242
7 Ocean Currents
range from a day, for a sea breeze, to quasi-decadal EI Nino. Near 15°S, a strong
sea breeze and a diurnal heating cycle cause a daily cycle of the evolution of
near surface turbulence. The mixed layer depth usually is about 20 m during
summer and autumn. The temperature in the upper 30 m or so is largely
governed by the wind with upwelling and deepening of the mixed layer being
important processes. Below about 50 m depth, the temperature is largely
controlled by coastal trapped waves. Although there is substantial spatial
variability, the basic characteristics of the pattern, mentioned above, seem to
persist alongshore over an extensive spatial scale and for a long time (Brink et
ai., 1983).
A poleward undercurrent dominates the entire flow on the shelf beneath the
surface Ekman layer and extends to several hundred metres in depth over the
slope. Such a poleward flow is a ubiquitous feature of the major coastal upwelling regions.
During the 1982-83 EI Nino event a tongue of unusually warm water was
observed (see Fig. 7.15b). Local winds off Peru remained normal for at least a
month after coastal sea levels and coastal temperatures began to rise in early
October 1982. At the EI Nino peak, from February through May 1983, winds
were stronger than normal at Callas (12°S) and much weaker than normal at
Talava (5
0
S). However, the winds which normally drive coastal upwelling in this
area collapsed for about a month in March and April 1983. A series of conductivity, temperature and depth (CTD) sections made at 5 0 S and 10 0 S (Huyer et
ai., 1987) showed that the apparent source depth of upwelling waters (50-100
m) remained constant. Outside of EI Nino events, this layer is normally below
the thermocline and upwelling waters are cool and high in nutrients. During
EI Nino it was at the top of the deeper and thicker thermocline. Therefore,
ascending water was warm and nutrient-poor.
There are many factors that can produce variations to the typical pattern of
upwelling, such as changes in the strength of the wind component parallel to
the shore, vertical structure of the water, variations in the bottom bathymetry,
and instabilities in the currents. These factors result in some differences in the
upwelling sites off Peru and those off the Spanish Sahara (the Canary Current),
off Oregon and California (the California Current), off the west coast of Southern Africa (the Benguela Current), and a rather anomalous Somali upwelling
in the western Indian Ocean. Descriptions of these upwelling systems are in
a series of papers by Richards (1981), Huyer (1983), Mittelstaedt (1983), Nelson and Hutchings (1983) and Schott (1983). Apart from these major coastal
upwellings, there are a number of upwellings which occur sporadically at the
western boundaries of the major oceans, some of which have been well documented, such as a summer upwelling off Nova Scotia, Canada, and that of
the northwest corner of Spain, near Cape Finistere. The physical environment
and biological production of some of these regions is reviewed by Tomczak and
Godfrey (1994) and Mann and Lazier (1996). We will return to the biological
consequences of coastal upwelling in Chap. 15.
7 Ocean Currents
range from a day, for a sea breeze, to quasi-decadal EI Nino. Near 15°S, a strong
sea breeze and a diurnal heating cycle cause a daily cycle of the evolution of
near surface turbulence. The mixed layer depth usually is about 20 m during
summer and autumn. The temperature in the upper 30 m or so is largely
governed by the wind with upwelling and deepening of the mixed layer being
important processes. Below about 50 m depth, the temperature is largely
controlled by coastal trapped waves. Although there is substantial spatial
variability, the basic characteristics of the pattern, mentioned above, seem to
persist alongshore over an extensive spatial scale and for a long time (Brink et
ai., 1983).
A poleward undercurrent dominates the entire flow on the shelf beneath the
surface Ekman layer and extends to several hundred metres in depth over the
slope. Such a poleward flow is a ubiquitous feature of the major coastal upwelling regions.
During the 1982-83 EI Nino event a tongue of unusually warm water was
observed (see Fig. 7.15b). Local winds off Peru remained normal for at least a
month after coastal sea levels and coastal temperatures began to rise in early
October 1982. At the EI Nino peak, from February through May 1983, winds
were stronger than normal at Callas (12°S) and much weaker than normal at
Talava (5
0
S). However, the winds which normally drive coastal upwelling in this
area collapsed for about a month in March and April 1983. A series of conductivity, temperature and depth (CTD) sections made at 5 0 S and 10 0 S (Huyer et
ai., 1987) showed that the apparent source depth of upwelling waters (50-100
m) remained constant. Outside of EI Nino events, this layer is normally below
the thermocline and upwelling waters are cool and high in nutrients. During
EI Nino it was at the top of the deeper and thicker thermocline. Therefore,
ascending water was warm and nutrient-poor.
There are many factors that can produce variations to the typical pattern of
upwelling, such as changes in the strength of the wind component parallel to
the shore, vertical structure of the water, variations in the bottom bathymetry,
and instabilities in the currents. These factors result in some differences in the
upwelling sites off Peru and those off the Spanish Sahara (the Canary Current),
off Oregon and California (the California Current), off the west coast of Southern Africa (the Benguela Current), and a rather anomalous Somali upwelling
in the western Indian Ocean. Descriptions of these upwelling systems are in
a series of papers by Richards (1981), Huyer (1983), Mittelstaedt (1983), Nelson and Hutchings (1983) and Schott (1983). Apart from these major coastal
upwellings, there are a number of upwellings which occur sporadically at the
western boundaries of the major oceans, some of which have been well documented, such as a summer upwelling off Nova Scotia, Canada, and that of
the northwest corner of Spain, near Cape Finistere. The physical environment
and biological production of some of these regions is reviewed by Tomczak and
Godfrey (1994) and Mann and Lazier (1996). We will return to the biological
consequences of coastal upwelling in Chap. 15.