Deep Sea Research Part I: Oceanographic Research Papers
CO2 outgassing off central Chile (31–30°S) and northern Chile (24–23°S) during austral summer 1997: the effect of wind intensity on the upwelling and ventilation of CO2-rich waters
Introduction
Wherever surface winds produce an oceanic divergence (in mid-ocean or off the coast), subsurface water is transported to the surface. In coastal areas this upwelled water is normally cold, rich in nutrients and CO2, and poor in oxygen. So, it could be expected that the initial effect of an upwelling pulse would be upward and downward air–sea fluxes of CO2 and O2, respectively. However, enhanced primary production due to the nutrient stimulus in the euphotic zone will tend to balance or even reverse the CO2 and O2 fluxes.
Since the release and sequestering of CO2 in the ocean depends on a range of complex processes and factors (e.g. photosynthesis, respiration, carbonate system chemistry, stratification, mixing, etc.), the resulting balance at any particular place and time can not be accurately predicted yet. Thus, the direct assessment of CO2 parameters is required to identify where, when and how these fluxes occur and to understand their variability. This is especially true for coastal upwelling systems, where observations of CO2 parameters are scarce and the spatio-temporal variability of fCO2 distribution is typically high (Simpson and Zirino, 1980; Copin-Montégut and Raimbault, 1994; Torres et al., 1999; Geen et al., 2000).
Among the most typical characteristics of eastern boundary current systems are coastal upwelling, a poleward undercurrent associated with the slope and a shallow oceanic oxygen minimum layer (OML). The strongest OML is located off the west coast of South America, extending commonly from the lower portion of the thermocline to more than 400 m depth (Antezana, 1978). Near the coast the OML coincides with the location of the Peru–Chile poleward undercurrent (Wooster and Gilmartin, 1961), whose large latitudinal range was early inferred by Brandhorst (1971).
The oxygen-deficient waters of the OML, which are also strongly CO2 supersaturated, are therefore advected polewards and at the same time transported towards the surface through deep upwelling off Peru (e.g. Copin-Montégut and Raimbault, 1994) and off Chile (e.g. Torres et al., 1999). However, the intensity of the upwelling varies in space and time, sometimes producing a weak upwelling of “thermocline waters” and in other cases producing deeper upwelling of the colder and saltier water (equatorial subsurface water, ESSW) that characterises the poleward undercurrent.
On a scale of weeks to months the thermohaline structure of the coastal water column can be determined by passing coastal-trapped waves (CTW) of remote origin (e.g. Shaffer et al., 1997), but the intensity of the local upwelling is strongly related to the alongshore wind stress on the surface ocean. Therefore, large latitudinal variations in the oceanic winds along the Chile–Peru coast (Bakun and Nelson, 1991; Shaffer et al., 1999) might be expected to produce an analogous variability in the upwelling and ventilation of CO2-rich waters. A full test of this hypothesis would involve a synoptic, long-term comparison of the CO2 air–sea fluxes in upwelling centres scattered along the Chile–Peru coast, corrected for the remote forcing of the upwelling that propagates polewards along the coast.
We present here a first exploratory test of this hypothesis based on the results of two surveys, one carried out near Antofagasta (northern Chile, 23°S) and one near Coquimbo (central Chile, 30°S), during January and February 1997.
Section snippets
Oceanographic stations
Discrete seawater samples and CTD data were collected onboard R.V. Abate Molina during austral summer 1997 off northern Chile (22.7–24.0°S, January 1997) and central Chile (29.9–30.7°S, February 1997) (Fig. 1A). The cruise off northern Chile, near Antofagasta, consisted mainly of the two grid surveys G1 and G2 (Fig. 1B). The grid surveys comprised 31 stations each arranged as five 140 km long east–west transects covering an area of approximately 21 000 km2 (Fig. 1B). The cruise off central Chile,
Time series of winds and hydrographic variables
In the two study areas (23°S and 30°S) both the coastal and oceanic winds were upwelling favourable during the study period (Table 1 and Fig. 3A and B), but stronger at 30°S than at 23°S (Table 1). The offshore intensification of the meridional wind components also promoted upwelling in both areas (Table 1). The coastal equatorward winds follow a prominent diurnal cycle, particularly in the austral summer, because they are partially forced by land–sea temperature differences. As expected, the
Latitudinal variation in the CO2 outgassing caused by coastal upwelling
Along north-central Chile, oxygen-poor and CO2-rich waters are located below the pycnocline. Thus the erosion of the pycnocline near the coast by upwelling and mixing cause a drastic CO2 outgassing near the coast. The average alongshore equatorward winds, which promote upwelling, have previously been found to increase from minimum values at about 20°S to maximum values at about 30°S (Bakun and Nelson, 1991; Shaffer et al., 1999), in agreement with our observations. This wind pattern explains
Acknowledgements
We thank Madeleine Hamamé, Carolina Parada and Alvaro Sotomayor for chlorophyll a analyses from the Coquimbo survey. We thank Oscar Pizarro and Nathalie Lefèvre for comments and suggestions. We thank the Department of Inorganic Chemistry and EULA-chemistry Laboratory at the University of Concepción for provision of laboratory facilities. We thank the Départment d’Oceanographie Spatiale IFREMER France, Sea Level Center at the University of Hawaii, and the Jet Propulsion Laboratory (JPL, NASA)
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