Abstract
Abstract. Two physical mechanisms can contribute to coastal upwelling in eastern boundary current systems: offshore Ekman transport due to the predominant alongshore wind stress and Ekman pumping due to the cyclonic wind stress curl, mainly caused by the abrupt decrease in wind stress (drop-off) in a cross-shore band of 100 km. This wind drop-off is thought to be an ubiquitous feature in coastal upwelling systems and to regulate the relative contribution of both mechanisms. It has been poorly studied along the central-northern Chile region because of the lack in wind measurements along the shoreline and of the relatively low resolution of the available atmospheric reanalysis. Here, the seasonal variability in Ekman transport, Ekman pumping and their relative contribution to total upwelling along the central-northern Chile region (∼ 30° S) is evaluated from a high-resolution atmospheric model simulation. As a first step, the simulation is validated from satellite observations, which indicates a realistic representation of the spatial and temporal variability of the wind along the coast by the model. The model outputs are then used to document the fine-scale structures in the wind stress and wind curl in relation to the topographic features along the coast (headlands and embayments). Both wind stress and wind curl had a clear seasonal variability with annual and semiannual components. Alongshore wind stress maximum peak occurred in spring, second increase was in fall and minimum in winter. When a threshold of −3 × 10−5 s−1 for the across-shore gradient of alongshore wind was considered to define the region from which the winds decrease toward the coast, the wind drop-off length scale varied between 8 and 45 km. The relative contribution of the coastal divergence and Ekman pumping to the vertical transport along the coast, considering the estimated wind drop-off length, indicated meridional alternation between both mechanisms, modulated by orography and the intricate coastline. Roughly, coastal divergence predominated in areas with low orography and headlands. Ekman pumping was higher in regions with high orography and the presence of embayments along the coast. In the study region, the vertical transport induced by coastal divergence and Ekman pumping represented 60 and 40 % of the total upwelling transport, respectively. The potential role of Ekman pumping on the spatial structure of sea surface temperature is also discussed.
Highlights
In the eastern boundary current systems wind-induced upwelling has mainly been described using two primary mechanisms (Sverdrup et al, 1942; Gill 1982; Pickett and Paduan, 2003; Capet et al, 2004; Jacox and Edwards, 2012)
The simulations with higher resolution (12 and 4 km) show a cyclonic wind stress curl within the coastal band and within the Coquimbo Bay system that is associated with a positive Ekman pumping, while in the oceanic sector a less intense anticyclone wind curl predominates
The negative curl within the coastal band is the result of an onshore decay in wind intensity that is characteristic of eastern boundary upwelling systems (EBUSs) (Capet et al, 2004; Renault et al, 2012)
Summary
In the eastern boundary current systems wind-induced upwelling has mainly been described using two primary mechanisms (Sverdrup et al, 1942; Gill 1982; Pickett and Paduan, 2003; Capet et al, 2004; Jacox and Edwards, 2012). Starting in the mid-1970s, a series of studies began assessing the contribution of Ekman pumping on coastal upwelling for the California Current system (Halpern, 1976; Nelson, 1977), which later expanded to the other four upwelling systems (Bakun and Nelson, 1991). In one of these four regions, the coast of north and central Chile, this mechanism has been poorly evaluated, primarily due to the scarcity of in situ data, limitations in diffusiometer winds that have a “blind zone” near the coast and the relatively low spatial resolution of the atmospheric reanalysis. This has caused a limited progresses in the understanding of the upwelling dynamics and the coastal circulation of the region, among other factors
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