Seasonal variability of coastal upwelling and the upwelling front off central Chile

Abstract

[1] We analyze sea surface temperature (SST) and chlorophyll a (Chl a) satellite data to study the seasonal variability of the upwelling off central Chile. Data from an oceanographic cruise are used to illustrate the vertical characteristics of the upwelling front and the geostrophic flow. The mean offshore extension of the SST front in summer is $110 km. There, SST changes from $13.5°C near the coast to 14.5°-15°C offshore. Hydrographic data show that the thermocline becomes progressively shallower toward the coast but that at a certain distance it abruptly bends to form a horizontal front that extends until $50 m depth. Satellite data show that the Chl a decays exponentially offshore, and a concentration of $1 mg m À3 indicates the offshore limit of the frontal region. The SST front observed most of the year is absent in winter, when the coastal water is only slightly colder than the oceanic one; however, relatively higher Chl a values remain present near the coast during winter. Northwest of Punta Lavapie (37°15 0 S), the frontal region is located progressively farther offshore, suggesting that a branch of an equatorward surface jet separates from the coast to form a meander, which is associated with a filament of cold water and an anticyclonic eddy observed frequently in this zone during summer. Hydrographic data show that isotherm and current disturbances related to mesoscale eddies can extend several hundreds of meters below the surface. In contrast, the coastal jet is quite shallow, extending until $150 m depth.

Time-Space Variability of Chlorophyll-a and Associated Physical Variables within the Region off Central-Southern Chile

“…The general pattern of Chl-a distribution described in this study is consistent with previous satellite-based results using coarser time-space resolution data for the region off central-southern Chile [6,22]. The analyses of SeaWiFS time series data (1998-2005, monthly means) in the region between 33 and 40°S [22] indicated that, albeit the spatial variability found between seasons, the mean offshore location of the Chl-a isoline of 0.5 mg•m −3 ranged between ~100 and 200 km from the coast, further offshore in the area north of P. Lavapié (~37°S).…”

“…The analyses of SeaWiFS time series data (1998-2005, monthly means) in the region between 33 and 40°S [22] indicated that, albeit the spatial variability found between seasons, the mean offshore location of the Chl-a isoline of 0.5 mg•m −3 ranged between ~100 and 200 km from the coast, further offshore in the area north of P. Lavapié (~37°S). Furthermore, this study suggested that the Chl-a isoline of 1 mg•m −3 represents the offshore limit of the upwelling thermal front; in turn, this isoline is mostly found within the first 100 km of the coast [10].…”

“…In the offshore region, wind stress curl is mostly anticyclonic (downwelling favorable) whereas in the coastal zone is mostly cyclonic (upwelling favorable) and is intensified during the spring-summer period [27]. The alongshore circulation pattern in the region includes an equatorward coastal current with characteristics of a jet (>10 cm•s −1 ), especially strong during the spring-summer period [22,28,29]. Mesoscale activity amplifies the annual cycle of satellite Chl-a in the region off central-southern Chile [10].…”

“…In the HCS, the patterns of spatial and temporal variability of surface Chl-a have been described in previous studies using different satellite datasets and time-space resolutions, as well as different periods of analysis and/or regions of study [3,6,8,21,22]. In the most recent satellite color time series study for the HCS (11 years: 1997-2007; 10-40°S), the annual and interannual scales of variability in Chl-a were found to be dominant [10].…”

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

“…Coastal upwelling and solar radiation promote the generation of an upwelling front in this region. Variations in the coastline orientation, bathymetry, general circulation, and mesoscale activity also influence front and eddy generation [56][57][58]. In the area off Point Lavapié, several cyclonic and surface/subsurface anticyclonic eddies are persistently generated in association with higher levels of eddy kinetic energy, and these features move seaward through the CTZ at a relatively low speed (~1.7 km d −1 ) [15,30,32,59].…”