410
Chapter 11: The Pacific Ocean
Thus, the pattern of wind stress and its curl over the province resembles the general
pattern established for eastern boundary currents by Bakun and Nelson (1991), but with
some special characteristics due to the alignment of the coast and coastal mountains.
Cyclonic curl adjacent to the coast takes maximal values in two areas in austral winter:
off Peru from 9
S to 15
S and off Chile at about 25
S. The Peru maximum is seasonally
stable, whereas off Chile it moves poleward in summer to about 32
S and simultaneously
strengthens so that the Peruvian and Chilean upwelling centers are seasonally out of
phase. In the bight at 18–20
S (Arica to Iquique), there is anomalous and persistent
anticyclonic curl, probably because the bight is open to wind from the south.
As a consequence of the specific characteristics of the wind field, the regional partitioning of positive and negative wind-stress curl is significantly less organized into
inshore cyclonic and offshore anticyclonic areas, especially during austral winter (April–
September). As Bakun and Nelson point out, wind stress is maximal not at the coast
itself, but farther offshore, and this leads to offshore upwelling; right at the coast, local
land-sea breezes may significantly reduce the potential effect at the sea surface of the
regional wind systems. Such local effects are particularly strong at capes that enclose a bay
open to the north, a conjunction (as at Punta Aguja) that may lead to the development
of a local coastal desert region.
The interaction between the gyral flow, wind forcing at the coast, and coastal topography is very complex and involves both poleward and equatorward motion in the coastal
currents; these have been very variously named, and I endeavor to use a conservative
selection in what follows. Well offshore, the circulation around the eastern edge of the
subtropical gyre from the divergence of the West Wind Drift (WWD) is weak, broad
and continuously equatorward; I shall follow what appears to be common usage in using
the term Humboldt Current for this flow. The offshore, poleward flow south of the
divergence of the WWD is termed the Cape Horn Current by Strub et al. Neither of
these flows are involved in the coastal effects in which we are interested.
Flow at the coast is equatorward during austral summer from Chiloe Island at 42
S
right up to Cape Aguja at about 6
S, but, in winter, equatorward flow is initiated only at
about 20
S as the Peru Coastal Current, while fluctuating flow obtains from here down to
about 32
S and, even further south, flow becomes poleward in the Chile Coastal Current.
A subsurface Poleward Undercurrent flows along the entire outer shelf and upper slope,
transporting high-salinity, nutrient-rich water just as occurs also in the CALC province.
Thus, off Peru, equatorward flow is shallow (∼25 m), above poleward flow that has
maximum velocities at about 100 m in the upper layers of an oxygen-depleted zone but
shallower than the depth of maximum nitrate depletion. Because of the shallow depth of
the equatorward flow, it is water of the Poleward Undercurrent that is upwelled at the
coast (Brockman et al., 1980); thus, at the upwelling near Concepción (37
S) high-salinity
upwelling plumes are readily distinguished from low-salinity plumes of fjord water that
emerge from the island passes to the south.
Upwelling water, and hence nutrient advection, differs in the Peruvian and Chilean
sectors of the Humboldt Current system: off Peru, equatorial subsurface water flowing
south in the subsurface countercurrent is upwelled, whereas off Chile it is subantarctic
water of the equatorward coastal current that upwells, though countercurrent upwelling
has been observed at Valparaiso (Johnson et al., 1980). The boundary between these two
source-water regimes lies at about 15
S according to Wyrtki (1963).
Though there are apparently regional differences in the depth of the source water
in upwelling cells, the canonical view is that upwelling is restricted to relatively shallow
depths and that there is an onshore flow of bottom water over the continental shelf (e.g.,
Codispoti et al., 1982). Upwelling occurs as numerous small coastal upwelling cells, from
which relatively small plumes (<100 km) of cool water may be entrained offshore. These
cluster in the following preferred locations on the Peruvian coast:
Chapter 11: The Pacific Ocean
Thus, the pattern of wind stress and its curl over the province resembles the general
pattern established for eastern boundary currents by Bakun and Nelson (1991), but with
some special characteristics due to the alignment of the coast and coastal mountains.
Cyclonic curl adjacent to the coast takes maximal values in two areas in austral winter:
off Peru from 9
S to 15
S and off Chile at about 25
S. The Peru maximum is seasonally
stable, whereas off Chile it moves poleward in summer to about 32
S and simultaneously
strengthens so that the Peruvian and Chilean upwelling centers are seasonally out of
phase. In the bight at 18–20
S (Arica to Iquique), there is anomalous and persistent
anticyclonic curl, probably because the bight is open to wind from the south.
As a consequence of the specific characteristics of the wind field, the regional partitioning of positive and negative wind-stress curl is significantly less organized into
inshore cyclonic and offshore anticyclonic areas, especially during austral winter (April–
September). As Bakun and Nelson point out, wind stress is maximal not at the coast
itself, but farther offshore, and this leads to offshore upwelling; right at the coast, local
land-sea breezes may significantly reduce the potential effect at the sea surface of the
regional wind systems. Such local effects are particularly strong at capes that enclose a bay
open to the north, a conjunction (as at Punta Aguja) that may lead to the development
of a local coastal desert region.
The interaction between the gyral flow, wind forcing at the coast, and coastal topography is very complex and involves both poleward and equatorward motion in the coastal
currents; these have been very variously named, and I endeavor to use a conservative
selection in what follows. Well offshore, the circulation around the eastern edge of the
subtropical gyre from the divergence of the West Wind Drift (WWD) is weak, broad
and continuously equatorward; I shall follow what appears to be common usage in using
the term Humboldt Current for this flow. The offshore, poleward flow south of the
divergence of the WWD is termed the Cape Horn Current by Strub et al. Neither of
these flows are involved in the coastal effects in which we are interested.
Flow at the coast is equatorward during austral summer from Chiloe Island at 42
S
right up to Cape Aguja at about 6
S, but, in winter, equatorward flow is initiated only at
about 20
S as the Peru Coastal Current, while fluctuating flow obtains from here down to
about 32
S and, even further south, flow becomes poleward in the Chile Coastal Current.
A subsurface Poleward Undercurrent flows along the entire outer shelf and upper slope,
transporting high-salinity, nutrient-rich water just as occurs also in the CALC province.
Thus, off Peru, equatorward flow is shallow (∼25 m), above poleward flow that has
maximum velocities at about 100 m in the upper layers of an oxygen-depleted zone but
shallower than the depth of maximum nitrate depletion. Because of the shallow depth of
the equatorward flow, it is water of the Poleward Undercurrent that is upwelled at the
coast (Brockman et al., 1980); thus, at the upwelling near Concepción (37
S) high-salinity
upwelling plumes are readily distinguished from low-salinity plumes of fjord water that
emerge from the island passes to the south.
Upwelling water, and hence nutrient advection, differs in the Peruvian and Chilean
sectors of the Humboldt Current system: off Peru, equatorial subsurface water flowing
south in the subsurface countercurrent is upwelled, whereas off Chile it is subantarctic
water of the equatorward coastal current that upwells, though countercurrent upwelling
has been observed at Valparaiso (Johnson et al., 1980). The boundary between these two
source-water regimes lies at about 15
S according to Wyrtki (1963).
Though there are apparently regional differences in the depth of the source water
in upwelling cells, the canonical view is that upwelling is restricted to relatively shallow
depths and that there is an onshore flow of bottom water over the continental shelf (e.g.,
Codispoti et al., 1982). Upwelling occurs as numerous small coastal upwelling cells, from
which relatively small plumes (<100 km) of cool water may be entrained offshore. These
cluster in the following preferred locations on the Peruvian coast: