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2 Cultural, Social and Economic Relevance

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164



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



165



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).



5.3



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,



166



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).



5.4



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

fjords.

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



167



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

year round.



5.5



Physical Oceanography



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,



168



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



169



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



170



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.



5.6



Regional Aspects



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



171



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

species.



5.7

5.7.1



Seasonality

Ekman Transport



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.



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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

south.

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

events.



5.7 Seasonality



173



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.



5.7.2



Primary Production and Influences of Sub-Surface

Currents



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)



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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

physical processes.



5.7.3



Phytoplankton Blooms and Anchoveta Spawning

off Peru



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:



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