Students also need to be
aware of periodic variations in climate both over the long and the short
term. Studying short term changes like the El Nino / La Nina will allow
students to appreciate periodic changes in climate.
El
Nino is the name given to the occasional
development of warm ocean surface waters along the
coast of Ecuador and Peru. When this warming occurs
the usual upwelling of cold, nutrient rich deep ocean
water is significantly reduced. El Nino normally
occurs around Christmas and usually lasts for a few
weeks to a few months. Sometimes an extremely warm
event can develop that lasts for much longer time
periods. In the 1990s, strong El Ninos developed in
1991 and lasted until 1995, and from Autumn 1997
to Spring 1998.
Normal conditions - "Walker Cell"
(La Nada?) The formation of an El Nino is linked with
the cycling of a Pacific Ocean circulation pattern known
as the southern
oscillation. In a normal year, a surface low
pressure develops in the region of northern Australia
and Indonesia and a high pressure system over the coast
of Peru. As a result,
the trade winds over the Pacific
Ocean move strongly from east to west. The easterly flow
of the trade winds carries warm surface waters westward,
bringing convective storms to Indonesia and coastal Australia.
Along the coast of Peru, cold bottom water wells up to
the surface to replace the warm water that is pulled
to the west.
The Walker Cell and a
cross-section of the Pacific Ocean.
Source: Waugh
El Nino conditions: In an El Nino year, air pressure drops
over large areas of the central Pacific and along the coast of South
America. The normal low pressure system is replaced by a weak high in
the western Pacific (the southern oscillation). This change in
pressure pattern causes the trade winds to be reduced. This reduction
allows the equatorial counter current (which flows west to east
to accumulate warm ocean water along the coastlines of Peru and Ecuador.
This accumulation of warm water causes the thermocline to drop in
the eastern part of Pacific Ocean which cuts off the upwelling of cold
deep ocean water along the coast of Peru. Climatically, the development
of an El Nino brings drought to the west Australia, rains to the Ecuador coast of South America, and convective storms and hurricanes
to the central Pacific.
The white streak edged by red shows is
the presence of the El Nino causes the height of the ocean along the
equator to increase from the middle of the image to the coastline of
Central and South America. The colours indicate the height of the water
above normal.
La Nina - "Low pressure over
'Australian' Pacific excessively strong"
After an El Nino event weather
conditions usually return back to normal. However, in some years the
trade winds can become extremely strong and an abnormal accumulation of
cold water can occur in the central and eastern Pacific. This event is
called a La Nina. A strong La Nina occurred in 1988 and
scientists believe that it may have been responsible for the summer
drought over central North America. The most recent La Nina began
developing in the middle of 1998 and was persistent into the winter of
2000. During this period, the Atlantic Ocean has seen very active
hurricane seasons in 1998 and 1999. In 1998, ten tropical storm
developed of which six become full-blown hurricanes. One of the
hurricanes that developed, named Mitch, was the strongest October
hurricane ever to develop in about 100 years of record keeping. Some of
the other weather effects of La Nina include abnormally heavy
monsoons in India and Southeast Asia, cool and wet winter weather in
southeastern Africa, wet weather in eastern Australia, cold winter in
western Canada and northwestern United States, winter drought in the
southern United States, warm and wet weather in northeastern United
States, and an extremely wet winter in southwestern Canada and
northwestern United States.
The presence of the La Nina causes the
height of the ocean either side of the equator to decrease from the
middle of the image to the coastline of North, Central, and South
America.
