eastward protruding high-salinity tongue near the equator which is later seen as an isolated core along the coast at 4;S and 6°S. Pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the EUC, and ultimately the Peru-Chile Undercurrent. The pressure gradient reversal along the equator occurs prior to the annual wind reversal, suggesting that remote forcing plays a role in the seasonal pulse of the EUC in the eastern Pacific.

CONTENTS I. Introduction 2. The Data and their Processing 3. Hydrographic Signatures of the Equatorial Undercurrent 3.1. The thermostad 3.1.1. Mean 3.1.2. Seasonal variation 3.2. The high-salinity core of the EUC 3.2.1. Mean 3.2.2. Seasonal variation 3.3. Dissolved oxygen 3.3.1. Mean 3.3.2. Seasonal variation

*Hawaii Institute of Geophysics contribution No. 1657. 63 J~'O

16/2-A

64 66 69 69 69 71 74 74 75 79 79 81

64

R. LUKAS

3.4. Dynamic topography and geostrophic flow 3.4.1. Mean 3.4.2. Seasonal variation 4. Conclusions Acknowledgements References

81 81" 86 87 88 89

1. INTRODUCTION TIlE TERMINATIONof the Pacific Equatorial Undercurrent as it approaches the west coast of South America is of great theoretical and practical interest (BJERKNES, 1961; RODEN, 1962; YOSHIDA, 1967). The Undercurrent seems to supply the upwelling region along the coast of Peru with nutrient-rich waters (WOOSTER and GILMARTIN, 1961; WYRTKI, 1963, 1967), and it may play a large part in El Nifio (STEVENSON and TAFT, 1971; WOOSTERand GUILLEN, 1974). This study confirms the connection of the Equatorial Undercurrent and the Peru-Chile Undercurrent which flows poleward along the South American coast. A schematic illustration of the mean circulation in the eastern equatorial Pacific constructed by WYRTKI(1967) is shown in Fig. I. The major surface currents are the westward flowing South Equatorial Current (SEC) supplied by the Peru Current (PC) and by recirculation from the North Equatorial Countercurrent (NECC). Water from the Equatorial Undercurrent (EUC) must recirculate into the SEC to account for the apparent downstream increase in transport of the SEC, but some EUC flow is shown contributing to the Peru-Chile Undercurrent (PCUC). The EUC is generally found between about 2°N and 2°S at depths from 30 to 300 m. This current often has a maximum speed greater than 100cm sec -t, and an eastward transport of about 30 Sverdrups in the central Pacific (LUKAS and FIRING, 1984; WYRTKI and KILONSKY, 1984). The EUC weakens noticeably from the central Pacific to the eastern Pacific. Just to the west of the Galapagos Islands its maximum speed is usually about 70cmsec -t (KNAUSS, 1960, 1966; TAFT and JONES, I973).

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SEE

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FIG. I. Schematic illustration of the major surface and subsurface currents of the eastern Pacific according to WYRTKI 0967). Numbers beside arrows indicate transport of the various components of the currents in Sverdrups (10~m ~sec- ~).

The termination of the Equatorial Undercurrent

65

The present knowledge of the seasonal fluctuations of the EUC leaves much to be desired, but we do know that in the central Pacific the current is strong and shallow from March to September, and relatively weak and deep from November through January (SAKAI, 1972; WYRTKI, MEYERS, MCLAIN and PATZERT, 1977; FIRING, FENANDERand MILLER, 1981). There seems to be a surfacing of the current in boreal spring (JONES, 1969), but this is probably due to passage of seasonally occurring first baroclinic mode Kelvin waves having an eastward current maximum at the surface (Hayes and Halpern, 1984). (The EUC itself is actually displaced downward with the thermocline during this process.) The seasonal variability of the EUC in the eastern Pacific has not be studied previously. The current observations of KNAUSS(1960, 1966) are evidence that the Galapagos Islands can block the EUC, although WVRTKI (1967), WHITE (1969), and STEVENSON and TAFT (1971) all found hydrographic evidence that the EUC penetrates past the Galapagos to the coast. The direct current measurements of CHRISTENSEN (1971) in the vicinity of the Galapagos Islands during February and March of 1969 also showed that there can be subsurface eastward flow on the equator east of the Galapagos. Extensive current measurements by LEETMAA(1982) show that the speed of the EUC is reduced and the current is displaced southward as it approaches the Galapagos, and that the EUC is sometimes found east of the Galapagos. WOOSTER and GILMARTIN (1961) made direct current measurements confirming the existence of the Peru-Chile Undercurrent which was hypothesized by GL,~THER(1936) on the basis of the observed high-salinity tongue. The coastal undercurrent appears to originate in the vicinity of 5°S and is stronger there than at 15°S (BROCKMANN,FARHBACH, HUVER and SMITH, 1980). This current parallels the continental shelf; the inshore edge is between l0 and 50 km offshore. The source waters for the northern portion of the PCUC are probably of equatorial origin, possibly from the EUC (WoosTER and GILMARTIN, 1961; WVRTKI, 1963, 1967; ZUTA, RIVERA and BUSTAMENTE, 1978). This implies a southward extension of the EUC from the equator. Such a flow is inferred by WHrrE (1969, 1971) from the EASTROPAC data. Evidence of southeastward extension of EUC water in the eastern Atlantic (RINKEL, SUND and NEUMANN, 1966; HISARD, 1973; HISARDand MOLIERE, 1973), suggests that similar dynamics may hold near the eastern boundary of both oceans. Eastward surface and subsurface geostrophic flow between the equator and about 3°S in the eastern Pacific has been inferred from the hydrographic structure or directly measured (BJERKNES, 1961; WVRTKI, 1963, 1965; STROUP, 1969; TSUCHIYA, 1974; LEETMAA,1982), but it is unclear whether this flow is a persistent circulation feature and whether it is related to the EUC in some way. LEETMAA(1982) suggests that the local wind stress curl may in fact drive this flow. SCHOTT(193 l) notes occasional observations of eastward surface flow to the west of the Galapagos and south of the equator during February and March. Schott also found evidence of eastward surface flow between l°N and 3°S extending eastward and south through the islands in March 1891, during an El Niho of major proportions. In this paper, I show that the circulation features described above are linked, and that the EUC flows into the PCUC. Water mass properties are used to show this link and to trace a seasonal 'pulse' of the current system from west of the Galapagos Islands to the coast of South America. In Section 2, the data and data processing procedures used in this study are described. Section 3 describes three hydrographic features associated with the EUC, using them to trace the path of the EUC as it approaches the eastern boundary and to illustrate aspects of the seasonal variation of the current. The conclusions are summarized in Section 4.

66

R. LUg.AS

2. THE DATA AND THEIR PROCESSING The data used in this study consist of temperature, salinity and dissolved oxygen data from historical hydrographic stations in the area bounded by 5°N and 10°S, and 100°W and 80°W. The primary source for these data was the National Oceanographic Data Center (NODC) hydrofile. A few stations taken as recently as 1975 were present in the hydrofile, but at the time this study was started the NODC hydrofile did not include the hydrographic data obtained during the 1975 El Nifio Watch Expedition (WYRTKI, STROUP, PATZERT, WILLIAMSand QUIN~r, 1976), so these data were added. Since this study was completed, many new measurements in the eastern equatorial Pacific have been made as part of NOAA's EPOCS program. It will be most interesting to compare these new measurements with the historical data studied here. Annual mean and seasonal mean vertical sections of temperature, salinity, dissolved oxygen and geostrophic velocity were derived from these historical data. From the vertical sections, maps of properties were prepared. The vertical sections and maps are used to make inferences about the termination of the EUC in the eastern Pacific. The choice of sections was dictated by the geographic distribution of the stations. The preference for certain cruise tracks as well as the large number of coastal stations is apparent in the map of station locations (Fig. 2). Fortunately, the region of greatest density of hydrostation data is where equatorial and coastal dynamics overlap (YosHIDA, 1967, see especially his Fig. 9) and where we expect the EUC to flow into the coastal undercurrent. Six meridonal and four zonal sections were chosen; these sections overlap in such a way that the three-dimensional circulation in this region can be illustrated. In this study, we concentrate on the mean circulation and certain aspects of the seasonal variation. Because the temporal resolution of the historical data is spatially variable, detailed analyses of the fluctuations in hydrographic structure are made only in selected areas. Also, insufficient data exist to analyse the important interannual variability associated with the El Nifio phenomenon, except in a very limited way (ENFIELD, 1981). Some El Nifio observations during 1965 and 1972 were made near the coast of South America, but these are really insufficient to stratify the mean by 'normal' and "El Nifio' conditions. Observations made during the YALOC Expedition in February, 1969 and during the PIQUERO Expedition (STEVENSON and TAFT, 1971) in June, 1969 may be unusual; there is no agreement among oceanographers on the intensity of the relatively weak E! Nifio that occurred in this year. There is however no question that the observations of the Dolphin Expedition between April and June, 1958 (KNAuSS, 1960) were made during the later stages of a major El Nifio event. The extent to which the results of this study are biased by unresolved interannual variability is unknown. I believe that it is no worse than the problems introduced by failure to resolve energetic high frequency variability characteristic of equatorial regions. Recent observations during the 1982-83 El Nifio event should lead to a much better understanding of the circulation changes during El Nifio. The quantity and quality of data depends on location, depth and the variable in question. In general, temperature observations are plentiful and so temperature is well-determined (small standard error of the mean), but salinity and especially oxygen concentration data are fewer and less reliable. Some standard deviations are given in Table 1 which are characteristic of various regions in the study area. Temperature standard deviations are large due to high variability in the thermocline, while the standard deviations of dissolved oxygen concentration are erratic due to variable quantity and quality of the data.

The termination of the Equatorial Undercurrent

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F I G . 2. Distribution ofhydrographic stations used in this study. The upper panel shows all stations. and the meridional sections used in this study are outlined. The lower panel shows only stations with maximum depth deeper than 500m, and the zonal sections (outlined) that were used in this study.

Because these historical data are o f variable quality, the detection and removal o f errors is very important. The quality control procedures applied were similar to those used by ROBINSON (1976). First a pilot analysis was made by averaging temperature, salinity and o x y g e n data at standard depths for each o f the sections outlined in Fig. 2. These sections were contoured to assess the general quality o f the data and to locate gross errors. The preliminary c o n t o u r plots were also used to define regions with similar hydrographic conditions. D a t a from each o f these regions were extracted from

68

R. Lur,.xs

TABLE 1. REPRESENTATIVE MEAN VALUES AND STANDARD DEVIATIONSFOR LOCATIONSALONG THE EQUATOR AND SOUTH AMERICANCOAST IN TIlE EASTERN PACIFIC T ( 100 m) 98°W EQ 95-W EQ 92~W EQ 88~W EQ 85~W EQ 82:W EQ 1.5 ~ 81.5~W 4:S 81 ~W 6:S 81~W

13.90 13.91 14.86 14.65 14.96 15.00 15.35

+ + 4+ 444-

0.12 0.14 0.23 0.22 0.17 0.24 0.18

SMAX 34.99 34.99 35.01 35.00 34.99 34.99 35.02

+ 0.05 __+ 0.04 + 0.02 + 0.02 4- 0.01 4- 0.02 4- 0.01

O., ( 125 m) 2.57 2.44 2.46 2.07 1.92 1.63 1.54

+ + + 4444-

0.07 0.11 0.04 0.20 0.06 0.10 0.04

15.88 4- 0.20

35.00 + 0.01

1.50 4- 0.04

14.90 + 0.13

35.00 __+ 0.01

1.16 4- 0.08

the data base, and T-Z, T-S, and T-02 plots were made to visually identify anomalous data. The data from each region were also subjected to a statistical filter. Salinity and oxygen data were averaged over one-degree temperature intervals and standard deviations were computed. Values that exceeded two standard deviations from the mean were flagged. The statistics were recomputed without the flagged data, and the same flagging procedure was repeated. This two-pass procedure is necessary because some regions contained stations with grossly erroneous data that had to be weeded out before the smaller errors could be detected (ROBINSON, 1976). All stations flagged by the filter or from the plots were subjected to a detailed inspection. Note that the statistical filter was only used to flag data as questionable, as opposed to procedures where values that failed the tests were rejected automatically. Several stations were discovered where the data were essentially worthless, yet values at only a few depths were flagged by the filter. When problem data were detected, the action taken was either to delete all or part of the station, or to change the values at one or more points. The particular solution for each of several types of problem is discussed by LUKAS (1981). The vertical sections of water mass properties were constructed by 'binning' the data and averaging within bins. The dimensions of the bins varied from section to section in order to resolve important features while insuring that each bin had at least eight stations in it. The width of each section determined one dimension of each bin; these are shown in Fig. 2. The other dimension of the bins for each section are as follows. All of the meridional sections were constructed using bins 1° of latitude long. Bins 2 ° of longitude in length were used on the zonal section along the equator, while 1° longitude bins were used for the zonal section along 1.5°S, and 0.5 ° longitude bins were used along the 4°S and 6~S zonal sections. The bins were overlapped by half their length. Thus, a section constructed with bins 1: long has a resolution of 0.5 °, and neighboring grid points have some stations in common. This overlapping of the bins is similar to taking a running mean of regularly sampled data, and decimating the result to a wider sampling interval. Although the data are insufficient to investigate the annual cycle in detail, the data in the equatorial and coastal regions are dense enough to form four seasonal averages. The seasons that were chosen were January-March (JFM), April-June (AM J), July-September

The terminationof the Equatorial Undercurrent

69

(JAS), and October-December (OND). This choice minimizes blurring of the seasonal cycle, as the transition from the cold OND period to the warm JFM period is rather sharp, as is discussed below. In a few regions, the data are of sufficient quantity to allow bi-monthly averaging, and the annual cycle in these regions is investigated in more detail. 3. HYDROGRAPHIC SIGNATURES OF THE EQUATORIAL UNDERCURRENT It is possible to make inferences about the path of the EUC without direct current measurements because of the distinct signatures of the current to the hydrography (TsucHVlVA, 1968; S'rRouP, 1969). These signatures include (1)the thermostad of equatorial 13°C water, (2) the high-salinity core, and (3) a tongue of water with a high concentration of dissolved oxygen. In this paper, dynamic topography is used to infer geostrophic flow to demonstrate the coincidence of these characteristic features of the EUC with eastward flow. 3.1. The thermostad

The thermostad of the Equatorial 13°C Water (MONTGOMERYand STROUP, 1962) is a water mass found near 200m depth in the eastern and central Pacific, with its volume increasing to the east. The thermostad is apparent in the rather large separation of isotherms and isopycnals just below the thermocline. TSUCHIVA(1968) presented maps of the separation of the 155 to 165 cl ton- ~thermosteric anomaly surfaces, suggesting that the distribution of this quantity is related to the structure of the whole equatorial current system. A comprehensive study of the thermostad was made by S'rRouP (1969), who first suggested that zonal convergence in the lower portion of the EUC might explain the increase in volume of this water mass toward the eastern Pacific. It is also possible that the large volume of this water mass in the east is due to formation in situ by vertical mixing associated with the high shear below the core of the EUC. Stroup concluded that the data were insufficient to discern between these two explanations. TSUCHIVA (1981) presented additional evidence showing that convergence of eastward flow is primarily responsible for the existence of the thermostad. This zonal convergence of eastward flow is due to the presence of the eastern boundary and to zonally varying wind stress (cf. ROTHSTEIN, 1984). MCPHADEN(1984) modelled the subsurface countercurrents and concluded that the subsurface countercurrents and the thermostad were dynamically related to the EUC, and that zonal convergence of momentum is important. In this study, the 160 and 180 cl ton-~ surfaces are used to delimit the thermostad. The appropriateness of these values in the eastern Pacific is discussed by STROUP (1969) and JONES 0973). The 180clton -t surface corresponds to a temperature of about 14~C and salinity of 35%0, while 160 cl ton -t corresponds to 13°C and 34.95%0. 3.1.1. Mean. Maps of the mean depth of the 160 and 180ci ton -~ surfaces are shown in Fig. 3. Both maps show indications of eastward geostrophic flow reaching the coast of South America at about 5°S, assuming that depression of an isopycnai implies higher pressure. There is an apparent connection between the Galapagos Islands and the coastal region near 5°S in the depth distribution of the 180 cl ton- t surface [Fig. 3(b)]. The depression of this surface in the 'corner' region (the triangular region bounded by the equator east of

70

R. LUKAS

MERN DEPTH OF 160 C L / r O N

(METERS)

5o

N

a EQ

5 <'

I0 = s IO0OW

95 °

90 °

MERN DEPTH OF 180 CL/TON

85 °

80ow

85 °

8(~'W

[METERS)

EQ

5*

iO o S

IO0OW

95 °

90 °

FIG. 3. Maps of the mean depth of the 160 cl t o n - ' (a) and 180cl ton-t (b) surfaces.

92°W, and the coastline from the equator to about 5°S) results in a geostrophic current flowing from the Galapagos Islands southeast to Peru. Some of the water from the EUC appears to be recycled northward and southward into the South Equatorial Current near the Galapagos Islands, as suggested by previous investigators (Knauss, 1966; Wyrtki, 1966; Stroup, 1969). Further evidence for this recycling of Undercurrent water is found in the distribution of salinity and dissolved oxygen to the northwest of the Galapagos Islands, which is discussed later. The distribution of depth of the 160 cl ton-~ surface [Fig. 3(a)] also shows the tendency for a depression in the corner region. Again, the Galapagos Islands seem to be associated with a major feature of the pattern, namely the large depression around the islands indicated by the 260 m depth countour. Unlike the shallower isopycnal, the 160 cl ton-

The termination of the Equatorial Undercurrent

71

MEAN THERMOSI'AO THICKNE55 CMETERS)

1

°"

EQ -

I0 o S IO0*W

~

~

95*

90*

85*

80'='W

FIG. 4. Mean thickness of the thermostad as defined by the vertical separation of the 160 and 180 cl t o n - x surfaces.

topography indicates an eastward flow between about 3° and 6°S, approaching the coast of South America at about 8°S. White (I 971) studied the acceleration potential and oxygen distribution observed on this surface during February-March 1967, concluding that a similar eastward flow was the South Equatorial Countercurrent (SECC). However, it is more likely that the eastward flow at 160clton -~ is the Southern Subsurface Countercurrent, as it is too deep to be the SECC (TsucHIVA, 1975). Figure 4 shows the resulting distribution of mean thermostad thickness. The maximum thickness of just over 170 m occurs near 98°W between 1° and 2°S. This maximum extends eastward to the vicinity of the Galapagos Islands, then southeastward to coast near 7°S. Because the EUC and SSCC are found along theflanks of this maximum, it seems unlikely that the large vertical extent of the thermostad in the eastern equatorial Pacific is the result of enhanced vertical mixing due to large vertical shears. Instead, the mean thermostad distribution supports the idea that the thermostad is due to the convergence of eastward flow near the eastern boundary, with maximum convergence south and west of the Galapagos Islands.

3.1.2. Seasonal variation. Along the equator west of the Galapagos Islands, the thermostad is found throughout the year but there is a rapid thinning of this layer in the JFM period. This thinning, found by WYRTKI(1964) on the equator at I15°W from December through February (Fig. 5), is due to the depression of all isotherms above 12°C. Figure 6 shows the meridional structure associated with this dramatic change in the thermal structure near the Galapagos Islands. This annual downwelling event is of great importance in the seasonal variation of the hydrographic structure of the eastern equatorial Pacific, as will be shown below. Prior to the downwelling event, there is an equally remarkable upwelling of the thermocline, with deeper isotherms reaching their shallowest depth before the shallower isotherms. JPO I & I 2 - B

72

R. LUKAS

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MONTHS

FIG. 5. Annual variation of the thermal structure on the equator at 115°W (after WYRTKI, 1964). Note the rapid depression of the isotherms above 12°C in the early part of the year, and the associated thinning of the thermostad. Also note the vertical delay in the time of minimum isotherm depth during boreal Fall.

East of the Galapagos Islands the thermostad is poorly developed during most of the year, and is thickest during September-October near the equator at the coast (Table 2). This increase in thickness is mostly due to the shoaling of isotherms usually found in the lower part of the thermocline and n o t to changes in the depth of the base of the thermostad. In particular, the 14~C isotherm is about 50m shallower in this area during OND than during the rest of the year. (This pattern of thermostad variation is seen to the west of the TABLE 2. SEASONAL VARIATION OF THERMOSTAD THICKNESS (SEE TEXT FOR DEFINITION) WITHIN INDICATED REGIONS ALONG THE COAST OF SOUTH AMERICA. BIMONTH OF MAXIMUM THICKNESS IS UNDERLINED

Jan/Feb

Mar/Apr

May/Jun Jul/Aug Sep/Oct

Nov/Dec

Average

Range

I°N-I°S 81ow-83°W

89

73

69

55

110

100

83

55

1°S-2°S 81 °W-83°W

80

66

70

57

108

104

81

51

3.5°S-4.5-S

116

72

63

70

95

125

90

62

124

93

93

88

109

102

102

36

81 °W-83°W 5.5°S-6.5°S 81°W-83°W

The termination of the Equatorial Undercurrent

ONO TEMPERQTURE

73

92W

0

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12 -1- 3 0 0 n L~

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{I 500

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FIG. 6. Mean temperature sections along 92°W for October-December (a) and January-March (b). Note the apparent absence of the Equatorial Undercurrent during October-December, and its obvious presence during January-March, which is evidenced by the stong isotherm spreading.

Galapagos Islands also [Figs 5 and 6], but with different phase.) Thermostad thickness is greatest next to the coast along 1.5°S during September-October again due to uplifting of the base of the thermocline. The thermostad is also not well-developed along 4°S, but now the maximum thickness near the coast is during the November-December period. At 6°S, the thermostad is well-developed (Fig. 4), there is less variation in its thickness, and the maximum thickness occurs even later than locations closer to the equator (Table 2). Why does the thermostad appear between the Galapagos and the coast of South America during the latter part of the year? This change in the stratification is evidence of an increase in transport in the lower portions of the EUC which include the temperature-salinity classes of the thermostad (TAFr and JoN~, 1973). If the EUC core speed increased at this time, we would see a spreading of the thermocline [as in Fig. 6(b)] or an increase in the salinity of

74

R. Let.as

the high-salinity core; in fact, such events do occur, but not until some months later. Why doesn't the zonal current increase throughout the whole EUC simultaneously? Recent theoretical work by McCREARV, PICAUXand MOORE (1984) shows that upward phase propagation of the annual cycle of density, pressure and velocity perturbations could account for the present results. In their continuously stratified model, an equatorially trapped Kelvin wave generated by remote periodic changes in wind stress carries energy downward into the ocean while its phase propagates upward. The result is that the EUC perturbations are seen first at depth. This situation is exactly what occures in the eastern equatorial Pacific (LtJKAS, 1981), and is illustrated by the vertical delay of the time of minimum isotherm depth which is seen in Fig. 5. 3.2. The high-salinity core of the EUC The high-salinity core of the EUC is a subsurface feature most often found slightly south of, and above, the speed maximum of the EUC. The core is seen in meridional sections as an isolated salinity maximum near the 300 cl ton-~ thermosteric anomaly surface. 3.2. I. Mean. TSUCHIYA(1968) traced this tongue of high-salinity water along the equator from the western Pacific to the eastern Pacific, concluding that this water is advected eastward in the EUC. KNAUSS(1966) suggested that this high-salinity core was supplied from the southern hemisphere subtropics by convergent meridional flow at the depth of the thermocline, and then entrained in the eastward flow. TAFT and JONES (1973) and TAFT, HICKEY,WUNSCH and BAKER(1974) investigated the question of meridional convergence at the depth of the core but the observations are not definitive. Conservation of mass requires a meridional and/or zonal convergence to balance the Ekman divergence due to the easterly trades along the equator. East of the Galapagos Islands, the zonal component of the wind stress is weak and often directed to the east, so the strong Ekman divergence common to the central Pacific is not found there. In addition, the zonal convergence in the EUC is such that meridional flow at the depth of the core is divergent near the eastern boundary (LuKAs, 1981). Thus, the high-salinity core in the eastern Pacific must be due to advection from the west. This isolated near-equatorial high-salinity core, found in every section where the EUC is found, is an excellent tracer of the upper portions of the EUC (RINKEL, SUND and NEUMANN, 1966; TSUCHIYA, 1968). The high-salinity core is prominent in each of the mean salinity sections from 98°W to near the coast at 82°W (Fig. 7). The core is always found to the south of the equator and at a depth of 50 to 100m, in the thermocline. Also seen in each of the sections is a tongue of high-salinity water (also at the depth of the thermocline) near 3° to 5°S. Except at 92°W, the high-salinity core of the EUC is clearly separated from this tongue by water of lower salinity. The mean salinity in the core decreases from 35.08%o at 98°W to 35.00%o at 92°W. Between 92 ° and 88°W, the EUC apparently veers far enough south to intersect the" southern high-salinity tongue, thus replenishing the salinity core of the EUC, which at 88°W has a value over 35.04%ooo.At 82°W, the core salinity is about 35.02%o. Because of the poor temporal sampling, some of these differences may not be significant. An interesting feature that appears in the mean salinity sections is a region of isolated high salinity from 2° to 4°N at about 75 to 100 m in the sections along 95 ° and 92°W. The salinity in this feature decreases toward the west, suggesting that it is EUC high-salinity core water deflected northward at the Galapagos Islands and entrained into the South Equatorial

The termination of the Equatorial Undercurrent

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76

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MEAN SALINITY

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in the core increases during this period, reaching values of about 35.05%o. This pattern confirms that the higher salinity during the early part of the year is due to increased eastward advection in the EUC. Further evidence of increased advection from the west can be seen in the seasonal vertical salinity sections along the equator (Fig. 10). Note that these sections are composed of averages of data between I°S and I°N, and do not accurately represent the maximum salinity in the high salinity core. Starting with the OND period, a tongue of water with salinity greater than 35.00%0 protrudes eastward near 75 m at 98°W. Salinity greater than 34.95%0 is found as far east as 95°W in this tongue. (The salinity greater than 34.95 nearer the coast is a residual of the previous cycle. See Fig. 10[d].) There is no isolated salinity maximum along the coast at 4°S [Fig. l l(a)]. The JFM period is substantially different from the preceding season. High-salinity water now extends as far east as 88°W along the equator, and to 84°W along 1.5°S (not shown). Salinity exceeds 35.14%o in part of this tongue. Near the Galapagos Islands, this tongue is thicker by a factor of about four, and it thins to the east. This situation illustrates the convergence caused by the impact of the EUC against the Galapagos Islands, and the consequent vertical spreading of EUC waters. Note that the salinity maximum along 4°S is still well offshore. In AM J, there is a clear separation between the high-salinity tongue at 98°W and the tongue protruding towards the coast from the region of the Galapagos. This separation could be due to a meridional excursion of the EUC between about 97 ° and 92°W, or perhaps the tongue has separated from its source and is now a distinct lens. (It is also possible that this is an artifact due to relatively sparse sampling near 95°W on the equator.)

The termination of the Equatorial Undercurrent

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This tongue now reaches the coast of South America, and the highest satinities are found between the islands and the coast. Now a distinct high-salinity core is found right along the coast at 4°S [Fig. 1 l(c)]. During JAS, water with salinity greater than 35°/oois found only between 89 ° and 82°W along the equator, and there is a westward thinning of the layer greater than 34.95%0. Along 1.5°S, the high-salinity water still extends out to the islands, but the highest salinity is found at the coast of South America. Along 4°S, the salinity has increased in the tongue found along the coast, and now the water of salinity greater than 35°/oois found offshore as far as 85°W at depths between 30 and 100m [Fig. 1 l(d)]. The seasonal cycles of temperature and salinity at depths near the EUC core on the

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equator at 92 ° and 82°W were constructed from two-month running averages and they illustrate this seasonal pulsation o f the E U C (Fig. 12). At 100m, the salinity and temperature maxima are in phase, whereas at 50 and 7 5 m the salinity maxima lag the temperature maxima. This is because the annual downwelling event is important in changing

The termination of the Equatorial Undercurrent OCT NOV OEC ~

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FIG. 12. Annual cycle of temperature and salinity on the equator at 92°W (solid lines) and 82:W (dashed lines) at 50 m, 75 m and 100 m. The values are two-month runningmeans. salinity near 100 m, whereas advection is more important at 50 and 75 m (LuKAs, 1981). Note that in the three-month seasonal averages of Fig. 10 it is not possible to isolate the contribution of vertical advection from that of zonal advection. Summarizing the results of this section, we find that the high-salinity core of the EUC can be traced to the coast of South America. Seasonal changes in the salinity maximum and its location suggest an increase in transport of the EUC near the Galapagos during the JanuaryMarch period. This transport increase appears along the coast in the April-June period. 3.3. Dissolved oxygen

Relatively high values of dissolved oxygen concentration (oxyty) in the equatorial region of the eastern Pacific have been attributed both to downwelling of water below the high-speed core of the EUC and to eastward advection of water with higher oxyty (KNAuSS, 1960, 1966; WVRTKI, 1967; TSUCHIYA, 1968; STROUP, 1969; TAFT and Jo~'rs. 1973). Although the concentration of dissolved oxygen is not a conservative property, when used in conjunction with other properties it has been shown to be a useful indicator of circulation. High concentrations of dissolved oxygen in subsurface layers will be taken as evidence of the presence of eastward flow in the eastern tropical Pacific, since in situ consumption is large. 3.3.1, Mean. In each of the six meridional sections spanning the equator, relatively high concentrations of dissolved oxygen are found within a few degrees of the equator between JPO 1612~C

80

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MEQN OXYGEN (MLIL] 0

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FIG. 13. Verticalsection of mean dissolvedoxygenconcentrationalong 95~W. about 50m and 200m depth (Fig. 13). A map of dissolved oxygen concentration at 100m depth (Fig. 14) shows the high oxyty tongue along the equator intersecting the Galapagos Islands. At the islands a secondary tongue is formed which protrudes southward but is quickly lost as a distinct feature. The main tongue of high oxyty along the equator is eroded to the east of the Galapagos; at 98°W the concentration at 100 m exceeds 2.3 ml I- ', whereas at 82°W it is about 1.8 ml 1-~. Because isopycnals in this depth range slope down to the east, the reduction of dissolved oxygen along isopycnals is even greater than found at constant depth. At this depth, it is evident that the high oxygen concentration found near the coast of South America derives from a source to the west. As noted by KNAUSS(1966) and TSUCHIYA(1968), the high oxyty tongue is not symmetric about the equator, and the volume of high oxyty water is larger north of the equator. The section at 92°W is an exception, showing water with greater than 2mll -~ of dissolved MEAN OIS50LVEO OXYGEN (ML/L) AT

EO

~ z -

10° ~ S IOO"W

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,t

95°

90°

85°

FIG. 14. Map of dissolvedoxygenconcentrationat 100m.

80°W

The termination of the Equatorial Undercurrent

81

oxygen at 100m as far as 3.5=S. This maximum is further evidence of the vertical and meridional divergence of the EUC as it strikes the Galapagos. The asymmetry of the high oxyty water between the Galapagos Islands and the coast of Ecuador is slight compared to that found in the sections on 95 ° and 98°W. A separate depression of the 2 ml 1- I contour north of the equator on the 95°W section (Fig. 13) coincides with a distinct tongue of high-salinity water [Fig. 7(b)] discussed in the previous section. The asymmetric distribution of the band of high oxyty in the eastern Pacific is further evidence that the horizontal recirculation of EUC water into the SEC takes place primarily to the north of the Galapagos. The two zonal sections along 4: and 6°S (not shown) have higher mean values of dissolved oxygen adjacent to the coast than offshore. Below 75 m, all of the isolines of dissolved oxygen slope downward toward the coast, which means that the high oxyty water must flow in from the north or south. Along 6°S, the contours are found at shallower depths than along 4°S, indicating further erosion of the high oxyty tongue that originates in the near-equatorial region. The relatively high oxyty water along the coast cannot be due to the presence of high oxyty Antarctic Intermediate water, because a distinct oxygen minimum separates the Equatorial Subsurface waters from the deeper polar waters (WYRTKI, 1967). 3.3.2. Seasonal variation. In all of the meridional sections, the tongue of high oxyty appears to be most weakly developed during the JAS and OND periods. This minimum might be the result of diminished EUC flow during those periods, but might be due to greater in situ consumption of dissolved oxygen. Coastal and equatorial upwelling is maximum during this time of year, and the greater primary productivity in the surface layer supplies increased quantities of organic matter to the deeper levels, resulting in 02 consumption. However, OND is the period of lowest salinity in the salinity maximum along the coast, suggesting that the EUC is indeed weaker at these times. 3.4. Dynamic topography and geostrophic flow

3.4.1. Mean. The mean dynamic topography of the sea surface relative to 500 decibars (Fig. 15) illustrates the strong influence of the Equatorial Front on the circulation in the eastern Pacific. The front runs in an approximately zonal direction along I°N toward the east, crosses the equator at about 90=W and intersects the coast of South America near 5°S. As a result, a strong westward geostrophic flow exists north of the equator and west of 85°W which is seen in the historical ship drift (WVRTKI, 1965). South of the equator between the Galapagos Islands and the coast, geostrophic flow is southeastward along the front toward the coast. The surface Ekman flow associated with the mean wind stress is generally northwestward, weakening the eastward flow. To the west of the Galapagos, a minimum in the westward surface current between the equator and about 2°S is due to the southward displacement of the equatorial trough caused by the southerly component of the wind stress (CANE, 1979). Figure 16 shows the distribution of dynamic height at 75 dbar relative to 500 dbar. At this depth, the geostrophic flow is southward and eastward almost everywhere to the south of the equator. The broad band of southeastward flow near the coast at 5°S is apparently supplied from two sources, one near the equator off Ecuador and the other from the Galapagos Islands. There is clear evidence of the EUC impinging on the Galapagos Islands.

82

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OTNRMIC.HEIGHT (OTN. CM.l

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FIG. 15. Mean dynamic topography of the sea surface relative to 500dbar. Contour interval is 1 dynamic cm. Arrowheads indicate the direction of geostrophic flow.

DYNAMIC HEIGHT

(OYN. CM.}

7S/50~

EQ

S

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95 °

90 °

85 °

80"W

FIG. 16. Mean dynamic topography of 75dbar relative to 500dbar. C o n t o u r interval is 0.5 dynamic cm. Arrowheads indicate the direction of geostrophic flow,

The geostrophic flow immediately to the west of the islands is northward and southward, creating a geostrophic divergence along the equator. A portion of the northward branch turns westward with the South Equatorial current, as suggested in previous sections on the basis of salinity and dissolved oxygen distributions. The rest of this branch flows eastward

The termination of the Equatorial Undercurrent

83

just north of the equator to the east of the Galapagos. The southward branch flows toward the coastal upwelling region after some meandering. There is also southward geostrophic flow along the coast extending southward from the equator. At 150 dbar (Fig. 17), the pattern is similar to that at 75 dbar, but now an eastward flow is found between about 3° and 7°S in the western region. This flow intersects the southward branch of the EUC near 4°S along 92°W, joining in the meander before connecting with the coastal circulation. This is the Southern Subsurface Countercurrent (TsucHIYA, 1975) mentioned earlier in the discussion of the distribution of thermostad thickness. The northern branch of the EUC at this depth does not recirculate into the SEC, but instead continues eastward north of the equator, nearly coincident with the high oxyty tongue seen in Fig. 14. The zonal component of the geostrophic current for each of the mean vertical sections was calculated, using the meridonally differentiated form of the geostrophic balance on the equator (JERLOV, 1985; TSUCaIVA, 1955). LUCASand Fmtr~G (1984) show that this method gives a good estimate of the mean flow of the EUC in the central Pacific, and HAVES(1982) has shown its usefulness in the eastern Pacific. Where the Equatorial Front is very close to the equator, unreasonably high geostrophic speeds are computed for the surface layer, because neglected nonlinearities and mixing must be important there. The ship drift analysis of WvRTKI(1965) does show high speeds along the frontal region however. Figure 18 shows the results of these calculations. Only speeds greater than 5cmsec -~ and less than 100cmsec -~ are contoured. Because the zero contours are not shown contours at + 5 cm sec- ~ have been added to define the limits of eastward and westward flows. Along 98°W, two distinct branches of the SEC exist. The branch north of the equator is more intense and deeper than the one in the south. These two branches are separated by the eastward flow of the EUC which has a subsurface maximum at 50 to 75 m near 0.5°S.

OYNRMIC HEIGHT [OYN. CM.I

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5* N

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5°

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IO0"W

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90 °

85 °

80=W

FIG. 17. Mean dynamic topography of 150dbar relative to 500dbar. Contour interval is 0.2 dynamic cm. Arrowheads indicate direction of geostrophic flow.

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FIG. 18. Meridional sections of mean zonal geostrophic current relative to 500dbar at 98:W (a), 95°W (b), 92°W (c), 88:W (d), 85=W (e) and 82°W (f). Eastward flow is indicated by dashed contours• Contour interval is 10 cm sec-*, except that + 5 cm sec-t contours are included instead of the zero contour, and values greater than 100 cm sec-t are not contoured,

At 95°W the SEC is not much different, although the northern branch extends to a greater depth near the equator and is shallower farther north. The eastward flow is still centered south of the equator but no longer has a subsurface maximum, and the 5 cm sec- z contour does not extend as deep as on 98°W. (Not shown in these figures is the Southern Subsurface Countercurrent, found between 100 and 200 m depth between 5°S and 7°S, with a maximum speed slightly more than 5cmsec-~.) AT 92°W [Fig 18(c)], the EUC splits into two branches. The southern branch is about the same intensity as on 95°W and does not have a subsurface maximum. The northern branch is less intense, has a subsurface maximum at about I00 m, and reaches to almost 400 m. Eastward flow through this section extends from 2°N to 2°S. A weak westward flow at about 0.5°S between 200 and 300 m may be related to the weak westward flow found at depth at the equator in the sections along 95 ° and 98°W. The section along 88°W illustrates the effect of the Galapagos Islands on the flow. The simple pattern of two westward flows separated by an eastward flow is replaced by a more complicated pattern of eastward and westward flows. The southern branch of the EUC is

The termination of the Equatorial Undercurrent

85

apparently weakened from 92°W and is centered at about 2°S instead of l°S. A narrow eastward flow on the equator is separated from the eastward flow at 2°S by a westward current that has a subsurface maximum between 30 and 50m. A separate and weaker westward flow is found north of the equator between 100 and 200 m. The area of eastward flow near 2:N may be connected with the eastward flow centered on the equator, and these are probably connected with the EUC flow around the north side of the islands. Note the coincidence of these eastward flows north of the equator with the relatively high salinity waters pointed out earlier in Section 3.2.1. The significant geostrophic currents along 85°W are eastward except for the westward surface flow between 2 and 5°N. Again there are three branches of eastward flow, though they are no longer separated by westward flow. The southernmost branch centered on 2°S has weakened, while the others are stronger, especially the northern branch. Along 82-~W the situation is considerably different from the other sections. This section is close to the coast between the equator and 5°S, and we do not expect strong flows normal to the coast. A subsurface westward current centered on 1.5°S between 50 and 300 m depth is connected to westward surface flow found north of the equator. Eastward currents are found between the equator and 4°S, but only in the upper 50 m. A weak eastward flow between 6 ° and 7°S extends from the surface to about 150 m. Along the 4 ° and 6°S sections there are two separate branches of southward flow: one near the coast and the other offshore near 84°W (Fig. 19). These are both continuous with similar, but stronger, features found in the geostrophic current section along !.5°S (not shown). The two southward flows are separated by a weak northward current. All of these flows weaken toward the south. The coastal branch has a surface maximum at 4°S ( - 29 cm sec- t ) and a subsurface maximum at 6°S ( - 7 cm sec- ~, 60 m). This is the origin of the Peru-Chile Undercurrent (PCUC). The offshore current, called the Peru Countercurrent (PCC) by WVRTKI (1963), also supplies EUC water to the coastal region. These two southward currents are not separated by significant northward flow along 6°S, and there is southward flow over most of the section.

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FIG. 19. Zonal sections of mean meridional geostrophic current relative to 500 dbar along 4°S (a), and 6-'S (b). Northward flow is indicated by dashed contours. Contour interval is 5 cm sec-t.

86

R. LUKAS

3.4.2. Seasonal variation. As indicated in Sections 3.1 and 3.2, the seasonal variation of thermal structure and salinity consistently suggest a seasonal pulsation of the EUC. This pulse appears first in the western portion of the study area, and then later along the coast of South America. The dynamic height at 75 dbar relative to 500 dbar is used to examine this pulse and its effect on the pressure gradient along the equator and along the coast of South America. These sections fortunately have the best data coverage. Figure 20 shows the dynamic height at 75 dbar along the equator for the four seasons and the mean. In the mean, dynamic height is highest at the coast, diminishing almost linearly to the west. This westward pressure gradient force is not balanced by the weak mean eastward wind stress because the convergence of momentum associated with the EUC impinging on the boundary is important (LUg:AS, 1981). During the JFM period, elevated dynamic height is seen near the Galapagos Islands. By the following season, dynamic height has diminished somewhat in that region, while having increased substantially near the coast. During the subsequent two seasons, the dynamic height diminishes everywhere east of 92°W. This dramatic reversal of the zonal pressure gradient between Ecuador and the Galapagos Islands during the early part of the year is also seen in the bimonthly dynamic height of the surface and 50dbar relative to 500dbar (Fig. 21). The reversal of the zonal pressure gradient occurs before the reversal of the zonal component of the wind stress in this region, suggesting that this reversal is remotely forced. The effect of this annual pulse is to cause an enhanced southward pressure gradient force at 75 dbar near the coast, extending all the way from the equator (Fig. 22). Normally, the southward longshore pressure gradient necessary to drive the PCUC is only found to the south of about 5°S (recall that the mean southward flow at 4~'S was surface trapped). Thus,

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FIG. 20. Seasonaland annual mean distribution of dynamic height of 75 dbar relative to 500dbar along the equator. The solid line through the annual mean is the least squares fit.

The termination of the Equatorial Undercurrent

87

4 ~'DYNAMICHEIGHT |.0

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FIG. 21. Annual cycle of zonal wind stress and dynamic height difference (relative to 500dbar) along the equator between the Galapagos Islands and South America. 53

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FIG. 22. Seasonal and annual mean distribution of dynamic height of 75 dbar relative to 500 dbar along 82:W. the flow o f E U C waters into the P C U C from the equator along the coast (indicated by the dashed line in Fig. 1) seems to occur during the AMJ season, which is when the isolated high salinity core appears along the coast at 4°S. During other times o f the year, E U C waters appear to flow along the more direct path from the Galapagos Islands to the South American coast near 5°S. 4. C O N C L U S I O N S The Galapagos Islands cause a significant perturbation o f the Equatorial Undercurrent but do not prevent the E U C from penetrating east o f the islands and to the coast o f South JPO 1612-D

88

R. LCKAS

America. The distributions of the high-salinity core and the high oxygen tongue show that the EUC flows both north and south of the Galapagos, though the flow to the south of the islands is much stronger in the mean. Some o f the waters from the EUC that are deflected north and south by the islands are recirculated in the South Equatorial Current as shown by WYRTKI (1967). Water from the EUC reaches the coastal upwelling area by two paths. One is along a line from the Galapagos Islands to the coast at 5°S (the Peru Countercurrent), and the other is along the equator to the coast, and then along the coast as the Peru-Chile Undercurrent. The Peru-Chile Undercurrent begins near 5°S; north of this latitude, the maximum southward current is at the surface. Eastward flow between 3°S and the equator is a feature of the mean circulation in the eastern Pacific. The passage of the EUC primarily to the south of the Galapagos Islands is a result of the mean southward displacement of the current to the west of the islands, which is most likely caused by the southerly component of the mean wind stress. Near the coast of South America, the current turns southward. A linear model of the EUC suggests that this shift is primarily due to boundary effects rather than to local wind curl (ROTHSTEIN, 1984). The EUC between the Galapagos Islands and the coast of South America goes through a pronounced seasonal cycle. In general, the EUC is strongest during the January-June period, and weakest during July-November, though the times of strongest and weakest flow depends on location and depth. When the current is weak, it flows southeastward from the Galapagos directly toward the main upwelling area at the coast. When it is strong, the EUC penetrates all the way to the coast along the equator, flowing southward along the coast and into the PCUC. The seasonal cycle is pronounced in the areas influenced by the EUC, because of the seasonal pulse of the current. The stratification of the upper 300 m between the Galapagos Islands and Ecuador is variable because of fluctuations in the zonal convergence of mass near the boundary. An increase in the thickness of the thermostad in this region precedes the arrival of a lens of high-salinity water advected eastward in the core of the EUC. The appearance of this high-salinity lens is accompanied by a downwelling of the upper-layer thermal structure. Lukas (1981) has shown that this downwelling propagates eastward and upward, and that its meridional profile might be explained by the passage of an equatorial Kelvin wave generated by seasonal trade wind fluctuations. Since this study was concluded, numerous observations of the near-equatorial eastern Pacific have been made under the auspices of NOAA's EPOCS program and by NSF funded investigations. Many hydrographic stations have been occupied, and more importantly, many direct current measurements have been made, both by profiling and by moorings. The climatology of this region will be much improved from the sparse description provided here when these new measurements are analysed. The large 1982-83 E1 Nifio anomaly was well-documented, and will provide a good contrast with seasonal cycles observed during the more normal years. Acknowledgements--During the course of this work, the author was supported as a graduate research assistant under National Science Foundation grants OCE78-29792 and OCE79-21789. The support of JIMAR under CooperativeAggrement NAg0RAH00002 (NOAA) is also appreciated in the final preparation of this paper. The support and guidanceof ProfessorKlaus Wyrtkias my thesis advisoris gratefullyacknowledged.MizukiTsuchiya offered valuable suggestions and encouragement.Jay McCreary and Gary Meyers offered their encouragement and knowledgeduring our stimulating discussions about equatorial dynamics. Joel Picaut, Mike McPhaden and

The termination of the Equatorial Undercurrent

89

anonymous reviewers offered constructive criticism of an earlier version of this manuscript. Thanks are also due to Lew Rothstein for making model calculations that confirmed the effect of the eastern boundary on the Equatorial Undercurrent.

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