the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.is seen with increasing latitude. The coast can be divided into general zones based on physicalcharacteristics. The shelf (200 m) off Peru is up to 100 km wide. Along the Chilean coast, fourzones (Figueroa 2002) include the northern region (north of ~32°S) with little freshwater influenceand an extremely narrow shelf (42°S) with a wider topographically complex, fjord-indented coastline and with strongfreshwater influence.Wind forcing in the southeast Pacific is dominated by the influence of the southeast Pacificsubtropical anticyclone that creates equatorward, upwelling-favourable winds along most of theSouth American Pacific coast with latitudinally varying seasonality. Between 5°S and ~35°S,monthly mean winds remain upwelling favourable throughout the year and are weakest along thenorthern Chilean coast ~17–23°S (Shaffer et al. 1999, Thomas et al. 2001a). Distinct phase differencesin the annual maxima are evident as a function of latitude (Thomas et al. 2001a), wheremaximum upwelling strength shifts from austral autumn–winter off Peru (Bakun & Nelson 1991,Carr et al. 2002), to austral spring–summer off northern Chile (Blanco et al. 2001) and australsummer south of ~30°S (Shaffer et al. 1999). Upwelling is augmented by a decrease in alongshorewind stress near shore that creates a region of cyclonic (negative) wind stress curl in the coastalzone, lifting isotherms. Two regions of maximum alongshore wind stress are evident, centred at~15°S and ~30°S (Shaffer et al. 1999, Thomas et al. 2001a, Hormazábal et al. 2004). North of30°S, strong land–sea thermal contrast in austral summer enhances upwelling-favourable alongshorewind stress (Rutllant et al. 2004a). At the very lowest latitudes (north of ~5°S), the annual meridionalmigration of the Inter-Tropical Convergence Zone (ITCZ) creates both wind and precipitation (andtherefore stratification) seasonality. Beginning at ~35°S and extending poleward, seasonality in thestrength of the subtropical anticyclone creates seasonal reversals between an austral summerupwelling maximum and mean winter conditions of poleward, downwelling-favourable windsdriven by winter storms associated with the polar front (Shaffer et al. 1999, Rutllant et al. 2004a).The strength and duration of these winter conditions increases with latitude. South of ~45°S, eventsassociated with polar front activity create poleward alongshore monthly mean winds, downwellingconditions and strong precipitation in coastal regions throughout the year.The large-scale equatorward surface flow of the HCS within the basin-scale gyre belies acomplicated and still relatively poorly sampled flow structure closer to the coast. The followingdescription is a general overview of commonly observed characteristics reviewed in more detailby Strub et al. (1998). The northern latitude at which the WWD approaches the continent shiftsseasonally from 35°S to 40°S in austral winter to ~45°S in austral summer. Flow from the WWDbranches north and south (Figure 2). The southern arm joins poleward coastal flow of the CapeHorn Current, a buoyancy-driven coastal current with input from local river runoff, which isstrongest during austral winter. The northern arm forms the main flow of the Humboldt Currentthat proceeds equatorward along the basin margin and joins the westward-flowing South EquatorialCurrent at lowest latitudes, off northern Peru-Ecuador. This main flow of the Humboldt Current islocated seaward (~75–85°W) of a system of narrower coastal flows. Closest to the coast and coupledto local wind forcing and coastal upwelling, the Chile Coastal Current (CCC) flows predominantlyequatorward. In summer, equatorward flow is most latitudinally extensive, traceable to 35–40°S(Atkinson et al. 2002) and flowing along the entire central and northern Chilean coast, continuouswith the equatorward flowing Peru Coastal Current (PCC), also forced by local winds, as far as~5°S. In winter, the PCC strengthens in association with the winter maximum in alongshore windstress. Off northern Chile (18–24°S) the seasonal maximum of the CCC is in the autumn (Blancoet al. 2001), while south of ~25°S the CCC becomes increasingly seasonally variable. South of~35°S, in association with the seasonal reversal in alongshore wind stress, the CCC is predominantly202
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILE−92° −88° −84° −80° −76° −72° WSEC 0°PCC−10°PCCCCCC−20°PCCCHCPCCC−30°Southern latitudeWWDCCC−40°CHC−50°Figure 2 Overview of the surface currents in the eastern South Pacific that influence the north-central Chileancoast, showing the West Wind Drift (WWD), the main flow of the Humbolt Current (HC), the Cape HornCurrent (CHC), the Chile Coastal Current (CCC), the Peru Coastal Current (PCC), the Peru-Chile Countercurrent(PCCC) and the South Equatorial Current (SEC).poleward in winter. Between these seasonal coastal currents and the more continuous equatorwardflow of the main Peru-Chile Current is the poleward-flowing Peru-Chile Countercurrent located~100–300 km offshore, originating in equatorial regions likely as Equatorial Undercurrent waterand continues to ~35°S. This flow is poorly resolved in drifter data (Chaigneau & Pizarro, 2005)but evident in altimeter data (Strub et al. 1995) and temperature-salinity characteristics (Strub et al.1998). Beneath the coastal surface currents, a Poleward Undercurrent is continuous from northernPeru to latitudes as high as 45–50°S (Silva & Neshyba 1979). This undercurrent, located over thecontinental slope (Zuta & Guillén 1970, Shaffer et al. 1995, Leth et al. 2004), delivers relativelysaline, nutrient-rich, low-oxygen water (equatorial subsurface water, ESSW) to coastal regions; it203
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the humboldt current system of northern and central chile - figema

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