What is El Niño ?
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 Niño. 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.
In the figure above, low atmospheric surface pressure (L) is located within the warmer water ocean surface .In the absence of El Niño 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.
Typically, El Niño 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 Niño of the same intensity may not give rise to exact replicate of climate patterns.
How is El Niño linked with the atmospheric conditions ?
During El Niño 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.
This alternating atmospheric surface pressure pattern in the tropical Pacific ocean as the oceanic condition changes from El-Niño to normal and vice versa is called the Southern Oscillation. The coupling relationship between the atmosphere and the ocean during El Niño events is known as El Niño-Southern Oscillation (ENSO).
Is there an opposite phase to El Niño (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 Niña – the opposite of El Niño. 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 Niño 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-Niño condition. The more negative the index the more intense the El Niño. On the other hand, high positive values of the SOI indicate a La Niña
A strong El Niño/La Niña usually corresponds to the persistent SOI of 1.5 (negative for El Niño and positive for La Niña) and above while the moderate one fluctuates between 0.8 and 1.5. A weak El Niño hovers between 0.4 and 0.8 of the SOI values.
Frequency of occurrence of El Niño/La Nina
In the last 50 years, El Niño had occurred 12 times. The two strongest El Niño of the last century occurred in 1982-83 and 1997-98. The following table lists the El Niño years:
On the other hand, the frequency of the occurrence of La Niña is much less than that of El Niño. The years of occurrence of La Niña 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.
What are the typical impacts in Malaysia ?
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.
The Malaysian Meteorological Service 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.