Pacific Coastal Biome
409
Humboldt Current Coastal Province (HUMB)
Extent of the Province
This province has by far the greatest north-south extent of any of the coastal boundary
current systems, and for that reason alone it would be advisable to subdivide it into
Chilean and Peruvian provinces, separated at about the change in the orientation of the
coast near 18
S, or (perhaps better) into tropical and subtropical provinces at about 40
S
where the west wind drift encounters the coast and diverges north-south. The argument
for this ideal arrangement is the same as for the SPSG province discussed earlier; the
reason that I have not chosen the ideal partition is the same for the two provinces.
Because of its size, and because it covers both Peruvian and Chilean regions, I revert to
the time-honored name of Humboldt for this province. In the north, the boundary is the
equatorial front that separates the equatorward flow of the coastal current from tropical
water at about 5
N, where the Peru Current turns offshore into the flow of the SEC.
To the south, it is at Tierra del Fuego and, seaward, the edge of eutrophic conditions
that lies at varying distances from the coast, depending on local eddy formation.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf is generally very narrow along the whole west coast of South
America because this is the leading edge of a drifting continent. The break of shelf occurs
at 200 m, and only off northern Peru, from 6
S to 10
S (Trujillo and Chimbote), is the
shelf somewhat wider, reaching a maximum of 125 km. From Callao (12
S) in Peru to
Valparaiso (33
S) in Chile the 200-m isobath lies within 10–20 km of the shoreline. From
Valparaiso southward, the shelf again widens and reaches 50 km off Concepción and
Chiloe Islands, but over this region the continental edge at 200 m is as topographically
complex as the coastline itself that here resembles the fjord coastline of Alaska or Norway.
The coastal topography of the entire province is dominated by the range of the Andes
mountains that everywhere follow the coastline, so that the 1000-m contour lies no more
than 100–200 km inland. Coastal deserts are induced (see later discussion) by interaction
between the wind regime and the mountains.
Defining Characteristics of Regional Oceanography
This is a very complex and much-misunderstood region and we are fortunate in having
a recent critical review of circulation by Strub et al. (1998); I lean heavily on it in what
follows.
The regional winds are dominated by the presence of the South Pacific High, centered
at about 30
S 95
W, which migrates meridionally with the seasons in step with the
similar motion of the ITCZ. Winds at the coast are therefore equatorward everywhere
north of 30
S, with some local exceptions, and poleward from 35
S down to Cape Horn.
Two regions have very heavy precipitation: below the seasonally migrating ITCZ, as
everywhere at low latitudes, and also off southern Chile. The southern wet region is—as
in the Pacific Northwest (Oregon to Alaska)—the consequence of westerly winds which,
on encountering a mountain barrier, generate sequential coastal low-pressure systems
that propagate poleward, as off Alaska; these systems are most frequent and strongest in
austral winter. Here, these winds induce downwelling at the coast in the same manner as
in the Gulf of Alaska. However, winds are perennially upwelling favorable, from central
Chile (35
S) to Pieta (5
N) where the coast changes orientation, and this effect is maximal
in austral winter. For these reasons, the HUMB province, as pragmatically constituted
here, comprises parts of two biomes: a northern region where upwelling is permanent or
seasonal, and a southern region where winds are primarily downwelling and where heavy
precipitation results in large surface buoyancy fluxes.
409
Humboldt Current Coastal Province (HUMB)
Extent of the Province
This province has by far the greatest north-south extent of any of the coastal boundary
current systems, and for that reason alone it would be advisable to subdivide it into
Chilean and Peruvian provinces, separated at about the change in the orientation of the
coast near 18
S, or (perhaps better) into tropical and subtropical provinces at about 40
S
where the west wind drift encounters the coast and diverges north-south. The argument
for this ideal arrangement is the same as for the SPSG province discussed earlier; the
reason that I have not chosen the ideal partition is the same for the two provinces.
Because of its size, and because it covers both Peruvian and Chilean regions, I revert to
the time-honored name of Humboldt for this province. In the north, the boundary is the
equatorial front that separates the equatorward flow of the coastal current from tropical
water at about 5
N, where the Peru Current turns offshore into the flow of the SEC.
To the south, it is at Tierra del Fuego and, seaward, the edge of eutrophic conditions
that lies at varying distances from the coast, depending on local eddy formation.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf is generally very narrow along the whole west coast of South
America because this is the leading edge of a drifting continent. The break of shelf occurs
at 200 m, and only off northern Peru, from 6
S to 10
S (Trujillo and Chimbote), is the
shelf somewhat wider, reaching a maximum of 125 km. From Callao (12
S) in Peru to
Valparaiso (33
S) in Chile the 200-m isobath lies within 10–20 km of the shoreline. From
Valparaiso southward, the shelf again widens and reaches 50 km off Concepción and
Chiloe Islands, but over this region the continental edge at 200 m is as topographically
complex as the coastline itself that here resembles the fjord coastline of Alaska or Norway.
The coastal topography of the entire province is dominated by the range of the Andes
mountains that everywhere follow the coastline, so that the 1000-m contour lies no more
than 100–200 km inland. Coastal deserts are induced (see later discussion) by interaction
between the wind regime and the mountains.
Defining Characteristics of Regional Oceanography
This is a very complex and much-misunderstood region and we are fortunate in having
a recent critical review of circulation by Strub et al. (1998); I lean heavily on it in what
follows.
The regional winds are dominated by the presence of the South Pacific High, centered
at about 30
S 95
W, which migrates meridionally with the seasons in step with the
similar motion of the ITCZ. Winds at the coast are therefore equatorward everywhere
north of 30
S, with some local exceptions, and poleward from 35
S down to Cape Horn.
Two regions have very heavy precipitation: below the seasonally migrating ITCZ, as
everywhere at low latitudes, and also off southern Chile. The southern wet region is—as
in the Pacific Northwest (Oregon to Alaska)—the consequence of westerly winds which,
on encountering a mountain barrier, generate sequential coastal low-pressure systems
that propagate poleward, as off Alaska; these systems are most frequent and strongest in
austral winter. Here, these winds induce downwelling at the coast in the same manner as
in the Gulf of Alaska. However, winds are perennially upwelling favorable, from central
Chile (35
S) to Pieta (5
N) where the coast changes orientation, and this effect is maximal
in austral winter. For these reasons, the HUMB province, as pragmatically constituted
here, comprises parts of two biomes: a northern region where upwelling is permanent or
seasonal, and a southern region where winds are primarily downwelling and where heavy
precipitation results in large surface buoyancy fluxes.