Humboldt Current

The current extends from southern Chile (~45th parallel south) to northern Peru (~4th parallel south) where cold, upwelled, waters intersect warm tropical waters to form the Equatorial Front. Sea surface temperatures off the coast of Peru, around 5th parallel south, reach temperatures as low as 16 °C (61 °F). This is highly uncharacteristic of tropical waters, as most other regions have temperatures measuring above 25 °C (77 °F). Upwelling brings nutrients to the surface, which support phytoplankton and ultimately increase biological productivity.

The Humboldt Current is a highly productive ecosystem. It is the most productive eastern boundary current system. It accounts for roughly 18-20% of the total worldwide marine fish catch. The species are mostly pelagic: sardines, anchovies and jack mackerel. The system's high productivity supports other important fishery resources as well as marine mammals (eared seals and cetaceans) and seabirds. Periodically, the upwelling that drives the system's productivity is disrupted by the El Niño-Southern Oscillation (ENSO) event, often with large social and economic impacts.

The Humboldt has a considerable cooling influence on the climate of Chile, Peru and Ecuador. It is also largely responsible for the aridity of the Atacama Desert in northern Chile and coastal areas of Peru and also of the aridity of southern Ecuador. Marine air is cooled by the current and thus is not conducive to generating precipitation (although clouds and fog are produced).

Physical oceanography

The trade winds are the primary drivers of the Humboldt Current circulation. Variability in this system is driven by latitudinal shifts between the Intertropical Convergent Zone and the trade winds in the north. Shifts within the South Pacific High at mid-latitudes, as well as cyclonic storms and movement of the Southern Westerlies southward also contribute to system changes. Atmospheric variability off central Chile is enhanced by the aggravation of coastal low pressure systems trapped between the marine boundary layer and the coastal mountains. This is prominent poleward from 27th parallel south to 42nd parallel south.

The Humboldt current, occupying the upper ocean, flows equatorward carrying fresh, cold Sub-Antarctic surface water northward, along the outskirts of the subtropical gyre. The main flow of the current veers offshore in southern Peru, as a weaker limb continues to flow equatorward. Around 18th parallel south the fresh, cold waters begin to mix with the warm, high salinity Subtropical Surface waters. This collision causes partial subductions. Within this region, the equatorial undercurrent (EUC) flows eastward along the equator, feeding the Peru-Chile undercurrent (PCU) that moves poleward.

Off the coast of central Chile, there is a coastal transition zone (CTZ), which is characterized by high eddy kinetic energy. This energy forms mesoscale eddies which extend 600–800 km (370–500 mi) offshore. The CTZ has three distinct regions within its boundaries:

1. high chlorophyll-a concentrations in wide regions off the coast of Peru (10–15°S),
2. high chlorophyll-a concentrations in wide regions off the coast of Chile (30°S), and
3. high chlorophyll-a concentrations in narrow regions off the coast of northern Chile (Montecino and Lange 2008). High chlorophyll-a concentrations are generally found within 50 km of the coast.

The limb of the Humboldt Current System that veers off the coast of Peru creates a decrease in ventilation within the system. This lack of ventilation is the primary driver of an intense oxygen minimum zone (OMZ) which is formed in the sub-surface to intermediate depths. In the north, the EUC ventilates the OMZ, and in the south the PCU advects low oxygen waters southward towards northern Chile. This OMZ is the fourth largest permanent hypoxic zone in the world's oceans. It occupies an area about 2.18 ± 0.66 million km³. The core of this zone is centered off Peru, creating a shallow upper boundary that reaches from about 100 m (330 ft) down to 600 m (2,000 ft). Another factor contributing to the OMZ is sinking and decay of primary productive resources.

Consequently, the OMZ forces many organisms to stay near the surface where nutrients and oxygen are obtainable. The presence of a shallow OMZ restricts the migration of zooplankton within the water column. Between 0 and 600 m (0–1,969 ft), many species of zooplankton occupy this space within the OMZ. This allows for a substantial exchange of carbon between the euphotic layer and the OMZ. 75% of the total zooplankton biomass move in and out of the OMZ. The OMZ also serves as a refuge for organisms that can live in hypoxic conditions.

Coastal upwelling is the main factor contributing to the high biological productivity of the Humboldt current. Upwelling within the current is not uniform across the entire system. Three notable upwelling subsystems are produced by this current:

1. seasonal upwelling in Chile only during the spring and summer, because of the displacement of the subtropical center of high pressure during the period January–March,
2. upwelling "shadow" that is less productive, but still large in northern Chile and Southern Peru, and
3. highly productive year-round upwelling in Peru. The upwelling shadow identified between 35°S and 15°S is caused by the oligotrophic subtropical gyre impinging on the coast. This creates a narrow, but highly productive, upwelling zone.