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GEAN CIRCUI.ATIOII IN TI.E EASTERN PACIFIC AhD EL NTNO
Pablo Lagc
Inetituto Geofigico del peru
Lima, Peru
ABSTRACT
Recent atmospherie and oceanographic studies show that sea slrface
temperature (SST) anomalies in the tropical Pacific plays an imporiant mle in the
global etmospheric circulation and climate. The SST- anomaliee, in turn, are a
consequence of anomalous fluetuations of surface winds that drive the circulation
in the tropicel Pacific. When the SST is anomalously large along the coast of
Ecuador and Peru as far south as 12 degrees, the name "El -Nino plienomenon,, has
been given. Tfp causes of El Nino sre not fully understood. However, the paper
will describe the recent scientific idees rclated to the warming of watere off
western South America based m observations and nunericel rnodel resulte.
Normally the climale m the coasL of Peru is cool end foggy, unlike the hot and humid
climate of the east cosst of South Arnerica at the same latituOe-.--frris is due to the presence
of cold water derived partly from the Peru current, which flows from south to north parallel
to the coast bringing water from the Antarctic, and partly from the upweliing of
low-temperature sr.rbsurface water from a depth of r-00 nretres or more, resulting from- ttre
action of the winds along the coast. The biological effect of the upwelling is extremely
important. The constant renewal of the nutrientJ enabbs the phytoplankton a-nO zooplankton
to develop as part of a food cfiain crrlminating in lhe anchovy. As a result, anchovy catches
are abundant. In 1970, Peru u.rpplied over me-fifth of the worldrs fish protein. During t5e
sljlTuner months, the climate alters slightly as a result of Lhe presence of warmer tropical
waier.
"El Nino" ig an atmosphere-regulated ceanographic phenomenon which sppears ffy1re or
less periodically in the form of very warm water oir lne Plruvian coast and torrential rain in
northern Peru, eausing the deaths of millions of anchovies and guano-producing birds. TIE
presence of warm water alters the climate along the coast of Peri md ihr;;ghout the world.
The clear skies typical of summer last an extralew months. In lhe United StJtes and Europe,
the winter is colder. . In India, the monsoon is less pronounced. In the pecific, rainfall
increases and abnormal hurricanes develop. During tha last 25 years, this phenomenon has
occurred in 1957-58, 1965, 1972-71 and 1976, the last period Uut onl navi'nl the greateet
intensity.
Theories Reqardino El Nino
The ceuses of the El Nino phenomenon are still not entirely ctear. The most recent
theories link the presence of werm water off the coasts of Ecuador and peru to atmospheric
events lhousands of kilometres to Ehe west, in the lropical central pacific. Dr. Klaus Wyrtki,
oceanographer et the University of Hawaii, maintains that El Nino is *re result of the action
of easterly and south-easterly winds in .the cenbral equatorial region of the pacific Ocean
(wyrtki, .\975; ivyrtki and Meyers, Lg76). wind flucruations generate a disturtance in the
Tgan which spreads eastwards in the form of a wave. As the i"u" gr""d"; 6" thermocline
sinks' The thermocline is the layer separeting the warmer and less dense zurface water from
the colder and denser deep water. when the thermocline lies deep, the votunre of warmer
water increeses. Dr. Wyrtki has docunented his observations of lne winds, sea tevel and
thermocline depth throughout the tropical Pacifie for the period 1950-1978. On the basis of