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The Peru Current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). Click here for example plots of seasonal averages. | |
Among the eastern boundary currents, the Peru Current is the largest upwelling system. The north-west alignment of the Andes mountains along the Peruvian coast forces the south-east trade winds to blow northward (Strub et al., 1998; Gunther, 1936) at a speed of around 8ms-1, causing an offshore flow in the surface layer (10-30 m) (Brink et al., 1983). Although modulated by a strong sea breeze (Brink et al., 1983; Gunther, 1936), the wind is almost constantly favorable to upwelling, presenting an alongshore maximum in the austral winter (Strub et al., 1998). This makes it one of the most productive upwelling systems of the world, causing cold nutrient rich water to appear along the coast, supporting an extraordinary abundance of marine life (Chavez, 1995; Wooster and Reid, 1963). The Peru Current System accounts for approximately 18-20% of the total fish catch worldwide (Penven et al., 2005).
The source water of the Peru Current results from some fraction of the Antarctic Circumpolar Current (ACC) turning north along the west coast of South America while it flows through the Drake Passage at approximately 40°S (Pickard and Emery 1990). In the north, the Peru Current contributes to the South Equatorial Current, by turning westward and forming a cold tongue around 4°S (Penven et al. 2005). The mean width of the Peru Current is approximately 800 km (Idyll, 1973) and the current speeds on the surface do not exceed 30 cm s-1 and decrease rapidly with depth. Surface flow directions are generally north to north-northwest (Gunther, 1936).
The Peru Current System (PCS) is characterized by the two northward flowing currents: the Peru Coastal Current (PCC) and the Peru Oceanic Current (POC), and the two opposing currents: the Peru Chile Under Current (PCUC) and the Peru Chile Counter Current (PCCC) (Penven et al. 2005). The surface flow is dominated by the equatorwards currents (POC and PCC). The PCC flows over the narrow shelf of South America from Valpariso, Chile, to north of Chimbote, Peru with a typical depth of ~200m (Batteen et al. 1995). It is the PCC which is associated with the coastal upwelling (Penven et al. 2005). The POC is wider than the PCC with widths up to 625km and reaching a maximum depth of ~700m. The net northward transport due to the PCC and the POC is approximately 19 Sv (Sverdrup, 1 Sv = 1 x 106 m3s-1) (Wooster and Reid, 1963). The PCUC dominates the poleward flow on the shelf, below the Ekman layer and extending a few hundred meters of depth over the slope (Brink et al. 1983). The mean flow in the core (220m) of the PCUC ranges around 13cms-1 (Shaffer et. al. 1999). As will just be described in this article, the sporadic weakening of the coastal current can allow the countercurrent to move south, thus disrupting the coastal upwelling that normally occurs along the coast and creating a condition known as the El Niño.
Along the coast of Peru, the temperature of its waters is nearly 15°F (8°C) colder than the normal for the surface of the Pacific in that latitude (Merill, 1990). The typical temperature of water mass along the coast is constantly modified by the horizontal mixing of cold upwelling waters and is characterized by temperatures between 15°C and 19°C (Wyrtki 1966, Stevenson & Taft 1971, Enfield 1975). The Peru Current is of relatively low salinity compared to the other eastern boundary currents. The primary mechanism that controls the salinity in the coastal region is the upwelling (Pickard and Emery 1990). On the other hand, in the open ocean salinity is controlled mainly by the difference of evaporation and precipitation, which is positive (4-6 mm/day) throughout the year (ECMWF ERA 40). The annual range of the temperature varies from about 4°C at 12°S and about 6°C at 4°S (Wooster, 1961). The Peru Current is largely responsible for the aridity that prevails in northern Chile and coastal areas of Peru and southern Ecuador. The reduction in coastal temperatures causes dry conditions to persist along the Peruvian coastal zone, which is responsible for the presence of one of the world's most arid deserts within the Andes and the Pacific Ocean (Lowenstein et al., 2003).
Another feature that controls the flow field, the salinity distribution and the energy budget of the underlying ocean is the Marine Boundary Layer Stratocummulus clouds. These clouds cover the entire Southeast Pacific and have an extremely high frequency of occurrence, which peaks in the September-November period (72%) and is reduced to minimum values (42%) in the winter of the N. Hemisphere (Klein and Hartman, 1993). As has been noticed by Hartman (1992) these clouds constitute a negative forcing, because they reduce the incoming solar radiation more than they trap the outgoing earth radiation. Because of this, it is no surprise that the heat content of the open water part of the Peru current, is controlled mainly by the annual cycle of Marine Stratocummulus clouds (Takahasi, 2005), which contribute to the reduced SST (Yu and Mechoso, 1999), especially in September-October, when the cloud coverage is at its maximum. These cloud conditions can be attributed to a high atmospheric pressure system (Fedorovich, 2004), which is driven by the low SSTs, a fact that highlights the strong ocean-atmosphere interaction of the Peru Current. Furthermore, this high pressure system generates strong surface SSW winds (8 m/s), which are the driving forcing of the surface component of the current.
The seasonal variability of the Peru Current is not well studied. Seasonal upwelling reaches its maximum intensity near 14°S-16°S, and becomes weaker around 6°S (Echevin et al. 2004). This upwelling is modulated by intense winds events that last for a few days up to a week in the spring and summer and are more frequent and stronger during the winter months (Echevin et al. 2004). Studies have shown that there is a maximum velocity of flow in the winter (Cucalon 1987). The PCUC is strongest in the spring and fall and weakest in the winter (Shaffer et al. 1999), transporting saline (35.0-35.1 psu) equatorial waters poleward (Echevin et al. 2004). The PCCC flows poleward transporting tropical warm saline waters (Lukas 1986). Lukas (1986) has shown both the undercurrent and countercurrent off Peru at 6°S to be connected to the equatorial undercurrent. The bottom and coastal topography largely influences the PCC and other currents in the system. There is also variability on the order of 5 to 20 days, related to equatorial trapped coastal waves that propagate along the Peru coast from approximately 5°S to 15°S (Hill et al. 1998; Brink et al. 1983).
On interannual time scales, ENSO is the major variability in the PCS. The thermocline deepens and the poleward currents strengthen, especially the PUCC. In the 1982-1983 El Niño, Huyer et al. (1991) noted that at 5°S the maximum poleward velocity of PCUC at 100m reached 10cms-1, while the normal value is about 4cms-1 (Blanco et al. 2002). The poleward transport of warm, saline water at the surface is also strengthened (Blanco et al 2002) during an El Niño. During El Niño conditions, positive sea surface anomalies extend from the equator to south of 20°S off Chile (Huyer et al. 1987; Strub et al 1998; Blanco et al. 2002). Upwelling ceases (Strub et al. 1998, Carr et al. 2002) off the Peruvian coast as the positive sea surface anomalies propagate across the Pacific basin. Surface values of salinity and temperature during the 1997-1998 El Niño were larger than 4°C and 0.6 psu around 20°S. Such variations during El Niño events have a major impact on the local biology (Thomas et al. 2001).
Batteen et al. (1995) models the effects of wind driven forcing on the generation of currents, eddies and filaments in the Peru Current. The coastal current becomes unstable which results in current meanders. As upwelling proceeds the meanders intensify, where cold upwelling filaments develop along the coast (Batteen et al. 1995). The meanders result in the formation of both warm core and cold core eddies. The horizontal fluxes from these eddies may be significant in the heat and salt budgets of the upper ocean on a longer time scale (Chaigneau and Pizarro 2005).