Modeling study of seasonal and inter-annual variability of circulation in the coastal upwelling site of the El Loa River off northern Chile

Estuarine, Coastal and Shelf Science 67 (2006) 93e107 www.elsevier.com/locate/ecss

Modeling study of seasonal and inter-annual variability of circulation in the coastal upwelling site of the El Loa River off northern Chile Winston Palma a,b, Ruben Escribano c,*, Sergio A. Rosales c a

Programa de Doctorado en Oceanografı´a, Departamento de Oceanografı´a, Universidad de Concepcio´n, Chile b Departamento de Ciencias del Mar, Universidad Arturo Prat, Iquique, Chile c Center for Oceanographic Research in the South Eastern Pacific (COPAS), Department of Oceanography, Universidad de Concepcio´n, P.O. Box 42, Dichato, Concepcio´n, Chile Received 7 July 2004; accepted 7 November 2005 Available online 19 January 2006

Abstract A three-dimensional numerical model using the Finite Element Method (FEM) was used to diagnose coastal currents off the El Loa River (21
S) in the northern upwelling region of Chile. This site has been recognized as an important spawning zone of the southern anchovy Engraulis ringens. Diagnostic current fields were obtained for summer and winter during the 1997e1998 El Nin˜o conditions and during a ‘‘normal’’ upwelling year. The results show this site as an efficient retention area in the nearshore, because of a reduced cross-shelf flow, a strong alongshore flow and presence of several anticyclonic eddies. A simulated Lagrangian experiment indicated that retention within the nearshore (<10 km) may last for more than 4 days under a steady-state wind condition. Wind regimes and water density fields during 1997e1998 (El Nin˜o) and 1995e1996 (‘‘normal’’ upwelling) did not cause differences in the general pattern of coastal circulation. However, the magnitudes of both the alongshore and the cross-shelf flows are substantially reduced during El Nin˜o in the nearshore spawning zone, possibly as a consequence of an anomalous water mass in the coastal area. This altered condition may limit the transport and dispersion of anchovy spawning products. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: circulation; eastern boundary currents; fish spawning; numerical modeling; upwelling

1. Introduction The upwelling region of northern Chile is a highly productive ecosystem that supports the strong fisheries of anchovy and sardine (Mann and Lazier, 1991; Walsh, 1991; Alheit and Bernal, 1993). Biological production in this area is restricted to a very narrow continental shelf, within which coastal upwelling takes place (Fonseca and Farı´as, 1987; Marı´n et al., 2001). Previous works have described the large-scale hydrographic characteristics of this region (Strub et al., 1998; Blanco et al., 2001), but physical processes that govern * Corresponding author. E-mail addresses: [email protected] (W. Palma), [email protected] (R. Escribano). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.11.011

circulation in the nearshore are poorly known. This is a critical issue to understand the physicalebiological interactions that take place in the clupeid spawning areas, which are often localized within a narrow (<20 km) band along the coast. One of the key spawning areas for anchovy in northern Chile is that off El Loa River (21
S). Annual ichthyoplanktonic surveys since 1992 have shown that anchovy eggs persistently aggregate in this area during the austral-winter spawning season (JulyeAugust) (Oliva et al., 2000). This particular site, however, has not been studied, making difficult the identification of factors and mechanisms that may favor anchovy spawning. This also limits our knowledge on processes that affect the spawning area and gives rise to strong interannual variability in anchovy recruitment and biomass (Alheit and Bernal, 1993).

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The physical heterogeneity near the coast may certainly result from the dominant circulation controlled by both baroclinic and barotropic factors, whose combined effects on circulation have not yet been examined for this important upwelling center. The area is exposed to the ocean, but the morphology of the coast is not uniform and this may affect the circulation nearshore. The bottom topography may also play a significant role determining coastal circulation in shallow areas (Rodriguez and Lorenzetti, 2001). All these factors may interact and determine the observed circulation. Superimposed over such factors there is also external forcing. In coastal areas off northern Chile the wind seems a key component determining near-surface circulation during upwelling (Sobarzo and Figueroa, 2001; Escribano et al., 2004a). In addition, coastal trapped waves can impose remote forcing on pattern of circulation in the coastal zone. A large-scale process that should also be considered is the well-known El Nin˜o phenomenon. Little is known about the impact of the El Nin˜o on circulation in the upwelling region off Chile (Blanco et al., 2002) and this lack of knowledge is even more pronounced in the nearshore zone. It is likely that annual variability in location and extension of anchovy spawning areas is related to changes in circulation patterns that modify the advective environment, inducing the mixing and transport of eggs and larvae, as suggested for another upwelling site in northern Chile (Rojas et al., 2002). This possibility has not been studied in the El Loa site, and as a first step it is important to examine the pattern of circulation and assess the role of some physical forces. The El Loa area may thus prove as a suitable site to analyze these processes, mainly because of its importance as a persistent spawning zone. In this work, we used a 3-D diagnostic model to study the coastal circulation under different seasonal regimes. Because of persistent aggregations of anchovy eggs and larvae in the nearshore area, we further test the hypothesis that dominant circulation may favor retention nearshore rather than offshore advection during active upwelling.

2. Data and methods 2.1. The study area The selected site comprises the coastal zone off El Loa River in the northern upwelling region of Chile (Fig. 1). This is a rather exposed coastal area facing the open sea and nearly aligned northesouth. Studies from satellite data have shown that upwelling occurs year-round near the mouth of El Loa River (Fonseca and Farı´as, 1987) and that cold-upwelling plumes are often oriented northward. The zone presents extremely arid conditions and southerly winds predominate at all seasons, although they become more intense and persistent during January (Thomas et al., 1994). These favorable-upwelling winds cause the ascent of deep (w200 m) equatorial subsurface waters (ESSW), which are associated with the oxygen minimum zone (OMZ) (Strub et al., 1998; Morales et al., 1999). Thus, the ESSW along with the OMZ may enter the

photic layer in shallow waters (<50 m) close to the shoreline, which is constrained by an abrupt topography (Fig. 1). Blanco et al. (2001) described the seasonal climatology of the northern upwelling region off Chile. The entire zone is dominated by near-surface (40e80 m) sub-Antarctic water (SAW), interacting with subtropical surface (0e40 m) water (SSW), which during El Nin˜o conditions may approach the coastal area. In the nearshore, the contribution of ESSW to the upper layer may vary depending on upwelling intensity. The ESSW is normally present between 100 and 300 m, but its presence on the surface layer is noted by low oxygen water observed mainly during the upwelling season (austral spring/ summer) (Morales et al., 1996). The SAW is colder (<14
C) and less saline (34.3e34.8) than the SSW with temperatures greater than 18.5
C and salinity in the range of 34.9 and 35.7. Major currents are the cold ChileePeru Current that flows to the Equator in offshore areas, but in the nearshore zone the poleward PerueChile counter current may dominate (Strub et al., 1998), interacting with upwelling plumes and altering coastal circulation, as shown near Antofagasta (23
S) (Escribano and Hidalgo, 2001; Marı´n et al., 2001). 2.2. Data sources The density fields for two seasonal conditions, summer (January) and winter (August), for two different years, were calculated: during the 1997e1998 El Nin˜o and during a cold ‘‘normal’’ year 1995e1996. For these four periods, there were data available from cruises carried out by the Institute for Fishery Research of Chile (IFOP). The cruises included CTD profiling, using a Neil Brown and a SeaBird SBE 19 CTDs deployed down to 500 m depth, or near the bottom over cross-shelf transects in the study region. Details on sampling methods are described in Blanco et al. (2002). A time series of wind data was available from a meteorological station located at the airport Diego Aracena, near Iquique (20
32# S), indicated in Fig. 1. We used a database of hourly winds since January 1995eDecember 1998. From these, modal wind directions for each of the study periods and their associated mean values of intensity were calculated. These parameters were then used to impose wind forcing on the circulation model for each of the study periods. 2.3. Numerical modeling We adapted a three-dimensional diagnostic model for baroclinic, wind-driven circulation of shallow seas. The model is a finite element solution of the 3-D shallow water equations, described in Lynch (1990) and Lynch et al. (1992). This model uses an externally specified density field and can be forced by tide or wind. The solutions for the linearized 3-D shallow water equations are found with conventional hydrostatic and Boussinesq assumptions, and eddy viscosity closure in the vertical. The density field is assumed as known and becomes a fixed baroclinic pressure gradient. The velocity fields upon this forcing, under the effect of wind and barotropic forcing at open water boundaries, are examined on the basis of

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Longitude °W Fig. 1. The coastal upwelling zone off El Loa River at northern Chile in the eastern south Pacific. This region was used to diagnose coastal circulation by numerical modeling. The topography is illustrated and location of the local airport from which wind data were available for the study period.

a well-detailed topography. A periodic-in-time solution is assumed of the form q(x, t) ¼ Re(Q)(x)e jut, with Q the complex amplitude of q and u the frequency. The steady responses are then the limiting case u ¼ 0. The horizontal momentum equation is:
 v vV juV þ fxV  N ¼GþR vz vz N

vV ¼ hj ðz ¼ 0Þ vz

N

vV ¼ kV ðz ¼ hÞ vz

where Rðx; y; zÞh 

g r0

Z

0

Vr dz Rðx; y; zÞ z

is the baroclinic pressure gradient (assumed known), Gðx; yÞh  gVx is the barotropic pressure gradient (assumed unknown), r(x, y, z) is the fluid density, V(x, y, z) is the horizontal velocity, with components u and v,ffi u is the radian pffiffiffiffiffiffi frequency, j is the imaginary unit ( 1), h(x, y) is the bathymetric depth, f is the Coriolis vector, N(x, y, z) is the

vertical eddy viscosity, g is gravity, hj(x, y) is the atmospheric forcing, and k is the linear bottom stress. The numerical solution for the equations is described in detail in Lynch et al. (1992). Tidal effects were assumed to be negligible for this particular area (Escribano et al., 2004a) and the model was then forced by wind alone, and without heading forcing. As a closure in the vertical axis, a constant dimensionless eddy viscosity (Nz) was used as 0.05, with a linearized partial-slip condition forced at the bottom. A dimensionless linear bottom stress (k) of 0.04 was applied and the proportion for sinusoidal spacing over 21 vertical layers was 2.5. In a first step, a 3-D mesh was constructed. This mesh comprised a grid of about 180  300 km (Fig. 2). The 3-D mesh contained 547 nodes with 1090 finite elements allocated for 21 vertical layers. As shown in Fig. 2 the resolution of the 3-D mesh ranges between 1 and 31 km, increasing in the nearshore to account for abrupt changes in topography (shown in Fig. 2). To obtain the mass field, the distribution of water density for each period was calculated from temperature and salinity data from surface to 500 m. We forced the model to diagnose current fields only down to 500 m, because density fields were available for that layer. Below this depth unexpected accelerations of currents may result while modeling, because

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Longitude °W Fig. 2. The 3-D mesh used to diagnose coastal currents by a numerical model. This grid contains 547 nodes with 1090 finite elements allocated for 21 vertical layers.

of the extremely abrupt slopes and uncertainties for density fields (Escribano et al., 2004a). The analysis of diagnosed circulation was thus emphasized in the near-surface layer which was expected to be mostly driven by wind. In order to assess the capacity of the coastal area to retain the water body and hence potentially the eggs and larvae of fishes a Lagrangian experiment was performed using the Drodge 3 program developed by Blanton (1995). The trajectories of five drifters were simulated for 4 days under steadystate current fields diagnosed by the 3-D model. The influence of changing conditions derived from seasonality, the El Nin˜o and inshore vs offshore locations was tested by analysis of variance applied on the magnitudes of the v and u components of currents obtained from the model outputs.

3. Results 3.1. Oceanographic conditions Wind data showed the dominance of southerly winds (positive values) for all periods and having most of the energy concentrated in the alongshore component (v-component)

(Fig. 3). The time series also revealed the seasonal pattern, characterized by a decline of winds during the winter (August) and a marked intensification during the summer reflected mostly in the v-component. Such pattern is much less marked in the u-component. There were no apparent trends or greatly altered inter-annual changes in the wind regimes during the whole period, although in August 1997 and January 1998 the mean intensity of the most frequent winds appeared slightly reduced (Table 1). The analysis of hydrographic conditions showed marked changes in density fields for the four months, possibly associated with upwelling variability (Fig. 4). August 1995, assumed as a winter-normal condition, was much colder than the other months, and characterized by stronger horizontal gradients when compared to August 1997 at the time the El Nin˜o was present. Under conditions of January, surface waters are subjected to summer warming and strong cross-shelf gradients develop in association with nearshore upwelling of cold subsurface water. January 1996 was colder than January 1998, judging by greater values of surface density. In fact, in January 1998 the El Nin˜o reached its maximal temperature anomaly in the region and sea surface temperature was nearly 24
C (Ulloa et al., 2001; Escribano et al., 2004b).

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v - component (m/s)

12 8 4 0 -4 -8

u - component (m/s)

12 8 4 0 -4 -8 Apr

Aug

Dec

Apr

1995

Aug

1996

Dec

Apr

Aug

1997

Dec

Apr

Aug

1998

Fig. 3. Time series of the north (v-component) and east (u-component) of winds from January 1995 through December 1998. Components were derived from daily winds at 14:00 hours local time. The solid line represents a smoothed low-pass filter to illustrate seasonal and inter-annual variation.

3.2. Numerical modeling Diagnostic circulation, under a steady-state wind forcing, revealed important alongshore flows in the near-surface layer, although with a number of meso-scale (30e100 km) eddies (Fig. 5). Associated with changes in density fields, circulation patterns also showed distinct seasonal features, as well as the influence of the 1997e1998 El Nin˜o. The general pattern of circulation remained about the same for all periods under study, but the velocity fields exhibited strong differences between seasons and during the El Nin˜o vs normal upwelling months. For instance, in August 1997 (El Nin˜o condition) the alongshore water flow was greatly diminished and well restricted to the coastal area. These El Nin˜o effects were also observed when comparing summer conditions, i.e. January 1996 (non-El Nin˜o) vs January 1998 (El Nin˜o). In this case, the large (>100 km) anticyclonic eddy observed in January 1996 becomes dispersed and diminished in magnitude during January 1998 (Fig. 5). The width of the coastal band with alongshore currents also appear to be constrained nearshore during the El Nin˜o (Fig. 5). Table 1 Dominant wind direction and speed estimated at 02:00 PM (local time) at the meteorological station in Airport Diego Aracena of Iquique (northern Chile). Modal values of direction and mean values of speed were estimated for each month from an hourly time series from 1995 through 1998 Date

Wind direction (
)

Wind speed (m/s)

August 1995 January 1996 August 1997 January 1998

210 205 210 214

7.2 7.9 6.6 5.7

There were also remarkable changes in the alongshore flow under different seasonal conditions. Normal conditions in winter (August 1995) were characterized by a northward flow, which became stronger in a narrow (<30 km) coastal band, whereas in the summer (January 1996) this coastal band was more extended offshore and the northward flow is even stronger (Fig. 6). The El Nin˜o conditions affected differently the alongshore flow depending on season. In the winter (August 1997) the alongshore flow becomes drastically reduced in the complete water column, while in the summer (January 1998) the reduction in the northward flow is less intense (Fig. 6). Simulated drifters may illustrate the influence of seasonal and inter-annual variability in circulation. Winter conditions showed a strong capacity of the zone to retain the water body and particles in the nearshore during 4 days under steady-state conditions in the absence of winds. This capacity is even incremented during the El Nin˜o conditions (Fig. 7). In the summer, even though drifters are still maintained within 30 km from the shore, there is a clear increase in alongshore transport, both under normal and the El Nin˜o conditions. During 4 days, passive particles can be transported as far as 200 km northward depending on their initial position with respect to distance to the shore. The trajectories of drifters may not be greatly modified by the effect of winds, but northward flow is more intense at all conditions when wind forcing is imposed (Fig. 8). Seasonal and El Nin˜o effects on Lagrangian drifters can be examined through changes in total distance of their trajectories and mean speed. During the winter (August), the mean speed of drifters decreased from inshore to offshore, both under normal upwelling and during the El Nin˜o. The opposite occurred in the normal summer (January 1996), when speeds

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Normal Upwelling

El Niño conditions

Latitude °S

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August 1997

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January 1996

Latitude °S

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72°

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Fig. 4. Surface density fields (Sigma-T) for four different months in the coastal area of the El Loa River in northern Chile. Contours were constructed from CTD data obtained by oceanographic surveys.

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25 [cm/s] Fig. 5. Diagnostic circulation of the near-surface layer under four monthly regimes under the effect of steady-state dominant wind for each period.

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Fig. 6. Cross-shelf sections of the north-component of currents derived from a 3-D numerical model in the coastal area of the El Loa River in northern Chile. The sections were obtained across the El Loa River location indicated in Fig. 1. Positive values represent northward flow.

notoriously increased from the shore to more oceanic areas. Summer under the El Nin˜o appears to alter this pattern and drifters are accelerated in the nearshore and reduced in speed in the offshore (Table 2). Since simulated drifters move over 3-D current fields, we can estimate the final depth of arrival after 4 days. During winter conditions, except for drifter 1 initially located in the nearshore, the rest of them did not suffer important changes in their initial depth (10 m) after 4 days. In the normal summer, drifters tend to sink notably during their trajectories. During the summer under the El Nin˜o condition, only one drifter was subject to sinking, while the others remained in the upper 20 m (Table 3). The application of the numerical model allowed us to obtain the v and u components and derive the magnitude of the resultant vector for each of the spatial nodes under the four conditions forced by wind. The inshore nodes were assumed as those within 40 km from shore and the rest considered as offshore ones. Mean values for all these parameters and their variation are shown in Table 4. These data clearly showed that the alongshore component controls most the energy of the currents. This component was also significantly stronger at inshore compared to offshore zones (Table 5). Winter and summer conditions significantly differ in magnitude of the vcomponent of currents, but they do not differ in the u-component (Table 5). These differences come from a sharp increase in the alongshore component of currents in the nearshore in association with changes in the mass field, which exhibited stronger cross-shelf gradients in summer conditions (Fig. 4). Increased wind forcing in the summer may also accelerate the currents over the alongshore axis.

According to the analysis of variance, the El Nin˜o influenced differently the alongshore coastal currents depending on season. In winter under the El Nin˜o (August 1997) the magnitudes of the v-component of currents were significantly reduced as compared to a normal winter (August 1995), but in the summer no differences in the v-components were found between January 1996 (normal upwelling) and January 1998 (El Nin˜o). The El Nin˜o also significantly influenced the cross-shelf flow (u-component) (Table 5). This effect is reflected in the offshore advection, which becomes reduced or even reversed during both winter and summer conditions in the inshore area upon El Nin˜o event. 4. Discussion Geostrophic calculations have been a useful tool to obtain patterns of large-scale circulation of major currents along the Chilean coast (Strub et al., 1998; Blanco et al., 2002). These approaches, however, may not yield much resolution for expected currents in the coastal zone, where bottom topography and morphology of the coastline may impose constraints for geostrophic estimates. As demonstrated by numerical modeling (Rodriguez and Lorenzetti, 2001), abrupt topography and coastline geometry might be major causes for vector accelerations near the coast. Despite the above shortcomings, geostrophic estimates can be used to compare our model outputs, because they have provided the only information available on patterns of coastal upwelling circulation for northern Chile. In this respect, the estimates of geostrophic flow in the region show variable patterns of circulation according to season for the same period as

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Fig. 7. The trajectory of five drifters under absence of wind. Drifters were simulated using the Drodge 3 program and the current outputs obtained from a 3-D numerical model. The numbers indicate the initial points (at 10 m depth). Drifters were simulated for 4 days under steady-state conditions.

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Fig. 8. The trajectory of five drifters under the effect of dominant wind for each condition. Drifters were simulated using the Drodge 3 program and the current outputs obtained from a 3-D numerical model. The numbers indicate the initial points (at 10 m depth). Drifters were simulated for 4 days under steady-state conditions.

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Table 2 Total elapsed distance of trajectories of five simulated drifters running for 4 days under steady-state conditions of winds in the area of El Loa River (northern Chile), for winter and summer periods during El Nin˜o (1997e1998) and non-El Nin˜o (1995e1996) conditions. Lagrangian drifters were simulated with Drodge 3 program and the current fields were obtained with a 3-D numerical model Drifter

1 2 3 4 5

August 1995

January 1996

August 1997

January 1998

Distance (km)

Speed (cm s1)

Distance (km)

Speed (cm s1)

Distance (km)

Speed (cm s1)

Distance (km)

Speed (cm s1)

202.4 58.4 19.7 19.8 51.2

59.6 16.9 5.7 5.7 14.8

10.9 41.3 51.4 198.3 209.7

3.2 11.9 14.9 57.4 60.7

51.3 57.4 53.0 34.8 30.6

14.8 16.6 15.3 10.1 8.7

101.3 178.9 224.1 80.8 39.0

29.3 51.8 64.8 23.4 11.3

causing substantial changes in the subsurface thermal structure (Blanco et al., 2002). Subsurface alterations of the density field, associated with intensification of the undercurrent, might explain the remarkable changes in subsurface circulation when comparing El Nin˜o vs normal upwelling patterns, as those illustrated in Fig. 6. Presence of a strong alongshore flow and meso-scale eddies in the coastal zone deserves special attention, because they may greatly determine transport and retention of passive particle and the water mass with its properties. In this respect, upwelling circulation might play an important role. The northward flow may result from the water motion along upwelling plumes, which tend to orient northwest in northern Chile (Sobarzo and Figueroa, 2001), but because they interact with the Peruvian southward current, the resulting orientation becomes northward (Marı´n et al., 2001). Therefore, the northward flow can be mostly maintained by upwelling circulation acting over the Ekman layer. To assess such possibility we constructed vertical profiles of the u and v components of currents from one of the nodes located at 18.5 km from the shore across the El Loa River. The resulting profiles confirm that in most cases cross-shelf currents are weaker than alongshore ones. Also the northward flow is stronger in the upper layer (50 m), where winds’ effects are noticeable and more remarkable in the u-component (Fig. 9). These profiles, however, also show that much spatial variability is expected in the current fields, depending on position over the grid, and this variability probably results from presence of eddies. Therefore, northward transport and recirculation through coastal eddies seem to be the major components for water retention nearshore off the El Loa River. Similar observations

in our study. For example, Blanco et al. (2002) estimated a potential strong southward flow for December 1997 in the coastal zone from 18
S to 24
S, but thereafter they showed a northward flow in March 1998. Our model shows a northward flow in January 1998, but monthly variability makes difficult the comparisons between December 1997 and January 1998, at the time when the strongest Kelvin wave arrived at the zone (Ulloa et al., 2001). Meantime, the circulation pattern of August 1997 from our model coincides with geostrophic estimates of Blanco et al. (2002), who also pointed out that August 1997 was a relaxation period of the 1997e1998 El Nin˜o. Even so, oceanographic anomalies in terms of deepened thermocline, oxycline and high water-column temperature strongly persisted that month in the nearshore, as shown in Ulloa et al. (2001). During the study period, much of the variability in hydrographic conditions, including sea level, SST and presence or disappearance of cross-shelf gradients was related to a progression from a cold (La Nin˜a) situation to a warm event (El Nin˜o) developed between May 1997 and January 1998 in the northern upwelling region off Chile (Escribano et al., 2004b). According to Blanco et al. (2002) coastal currents were characterized by northward flows during normal upwelling conditions, and strong poleward transport during the passages of Kelvin waves upon El Nin˜o, but even during the El Nin˜o (May 1997eJanuary 1998) there was strong intra-seasonal variability reflected in changing patterns of circulation, especially in the nearshore zone (Blanco et al., 2002). Another important hydrographic component affected by the El Nin˜o was the undercurrent. This current associated with the equatorial subsurface waters became much stronger during the El Nin˜o,

Table 3 Depth of simulated drifters after 4 days running under steady-state conditions in the area of El Loa River (northern Chile) and for winter and summer periods during El Nin˜o (1997e1998) and non-El Nin˜o (1995e1996) conditions. Lagrangian drifters were simulated with Drodge 3 program and the current fields were obtained with a 3-D numerical model. NoW and W indicate absence and presence of wind forcing, respectively Date Drifter

1 2 3 4 5

August 1995

January 1996

August 1997

January 1998

NoW

W

NoW

W

NoW

W

NoW

W

55 6 6 6 5

8 6 8 8 8

50 298 136 150 62

19 340 20 271 15

6 11 8 10 10

4 6 6 10 10

9 499 10 8 8

16 242 20 8

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Table 4 Mean values and standard deviation of current parameters off El Loa River (northern Chile), as estimated from a 3-D numerical model for winter and summer periods during El Nin˜o (1997e1998) and non-El Nin˜o (1995e1996) conditions. Parameters u and v are the east (cross-shelf) and north (alongshore) components of currents, respectively. Mean values were obtained from the total nodes of the grid used by the model Period

Inshore (20 nm), n ¼ 328

Offshore (>20 nm), n ¼ 219

Component u

August 1995 January 1996 August 1997 January 1998

v

Mean

SD

Mean

SD

0.11 0.17 0.00 0.09

0.741 1.497 0.135 0.652

0.32 0.68 0.13 0.69

0.432 0.836 0.227 0.837

Magnitude mean

Direction mean

0.34 0.70 0.13 0.69

108.189 103.775 89.913 82.477

were made in other locations at northern Chile, such as in Antofagasta (23
S) (Marı´n et al., 2001; Escribano et al., 2004a). This retention mechanism differs from those described for other upwelling regions, in which water retention nearshore may be favored by opposing vertical flows expressed as an offshore Ekman transport and a subsurface compensatory flow. This vertical flow-structure may even allow recirculation of plankton advected offshore from the upwelling centers (Hutchings et al., 1995). Although plankton may be subject to strong spatialetemporal variability promoting either retention or advection in the nearshore off Chile during upwelling (Giraldo et al., 2002; Narva´ez et al., 2004). In any case, coastal recirculation in surface well-oxygenated waters may be more advantageous in the coastal zone off northern Chile subjected to an intense and shallow (<50 m depth) oxygen minimum zone that constraints the vertical distribution of plankton and fish larvae (Morales et al., 1996, 1999). Our model revealed the persistence of anticyclonic gyres in the coastal area off the El Loa. These hydrographic features may allow recirculation in the nearshore zone. Passive particles that are transported northward might become accumulated in the northern portion of our study zone under the action of these eddies, because from there they are forced to return southward. However, the alongshore flow may also promote particle dispersion along the coastal band. Thus, retention over the entire coastal area should result from the interplay

Table 5 Analysis of variance to test the effects of season, the El Nin˜o and location (inshore vs offshore) on the magnitude of coastal currents estimated by a 3-D numerical model for winter and summer periods during El Nin˜o (1997e1998) and non-El Nin˜o (1995e1996) conditions. Mag is the vector resultant magnitude, whereas v and u are magnitudes of the alongshore and cross-shelf components of currents, respectively F-ratio

P-value

Source of variation

Variable

El Nin˜o

Mag v u

21.203 6.400 10.127

0.000 0.011 0.001

Season

Mag v u

128.217 89.304 0.287

0.000 0.000 0.593

Location

Mag v u

218.771 470.007 1.634

0.000 0.000 0.201

Component u

v

Mean

SD

Mean

SD

0.02 0.01 0.01 0.01

0.110 0.214 0.034 0.164

0.01 0.02 0.01 0.07

0.087 0.320 0.039 0.146

Magnitude mean

Direction mean

0.02 0.03 0.01 0.07

143.66 112.04 159.01 77.73

between alongshore dispersion and recirculation. This process allows the existence of an efficient mechanism to retain spawning products of fish and hence explain dense aggregations of fish eggs and larvae in the coastal zone off El Loa River, as reported by several ichthyoplanktonic surveys performed over a 10-year-period (Oliva et al., 2000). However, not only can spawning products be maintained nearshore, but also phytoplankton and zooplankton which sustain larval feeding, thus assuring larval survival in terms of food availability. All these conditions make this fish spawning area highly favorable for larval survival and recruitment success. One of the main fishery resources in northern Chile is the anchovy, Engraulis ringens (Alheit and Bernal, 1993). The main spawning period takes place during late winter (August), under conditions of reduced advection, although spawning may keep going through the spring and early summer (Castro et al., 2000). Once spawned, anchovy eggs develop to hatching in about 3e4 days, depending on water temperature (Lo, 1985). Our model predicts that during that time eggs can be transported as far as 200 km northward, although still maintained nearshore. In winter conditions, the eggs are also maintained in the upper layer, in contrast to a summer situation during which they tend to sink while drifting. Lower temperature and presence of very low oxygen concentration at depths greater than 20 m in the nearshore, because of the shallow oxygen minimum zone (Morales et al., 1999), may negatively impact larval survival in the summer. Thus, winter spawning seems more favorable from the viewpoint of eggs and larval survival. All these predictions, however, should be considered with caution, because of time-dependent variability. Potential sources of short-term variability may arise from changing wind conditions, or tidal forces. The use of dynamical models might account for this variability (e.g. Lynch et al., 1998; Mesı´as et al., 2003). Nevertheless, the temporal scale to impose time-dependent variability is a matter of further studies and this kind of models may represent a second step for analyzing upwelling circulation in a region where wind-driven upwelling is nearly permanent year-round (Escribano, 1998; Blanco et al., 2001). The influence of wind forcing in our model outputs may deserve some further considerations. The intensity and direction of winds are subject to high variability in northern Chile on various time scales, from hours to seasons (Vergara, 1991; Escribano and Hidalgo, 2001). Thus steady-state conditions

W. Palma et al. / Estuarine, Coastal and Shelf Science 67 (2006) 93e107

Depth (m)

u - component

v - component

0

0

-40

-40

August 1995

-80

Normal upwelling Depth (m)

-0.1

-0.05

0

0.05

0.1

0

0

-40

-40

0

0.4

0.8

0

0.4

0.8

0

0.4

0.8

January 1996 -80

-80

-120

-120

-0.1

Depth (m)

-80

-120

-120

El Niño conditions

105

-0.05

0

0.05

0.1

0

0

-40

-40

August 1997 -80

-80

-120

-120

-0.1

-0.05

0

0.05

0.1

0

0

Without wind With wind

Depth (m)

-40

-40

January 1998 -80

-80

-120

-120

-0.1

-0.05

0

0.05

0.1

0

0.4

0.8

Component (m/s) Fig. 9. Vertical profiles of the east (u-component) and the north (v-component) components of currents at 18 km from shore across the El Loa River in northern Chile. Currents were derived from outputs of a 3-D numerical model under four seasonal and inter-annual conditions. Continuous and dotted lines represent conditions under absence and presence of dominant wind, respectively.

106

W. Palma et al. / Estuarine, Coastal and Shelf Science 67 (2006) 93e107

of winds are hardly expected. Winds may also vary in the space and variation in the wind field could impose spatial variability in circulation of the upper layer. The use of wind fields, however, did not produce different circulation patterns in the coastal zone off Antofagasta (23
S), northern Chile when comparing results with the use of a constant dominant wind (Escribano et al., 2004a). Therefore, it seems that wind data from the fixed station at the airport may well represent the more general wind conditions for the coastal zone. Temporal variability of winds may be more difficult to account. The seasonal cycle seems more stable from year-to-year, but day-today and intra-seasonal variability can be substantial (Fig. 3). Over short-term scales (daily) wind variation may affect upwelling circulation in a common scale of 3e7 days in northern Chile (Marı´n et al., 2001), allowing potential steady-state situations not longer than this time over the meso-scale spatial extension. Therefore, 4-day drifting, assuming a steady-state condition seems reasonable to assess potential transport of passive particles. Inter-annual variability has also been shown to cause changes in wind regimes in the eastern south Pacific (Blanco et al., 2002; Mesı´as et al., 2003). Our data showed a slight decrease in wind intensity during El Nin˜o, during both months considered (August 1997 and January 1998), but a more global analysis of wind patterns in the eastern south Pacific from satellite data (ERS1 and ERS2) has recently shown that wind anomalies during the 1997e1998 El Nin˜o were present mainly at the equatorial region, but not noticeable at 20
e23
S (Escribano et al., 2004b). Blanco et al. (2002) also found weak positive and negative anomalies of alongshore wind during 1997e1998 using data from coastal stations. Decreased wind during this El Nin˜o mostly occurred farther south, at central/south of Chile (Mesı´as et al., 2003). In fact, our wind data from the fixed station did not clearly reveal a decreased wind pattern during 1997e1998, coinciding with the analysis of satellite wind data for this zone described in Escribano et al. (2004b). Thus, it is likely that a slight reduction of wind intensity during August 1997 and January 1998 shown in Table 1 was not important for the seasonal and annual regimes. In any case, wind forcing did not clearly affect model outputs of circulation and resulting differences from a normal upwelling year were apparently caused by changes in water mass structure, probably in terms of altered temperature gradients. Although winds acting on the surface layer may also modify the density fields and temperature gradients, a major cause of the altered water mass during the El Nin˜o is probably due to advection driven by remote-equatorial forcing. This phenomenon causes the intrusion of oceanic warm SSW into the coastal area of northern Chile, inhibiting the upwelling of cold subsurface water (Arntz and Fahrbach, 1996; Escribano et al., 2004b). If this situation occurs during the winter, then alongshore transport becomes diminished according to our results. We do not know the consequences of reduced northward transport for fish eggs and larvae, but it may be that larval dispersion along the coast is necessary to enhance survival. In the same context, cross-shelf flow becomes reduced under El Nin˜o effect as well, and this can also result from greatly diminished

density gradients upon presence of oceanic waters in the nearshore. Nevertheless, the impact of reduced offshore advection during El Nin˜o on anchovy spawning and larval success may need further studies. Numerical modeling has proven of high utility to diagnose coastal currents (Lynch et al., 1992), and examine the role of physical forcing in modifying coastal flows (Rodriguez and Lorenzetti, 2001; Escribano et al., 2004a). Further applications have dealt with physicalebiological coupled modeling to study the dynamics and ecology of populations in the coastal ecosystem. Studies in zooplankton populations (Miller et al., 1998; Lynch et al., 1998) and in fish larvae ecology (Werner et al., 1996) are some examples. Larval ecology of economically important fishes at the upwelling region of northern Chile is to be considered in the near future to develop biophysical modeling. In that context, our model should be considered as a first step to evaluate the role of the physical environment in providing conditions for a highly advantageous fish spawning area. Acknowledgements Oceanographic data were obtained from cruises carried out by the Fisheries Research Institute IFOP (Instituto de Fomento Pesquero de Chile) through the projects ‘‘Bio-oceanographic Monitoring of Northern Chile’’ and the ‘‘Anchovy Recruitment Project’’. Research of R. Escribano and S. Rosales is being supported by the Center for Oceanographic Research in the eastern south Pacific (COPAS) funded by the FONDAP program of CONICYT Chile. Three anonymous reviewers have greatly helped clarify ideas to improve earlier versions of this work. References Alheit, J., Bernal, P., 1993. Effects of physical and biological changes on the biomass yield of the Humboldt Current Ecosystem. In: Alexander, L., Gold, B.D. (Eds.), Large Marine Ecosystems. V: Stress, Mitigation and Sustainability. Advancement of Science press, USA, pp. 53e58. Arntz, W.E., Fahrbach, E., 1996. El Nin˜o Experimento Clima´tico de la Naturaleza. Fondo de Cultura Econo´mica, Mexico, DF, 312 pp. Blanco, J.L., Thomas, A.C., Carr, M.-E., Strub, P.T., 2001. Seasonal climatology of hydrographic conditions in the upwelling region off northern Chile. Journal of Geophysical Research 106, 11451e11467. Blanco, J.L., Carr, M.-E., Thomas, A.C., Strub, P.T., 2002. Hydrographic conditions off northern Chile during the 1996e1998 La Nin˜a and El Nin˜o. Journal of Geophysical Research 107. doi:10.1029/2001JC001002. Blanton, B.O., 1995. User’s Manual for 3-Dimensional Tracking Drogue Program on a Finite Element Grid with Linear Finite Elements. Ocean Processes Numerical Methods Laboratory Report UNC 95-1, USA, 15 pp. Castro, L.R., Salinas, G.R., Hernandez, E.H., 2000. Environmental influences on winter spawning of the anchoveta Engraulis ringens off Central Chile. Marine Ecology Progress Series 197, 247e258. Escribano, R., 1998. Population dynamics of Calanus chilensis in the Chilean Eastern boundary Humboldt current. Fisheries Oceanography 7, 245e251. Escribano, R., Hidalgo, P., 2001. Circulacio´n inducida por el viento en Bahı´a de Antofagasta, norte de Chile. Revista de Biologı´a Marina y Oceanografı´a, Chile 36, 43e60. Escribano, R., Rosales, S., Blanco, J.L., 2004a. Understanding upwelling circulation of Bahı´a Antofagasta (northern Chile): a numerical modeling approach. Continental Shelf Research 24, 37e53.

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