Deep-Sea Research I 48 (2001) 2699–2726

The physical structure of a cold filament in a Chilean upwellingzone (Pen!ıınsula de Mejillones, Chile, 231S)

Marcus Sobarzoa,*, Dante Figueroab

aCentro EULA-CHILE, Universidad de Concepci !oon, Casilla 160-C, Concepci !oon, ChilebDepartamento de F!ıısica de la Atm !oosfera y del Oc !eeano (DEFAO), Universidad de Concepci !oon, Casilla 160-C,

Concepci !oon, Chile

Received 5 January 2001; accepted 11 May 2001

Abstract

Cold filaments associated with Eastern Boundary Currents are typically narrower than 100 km but can beseveral hundred kilometers long, extending from the coast to the open ocean in upwelling areas. One suchstructure, observed off Pen!ıınsula de Mejillones (231S, Chile), was studied with both satellite images and two5-days hydrographic cruises carried out during January 1997. The study used a coastal grid of 31 stations inan area of 165 � 155 km2, approximately. The spatial distribution of the filament and its change betweencruises are described from the horizontal distributions of dynamic height, temperature, salinity anddissolved oxygen. The filament was a shallow feature (thickness o100m) and extended at least 165 kmtoward the open ocean. A meandering northward current flowed at the borders of the filament, separatingoceanic and coastal waters of different physical properties. Comparisons of cross sections of the filamentnear the coast and in the oceanic zone show the ascent of the shallow salinity minimum (SSM), and itsextension toward the ocean, bound to the filament. It is concluded that Subantarctic Water ((SAAW)distinguish by low salinity, high dissolved oxygen) and Equatorial Subsurface Water ((ESSW) high salinity,low dissolved oxygen, high nutrient content) form this filament, and that their relative proportions dependon the strength of the coastal upwelling. Thus, the knowledge of the dynamics of these structures isfundamental to better understanding of the spatial distribution of important biological variables, such asnutrients and chlorophyll, in the coastal ecosystem. r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Coastal upwelling; Cold filament; Chile; Peninsula de Mejillones (231S)

*Corresponding author. Fax: +56-41-242546.

E-mail addresses: msobarz@udec.cl (M. Sobarzo).

0967-0637/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 3 7 ( 0 1 ) 0 0 0 3 1 - 0

1. Introduction

The coastal area at 231S off Peninsula de Mejillones, Chile, is a part of the upwelling system ofthe southeastern Pacific, which extends from 51S (off Peru) to at least 371S (off south-centralChile). From a biological point of view this region has been recognized as one of themost productive of the world, providing nearly 15% of the estimated 80 million tons ofworldwide fishing (Hill et al., 1998). In spite of this importance, the waters located south of 151Sin the Eastern Pacific constitute one of the least known regions of the global ocean (Fonseca,1989; Shaffer et al., 1995). The necessity of understanding the factors that determine its highbiological productivity and the strong oceanographic and social effects caused by El Ni *nno eventsin this region (Fuenzalida, 1985; Blanco and D!ııaz, 1985; Kelly and Blanco, 1986; Fuenzalida,1992) have motivated new studies in this zone. These studies have generated relevant new insightsinto the dynamics of this region, especially in relation to coastal-trapped waves (Shaffer et al.,1997, 1999).The high productivity of the coastal area off Pen!ıınsula de Mejillones has been associated with a

poleward undercurrent, which transports water that is rich in nutrients (Equatorial SubsurfaceWater (ESSW)), and with the process of coastal upwelling, which causes the ascent of this water tothe surface (Fonseca, 1989). As a result, the water off Peninsula de Mejillones is normally cold andbiologically productive.The interaction between the subsurface upwelling water and the more oceanic surface water

has been the object of intensive studies in upwelling areas in other parts of the world,especially through the Coastal Transition Zone Program off California (Brink andCowles, 1991). According to Strub et al. (1991), several conceptual models of velocity structurescan be associated with cold filaments observed through satellite images in the California CurrentSystem and in other eastern boundary currents. The first model corresponds to ‘‘squirt-like’’ jets,which transport coastal upwelling waters to the open ocean, becoming a pair of vortexes thatprobably rotate in opposite directions. A second conceptual model incorporates a series ofmesoscale whirls imbedded in a slow equatorward current (Mooers and Robinson, 1984;Rienecker et al., 1987). A third conceptual model consists of a steady equatorward jet, whichmeanders toward and away from the coast. Because of flow instabilities closed whirls could beformed on both sides of the jet. However, in this model of a meandering jet, the jet is the primarystructure and source of energy. The biological implication of the structure of the meandering jet isthat the nutrients and the biomass of the coastal ocean tend to remain on the inshore side of thejet (Strub et al., 1991).Not always, however, does the presence of cold filaments guarantee that the coastal

zone is biologically productive. In the particular case of Peninsula de Mejillones, thedeepening of the ESSW caused by an El Ni *nno event, for example, due to the southwardmotion of warm subtropical surface water trapped to the coast, can significantly modify thepattern of productivity. Another alteration can be produced by the weakening of thesouthwesterly winds, which are upwelling favorable, or the presence of Subantarctic Water(SAAW), which, being located over the ESSW, prevents to ESSW from ascending to the surface.Thereby, although in some cases the upwelling process could occur, the water that is transportedto the surface does not necessarily contribute with the same intensity to the biological productivityof the system.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262700

In this study we characterize the structure of a cold filament of low salinity and high dissolvedoxygen content, detected during January 1997 in the coastal zone off Peninsula de Mejillones,Chile (231S). We also analyze the model of velocity structure that best fits to the describedhydrographic situation.

1.1. Water masses in the study area

Five water masses have been described off northern Chile between Arica (18.51S) andCoquimbo (301S) and between the coast and around 400 km offshore. Between the surface andapproximately 75m depth there is Subtropical Surface Water (SSW), which shows a clearlydefined surface nucleus, mainly north of Antofagasta (231S). The nucleus is characterized bytemperature between 16 and 241C, salinity between 34.9 and 35.3, and dissolved oxygen contentbetween 4.5 and 5.5ml/l (Silva and Konow, 1975; Robles et al., 1976).South of 301S (Coquimbo) there is SAAW between 0 and 120m. In this zone this water

mass has a surface nucleus characterized by temperature between 9 and 151C, salinity between33.0 and 34.3, and dissolved oxygen content between 5 and 7ml/l (Silva and Konow, 1975;Robles, 1976).The region where these two surface water masses interact is a transition zone located between

231S and 301S. Off Pen!ıınsula de Mejillones (231S) the surface salinity still keeps a maximum thatallows, together with the temperature, recognition of SSW, in spite of the weakening of itscharacteristics by mixing with water of subantarctic origin. In particular, during the australsummer of 1997 this water mass was practically not detected off Pen!ıınsula de Mejillones, as thesurface salinity was lower than 34.9. In this transition zone the SAAW lies under the SSW,forming a shallow salinity minimum (SSM) (Gunther, 1936; Wyrtki, 1963; Reid, 1965, 1973a, b;Brandhorst, 1971). As the SAAW moves northward, its nucleus sinks, until it reaches depthsaround 75–125m. North of 251S and to a typical depth of 100m, it is still possible to detectSAAW, recognizable through the SSM, with salinity values between 34.2 and 34.4 (Reid, 1973a;Silva and Konow, 1975).Between 150 and 350m, approximately, there is Equatorial Subsurface Water (ESSW),

characterized by a maximum of salinity and a minimum of dissolved oxygen. Temperaturebetween 8 and 121C, salinity between 34.4 and 34.9, and dissolved oxygen content between 0.2and 3ml/l characterize its nucleus (Reid, 1965; Silva and Konow, 1975). It has been recognizedthat this is the water mass, rich in nutrients, that contributes to the high biological productivity ofthe area.In the intermediate layer between 400 and 1000m depth there is Antarctic Intermediate

Water (AIW). Temperature between 4 and 6.51C, salinity between 34.2 and 34.5, anddissolved oxygen between 2 and 6ml/l characterize its nucleus (Reid, 1965; Silva andKonow, 1975; Sievers and Silva, 1975). Below 1000m depth is found Pacific Deep Water(PDW). South of 351S this water mass is characterized by temperature below 41C, salinity higherthan 34.4 and dissolved oxygen content higher than 4ml/l. North of 351S this water mass hassalinity higher than 34.5 and dissolved oxygen content larger than 2ml/l (Reid, 1965, 1973b; Silvaand Konow, 1975).

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2701

2. Materials and methods

In January 1997 (austral summer) two hydrographic cruises were carried out off Pen!ıınsula deMejillones by the Chilean R/V Abate Molina. The hydrographic grid, which consisted of 31stations, was sampled in two cruises called Hydrographic Cruise 1 (HC-1) and HydrographicCruise 2 (HC-2), carried out during 11–16 January and during 22–26 January (see Fig. 1).Maximum sample depths were alternatively 700m and 350m along every section. At everystation, temperature, salinity and dissolved oxygen content profiles were obtained with a GeneralOceanics CTDO with a sample frequency of 30 samples/s. The pressure, temperature,conductivity and dissolved oxygen sensors of the CTDO, having resolutions of 0.05 db,0.0011C, 0.01m and 0.01ml/l, were calibrated by the manufacturer in June 1996, guaranteeinguncertainty limits of 70.6 db (down to 600m depth), 70.0051C, 70.005m and 70.01ml/l.Water samples for salinity, temperature and dissolved oxygen calibrations were also obtained.In order to characterize the alongshore vertical hydrographic structure, two longitudinal

sections were selected (see lines Y and Z in Fig. 1).

Fig. 1. Study area. Peninsula de Mejillones (231S), Antofagasta, Chile.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262702

The hydrographic study was complemented with satellite images of sea surface temperature(SST) during the cruises, obtained from a NOAA satellite. Two of these images are shown here. Inorder to determine the local atmospheric forcing in the study area, surface winds of the coastalregion near Antofagasta bay (231S) were collected from January to September 1997 with anautomatic meteorological station.The intensity of the stratification of the water column was calculated from the potential energy

anomaly ðfÞ defined as (Bowden, 1983):

f ¼g

H

Z 0

�Hðrm � rÞz dz;

where r; H and rm are the density at any depth z; the water column depth, and the mean densityof the water column for each station. This parameter quantifies the deficit in potential energy dueto stratification, averaged over depth, compared with that of the fully mixed water column. Waterthat is vertically more homogeneous will present smaller f values.

3. Results

The satellite images of SST obtained during HC-1 show a cold filament, which extended fromthe southern limit of Peninsula de Mejillones to the west, having temperatures between 19 and201C (see upper panel of Fig. 2). During HC-2 waters with temperature around 18–191C werenearer to the coast, with a weaker westward projection than during HC-1, which could beidentified as the filament.

3.1. North–south wind stress

The hourly and the subinertial (period>40h) north–south wind stress measured off Peninsulaof Mejillones is shown in Fig. 3 for January 1997. The wind was favorable for coastal upwellingwith a pronounced daily cycle. The subinertial component indicated a more intense upwellingduring HC-2.

3.2. Horizontal distribution of the hydrographic variables during HC-1 (11–16 January 1997)

At 4m depth the distribution of the geopotential anomaly suggests a weak and meanderingsurface flow, directed toward the northwest and then to the coast (Fig. 4). The low values of thegeopotential anomaly near the coast are related to temperature between 18 and 211C, to lowsalinity and to high dissolved oxygen (5–5.5ml/l). According to these distributions the upwellingarea of cold waters (between 18 and 211C) includes 9 coastal stations (stations 1, 14, 15, 22, 23, 24,25, 26, 27 and 28). The T–S and S–O diagrams show that, at 4m depth, the dissolved oxygencontent diminishes and the temperature increases when the salinity increases (Figs. 5a and c).Although at this depth almost all stations show dissolved oxygen and temperature ranges thatcorrespond to SSW, the salinity (less than 34.9) indicates that the water is a mixture of SSW andSAAW. Only station 9 (outside of the upwelling area) corresponds properly to SSW. The coastalstations 15, 24 and 25 (within the upwelling area) correspond to SAAW, according to their ranges

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2703

of salinity and oxygen, although 1–4.51C warmer than usual, because of the mixing with SSW andperhaps due to solar heating. Accordingly, the water mass at the surface of the upwelling area isSAAW (after the ranges of salinity and dissolved oxygen), at a relatively high temperature due tothe mixing with SSW.At 50m depth the cold structure is observed extending offshore, characterized by the 131C-

isotherm (Fig. 4f ). In contrast to the 4m depth, at 50m low temperatures are associated with highsalinity and low dissolved oxygen (Figs. 4g and h). The horizontal distribution of salinity does notshow the same extension offshore that the temperature and the dissolved oxygen show, because ofthe SSM found offshore. At the surface level the T–S and S–O diagrams show a linear relation. At50m depth, on the contrary, the distribution of variables shows the interaction of three watermasses (Figs. 5b and d). The stations within or near the upwelling area show ESSW, in which casehigh salinity is associated with low values of dissolved oxygen (Fig. 5b). The temperature of the

Fig. 2. Infrared satellite images of sea surface temperature. Peninsula de Mejillones, Chile. January 11 and January 24,

1997.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262704

ESSW is however around 11C higher than usual, due to the interaction with warmer waters(Fig. 5d). At the stations outside of the upwelling area the S–O and T–S relationships areinverse, in the sense that high salinity is associated with high values of dissolved oxygencontent and temperature, corresponding to mixed waters with a prevalence of SAAW. The pointwhere both tendencies intersect in the T–S and S–O diagrams corresponds to pure SAAW,according to the temperature and salinity ranges (Fig. 5d). This SAAW has low oxygen due to itsmixing with ESSW (Fig. 5b). According to this, at 50m depth, in the upwelling area, the watercorresponds to ESSW that is 11C warmer than usual for this area. This water mass, enclosed bySAAW, has low dissolved oxygen (o3ml/l) due to mixing with ESSW, and salinity between 34.4and 34.9.At 100m depth the dynamical topography and temperature fields tend to be more

homogeneous than in the upper layers (Fig. 6a and b). In Figs. 6c and d the horizontal fieldsof salinity and dissolved oxygen show a filament toward the northwest with high salinity (>34.7)and low oxygen content (o1ml/l). In the T–S and S–O diagrams the data show again a linearrelation (straight lines), which indicates that only SAAW and ESSW interact (Figs. 5a and c). Inthe coastal zone, particularly in the upwelling area, there is ESSW, slightly warmer than usual.Far from the upwelling focus (stations 4, 5 and 31) it is still possible to find SAAW, but containingrelatively low dissolved oxygen due to mixing with ESSW.At 200m depth the dynamical topography shows a weak southward flow (Fig. 6e). The

temperature and salinity fluctuate slightly around 11.71C and 34.8 (Figs. 6f and g). The whole areashows particularly low values of dissolved oxygen. The S–O and T–O diagrams (not shown) showthe total prevalence of ESSW at this depth.

Fig. 3. Hourly and subinertial north–south wind stress off Peninsula de Mejillones during January 1997. Cruise periods

are indicated by hatched areas.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2705

Fig. 4. Maps of geopotential anomaly (relative to 350 db), temperature, salinity and dissolved oxygen at 4m (a, b, c, d)

and 50m (e, f, g, h) depth during HC-1 (January 11–16, 1997). Contours intervals are 0.05m2/s2, 0.51C, 0.05 and0.2ml/l, for geopotential anomaly, temperature, salinity and dissolved oxygen.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262706

We conclude that the jet associated with this upwelling area affects mainly the top 50malthough at 100m depth it is still possible to observe the displacement of the isohalines andcontours of dissolved oxygen associated with the ESSW, from the Peninsula de Mejillones to thewest.

Fig. 5. S–O and T–S diagrams for 4, 50, and 100m depth during HC-1 (January 11–16, 1997). (a) and (c) diagramsinclude the 4m (*) and 100m (� ) depths.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2707

Fig. 6. Maps of geopotential anomaly (relative to 350 db), temperature, salinity and dissolved oxygen at 100m and

200m depth during HC-1 (January 11–16, 1997). Contours intervals are 0.05m2/s2, 0.51C, 0.05 and 0.2ml/l, forgeopotential anomaly, temperature, salinity and dissolved oxygen.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262708

3.3. Horizontal distribution of the hydrographic variables during HC-2 (22–26 January 1997)

The horizontal distributions during the second hydrographic cruise show that the meanderingstructure observed during HC-1 is now more restricted to the coastal area, and directedtowards the north (Fig. 7). The cold water tongue is 1–51C colder than during HC-1, affectingthe whole Peninsula de Mejillones. The horizontal distributions of salinity and dissolvedoxygen are coherent with this pattern. The low values of geopotential anomaly found near thecoast are now related to temperatures between 15 and 191C, to low salinity and to high dissolvedoxygen (5–6.8ml/l). According to these distributions, the upwelling of cold waters affects thecoastal stations 1, 2, 3, 11, 12, 13, 14, 15, 16, 24, 25, 26, 27 and 28. The T–S and S–O diagrams forthis cruise at 4m depth are similar to those found during HC-1, with mixing of SAAW and SSWat the surface. In this mixed water SAAW prevails at the coastal stations, and SSW at the moreoceanic ones. According to their salinity and dissolved oxygen ranges the coastal stations 1, 2, 15,16, 25 and 26 (within the upwelling area) correspond to SAAW (Fig. 8a), although they are,because of the mixing with SSW, between 1 and 3.51C warmer than usual SAAW in this region(Fig. 8c).At 50m depth the cold waters (12.5–151C), with high salinity (34.60–34.80) and

moderately high dissolved oxygen (up to 5.4ml/l), are separated from oceanic water of similarcharacteristics by a warm tongue (15–171C). This tongue has a rather high and homo-geneous dissolved oxygen (between 5.4 and 5.6ml/l) and is approximately along-shoreoriented (right side of Fig. 7). As for HC-1, the distribution of hydrographic variables at 50mdepth shows the interaction of three water masses (Figs. 8b and d). The stations thatare affected by coastal upwelling show the presence of ESSW, with high salinity and lowdissolved oxygen. This ESSW, however, is about 11C warmer than usual in this region, dueto its interaction with warmer mixed water. The temperature varies between 12 and 131C,with a slight tendency to decrease when salinity increases. At the stations located outside ofthe upwelling area the S–O and T–S relationships change. High values of salinity in thiswater are associated with high values of dissolved oxygen and temperature. The intersectionpoint of the mixing lines SSW–SAAW and SAAW–ESSW of the T–S diagram correspondsto SAAW. According to this, at 50m depth the upwelling of cold waters affects practicallythe whole coastal region, including a small projection of cold water on the western side of thestudy area (Fig. 7f ).As in the case of HC-1, at 100m depth the dynamical topography and temperature

fields tend to be more homogeneous than at shallower depths, showing practically constantvalues (Fig. 9, left side: a, b). The low-salinity, low-oxygen tongue that extendedduring HC-1 from the coast toward the ocean is not observed during HC-2. Thedistributions of temperature, salinity and dissolved oxygen show lineal relationships (rightlines in Figs. 8a and c), showing the total prevalence of ESSW. The ESSW continueshaving temperatures about 11C warmer than the expected values for this water mass in thisregion.At 200m depth the dynamical topography and the distributions of T, S and O2 show very little

spatial variation (right side of Fig. 9). The whole area is associated with particularly low dissolvedoxygen and high values of salinity (between 34.70 and 34.80). The S–O and T–S diagrams (notshown) indicate the total prevalence of ESSW at this depth.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2709

Fig. 7. Maps of geopotential anomaly (relative to 350 db), temperature, salinity and dissolved oxygen at 4m and 50m

depth during HC-2 (January 22–26, 1997). Contours intervals are 0.05m2/s2, 0.51C, 0.05 and 0.2ml/l, for geopotentialanomaly, temperature, salinity and dissolved oxygen.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262710

Therefore, the surface jet observed during HC-1 is found nearer to the coast during HC-2,without the meandering character it had during the first cruise. It is possible that a ring of coldcharacteristics has separated from the meandering jet, drifting to the west, and that this ringcaused the horizontal distributions observed during HC-2.

Fig. 8. S–O and T–S diagrams for 4, 50, and 100m depth during HC-2 (January 22–26, 1997). (a) and (c) diagrams

include the 4m (*) and 100m (� ) depths.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2711

Fig. 9. Maps of geopotential anomaly (relative to 350 db), temperature, salinity and dissolved oxygen at 100m and200m depth during HC-2 (January 22–26, 1997). Contours intervals are 0.05m2/s2, 0.51C, 0.05 and 0.2ml/l, forgeopotential anomaly, temperature, salinity and dissolved oxygen.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262712

3.4. Geopotential anomaly and temperature–salinity relationships

Fig. 10 shows the strong relation between the surface distribution of geopotential anomalyrelative to 350 db, obtained from HC-1, and the distribution of surface temperature obtained fromsimultaneous satellite observations. The surface structure of the cold filament off Peninsula deMejillones is clearly related to the surface dynamical topography, which shows an equatorwardmeandering current that shifts at least one degree of longitude to the west. This meanderingcurrent separates the cold water near the coast from the warmer one observed west of the filament.The relationships between geopotential anomaly, temperature and salinity at 4, 50 and 100m

depth during HC-1 and HC-2 are shown in Fig. 11. During both cruises the temperature tends tobe lower as the geopotential anomaly diminishes, especially at 4 and 50m depth (Figs. 11a and b).The salinity at 4m depth also has a similar behavior (Fig. 11d), unlike what was found by Huyeret al. (1991) in the coastal transition zone off northern California. This means that the cold surfacewater that forms the filament off Peninsula de Mejillones is formed by low salinity water comingfrom the SSM. At 50m depth the salinity does not show the same tendency (Fig. 11e). At 100m

Fig. 10. Maps of geopotential anomaly (relative to 350 db, in m2/s2) during HC-1 and sea surface temperature (1C)from NOAA image (January 11, 1997; 19 : 25 GMT).

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2713

depth is found a slight tendency to have higher values of the geopotential anomaly as the salinitydiminishes (Fig. 11f ).

3.5. T–S diagrams for mean profiles

Using the horizontal structure of the geopotential anomaly (GA, relative to 350 db) of thefilament during HC-1 and HC-2, T–S diagrams were calculated from mean temperature andsalinity profiles obtained for the regions inshore of the jet (GAo6:0m2/s2), in the jet(6:0oGAo6:2m2/s2) and offshore of the jet (GA > 6:2m2/s2).Offshore of the jet, the first 50m of depth in both cruises are dominated by SSW with S > 34:5

and T > 16:01C (Fig. 12). This ‘‘local’’ SSW originates from the mixing of SSW and SAAW. Inthe jet, only the first 25m, approximately, correspond to SSW. The water lying around 50m depthcorresponds to SAAW with To131C and salinity near 34.45 during both cruises. Inshore of thejet and in the first 50m depth the SAAW prevails, especially during HC-2, with lower surfacetemperature and salinity.

Fig. 11. Scatter plots of temperature (top) and salinity (bottom) versus geopotential anomaly at 4, 50 and 100m depth

during HC-1 (triangles) and HC-2 (circles).

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262714

3.6. Vertical structure in across-filament sections

Across-filament sections were analyzed at the longitudes 711040W (Z-line, oceanic across-filament section) and 701620W (Y-line, coastal across-filament section), as shown in Fig. 1. Alongthe Z-line during HC-1 (Figs. 13a–d), the effect of the jet, up to some 65 km from the coast, ischaracterized by relatively cold water (between 14 and 181C) that affects the first 50m depth. Thiswater is associated with low salinity (o34.5) and high dissolved oxygen (>5ml/l). Along the Y-line, located at only 30 km from the coast, the 131C-isotherm is found between 25 and 50m depthat stations 25 and 14 (upwelling focus). The ascent of the SSM is particularly evident at stations 25and 14. At stations 3 and 27, on the other hand, the SSM is located between 40 and 100m depth(Figs. 13e–h).During HC-2 and along the Y-line the first 50m depth of the water column were colder than

during HC-1, particularly at stations 13 and 14 (Fig. 14e). In contrast to HC-1, when the SSM waslocated near the surface at stations 25 and 14 during HC-2 (Fig. 13f ) this salinity minimumpractically disappeared, with only some vestiges remaining at station 27 (Fig. 14f ). Water withhigher salinity (>34.7), having minimum dissolved oxygen values, has ascended to depths around

Fig. 12. Mean T–S diagrams for HC-1 and HC-2 according to the geopotential anomaly. GAo6:0: ‘‘Inshore’’ of thejet; GA > 6:2: ‘‘Offshore’’ of the jet; 6:0oGAo6:2: ‘‘In the jet’’. Arrows show the 50m depth level in each profile.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2715

Fig. 13. Across-jet sections of temperature, salinity, dissolved oxygen and sigma-t during HC-1 along the Z-line (left)

and the Y-line (right). Contours intervals are 0.51C, 0.05, 0.5ml/l and 0.1 for temperature, salinity, dissolved oxygenand sigma-t.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262716

Fig. 14. Across-jet sections of temperature, salinity, dissolved oxygen and sigma-t during HC-2 along the Z-line (left)and the Y-line (right). Contours intervals are 0.51C, 0.05, 0.5ml/l and 0.1 for temperature, salinity, dissolved oxygen

and sigma-t.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2717

25m near the coast (stations 13 and 14) by an increment of upwelling, transporting the SSMoffshore. In this case, the coastal water that ascends and occupies the first 50m depth is ESSW. Inthis way, the coastal upwelling off Peninsula de Mejillones causes a jet toward the north thatdeforms the structure of the SSM. The ascent of waters at stations 13, 14 and 15 carry the SSMtoward the surface layers. If the upwelling is intense the SSM disappears from the surface layersbecause of the ascent of waters of high salinity (S > 34:7; Fig. 14f ). The waters that surround thisupwelling focus maintain the depth of the SSM around 70–90m depth on the Z-line and around20–60m depth on the Y-line during both HC-1 and HC-2.

3.7. Vertical structure along the filament

During HC-1 the 13.51C isotherm is observed above 50m depth from the coast to around120 km offshore (Fig. 15a). This rising of cold waters produces a strong horizontal temperaturegradient between stations 20 and 21. At 25m depth the temperature changes from 151C to 191C inaround 45 km along the filament. During HC-2 the thermal front is found nearer the coast,between stations 17 and 18, where the temperature at 25m depth changes from 151C (station 17)to 191C (station 18, see Fig. 15c).During HC-1 the shallow salinity minimum (SSM), having values between 34.2 and 34.4, was

associated with the low temperature water found between 25 and 50m depth. This SSM hadadvected to shallower waters by the rising of saline waters (ESSW), becoming a part of thefilament. West of 130 km from the coast the SSM is found at its usual depth between 75 and 100depth. During HC-2 the SSM remains at this depth range up to ca. 110 km from the coast. Nearthe coast it ascends to ca. 25m depth (Figs. 15b and d).

3.8. Structure of the shallow salinity minimum (SSM)

As was shown in the previous section, depending on the strength of the coastal upwelling, thewater that rises near the coast can be SAAW, which produces the SSM, or ESSW, whichtransports high salinity water to the surface. If the upwelling is weak (as was the case during HC-1) the water that rises comes just from the first 100m of depth, causing the SSM to ascend to thesurface. On the other hand, if the upwelling is more intense (as was the case during HC–2), thewater that rises comes from depths greater than 100m, bringing high salinity water to the surfacelayers and displacing the SSM offshore. In the last case the nutrient concentration would be largerthan in the first case.Thereby, the location of the SSM in the study area is critical for the determination of the impact

that the upwelling will have on the biological productivity of the coastal ecosystem off Peninsulade Mejillones. In order to observe this location, Fig. 16 shows the depth of the SSM during thetwo hydrographic cruises. The SSM was calculated for every salinity profile, applying a verticalrunning mean of 5 terms (corresponding to 5m). Values between 34.18 and 34.40 were obtained.In the case of HC-1, the SSM ascends to depths between 10 and 15m at stations 25, 26, 14 and

15 (focus of the upwelling), and then it is found deeper, until ca. 55m, following the direction ofthe cold filament (compare with Figs. 4f and 7f ). This cold filament could transport the SSMoffshore through upwelling pulses, generating patches of SSM that follow the direction of the

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262718

Fig. 15. Temperature and salinity vertical sections along the filament during HC-1 and HC-2. The 12.71C-isotherm isdashed.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2719

filament. Station 21, for example, could represent a patch where the SSM would be located at only15m depth (Fig. 16a).During HC-2 the SSM reaches the surface in the coastal area, occupying a larger part of the

coastal border (Fig. 16b). The filament that extended to the northwest during HC-1 has been splitby warmer waters, which have higher salinity and higher concentration of dissolved oxygen

Fig. 16. Horizontal distribution of the depth of the Shallow Salinity Minimum (34.18–34.20). The depth is shown inmeters.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262720

(Figs. 7f and h). In this case a patch of SSM was formed in the northwest corner of the studyarea (stations 7 and 8), which maintains cold characteristics (131C) and low dissolved oxygen(o3ml/l), as one can observe in Figs. 7f and h.

3.9. Potential energy of the water column

In order to quantify the transition from vertically well-mixed zones to those where stratificationexists, the anomaly of the potential energy, f; was calculated (Fig. 17). Waters that are verticallymore homogeneous (small f) show a horizontal distribution of f similar to that of the coldfilament described in Fig. 4 and to that of the horizontal structure of the SSM (Fig. 16).Integrating from the surface down to 150m depth it is observed that along the main axis of thefilament three nuclei of waters are formed, which are vertically more homogeneous than thewaters outside the filament.In the case of HC-2 (Fig. 17, right side: e, f, g, h) the water column near the coast is much more

homogeneous than during HC-1. In this case the most homogeneous waters cover from thesouthern part of Peninsula de Mejillones to the north, and are distributed nearer to the coast thanduring HC-1. Integrating from the surface down to 150m depth it is observed that stratifiedwaters enter into the study area, separating more homogeneous, oceanic waters from those thatare located close to the coast.

4. Discussion and conclusions

The existence of structures in the form of filaments has been described in the literature forseveral coastal regions of eastern boundary currents. These filaments have cold characteristics andtypical sizes smaller than 100 km in width, but several hundreds of kilometers in length, extendingfrom the coast toward the open ocean (Brink and Cowles, 1991). There is evidence that these coldfilaments are structures associated rather with the upwelling season (spring and summer) thanwintertime, and that they are mainly located in some specific coastal regions (Lutjeharms andStockton, 1987). In these regions the coastal transition zone shows meandering currents thatseparate coastal and oceanic waters that are physically and biologically different. The physicalstructure of these transition regions has been studied by several authors (Brink and Cowles, 1991;Brink et al., 1991; Kosro et al., 1991; Strub et al., 1991; Huyer et al., 1991). Other authors haveanalyzed the biological effects that these structures have on the coastal ecosystem (Abbot andBarksdale, 1991; Hood et al., 1991; Mackas et al., 1991).In the case of eastern boundary currents of the Southern Pacific, and especially for the Chilean

coast between 201S and 381S, relatively few studies of the physics of upwelling areas have beencarried out, with poor temporal and space resolution (Strub et al., 1991). Some studies that utilizedsatellite images did identify structures similar to filaments, originating from Chilean locations thathave topographical peculiarities, like points and capes (C!aaceres and Arcos, 1991; Fonseca, 1989;Fonseca and Far!ııas, 1987; Kelly and Blanco, 1984). These studies usually do not describe thephysical characteristics of these filaments nor the impact that they have on the coastal ecosystem.The results obtained in this study show that the knowledge of the structure of the coastal

transition zone off Peninsula de Mejillones (filaments and meandering currents) can be essential

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2721

Fig. 17. Potential energy anomaly (in J/m3) during HC-1 and HC-2. Contours intervals are 10 (J/m3).

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262722

for the understanding of the spatial distributions of important biological variables associated withthe coastal ecosystem (nutrients, chlorophyll, etc.). The most important features of the structureof this zone, deduced from the present study, can be summarized as follows:

1. The upwelling area off Pen!ıınsula de Mejillones is located in a geographical zone where astrong interaction of surface water masses occurs. On one hand, SSW arrives from the north,having a thickness near to 80m and a clearly defined nucleus north of Antofagasta (231S). On theother hand, from the south flows SAAW, in the so-called Chile–Peru Current, of thickness near120m and very defined characteristics at the surface south of 301S. Between 231S and 301S alatitudinal transition zone exists, where the SAAW sinks under the SSW, originating a SSM(Gunther, 1936; Wyrtki, 1963; Reid, 1965, 1973a, b; Brandhorst, 1971). This SSM displaces theESSW to greater depths, between 80 and 560m. In the coastal stations, the SSM occupies thewater column between 5m and 80m depth.2. This distribution of water masses implies that it is necessary to modify the traditional

approach regarding the physical, chemical and biological characteristics of the water that upwellsoff the Peninsula de Mejillones. In the upwelling system of central Chile (30–381S) normally thewater that ascends near the coast is colder, more saline and with less concentration of dissolvedoxygen than the adjacent coastal waters, due to the presence of ESSW. If this is the case, ESSW,rich in nutrients, contributes to the biological productivity of the coastal ecosystem. In the case ofPeninsula de Mejillones, on the other hand, the relatively cold water found at the sea surface,caused by upwelling, does not necessarily imply more-saline water, less dissolved oxygen and largernutrient concentrations near the coast. As is shown in this study, if the upwelling is not strongenough to pump water from depths greater than 100m, the ESSW will not reach the subsurfacelayers of the sea and its contribution to the biological productivity will probably be less significant.3. The spatial distribution of the filament and its variation between the two cruises are described

in this study by means of horizontal distributions of dynamic height, temperature, salinity anddissolved oxygen. The horizontal temperature distribution shows the filament down to 50m depth(Fig. 4f ), but, on the other hand, the distributions of salinity and dissolved oxygen indicate thatthe structure is observed down to, at least, 100m depth (Figs. 6c and d). No explanation wasfound for this apparent difference. According to the figures, the filament extends to the northwestuntil at least 170 km from the coast.4. According to this study it is postulated that, depending on the strength of the coastal

upwelling, the water that forms the filament can be SAAW or ESSW or a mixture of both. Forthis reason it seems convenient to analyze two cases. The first case, which corresponds to whatoccurred during HC-1, shows a weak upwelling (Fig. 3), which pumps subsurface water in thecoastal area from depths above 100m. The water that rises in this case is SAAW, characterized bya SSM. This water rises up to 10m depth in the coastal zone, and then it is displaced toward theocean, advected in the filament. As this water is displaced toward the northwest, it sinks, reachingup 55m depth in the northwest boundary of the study area. In this case the SSM constitutes a verygood tracer of the filament (Fig. 16a). The second case corresponds to a stronger upwelling(Fig. 3), which pumps water from depths near or below 100m, causing the SSM to reach thesurface near the coast, even completely disappearing at some stations. In this case, whichcorresponds to what occurred during HC-2, ESSW rises near the coast, characterized by highersalinity and lower concentration of dissolved oxygen, what basically form the filament. During

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2723

HC-2, however, the SSM rapidly sinks deeper than 65m as it extends to the open ocean, andshallows at more oceanic stations. This situation suggests that the filament split in two partsbetween the two cruises, due to the intrusion of water having higher temperature, salinity, anddissolved oxygen content (Fig. 16b). If the process of upwelling were particularly strong, the SSMwould disappear near the coast, and this zone would be dominated by ESSW of high salinity.5. It is probable that during HC-2 the initial phase of a coastal upwelling event off Peninsula de

Mejillones was occurring. From this point of view, the cold water (isotherms of 12.51C and 131C)ascends in the coastal area because of Ekman pumping. On the other hand, the conditions foundduring HC-1 could represent an advanced stage of a previous upwelling event. In this case the coldwater had been advected due to Ekman transport, allowing the development of a cold filament.6. The horizontal distribution of dynamic height shows the presence of a meandering current

that has a thickness of ca. 50m. This current meanders from south to north following the borderof the filament. As in the case described by Strub et al. (1991) for the California Current System,this speed structure separates coastal and oceanic waters that have different physical and probablybiological properties. However, in the case of the Pen!ıınsula de Mejillones, and differently from thecase of the California Current System, the water that is located between the jet and the coast canbe ESSW (high salinity and nutrient concentrations, low dissolved oxygen content) or SAAWwith a subsurface minimum of salinity and with more dissolved oxygen and less nutrient contentthan the ESSW.7. Therefore, the study of the dynamics of the transition zone off Pen!ıınsula de Mejillones is of

fundamental importance for the better understanding of the spatial distributions of importantbiological variables associated with the coastal ecosystem (nutrients, chlorophyll, etc.). As thefertilization (N, P, CO2) depends on the depth from which the water upwells, the maximum valuesof the nutrients probably occur when ESSW reaches the surface (Zentara and Kamykowski, 1977;Torres et al., 1999). This study postulates that only in the case that the coastal upwelling succeedsin pumping water from below the depth of the SSM will the biological productivity of the coastalsystem be benefited by water that is rich in nutrients.8. The sample design used in the cruises of this study did not allow us to obtain definitive

conclusions about the temporal variability of the filaments. Cruises that need more than a coupleof days for obtaining a whole hydrographic description of the upwelling area are too long to‘‘capture’’ them, because of the high temporal variability of these structures. Studies having bettertemporal resolution are to be carried out in order to obtain a better understanding of thedynamics of these structures. In the next stage of this study we expect to combine the physicalinformation, shown in this study, with the biological information obtained during the samecruises to obtain more insight about the behavior of the ecosystem. It appears also important todedicate some attention to the seasonal variability that introduces the interaction of the SAAWwith the SSW in this zone because of the effects that this variability can have on the existence ofthe SSM in this coastal zone.

Acknowledgements

The Chilean National Fund for Scientific and Technological Research (FONDECYT)supported this work under grant Sectorial 5960002. One author (D.F.) was partially founded

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262724

in the final stage of this work by the Chilean National Fund for Priority Areas (FONDAP-Humboldt Program), also supported by FONDECYT, Chile. Jos!ee Rutllant (Universidad deChile) kindly provided data winds on the Antofagasta. We thank the captain and the crew of theR/V Abate Molina, especially Hern!aan Reyes for the CTD operation. Finally, we wish to thank thereviewers for their very helpful comments.

References

Abbot, M., Barksdale, B., 1991. Phytoplankton pigment patterns and wind forcing off Central California. Journal of

Geophysical Research 96, 14649–14667.Blanco, J., D!ııaz, M., 1985. Caracter!ıısticas oceanogr!aaficas y desarrollo de El Ni *nno 1982–1983 en la zona norte de Chile.

Investigacion Pesquera (Chile) 32, 53–60.

Bowden, K.F., 1983. Physical Oceanography of Coastal Waters. Ellis Horwood Limited, England. 302pp.Brandhorst, W., 1971. Condiciones estivales frente a la costa de Chile. Revista de Biologia Marina (Chile) 14, 45–84.Brink, K., Cowles, T., 1991. The coastal transition zone program. Journal of Geophysical Research 96, 14637–14647.Brink, K.H., Beardsley, R.C., Niiler, P.P., Abbot, M., Huyer, A., Ramp, S., Stanton, T., Stuart, D., 1991. Statistical

properties of near-surface flow in the California Coastal Transition Zone. Journal of Geophysical Research 96,14693–14706.

C!aaceres, M., Arcos, D., 1991. Variabilidad en la estructura espacio-temporal de un !aarea de surgencia frente a la costa de

Concepci !oon, Chile. Investigacion Pesquera (Chile) 36, 27–38.Fonseca, T., 1989. An overview of the poleward undercurrent and upwelling along the Chilean coast. In: Neshyba, S.J.,

Mooers, Ch.N.K., Smith, R.L., Barber, R.T. (Eds.), Poleward Flows Along Eastern Ocean Boundaries. Springer-

Verlag, New York, pp. 203–216.Fonseca, T., Far!ııas, M., 1987. Estudio del proceso de surgencia en la costa chilena utilizando percepci !oon remota.

Investigacion Pesquera (Chile) 34, 33–46.

Fuenzalida, R., 1985. Aspectos oceanogr!aaficos y meteorol !oogicos de El Ni *nno 1982–83 en la zona costera de Iquique.Investigacion Pesquera (Chile) 32, 47–52.

Fuenzalida, R., 1992. Anomal!ııas oceanogr!aaficas y meteorol !oogicas durante el desarrollo de El Ni *nno 1991–92 en la zonade Iquique (201180S). Investigacion Pesquera (Chile) 37, 67–72.

Gunther, E.R., 1936. A report on oceanographical investigations in Peru Coastal Current. Discovery Reports 13, 107–276.

Hill, E., Hickey, B., Shillington, F., Strub, T., Brink, K., Barton, E., Thomas, A., 1998. Eastern ocean boundaries

coastal segment (E). The Sea. Vol. 11. John Wiley & Sons, New York, pp. 30–67.Hood, R.R., Abbot, M., Huyer, A., 1991. Phytoplankton and photosynthetic light response in the Coastal Transition

Zone off Northerm California in June 1987. Journal of Geophysical Research 96, 14769–14780.

Huyer, A., Kosro, P.M., Fleischbein, J., Ramp, S., Stanton, T., Washburn, L., Chavez, F., Cowles, T., Pierce, S.,Smith, R., 1991. Currents and water masses of the Coastal Transition Zone off Northern California, June to August1988. Journal of Geophysical Research 96, 14809–14831.

Kelly, R., Blanco, J., 1984. Proceso de surgencia en Punta Nugurne, Chile (Lat. 361S), Marzo 1983. Investigacion

Pesquera (Chile) 31, 89–94.Kelly, R., Blanco, J., 1986. Caracter!ıısticas oceanogr!aaficas en el norte de Chile durante la declinaci !oon de El Ni *nno 1982–

1983. Investigacion Pesquera (Chile) 33, 77–90.

Kosro, P.M., Huyer, A., Ramp, S., Smith, R., Chavez, F., Cowles, T., Abbot, M., Strub, T., Barber, R., Jessen, P.,Small, L., 1991. The structure of the transition zone between coastal waters and the open ocean off NorthernCalifornia, winter and spring 1987. Journal of Geophysical Research 96, 14707–14730.

Lutjeharms, J.R.E., Stockton, P.L., 1987. Kinematics of the upwelling front off southern Africa. South African Journalof Marine Science 5, 35–50.

Mackas, D., Washburn, L., Smith, S., 1991. Zooplankton community pattern associated with a California Current cold

filament. Journal of Geophysical Research 96, 14781–14797.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–2726 2725

Mooers, C.N., Robinson, A.R., 1984. Turbulent jets and eddies in the California Current and inferred cross-shoretransports. Science 223, 51–53.

Rienecker, M.M., Mooers, C.N., Robinson, A.R., 1987. Dynamical interpolation and forecast of the evolution ofmesoscale features off northern California. Journal of Physiology and Oceanography 17, 1189–1213.

Reid, J.L., 1965. Intermediate waters of the Pacific Ocean. The Johns Hopkins Oceanographic Studies 2, 85.

Reid, J.L., 1973a. Transpacific Hydrographic sections at Lats. 431S and 281S, the SCORPIO Expedition III. Upperwater and a note on southward flow at mid depth. Deep-Sea Research 20, 39–49.

Reid, J.L., 1973b. The shallow salinity minima of the Pacific Ocean. Deep-Sea Research 20, 51–68.

Robles, F., 1976. Descripci !oon general de las condiciones oceanogr!aaficas en Aguas Chilenas. Instituto de FomentoPesquero, Santiago,Chile, 90pp.

Robles, F., Alarc !oon, E., Ulloa, A., 1976. Water masses at the northern Chilean zone and their variations in a coldperiod (1967) and warms periods (1969, 1971–1973). El Ni *nno Workshop, Guayaquil, Ecuador. December 1974.

FAO. Inf. Pesca, 185, 94–196.Shaffer, G., Salinas, S., Pizarro, O., Vega, A., Hormaz!aabal, S., 1995. Currents in the deep ocean off Chile (301S). Deep-

Sea Research 42, 425–436.

Shaffer, G., Pizarro, O., Djurfeldt, L., Salinas, S., Rutllant, J., 1997. Circulation and low frequency variability near theChilean coast: remotely forced fluctuations during the 1991–1992 El Ni *nno. Journal of Physiology and Oceanography27, 217–235.

Shaffer, G., Hormaz!aabal, S., Pizarro, O., Salinas, S., 1999. Seasonal and interannual variability of currents andtemperature off central Chile. Journal of Geophysical Research 104, 29951–29961.

Sievers, H., Silva, N., 1975. Masas de agua y circulaci !oon en el Oc!eeano Pac!ııfico Sudoriental. Latitudes 181S–331S.(Operaci !oon Oceanogr!aafica MARCHILE VIII). Ciencia y Tecnica del Marina (Chile) 1, 7–67.

Silva, N., Konow, D., 1975. Contribuci !oon al conocimiento de las masas de agua en el Pac!ııfico Sudoriental. Expedici !oonKRILL. Crucero 3–4 Julio-Agosto 1974. Revista de la Comisi !oon Permanente del Pac!ııfico Sur. 3, 63–75.

Strub, P., Kosro, M., Huyer, A., 1991. The nature of the cold filaments in the California Current System. Journal of

Geophysical Research 96, 14743–14768.Torres, R., Turner, D., Silva, N., Rutllant, J., 1999. High short-term variability of CO2 fluxes during an upwelling event

off the Chilean coast at 301S. Deep-Sea Research I 46, 1161–1179.

Wyrtki, K., 1963. The horizontal and vertical field of motion in the Peru Current. Bulletin of the Scripps Institute ofOceanography 8, 313–346.

Zentara, S.J., Kamykowski, D., 1977. Latitudinal relationships among temperature and selected plant nutrients along

the West Coast of North and South America. Journal Marine Research 35, 321–337.

M. Sobarzo, D. Figueroa / Deep-Sea Research I 48 (2001) 2699–27262726