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What is El Niņo ? Every
three to seven years, a warm current of water replaces the usual cold current
of water off the west coast of Peru, South America.
This observed oceanic phenomenon is called El Niņo.
This warming is now known to occur over a wider area covering the
central and eastern Pacific and has linkage with the occurrence of some major
unusual weather conditions in different parts of the world like severe floods
and prolonged droughts. In
Southeast Asia, Indonesia and Australia, drier than normal conditions occur
whereas central and eastern equatorial Pacific are unusually wet. Typically,
El Niņo lasts between 9 to 18
months. It usually begins to develop in the early part of the year,
peaks at the end of the year and
declines by early part of the following year.
El Niņo of the same intensity may not give rise to exact replicate of
climate patterns. How
is El Niņo linked with the atmospheric conditions ? During El Niņo period, warmer water in the central and eastern Pacific supplies the atmosphere immediately above it with additional heat and moisture. It favours strong rising motion and thus lowers the atmospheric surface pressure in the rising motion area. The rising moist air condenses to form large areas of thunderstorm clouds and heavy rainfall in the area. In the western part of the Pacific including the Malaysian region, atmospheric pressure increases, resulting in relatively drier weather.
120o E 80o W In the figure above, low atmospheric surface pressure (L) is located within the warmer water ocean surface .In the absence of El Niņo or under normal condition as shown in the figure below, the atmospheric surface pressure over the western Pacific is usually low while that over the central and eastern Pacific is high. Under such condition, the western Pacific is generally wet and the central and eastern Pacific are generally dry.
This alternating atmospheric surface pressure pattern in the tropical Pacific ocean as the oceanic condition changes from El-Niņo to normal and vice versa is called the Southern Oscillation. The coupling relationship between the atmosphere and the ocean during El Niņo events is known as El Niņo-Southern Oscillation (ENSO). Is
there an opposite phase to El Niņo (La Nina) ? At
times, though not frequent, the sea surface temperature over the central and
eastern Pacific can become very much lower than usual.
This phenomenon is termed La Niņa – the opposite of El Niņo.
Under such condition, the atmospheric surface pressure over the
equatorial western Pacific decreases, giving rise to more cloud formation and
heavy rainfall. In
La Nina conditions, high atmospheric surface pressure (H) is formed over the
central and eastern Pacific while the low pressure is located more towards the
western Pacific. How
do we monitor El Nino and the atmospheric response ? The essential parameters used to monitor El Niņo and the atmospheric response include sea surface temperatures in the equatorial Pacific, ocean sub-surface temperatures down to a 150 m depth, abnormal cloudiness and rainfall patterns in the Pacific. Since the atmospheric surface pressure and ocean temperature are closely related, an atmospheric index known as the Southern Oscillation Index (SOI) is also used to measure the atmospheric response. This index is computed using the monthly fluctuations in the air pressure difference between Tahiti (representing eastern Pacific) and Darwin (representing western Pacific). When there is a significant negative value of SOI lasting for at least 6 months as shown in the figure below, we have an El-Niņo condition. The more negative the index the more intense the El Niņo. On the other hand, high positive values of the SOI indicate a La Niņa.
A
strong El Niņo/La Niņa usually corresponds to the persistent SOI of 1.5
(negative for El Niņo and positive for La Niņa) and above while the moderate
one fluctuates between 0.8 and 1.5. A
weak El Niņo hovers between 0.4 and 0.8 of the SOI values. Frequency
of occurrence of El Niņo/La Nina In
the last 50 years, El Niņo had occurred 12 times. The two strongest El Niņo
of the last century occurred in 1982-83 and 1997-98.
The following table lists the El Niņo years:
On
the other hand, the frequency of the occurrence of La Niņa is much less than
that of El Niņo. The years of occurrence of La Niņa are listed below:
What
are the climate changes during an El Nino ? Within
the tropics, cloudiness and associated rainfall shifts from western Pacific to
the central and eastern Pacific, resulting in abnormally dry conditions over
Malaysia, Indonesia, the Philippines and
northern Australia during an El Nino.
Drier/warm weather is also
observed in southeastern Africa, India and northern Brazil.
Wetter weather occurs along the west coast of tropical South America
and the gulf coast of North America as shown in the figure below. In
the presence of moderate/strong El Nino,
the rainfall over East Malaysia is much below average during the
southwest (June – August) and northeast monsoons (November – February) but
is below average over Peninsular Malaysia only during the southwest monsoon
(June – August). Weak
El Nino event is noted to have minimal impact on rainfall in Malaysia.
In addition, below and above average rainfall can also occur during
non-El Nino/La Nina years. Further Information The
Malaysia Meteorological Department continually updates its seasonal forecasts on
its web site at http://www.met.gov.my. The
Department can be contacted at 03-79587422 and 03-79569422.
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