El Nino is associated by high air pressure in the Western Pacific and low air pressure in the Eastern Pacific. During El Nino, the trade winds decrease, ocean water piles up off South America, the sea surface temperature increases in the Eastern Pacific, and there is a shift in the prevailing rain pattern from the Western Pacific to the Western Central Pacific. El Nino cycle is known to occur at an interval of four years. Though, in the study of over a century, at times, the cycle has lasted for even seven years. Sometimes, the cycle period is short. The exact reason and life span of El Nino are not very well reasoned but the warm El Nino phase normally lasts just for about 9 months or so. Modern day research and reanalysis techniques have managed to find at least 26 El Nino events since 1900, with the 1982-83, 1997-98 and 2014-2016 events among the strongest on record.

Peru on the East Pacific coast of South America is well associated with El Nino. El Nino may have led to the demise of the Moche and other pre-Columbian Peruvian cultures. Human sacrifices to prevent excessive rains prevailed in Peruvian culture. In around 1525 when Francisco Pizarro made landfall on Peru, he noted rainfall occurring in the deserts. His, above description became the first written record of the impacts of El Nino. In an early recorded details from 1892, there is a mention of the term “El Nino” in respect of climate, where Captain Camilo told the geographical society congress in Lima that Peruvian sailors named the warm south-flowing current “El Nino”. ‘El-Ninyo’, as the pronunciation is supposed to be, means ‘the boy’ in Spanish and was named so by Peruvian fishermen after the Christ child, since its effects are generally first felt around Christmas.

La Nina normally follows an El Nino. It is sometimes, referred to as El Viejo, the ‘Little Girl’, anti-El Nino, or sometimes it is called ‘a cold event or a cold episode. This is because of unusually cold ocean temperatures in the Equatorial Pacific, compared to El Nino. The recurrence rate of La Nina is about half that of El Nino. During La Nina  air pressure is high in the eastern pacific and low in the western pacific. During La Nina, the trade winds increase, ocean water piles up in the Western Pacific, the sea surface temperature decreases in the Eastern Pacific, and the prevailing rain pattern also shifts farther west than normal. The impacts of La Nina on the global climate and ocean temperature is opposite those of El Nino.

Some of the initial signs of an El Nino are:
1.   Increase in sea level pressure over the Indian Ocean, Indonesia, and Australia.
2.   Drop of air pressure over Tahiti (Central And Eastern Pacific).
3.   Trade winds in the South Pacific tend to become weak or even change to eastwards.
4.   Rising warm air near Peru, causing rain in the northern Peruvian deserts.
5.   Warm sea water spreading from the West Pacific and the Indian Ocean to the East Pacific, taking the rain with it and causing extensive drought in the Western Pacific and rainfall in the normally dry Eastern Pacific.

El Nino and La Nina are parts of an oscillation in the ocean-atmosphere system called the El Nino-Southern Oscillation, or ENSO cycle. El Nino and La Nina are the most powerful meteorological phenomenon on the earth and are believed to affect the climate across more than half the planet. Mathematically, El Niño is sustained differences in Pacific Ocean surface temperatures when compared with the average value at least 0.5°C, averaged over the East-Central Tropical Pacific Ocean. When this happens for less than five months, it is classified as El Nino or La Nina conditions. If the anomaly persists for five months or longer, it is called an El Nino or La Nina episode.

Associated Theories
Easterly trade winds blowing towards the west, across over the Pacific, normally, pushes warm surface water away from the South American coast towards Australia and  Philippines providing cold and nutrient-rich waters on the Peruvian coast & thus supporting diverse marine ecosystems, and major fisheries. Thus, in normal, non-El Nino conditions, trade winds blow toward the west across the tropical Pacific, away from South America. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 30 to 60 cm higher offshore Indonesia than across the Pacific, offshore Ecuador. The sea-surface temperature is also about 8 degrees Celsius, warmer in the west. Cooler ocean temperatures dominate offshore northwest South America, due to an upwelling of cold water from deeper levels. The Southern Oscillation is the atmospheric counterpart of El Nino. It is an oscillation in air pressure between the tropical eastern and the western Pacific Ocean waters.

During El Nino however, the trade winds are much lighter in the Central and Western Pacific. Due to this warm water is stagnant on the surface. Activity of nutrients produced by the upwelling remains down, disrupting the normal eco system. The warmth spreads eastward, the trade winds, which are partly controlled by the temperature and pressure differences between the two sides of the Pacific, get even weaker. So there’s a double deteriorating effect; the chilly waters that usually help cool down the South American coast stay trapped deep below the surface, and the winds that would help cool things down are not there. These changes set up a feedback loop between the atmosphere and the ocean that boosts El Nino conditions. The location of tropical storms shifts eastward during an El Nino because atmospheric moisture is energy for thunderstorms, and the greatest amount of evaporation takes place above the ocean’s warm waters. The clouds and rainstorms associated with warm ocean waters also shift toward the east. The warm waters release so much energy into the atmosphere that weather changes all over the planet. An El Niño creates stronger wind-shear and more-stable air over the Atlantic, which makes it harder for hurricanes to form. However, the ‘warmer than average’ ocean temperatures boost Eastern Pacific hurricanes, contributing to more-active tropical storm seasons.

Strong El Ninos are also associated with above average precipitation in the southern tier of the United States from California to the Atlantic coast. The cloudier weather typically causes below average winter temperatures for those states, while temperatures tilt warmer than average in the northern tier of the United States. Rainfall is often below average in the Ohio and Tennessee valleys and the Pacific Northwest during an El Niño. Record rainfall often strikes Peru, Chile and Ecuador during an El Nino year.

A wide variety of disasters have been blamed on the El-Nino effect including,
1. A famine in Indonesia in 1983 and bush fires in Australia arising from droughts.
2. Rainstorms in California.
3. Destruction of anchovy fishery off the coast of Peru.
4. Floods & damage in Americas.
5. Storms hit China.

6. Forest fires in South-East Asia and Brazil.
7. Snow fall in Guadalajara.
8. Abnormal temperatures are seen in US during summers or winters.
9. higher than normal number of hurricanes in the Atlantic.
10. El Nino in 1876–77 gave rise to the most deadly famines of the 19th century. The 1876 famine alone in northern China killed up to 13 million people.

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