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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000300008 

  Gayana 68(2): 358-362, 2004

SPATIAL AND TEMPORAL VARIABILITY OF THERMAL FRONTS OFF CENTRAL CHILE (33-40S)

 

Letelier, J.1,3, O. Pizarro1, S. Núñez2 & D. Arcos2

1. Departamento de Física de la Atmósfera y el Océano (DEFAO) y centro FONDAP-COPAS, Universidad de Concepción, Chile.
2. Instituto de Investigación Pesquera, Talcahuano, Chile
3. Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, Chile.


ABSTRACT

During the upwelling season along the coast off central Chile (33-40S), cold upwelled water can be easily distinguished from warm oceanic water using satellite infrared images. The boundary between cold and warm surface waters is a relatively narrow region with large horizontal gradients, denominated upwelling front, which is also clearly visible in satellite images. Here, using daily NOAA satellite infrared images from the period 1998-2000, we described the spatial distribution of mesoscale features related to upwelling fronts. Together with infrared data, we use SeaWiFS's color images to explore the relationship between Chlorophyll concentration and thermal upwelling fronts. Our results show that thermal fronts have maximum intensities (>0.2C/km) in spring and summer and they are typically located between 30 and 300 km off the coast. The offshore extension of the upwelling front was usually larger between 35-37S, related to the presence of a large anticyclonic eddy in this region during the warm season. In general, chlorophyll was well correlated (inversely) to SST showing similar offshore variability scale and covariance in mesoscale features like eddies and filaments.


INTRODUCTION

Upwelling fronts are characterized by intense horizontal gradients in different oceanographic properties (Bowman & Esaias, 1978; Fedorov, 1985). These gradients result from the steep inclination toward the surface of the pycnocline near the coast, induced by offshore Ekman transport (Smith, 1968; Smith, 1994). Satellite infrared images have revealed that upwelling regions are associated with very rich mesoscale structures, such as eddies and filaments of cold water. Particular features in the coastline and bottom topography, together with spatial changes in the alongshore wind stress and alongshore current, have been related to these structures. Off central Chile (33-40 S) winds are upwelling favorable during the austral spring and summer. Through these seasons, several studies have shown the presence of large filaments and eddies covering spatial scales that span from a few kilometers to a few hundreds of kilometers (Fonseca & Farías, 1987; Cáceres, 1992; Sobarzo & Figueroa, 2001). Along this region, the coastal transition zone (CTZ), as well as the coastal ocean, are characterized by a very high biological productivity (Thomas, 1999; Hormazabal et al., 2004), probably related to eddies and filaments that transport coastal water offshore. Here, using satellite information of sea surface temperature (SST) and chlorophyll concentration, we analyze the coastal upwelling and the upwelling front variability and their typical offshore extension off central Chile.

METHODOLOGY

We use 379 high-resolution (1.1 km) infrared images from the AVHRR sensor from the NOAA satellite series of a region spanning 7 of longitude, from 33 to 40S during the period 1998-2000. These images were captured and georeferenced in the Instituto de Investigación Pesquera (INPESCA). Each image was filtered using a 2 dimensional triangular median filter of 5 weights to eliminate high frequency noise in the SST spatial distribution. Five pixels around the clouds were removed to discard transition pixels between clouds and valid SST data. We calculate the mean SST and variance in each location for the entire period, and for winter and summer. Thermal surface gradients were calculated for each image. Based on these daily gradient matrixes seasonal and total (for the entire study period) mean values were estimated. Meridional (Zonal) SST gradients were calculated by minimum-square slope fitting of a moving meridional (Zonal) box of 5x3 (3x5) pixels through the entire image. The gradient magnitudes were used to identify frontal zones within the study region.

RESULTS

The spatial distribution of the SST mean and variance for the entire period (1998-2000) are presented in fig. 1. From the SST distribution we can clearly differentiate the most important upwelling centers in the study zone, which are related to lower coastal temperature, namely Punta Topocalma (3409'S), Punta Nugurne (35°57'S), Punta Lavapié (37°08'S) and Punta Galera (4000'S). The observed SST range goes from values higher than 9C near the coast, and especially near upwelling centers, to values less than 18C in the oceanic area northwest of the zone of study. In general, isotherms are approximately parallel to the coast, but isotherms colder than about 14.5C are projected offshore near 35° 30' S - 37 S, while isotherms of 13 C are quite associated with the 200m isobath (the offshore limit of the continental shelf) in this region. The spatial distribution of the SST variance shows lower values near the coastal band, coinciding with lower SST mean values. The spatial distribution of the summer SST mean (Fig. 2, left) is similar to that observed in fig. 1, but warmer. In this case, the isotherms associated with the 200 m isobath is near 14C, while the isotherms denoting 15, 15.5 and 16C project offshore from the shelf. In the mean distribution of winter SST, the meridional SST contrast observed in summer, disappears and only a latitudinal SST is observed (Fig. 2, right) with lower temperatures toward the southern region. The mean SST during winter goes from 9 to 14C.


 
Figure 1. Mean sea surface temperature (C)(left)and variance of SST(right) for the 1998-2000 period in central Chile (33-40S)


 
Figure 2. Summer mean SST (C) )(left) and winter mean SST (C) (right) with main upwelling points in central Chile (33-40S).

The surface distribution of summer mean gradients (Fig. 3) shows maximum values of about 15 x 10-2 (C/km). Gradients larger than 9 x 10-2 (C/km) are usually observed near the coast; so they characterize well the limit of the coastal zone. North of 35 S gradients are close to the breaking of the continental shelf, while south of 35S, along the region where isotherm extend offshore, intense gradients are projected far offshore describing circles with diameters that range from 12 to 70 km.


 
Figure 3. Summer(left) and winter(right) mean thermic fronts (C/km) off central Chile (33-40S).

DISCUSSION

Intense fronts and filaments observed in the south-central region (33-40S) off Chile are associated with coastal upwelling processes that occur at various points along the coast (Kelly & Blanco, 1984; Fonseca & Farías, 1987; Cáceres, 1992; Sobarzo, 1994). These upwelling centers are related to the presence of irregular points along the coastline and they are characterized by having average SSTs lower than those observed in the rest of the zone during the upwelling season. SST in these centers is rather similar to common values observed during winter in the entire zone (Fig. 1, 2). Ekman transport generates a coastal band of cold water, which is characteristic of the eastern edges of oceans (Hill et al., 1998). Spatial SST variance distribution (Fig. 1, right), shows that in the upwelling areas, SST variability is less than that observed in the oceanic region, since this last region is strongly influenced by the seasonal cycle of the short wave radiation. In contrast, due to the enhancing of the upwelling of cold water during spring and summer, coastal areas remain relatively cold during the whole year.

In the south-central zone off Chile, like in other upwelling regions, upwelling fronts have a surface manifestation as thermal gradient. So they are observed in satellite infrared images as a narrow band that covers tens to hundreds of kilometers along the coast. Due to the seasonal cycle of the SST in the offshore region, thermal fronts are predominantly observed during the upwelling season (spring and summer), corresponding to the period of strengthening of the equatorward wind stress component along the coast (Bakun & Nelson, 1991).
The mean thermal intensity of these fronts is similar to those described in the upwelling zones of Canary, Peru, and Benguela currents (Table 1). In general, thermal magnitude of these fronts is influenced by the intensity of the upwelling and by the stratification during the upwelling season, which is intensified by the seasonal SST cycle.


Table 1. Typical surface gradient values for temperature, salinity, and sigma-t, modified by Fedorov (1985).

Upwelling Values
Areas °C/km Ppt/km Sigma-t/km

Oregon 1.0-4.0 0.7-2.0 0.1-1.0
South Africa 0.5-1.0 0.1-0.2 0.02-0.15
Canaria 0.05-0.5 0.01-0.1 0.01-0.12
Perú 0.1-0.2 0.01-0.02 0.01-0.03

The spatial distribution of mean summer thermal fronts (Fig. 3) considering gradients smaller than 6 x 10-2 (C/km) no significant, shows important latitudinal differences, which are also clear when we take average values among different latitudinal bands (Fig.4). North of 35S, thermal fronts (gradients > 6 x 10-2 (C/km)) are limited to a distance less than 130 km from the coast, forming an area without fronts in the intermediate oceanic zone (130-300 km from the coast, Fig. 3). Between 35-37S, fronts do not seem to be restricted to this zone; in fact, it is observed that they are significant along the entire meridional domain (Fig. 3, 4). Between 37 and 39S the large western extension of the fronts is also observed, although in a more irregular way. This latitudinal increase of the frontal zone extension away from the coast could be explained by changes in the orientation of the coastline , and by the increase of the width of the continental shelf (Fig. 4), which could also reinforce upwelling and therefore, offshore projection of the upwelling fronts (Crépon et al., 1984; Sobarzo, 1999). This influence increases from north to south, reaching a maximum near 36S-37S, where the fronts develop a larger extension toward the west.


 
     
 
     
 
     
Figure 4. Cross-shelf summer and winter mean SST(C-red), thermal fronts(C/km-blue) and Clorophyll-a (mg/m3-green) whit mean batimetry of the section. Section 33-35S (upon), section 35-37S (center) and section 37-39S (below). The values were computed using summer and winter data from the 1998-2000 period.

During summer thermal front intensity, estimated using gradients larges than a 9 x 10-2 (C/km), and chlorophyll concentration have similar offshore decay scales. The scales are similar to those estimated from the offshore SST increasing along the different latitude showed in Fig. 4, which seem to be related, in turn, to the width of the continental shelf. These length scales can be used to estimate the offshore limit of the upwelling region.

 

ACKNOWLEDGMENTS

We thank Mr. Ricardo Uribe for his help in the development of this paper and MSc. Gabriel Yuras for his help in the acquisition and processing of color images. Especial acknowledgments to the Instituto de Investigacion Pesquera for permitting the use of SST information.

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