El Nino

El Nino is a weather phenomenon that occurs in the Equatorial Pacific Ocean normally lasting between 6 to 18 months.  First we should understand the normal conditions in the Equatorial Pacific Ocean to better understand about an El Nino.

Normal Conditions in the Equatorial Pacific Ocean (ENSO neutral) & La Nina:

The Pacific Ocean stretches half way around the globe at the Equator from South American coast at 80W to Indonesia at 120 E. The Pacific ocean is very large and is divided into three parts viz. West Pacific, Central Pacific and East Pacific ocean. The ocean surface in the West Pacific is normally warm at around 29 C while the ocean surface temperature in the East Pacific is much cooler at around 22 C. to 26 C. depending upon the time of the year as shown in the seasonal changes of average ocean temperature map from CPC.

The ocean surface pressure is lower in the West Pacific compared to Central Pacific and East Pacific and hence Trade winds blow Westwards from the East Pacific towards the West Pacific. The ocean current also moves from East to West. Upwelling of cold water takes place near the coast of Peru/Equador. These deeper waters have nutrients that stimulate the growth of microscopic photosynthetic algae (“Phytoplankton“). Phytoplankton, in turn, serve as food for the zooplankton. Both phytoplankton and zooplankton serve as food for fish thereby increasing the fish production in the East Pacific.
Due to high ocean surface temperature in the West Pacific, there is evaporation and clouding and rain over West Pacific while the East Pacific there is dry condition.  Sun heated warm water piles up in the West Pacific ocean and this is the reason why the temperature of the West Pacific ocean is higher than that of the East Pacific ocean.

La Nina is essentially the strengthening of the Normal conditions. During a La Nina, the trade winds strengthens further compared to Normal conditions, which causes the thermocline to tilt further, causing warm water to pool against Australia. The rain follows the location of the warm water, so Australia experiences above average rainfall, while South America experiences below average rainfall.

El Niño:

El Niño is a naturally occurring climate phenomenon characterized by an abnormal warming of sea surface temperatures across the eastern and central equatorial Pacific Ocean. An El Niño event typically lasts between 6 and 18 months and develops when the normally prevailing easterly trade winds weaken or occasionally reverse.

Under normal conditions, these trade winds push warm surface waters westward toward Indonesia and northern Australia, allowing cooler, nutrient-rich waters to upwell along the coasts of Peru and Ecuador. During an El Niño event, the weakened trade winds permit the warm western Pacific waters to spread eastward across the Pacific Ocean. This suppresses the normal upwelling of cold water in the eastern Pacific and significantly alters ocean-atmosphere interactions.

The reduction in upwelling means that the waters off the coasts of Peru and Ecuador become warmer and contain fewer nutrients. As a result, the growth of microscopic photosynthetic algae (phytoplankton) and zooplankton declines, leading to reduced fish productivity and disruptions to marine ecosystems in the eastern Pacific.

The term “El Niño” is Spanish for “The Boy” or “Christ Child”. The name was given by Peruvian fishermen because the unusual warming of coastal waters often became noticeable around Christmas time.

Atmospheric Impacts

Changes in ocean temperatures also influence the atmosphere. Tropical thunderstorms are fueled by warm, moist air rising from the ocean surface. The warmer the ocean, the stronger and more widespread the thunderstorm activity.

As the warmest waters of the Pacific shift eastward during an El Niño event, the region of strongest tropical convection and thunderstorm development also moves eastward. Consequently, rainfall that normally occurs over the tropical rainforests of Indonesia and surrounding regions shifts toward the central and eastern Pacific Ocean. Increased rainfall can occur over parts of Peru and Ecuador, while regions such as Indonesia, Australia, and nearby areas often experience below-normal rainfall and an increased risk of drought.

Southern Oscillation (SOI)

The Southern Oscillation is a large-scale atmospheric phenomenon characterized by a see-saw pattern of surface air pressure fluctuations between the eastern and western tropical Pacific Ocean. When surface air pressure is higher than normal over the eastern tropical Pacific, it is typically lower than normal over the western tropical Pacific, and vice versa.

Because these atmospheric pressure changes occur in close association with variations in tropical Pacific sea surface temperatures, scientists refer to the coupled ocean-atmosphere phenomenon as the El Niño–Southern Oscillation (ENSO).

Southern Oscillation During ENSO Phases

During an El Niño event, higher-than-normal surface air pressure develops over Indonesia and the western tropical Pacific, while lower-than-normal pressure occurs over the eastern tropical Pacific. These pressure anomalies weaken the normal east-to-west pressure gradient across the Pacific.

During a La Niña event, the pattern reverses. Surface air pressure becomes lower than normal over Indonesia and the western tropical Pacific, and higher than normal over the eastern tropical Pacific. This strengthens the normal pressure gradient and enhances the trade winds.

Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is one of the primary indicators used to monitor the strength and phase of the Southern Oscillation. It is calculated from the difference in surface air pressure anomalies between Tahiti, French Polynesia and Darwin, Australia.

The SOI provides a measure of the atmospheric component of ENSO:

• Negative SOI values are associated with El Niño conditions.
• Positive SOI values are associated with La Niña conditions.

SOI During El Niño

During El Niño episodes, the SOI typically becomes strongly negative because:

• Air pressure at Tahiti is lower than average.
• Air pressure at Darwin is higher than average.

This reflects a weakening of the normal Pacific atmospheric circulation.

SOI During La Niña

During La Niña episodes, the SOI typically becomes positive because:

• Air pressure at Tahiti is higher than average.
• Air pressure at Darwin is lower than average.

This reflects a strengthening of the normal Pacific atmospheric circulation and trade winds.