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2 Cultural, Social and Economic Relevance
5 The Peruvian-Chilean Coastal Upwelling System
Fig. 5.2 Peruvian anchoveta (Engraulis ringens). Image source: http://perumaritimo.pe/uploads/
noticia105.jpg [accessed on 5 April 2016]
cessation of upwelling during El Niño events. In terms of their cultural context, El
Niños are believed to have affected pre-Columbian Incas and led to the demise of
the Moche and other pre-Columbian Peruvian cultures (Fagan 1999). Peru’s ﬁsheries contribution to the GDP varies annually, depending to a large extent on
oceanic conditions. Fishing, which is very dependent on the availability of anchoveta (Engraulis ringens, Fig. 5.2) “normally” accounts for roughly 3.5 % of the
country’s GDP and employs approximately 80,000 people. In El Niño years, such
as 1998, the sector’s contribution falls to below 1 % of GDP. Beyond their
immediate influence on ﬁsh landings, El Niños have other widespread socioeconomic impacts associated with severe flooding and extensive weather-related
damages throughout Latin America, and flood-related damages to infrastructure
also result in substantial economic losses in El Niño years. For instance, the
1997/98 El Niño caused damages of about USD 3.5 billion, or about 4.5 % of
GDP. Hence, the economies of Ecuador, Peru and Chile are particularly vulnerable
to El Niño events.
Environmental disturbances associated with El Niño have triggered socioeconomic and political reactions in Peru altering aspects of society. For instance, a
strong El Niño event caused the anchovy ﬁshery to collapse in 1993. This, coupled
with political change in Peru, led to a nationalization of the Peruvian ﬁshing
industry, resulting in massive layoffs and a restructuring of the industry (Glantz
1979). During the 1997/1998 El Niño, the ﬁshermen’s labor union was virtually
powerless, and unable to secure governmental aid beyond some provision of
foodstuffs (Broad 1999).
Chile’s ﬁsheries account for about 2 % of GDP and 11 % of its global exports.
In contrast to Peru, Chile’s seafood export market is strongly founded on aquaculture products (e.g., salmon, trout, oysters, abalone), which are externally fed and
are thus largely independent of oceanic nutrient sources. Chile’s dependence on
5.2 Cultural, Social and Economic Relevance
wild ﬁsheries is relatively low, and El Niño events have little impacts on the
country’s catch rates of anchoveta (0.3 Million tonnes per year), which are only
10 % of Peru’s decimated ﬁsh landings during El Niño (<3 Million tonnes).
History of Discovery
Alexander von Humboldt (1769–1859) and Aimé Bonplant (1773–1858) were the
ﬁrst to scientiﬁcally explore the flora, fauna, and topography of Latin America.
While on the west coast of South America, von Humboldt discovered and scientiﬁcally documented the Peru Current in 1802, which, over the objections of von
Humboldt himself, has been referred to as the “Humboldt Current” (Berghaus
1837–1847). Humboldt (1846), in his book Cosmos, wrote on page 301:
in the Southern Paciﬁc Ocean, … a current the effect of whose low temperature on the
climate of the adjacent coast was ﬁrst brought into notice by myself in the autumn of 1802.
This current brings the cold water of high southern latitudes to the coast of Chile, and
follows its shores and those of Peru northward.
At the congress of the Geographical Society in Lima in 1892, Peruvian Navy
Captain Camilo Carrillo’s account is the ﬁrst reference to El Niño, although it was
certainly known about long before this by the local ﬁshermen. He made the following statement (Carrillo 1892):
Peruvian sailors from the port of Paita in northern Peru, who frequently navigate along the
coast in small crafts, either to the north or to the south of Paita, named this current “El
Niño” without doubt because it is most noticeable and felt after Christmas.
Between the ﬁrst two World Wars, two important expeditions covered the
region: the Carnegie expedition from the USA in 1928–1931 (Sverdrup 1930), and
the William Scoresby expedition from Britain in 1931 (Gunther 1936). These
expeditions represented a major early contribution to the knowledge of the Chilean
coast sector of the Humboldt Current. The Scoresby expedition (May–September
1931) covered the region between 3°–48° S from the coast to 300 miles off Peru
and, in more detail, off Chile to 50 miles offshore, collecting data on temperature
and salinity, the effects of wind on water movement, and the consequent effect on
the phosphate content and, hence, marine life. Whereas Carnegie’s survey referred
to the Oceanic Peru Current, Gunther’s report dealt with the Coastal Peru Current,
although the upwelling system in this region had long before been described by von
Humboldt. Later, Wooster (1970) proposed using the name Humboldt Current to
describe the whole of the Chile-Peru Current system.
Since the late 1960s numerous research cruises have focussed on the Peru
upwelling regions. Noticeable is the Pisco cruise (e.g. Smith et al. 1971) from 20
March–3 May 1969 which collected almost daily surface maps of temperature,
salinity, nitrate, silicate, phosphate, and fluorescence. As part of the multi-national
Surface Ocean-Lower Atmosphere Study (SOLAS), which was established in 2004,
5 The Peruvian-Chilean Coastal Upwelling System
observational studies started to focus on air-sea gas fluxes, oxygen minimum zones
and the biogeochemistry in the Peru and other eastern boundary upwelling regions.
Studies of the ocean and weather conditions during the 1960s emphasized the
anomalous warm years, and referred to those episodes as El Niños. Bjerknes (1966,
1969) documented coherent warm equatorial SST anomalies from the dateline to
the coast of Ecuador, and related this feature to both the warming at the Peru coast
and to planetary-scale changes in the tropical atmosphere, the “Southern
Oscillation”. In subsequent years, the term “El Niño” has been used in the literature
to describe basin-scale equatorial Paciﬁc warmings, and this has blurred the distinction with the coastal phenomenon, which, while related to it, does not exhibit a
one-to-one correspondence with the basin-scale SST variability (e.g., Trenberth and
Stepaniak 2001). This usage has led to confusion and contradictions in the use of
terminology (Aceituno 1992). Trenberth (1977) explored possible deﬁnitions of
El Niño and concluded that the deﬁnition is still evolving and alternative criteria
might be used. In 1983, Working Group 55 of the Scientiﬁc Committee for Ocean
Research (SCOR) deﬁned El Niño as “the appearance of anomalously warm water
along the coast of Ecuador and Peru as far south as Lima (12° S)”. This implied a
normalized SST anomaly exceeding one standard deviation for at least four consecutive months at three of ﬁve Peruvian coastal stations. Nowadays, scientists
frequently use SST anomalies in the region 5° N to 5° S, 170°–120° W (referred to
as “Niño 3.4”) as an operational deﬁnition for monitoring and prediction purposes.
This index is based on SST anomalies (3-month average SST anomaly >=0.5°C
with respect to the 1971–2000 average) in the Niño 3.4 region. A brief history of El
Niño deﬁnitions is given by Stewart (2008).
Bathymetry and Atmospheric Forcing
Figure 5.1 shows the general bathymetry and topography for western South
America (see also Fig. 5.6). The continental region is complex near Panama (9° N),
where some wider shelf regions (width > 100 km) are found. The shelf becomes
narrow off Colombia and Ecuador, wider (up to 100 km) off Peru, very narrow off
northern Chile, and wider again off southern Chile. The shelf is virtually missing
entirely off parts of northern and central Chile, e.g., at 30° S it rapidly drops to
800 m by about 8 km offshore. Near Concepción (36°–37° S), the shelf expands to
widths of 20–60 km, similar to other upwelling regions. South of 42° S, the shelf is
several hundred kilometres wide and it is covered with scattered islands offshore of
The Peruvian coastline is relatively straight except north of about 5° N, where
the coastline orientation changes at it bends into Ecuador’s Gulf of Guayaquil. The
continental shelf near Pisco (13.7° S) is particularly productive and forms a distinct
upwelling centre. Here, upwelling is enhanced via the influence of the irregular
coastline of the Paracas Pensinula (located south of Pisco) and larger islands such as
5.4 Bathymetry and Atmospheric Forcing
Isla De Saugayan. Overall, the coastline of northern and central Chile (18° S
to *40° S) is also relatively straight, but in the nearshore region small-scale
geographic features produce a high spatial heterogeneity, which also influences
oceanographic conditions in this area. Several bay systems having complex internal
circulation are found along the coast of northern-central Chile. Headlands favour
the generation of powerful coastal flow structures (squirts) that transport surface
waters up to 100 km offshore (Marín et al. 2003b). In contrast, long stretches of
exposed outer coast without headlands or bays, as found for example in northern
Chile between 20° S and 22° S, favour alongshore currents leading to relatively
homogeneous conditions and downstream transport (Palma et al. 2006).
On land, the Andes Mountain Range extends along most of the length of Peru
and Chile at a distance of 100–200 km from the coast. The marine climate of the
region results from interactions of larger scale atmospheric systems and the
mountain range. The most influential atmospheric pattern is the South Paciﬁc High
(see Fig. 2.13), also known as the southeastern Paciﬁc Subtropical Anticyclone,
which drives equatorward winds along the coasts of Chile and Peru. The equatorward winds are the principle agent of coastal upwelling. The South Paciﬁc High
is bounded in the north by the Inter-Tropical Convergence Zone (ITCZ) and to the
south by the polar front and its disturbances. The ITCZ moves from *10° N in
austral winter (June–September) to 2°–5° N in austral summer (December to
March). When the ITCZ is farthest north, the southeast trade winds turn eastward
off northern Ecuador and Colombia, creating downwelling-favourable (poleward)
coastal winds (Strub et al. 1998). When the ITCZ is in its northernmost position in
austral winter, upwelling favourable winds exist north of 4° N.
South of 27° S in winter, the effects of polar front disturbances become more
pronounced, with time scales of 7–10 days. These synoptic-scale disturbances
trigger coastal atmospheric lows between 27° and 32° S that propagate poleward as
atmospheric coastal trapped waves and cause upwelling enhancement-relaxation
cycles (Rutllant 1993). Between 35° and 45° S, upwelling favourable winds display
a high degree of synoptic scale of variability (Djurfeldt 1989). South of 45° S,
polar-front synoptic disturbances create mean downwelling-favourable winds all
The coastal upwelling off Peru is embedded with several more or less independent
currents interacting in a rather complicated manner (Wyrtki 1966). In the shallow
upper ocean, the Peru-Chile upwelling system is dominated by an equatorward flow
of fresh, relatively cool subantarctic surface water, originating from the northern
branch of the Antarctic Circumpolar Current. This northward-flowing, cool surface
current is the Humboldt Current (or Peru Current). In the north (*4° S), the
Humboldt Current feeds the westward flowing South Equatorial Current.
Upwelling-favourable coastal winds lead to the formation of a coastal upwelling jet,
5 The Peruvian-Chilean Coastal Upwelling System
Fig. 5.3 Key currents of the Peru-Chile current system (from Codispoti et al. 1989)
known as Peru Coastal Current (Fig. 5.3). Generally, the Peru Coastal Current is
strongest from April to September. Gunther (1936) ﬁrst distinguished a poleward
countercurrent situated between these equatorward currents. This intermediate
current, called the Peru Countercurrent or Gunther Current, is a weak and irregular
southward flow along 80° W and is usually observed only as a subsurface current. It
is strongest near 100 m depth, but reaches to about 500 m.
Beneath the equatorward surface currents, a poleward flowing undercurrent
dominates the subsurface and the shelf (Brink et al. 1983; Penven et al. 2005;
5.5 Physical Oceanography
Montecino and Lange 2009). This so-called Peru-Chile Undercurrent is the
dominant undercurrent within the ﬁrst 180 km from the coast (Huyer 1980), and
serves as a major source of upwelled water (Penven et al. 2005; Kessler 2006;
Pennington et al. 2006). It originates from the eastward flowing Equatorial
Undercurrent, which splits at the Galapagos Islands into two branches, one propagating to the south to form the undercurrent, the other flowing southeastward
reaching the coast at around 5° N (Lukas 1986; Penven et al. 2005; Kessler 2006).
In addition to coastal upwelling, the trade winds cause oceanic divergence or
surface Ekman transport away from the equator. This divergence forces local
upwelling along the equator, which produces a region of shallow thermoclines and
above-normal productivity that extends westward along the equator from the coast
(see also Chap. 9).
The vertical displacement of the upwelling surface is typically less than 50 m
between 15° S and 34° S. Source water is primarily Equatorial Sub-Surface Water
(ESSW) north of 18° S and Sub-Antarctic Water (SAW) south of that; beyond 35°
S there is an influence of Antarctic Intermediate water (AIW) (Blanco et al. 2001).
The two water masses are clearly distinct in their oxygen levels. ESSW has an
oxygen concentration of <2 ml/l, whereas SAW has a concentration of 4 ml/l.
Due to its close connection to the eastern limit of the equatorial currents, the
Peru-Chile current system is strongly affected by large-scale, basin-wide interannual variability caused by ENSO events (Brainard and McLain 1987; Carr et al.
2002). During stronger El Niño events, wind anomalies reduce or eliminate the
east-west downward tilt of the equatorial thermocline. In conjunction with eastward
propagating equatorial Kelvin waves comes the increase in sea surface height and
the deepening of the thermocline, oxycline, and nutricline in the eastern equatorial
Paciﬁc Ocean (Pennington et al. 2006). As a result, the equatorward water flow in
the Humboldt Current decreases or even reverses, and warm and nutrient-depleted,
but oxygen-richer equatorial water appears on the Peruvian shelf (Thiel et al. 2007).
The deepening of the thermocline results in a substantial decrease in upwelling
intensity and, consequently, in reduced nutrient supply and primary production.
The other phase of ENSO La Niña, leads to a shallower thermocline and a
strengthened upwelling, associated with colder sea surface temperatures on the
Peruvian-Chilean shelf (Pennington et al. 2006; Thiel et al. 2007; Quinones et al.
2010). On the more local scale, seasonality and anomalies in local wind systems, as
well as other local factors, such as coastal topography, the width of the continental
shelf, and the source of the upwelled waters, produce strong seasonal and latitudinal
variability in primary production within the Peru-Chile current system (Thomas
et al. 2004). Coastally trapped waves appear especially energetic during El Niño
periods and weaker during La Niña periods and austral winter (Shaffer et al. 1999).
Coastally trapped waves frequently propagate poleward along the entire Peru
and Chile coastlines, traceable to wind fluctuations in equatorial regions
(Hormazábal et al. 2001). The waves raise and lower the pycnocline/nutricline,
influencing the effectiveness of upwelling, with dominant frequencies of days to
weeks off Peru (Enﬁeld et al. 1987) and *50-day periods off northern and central
Chile (e.g., Rutllant et al. 2004b). Ramos et al. (2006) show that variability in
5 The Peruvian-Chilean Coastal Upwelling System
equatorial source waters at both annual and semi-annual periods strongly modulates
isotherm depth along the coast. At the shorter time-/space scales, diurnal cycles in
wind stress are important contributors to forcing along the arid northern Chilean
coast, especially in summer (Rutllant et al. 1998), but become less important with
increasing latitude, where storm-mediated variability on 3- to 7-day cycles increases
(Strub et al. 1998), with a maximum in austral winter.
While the continental shelf near Pisco (Peru) is a region of enhanced upwelling,
there are a number of individual upwelling centres along the Chilean coastline.
South of 26° S, within the area where coastal upwelling is more seasonal,
Coquimbo Bay (30° S) and the Bay of Concepción (37° S) are important regional
coastal upwelling centres.
The Coquimbo Bay system is located between two capes (Punta Lengua de Vaca
and Punta Pájaros) and is the site of intensive ﬁsheries and eco-tourism. Filaments,
including bifurcated upwelling ﬁlaments (Moraga et al. 2001), are known to generate at Punta Lengua de Vaca, contributing cold, nutrient-high waters to the coastal
system (e.g., Montecino et al. 2006). Marín et al. (2003b) have shown that
cold-water squirts (i.e. ﬁlaments leading to counter-rotating eddy pair) generate at
the northern end of the bay system (near Punta Pájaros).
Satellite observations (Fig. 5.4) and Lagrangian drifter data (Marín and Delgado
2007) have shown that this squirt is a recurrent feature in the area, reaching distances on the order of 140 km offshore. Squirt speeds, estimated both through
feature-tracking analysis (Marín et al. 2003b) and Lagrangian drifters (Marín and
Delgado 2007), range between 0.2 and 0.3 m/s. Thus, considering that the lifetime
Fig. 5.4 SeaWiFS image (chlorophyll-a) from 20 January 1999 (left panel) and 19 January 2002
(right panel). Long arrows show the main axis of the squirt; short arrows correspond to the ‘‘tip of
the hammer.’’ Taken from Marín et al. (2003b)
5.6 Regional Aspects
of a single squirt is related to the active period of equatorward wind events, which
for the area range between 3 and 7 days (Rutllant et al. 2004a), coastal organisms
trapped within the squirt are likely to reach 100–200 km offshore in a period of less
than a week.
Another important upwelling focus in the northern zone is the Mejillones
Peninsula (23° S). Observational (e.g., Marín et al. 2003a) and modelling studies
(Escribano et al. 2004) have shown that the dynamics of the coastal ecosystems in
that area largely depend on the generation of upwelling ﬁlaments here. The generation of ﬁlaments in the northern tip of the peninsula (Punta Angamos) has been
identiﬁed as the main mechanism of nutrient enrichment in the surface layers
(Marín and Olivares 1999). Furthermore, upwelling shadows within Mejillones
Bay, an equator-facing bay located in the northern end of the peninsula, have been
dynamically linked to the generation of bifurcated ﬁlaments at Punta Angamos
(Marín et al. 2003a). This shadow is an important physical structure within the bay,
affecting primary production (Marín et al. 2003a) and the retention of planktonic
organisms (Olivares 2001).
An alternative mechanism, described as an upwelling trap by Castilla et al.
(2002) and also related to the coastal upwelling dynamics, generates higher temperatures inside Antofagasta Bay, a pole-facing bay at the southern end of the
Mejillones Peninsula. In this case also the physically generated structure contributes
to the retention of planktonic organisms. Thus, mesoscale flow features (upwelling
shadows and upwelling traps), associated with cold-water ﬁlaments, seem to play
an important role not only in relation to the biological productivity of coastal
upwelling regions but also as mechanisms for the retention of coastal planktonic
From the standpoint of coastal wind forcing, the Peru-Chile upwelling system can be
divided into two latitudinal areas with a transition near 26° S in northern Chile
(Figueroa 2002). From 26° S to the north, upwelling-favourable winds exist
year-round; south of this latitude greater seasonality is observed (Thiel et al. 2007).
Time series of the upwelling index (see Sect. 2.1) provides information on seasonal
and interdecadal variations of the alongshore wind conditions. To illustrate spatial
variations of wind conditions along the west coast of South America from 15° S to
45° S, we use monthly averaged upwelling indices for the period 1981–2002, provided by NOAA’s Paciﬁc Fisheries Environmental Laboratory (PFEL) (Fig. 5.5).
Note that this data set excludes the highly productive Peruvian upwelling region
located at 5°–15° S. Mendo and Castillo (1987) present upwelling-index data for the
period 1953–1985 for the Peruvian coast north of 15° S.
5 The Peruvian-Chilean Coastal Upwelling System
Fig. 5.5 Time series of monthly mean upwelling index at various locations along the coasts of Peru
and Chile for the period 1 January 1981 to 31/12/2001. Data source: NOAA, http://www.pfeg.noaa.
gov/products/PFEL/modeled/indices/upwelling/SA/upwell_menu_SA.html [accessed 5 April 2016]
From this time series, it is evident that the overall magnitude of upwelling
favourable wind stress strongly decreases southward along the coast. At the northernmost Peruvian location (15° S), the upwelling index peaks at values >5 m2/s
(corresponding to an offshore mass transport of >600 tonnes per 100 m of coastline).
The magnitude of the upwelling index reduces to *2.5 m2/s in the northern portion
of Chile (24°–36° S) and drops to relatively low values of <1 m2/s farther to the
At 15° S, upwelling-favourable alongshore winds are a largely continuous
phenomenon (Fig. 5.5a). Interestingly, the time series indicates climate shifts of
surface wind systems and the associated magnitude of the upwelling index. That is,
the periods 1983–1988 and 1996–2002 experienced stronger upwelling winds with
a upwelling index of *5 m2/s on average, but these periods were interrupted by a
period (1989–1995) during which the upwelling index was markedly reduced by
half to *2.5 m2/s.
At 21° S, weak but continuous upwelling-favourable winds occurred until 1996,
followed by a transition to a regime of markedly reduced alongshore winds. In this
northern region (15°–21° S), we can also clearly identify ENSO-related wind disturbances. Substantial weakening of the upwelling index occurred during the particularly strong 1982/83 and 1998/99 El Niño events. In contrast, alongshore
coastal winds farther away from the equator are not much influenced by such ENSO
The upwelling dynamics off the northern portion of Chile (24° S–36° S) have a
pronounced seasonal cycle (Fig. 5.5b). Upwelling-favourable winds develop regularly during austral summer, but tend to vanish during austral winter. This behaviour
is characteristic of seasonal coastal upwelling systems. The wintertime disappearance
of offshore Ekman drift generally supports the survival of anchoveta eggs off Chile.
While the continental shelf south of 36° S still occasionally experiences
upwelling-favourable wind events, the magnitude of such events is much smaller. At
times, this region is exposed to extended periods of reversed, downwelling favourable wind conditions, which explains the relatively low productivity of this region.
Primary Production and Influences of Sub-Surface
The continental shelf of the Peru-Chile coastal upwelling system is narrower
compared with the other eastern boundary currents (compare with Figs. 4.14, 6.16
and 7.14). The widths of both the inner and outer shelves are far less than
50 km between 15° and 35° S (Fig. 5.6, ﬁrst panel), and the continental slope is
very steep, especially between 20° S and 34° S. Between 7° and 10° S, both inner
and outer shelves broaden and extend beyond 100 km. The shelf width can be used
as a proxy for the availability of benthic iron, which for narrow shelves can limit
primary production (Carr and Kearns 2003).
Ekman transport is offshore year-round throughout most of this system. The
regions north of 15° S and between 22° and 40° S display the largest Ekman transports of 0.5–1 m2/s in July–September and >0.5 m2/s in October–December,
respectively (Fig. 5.6, second panel). Both transport and seasonal range are minimum
between 16° and 22° S (compare with Fig. 5.5). Maximum sea-surface-temperature
Fig. 5.6 Variation in shelf width and seasonal changes in offshore Ekman transport, chlorophyll
concentration, primary production, oxygen and phosphate concentrations with latitude in the
Peru-Chile upwelling system. The horizontal black lines divide the system into separate zones
(from Carr and Kearns 2003)
5 The Peruvian-Chilean Coastal Upwelling System
anomalies (not shown) occur in April to June in most of the region with maximum
values >5 °C north of 15° S.
Biomass peaks in January–March north of 22° S and in October–December to
the south. The maximum chlorophyll-a concentration exceeds 5 mg/m3 north of
20° S and south of 35° S; seasonal ranges are at maximum in these regions as well
(Fig. 5.6, third panel). Primary production has the same seasonal cycle as chlorophyll concentration and is at maximum (>4 g C/m2/day) north of 18° S in January–
March and south of 34° S in October–December) (Fig. 5.6, fourth panel). Primary
production attains a minimum at 27° S (*2 g C/m2/day).
As stated above, upwelling source water is primarily Equatorial Sub-Surface
Water (ESSW) north of 18° S and Sub-Antarctic Water (SAW) south of that.
Beyond 35° S there is an influence of Antarctic Intermediate Water (AIW) (Blanco
et al. 2001). The two water masses are clearly distinct in latitudinal variations in
oxygen concentration (Fig. 5.6, ﬁfth panel). The phosphate content of the source
water is the highest of all eastern boundary currents (Fig. 5.6, last panel; compare
with Figs. 4.14, 6.16 and 7.14) and exceeds 2 mM/m3 both north of 20° S and
south of 34° S. The seasonal variation in source water characteristics reflects the
poleward transport of ESSW in the undercurrent (maximum in April–June) and the
equatorward transport of SAW (maximum in July–September) as seen in seasonal
variations in oxygen. The minimum in nutrient concentration of the source water
near 27° S is reflected in the chlorophyll concentrations.
Friedrich and Codispoti (1981) located the front between SAW and ESSW
at *15° S and found maximum influence of low nutrient-high oxygen SAW in
July–September (which is also the strongest upwelling period). Carr and Kearns
(2003) identiﬁed a similar maximum in SAW influences in the same months, but
maximum variability was observed at 18° S rather than 15° S. Carr and Kearns
(2003) point out that this difference is due to different methodologies rather than
Phytoplankton Blooms and Anchoveta Spawning
Winds along the coastline of Peru are favourable for upwelling throughout most of
the year. Generally, the alongshore wind magnitude intensiﬁes during austral winter
months (June–September) (Fig. 5.7). Due to upwelling and incursions of warmer
tropical water, the coastal waters off Peru undergo a seasonal temperature cycle in
which the warmest (coldest) periods occur at the end of austral summer (winter).
The reader should note that seasonal atmospheric heat-flux variations, which are
less pronounced near the equator than at higher latitudes, are not the cause of this
pronounced seasonal temperature cycle. Instead, SST anomalies are rather controlled by two opposing processes: