the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.associated with cold-water filaments, seem to play an important role not only in relation to thebiological productivity of coastal upwelling regions but also as mechanisms for the retention ofcoastal planktonic species. If account is taken of the fact that Mejillones Peninsula is located inthe area where upwelling-favourable winds occur year-round, then those areas may constituterecurrent retention zones.Farther south, within the HCS area where upwelling is more seasonal, the Coquimbo BaySystem (30°S) is another important coastal upwelling centre. This system is located within twocoastal points (Punta Lengua de Vaca and Punta Pájaros) and is the site of intensive fisheries andeco-tourism (Ramírez 2005b). Filaments, including bifurcated upwelling filaments (Moraga et al.2001), are known to generate at Punta Lengua de Vaca, contributing with cold, nutrient-high watersto the coastal system (Montecino & Quiroz 2000, Montecino et al. 2005). A recent Lagrangianstudy conducted within the bay (Marín & Delgado 2007) shows that the dominant equatorwardflow is modulated at quasi-inertial frequencies, enhancing the coastal retention times. Furthermore,Marín et al. (2003a) have shown that cold-water squirts generate at the northern end of the baysystem (near Punta Pájaros). Squirts seem to be geographically anchored to locations meeting therequirements for their generation (Strub et al. 1991). Thus, it is highly likely that indeed the squirtsobserved in the vicinity of Punta Pájaros are normally generated within the same area, makingthem a recurrent mesoscale flow feature. As a test of this idea a simple model was built using theROMS model (Shchepetkin & McWilliams 2005) and initialised with the ROMSTOOLS package(Penven 2003). The study area was defined by longitudinal boundaries set at 74°W and 70°W andlatitudinal boundaries at 32°S and 24°S (see also Figure 3). This model has been run for 2 yr inother eastern boundary systems such as the California Current System (Marchesiello et al. 2003)to obtain a statistical equilibrium condition. However, in the present test case a squirt was generatedjust after 17 days of integration, which closely resembles the squirt observed in an SST imageobtained in January 2005 in shape, size and location (Figure 3). The interesting, and indeedserendipitous, observation resulting from this numerical exercise is that transient modes developedas perturbations (e.g., baroclinic instabilities) from climatological mean conditions generate coastalflow structures (squirts) which are recurrently found within the HCS. Satellite observations andLagrangian drifter data (Marín & Delgado 2007) have in fact shown that this squirt is a recurrentfeature in the area, reaching distances on the order of 140 km offshore. Squirt speeds, estimatedboth through feature-tracking analysis (Marín et al. 2003a) and Lagrangian drifters (Marín &Delgado 2007), range between 0.2 and 0.3 m s −1 . Thus, considering that the lifetime of a singlesquirt is related to the active period of equatorward wind events, which for the area range between3 and 7 days (Rutllant et al. 2004a), coastal organisms trapped within the squirt are likely to reach100–200 km offshore in a period of less than a week.The conclusions that may be offered as a result of this brief, and highly condensed, analysisof the prevailing mesoscale coastal features of the HCS are nevertheless far reaching. In the firstplace, if these features are intrinsic to the HCS (generated as a result of coastline geometry, bottomtopography and prevailing flow conditions including perturbations) and not dependent on largescale,low-frequency forcing (e.g., ENSO), then a whole new array of multiscale (nested) modelsare necessary to generate predictable bio-oceanographic coastal patterns for the HCS. Second,coastal upwelling areas can be described as a dynamic mosaic of nearshore retention/offshoreexpatriation patches or sectors. Thus, for a planktonic organism, remaining in a specific location(i.e., local populations) within the HCS coastal zone becomes a probabilistic process. If, forexample, larvae are entrained within an upwelling shadow then there is a high chance that theywill remain in the same sector for a period close to a week. If, on the other hand, the larvae areentrained within a squirt, then in a period close to 24 h they will be expatriated offshore. However,since the seascape is dynamic and depends upon the upwelling condition, it is not possible to ensurethat a given geographic location will always act as a retention or expatriation locality.206
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILE26°S73° 72° 71°28°S28°28°30°S29°29°73°W 72°W 71°W30°30°73° 72°71°60 0 60 120 180 KilometersFigure 3 (See also Colour Figure 3 in the insert following page 344.) Result of the squirt simulation by meansof the Regional Oceanic Modeling System (coloured image) for 17 January in comparison to the thermalsquirt (NOAA satellite, black-and-white image) observed on 15 January 2005. Temperature coding of thecoloured image goes from blue (lower temperature) to red (higher temperature) while for the NOAA imageit goes from lighter to darker tones of grey. White areas in the NOAA image correspond to clouds. The studyarea in the model was defined by longitudinal boundaries set at 74°W and 70°W and latitudinal boundariesat 32°S and 24°S. The resolution was 1/10°, approximately 10 km, with 20 vertical levels, a minimum depthof 50 m and a smoothed bathymetry. The baroclinic time step was set to 900 s. Surface forcing (wind stress,heat fluxes, freshwater flux, SST, sea-surface salinity, and short-wave radiation) was derived from the monthlyclimatology found in the COADS dataset (Da Silva et al. 1994) and interpolated into the model grid. Thetemperature and salinity initial conditions for the model were obtained for the month of January from the‘World Ocean Atlas 2001’ (Conkright et al. 2002). The Ekman and geostrophic velocities at the boundarywere obtained combining these data with COADS winds, following Marchesiello et al. (2001), with level ofno motion defined at 500 m (Marchesiello et al. 2003).The water column chemistryThe northern Chilean ocean margin (~18–30°S) presents distinctive characteristics in topography,climatic and oceanographic conditions, which modulate PP and water column chemistry. It featuresa comparatively low PP (for the HCS), and despite semi-permanent wind-driven upwelling someareas are considered as high-nutrient low-chlorophyll (HNLC) environments (Torres 1995, Daneriet al. 2000). These observations are in contrast to the localised prominent upwelling cells withrelatively high PP, such as off Iquique (21°S), Antofagasta (23°S) and Coquimbo (30°S) (0.5–9.3 gC m −2 d −1 ; González et al. 1998, Daneri et al. 2000, Thomas et al. 2001a).Studies carried out in the main upwelling centres indicate that highest PP foci occur close tothe coast over the narrow continental shelf and fuel major fisheries, with catches that represent207
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