Editing Humboldt Current (section)

== Physical oceanography ==
[[Image:Tropical cyclones 1945 2006.png|thumb|upright=1.9|The presence of the Humboldt Current and its associated wind shear makes for conditions that inhibit the formation of [[tropical cyclone]]s.<ref>{{cite journal|last=Ruminski|first=Mark|date=Jan 1991|title=Two Unusual Tropical Cyclones in the Southeast Pacific|journal=Monthly Weather Review|volume=119|issue=1|pages=218–222|doi=10.1175/1520-0493(1991)119<0218:TUTCIT>2.0.CO;2|bibcode=1991MWRv..119..218R |doi-access=free}}</ref><br/>(Worldwide tropical cyclone tracks, 1945&ndash;2006.)]]

The [[trade winds]] are the primary drivers of the Humboldt Current circulation.<ref name=":0" /> 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]].<ref name=":0" />

The Humboldt current, occupying the upper ocean, flows equatorward carrying fresh, cold Sub-Antarctic surface water northward, along the outskirts of the [[subtropical gyre]].<ref name=":0" /> 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 [[subduction]]s. Within this region, the [[Equatorial Undercurrent|equatorial undercurrent]] (EUC) flows eastward along the equator, feeding the [[Peru-Chile undercurrent]] (PCU) that moves poleward.<ref name=":0" />

Off the coast of central Chile, there is a coastal transition zone (CTZ), which is characterized by high eddy kinetic energy.<ref name=":0" /> This energy forms [[mesoscale eddies]] which extend {{convert|600-800|km|mi|abbr=on}} offshore. The CTZ has three distinct regions within its boundaries:

# high chlorophyll-a concentrations in wide regions off the coast of Peru (10–15°S), 
# high chlorophyll-a concentrations in wide regions off the coast of Chile (30°S), and
# 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&nbsp;km of the coast.<ref name=":0" />

The limb of the Humboldt Current System that veers off the coast of Peru creates a decrease in ventilation within the system.<ref name=":0" /> 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 [[Advection|advects]] low oxygen waters southward towards northern Chile.<ref name=":0" /> This OMZ is the fourth largest permanent hypoxic zone in the world's oceans. It occupies an area about 2.18 ± 0.66 × 10<sup>6</sup> km<sup>3</sup>. The core of this zone is centered off Peru, creating a shallow upper boundary that reaches from about {{convert|100|m|ft|abbr=on}} down to {{convert|600|m|ft|abbr=on}}.<ref name=":0" /> Another factor contributing to the OMZ is sinking and decay of primary productive resources.<ref name=":1" />

Consequently, the OMZ forces many organisms to stay near the surface where nutrients and oxygen are obtainable.<ref name=":1" /> The presence of a shallow OMZ restricts the migration of [[zooplankton]] within the water column. Between {{convert|0|and(-)|600|m|ft|abbr=on}}, 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 [[hypoxia (environmental)|hypoxic]] conditions.<ref name=":1" />

Coastal upwelling is the main factor contributing to the high biological productivity of the Humboldt current.<ref name=":0" /> Upwelling within the current is not uniform across the entire system. Three notable upwelling subsystems are produced by this current:

# 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, 
# upwelling "shadow" that is less productive, but still large in northern Chile and Southern Peru, and
# highly productive year-round upwelling in Peru.<ref name=":0" /> 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.<ref name=":0" />