El Niño and La Niña opposite phases of what is known as the El Niño-Southern Oscillation (ENSO). El Niño is the warm phase, with warmer oceanic temperatures in the Central and East-Central Equatorial Pacific, while La Nina is the cool phase. The ENSO has far-reaching consequences in the Western Hemisphere, affecting weather across the globe.
The El Niño-Southern Oscillation (ENSO)
The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific (approximately between the International Date Line and 120 degrees West).
El Niño and La Niña are considered the ocean part of ENSO, where changes in sea temperatures (both at the surface and at depth) occur, while the Southern Oscillation is its atmospheric changes (changes in air pressure and trade winds).
El Niño means The Little Boy, or Christ Child in Spanish. El Niño was originally recognized by fishermen off the coast of South America in the 1600s, with the appearance of unusually warm water in the Pacific Ocean. The name was chosen based on the time of year (around December) during which these warm waters events tended to occur.
The term El Niño refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific. El Niño is accompanied by high air surface pressure in the tropical western Pacific.
During an El Niño event, westward-blowing trade winds weaken along the Equator. These changes in air pressure and wind speed cause warm surface water to move eastward along the Equator, from the western Pacific to the coast of northern South America.
These warm surface waters deepen the thermocline, the level of ocean depth that separates warm surface water from the colder water below. During an El Niño event, the thermocline can dip as far as 152 meters (500 feet).
This thick layer of warm water does not allow normal upwelling to occur. Without an upwelling of nutrient-rich cold water, the euphotic zone of the eastern Pacific can no longer support its normally productive coastal ecosystem. Fish populations die or migrate. El Niño has a devastating impact on Ecuadorian and Peruvian economies.
El Niño also produces widespread and sometimes severe changes in the climate. Convection above warmer surface waters bring increased precipitation. Rainfall increases drastically in Ecuador and northern Peru, contributing to coastal flooding and erosion. Rains and floods may destroy homes, schools, hospitals, and businesses. They also limit transportation and destroy crops.
As El Niño brings rain to South America, it brings droughts to Indonesia and Australia. These droughts threaten the region’s water supplies, as reservoirs dry and rivers carry less water. Agriculture, which depends on water for irrigation, is threatened.
Stronger El Niño events also disrupt global atmospheric circulation. Global atmospheric circulation is the large-scale movement of air that helps distribute thermal energy (heat) across the surface of the Earth. The eastward movement of oceanic and atmospheric heat sources cause unusually severe winter weather at the higher latitudes of North and South America. Regions as far north as the U.S. states of California and Washington may experience longer, colder winters because of El Niño.
Climate records of El Niño go back millions of years, with evidence of the cycle found in ice cores, deep sea muds, coral, caves and tree rings.
The cycle begins when warm water in the western tropical Pacific Ocean shifts eastward along the equator toward the coast of South America. Normally, this warm water pools near Indonesia and the Philippines. During an El Niño, the Pacific’s warmest surface waters sit offshore of northwestern South America.
Forecasters declare an official El Niño when they see both ocean temperatures and rainfall from storms veer to the east. Experts also look for prevailing trade winds to weaken and even reverse direction during the El Niño climate phenomenon. These changes set up a feedback loop between the atmosphere and the ocean that boosts El Niño conditions.
What causes an El Niño?
How often do El Niños occur?
El Niños occur every three to five years but may come as frequently as every two years or as rarely as every seven years. Typically, El Niños occur more frequently than La Niñas. Each event usually lasts nine to 12 months. They often begin to form in spring, reach peak strength between December and January, and then decay by May of the following year.
Their strength can vary considerably between cycles. One of the strongest in recent decades was the El Niño that developed the winter of 1997-98.
What happens when El Niño is not present?
In normal, non-El Niño conditions, trade winds blow toward the west across the tropical Pacific, away from South America. These winds pile up warm surface water in the western Pacific, so that the sea surface is about 1 to 2 feet (0.3 m to 0.6 m) higher offshore Indonesia than across the Pacific, offshore Ecuador.
The sea-surface temperature is also about 14 degrees Fahrenheit (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. This nutrient-rich cold water supports diverse marine ecosystems and major fisheries.
El Nino Impacts
The warmer waters in the central and eastern tropical Pacific Ocean have important effects on the world’s weather. The greatest impacts are generally not felt until winter or spring over the Northern Hemisphere, L’Heureux said. The 1982-83 El Niño is estimated to have caused more than $10 billion in weather-related damage worldwide.
Strong El Niños 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 Niño year. Fish catches offshore South America are typically lower than normal because the marine life migrates north and south, following colder water.
Normally, strong trade winds blow westward across the tropical Pacific, the region of the Pacific Ocean located between the Tropic of Cancer and the Tropic of Capricorn. These winds push warm surface water towards the western Pacific, where it borders Asia and Australia.
Due to the warm trade winds, the sea surface is normally about .5 meter (1.5 feet) higher and 45° F warmer in Indonesia than Ecuador. The westward movement of warmer waters causes cooler waters to rise up towards the surface on the coasts of Ecuador, Peru, and Chile. This process is known as upwelling.
Upwelling elevates cold, nutrient-rich water to the euphotic zone, the upper layer of the ocean. Nutrients in the cold water include nitrates and phosphates. Tiny organisms called phytoplankton use them for photosynthesis, the process that creates chemical energy from sunlight. Other organisms, such as clams, eat the plankton, while predators like fish or marine mammals prey on clams.
Upwelling provides food for a wide variety of marine life, including most major fisheries. Fishing is one of the primary industries of Peru, Ecuador, and Chile. Some of the fisheries include anchovy, sardine, mackerel, shrimp, tuna, and hake.
The upwelling process also influences global climate. The warm ocean temperature in the western Pacific contributes to increased rainfall around the islands of Indonesia and New Guinea. The air influenced by the cool eastern Pacific, along the coast of South America, remains relatively dry.
El Niño also affects precipitation in other areas, including Indonesia and northeastern South America, which tend toward drier-than-normal conditions. Temperatures in Australia and Southeast Asia run hotter than average. El Niño-caused drought can be widespread, affecting southern Africa, India, Southeast Asia, Australia, the Pacific Islands and the Canadian prairies.
The Hurricane Season and El Nino Impacts to T&T
Over the Atlantic basin, the amplified trough is associated with stronger upper-level westerly winds and stronger lower-level easterly trade winds, both of which increase the vertical wind shear and suppress hurricane activity. In addition to enhanced vertical wind shear, El Niño suppresses Atlantic hurricane activity by increasing the amount of sinking motion and increasing the atmospheric stability.
La Niña means The Little Girl in Spanish. La Niña is also sometimes called El Viejo, anti-El Niño, or simply “a cold event.“
La Niña episodes represent periods of below-average sea surface temperatures across the east-central Equatorial Pacific. La Niña is also accompanied by low air surface pressure in the tropical western Pacific
Global climate La Niña impacts tend to be opposite those of El Niño impacts. In the tropics, ocean temperature variations in La Niña also tend to be opposite those of El Niño.
During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
What causes a La Nina?
How often does a La Nina occur?
La Nina Impacts
The Hurricane Season and La Nina Impacts to T&T
During La Niña, the area of tropical convection and its Hadley circulation is retracted westward to the western Pacific and Indonesia, and the equatorial Walker circulation is enhanced. Convection is typically absent across the eastern half of the equatorial Pacific.
In the upper atmosphere, these conditions produce an amplified trough over the subtropical Pacific in the area north of the suppressed convection, and a downstream ridge over the Caribbean Sea and western tropical Atlantic. Over the central and eastern subtropical Pacific, the enhanced trough is associated with stronger upper-level winds and stronger vertical wind shear, which suppress hurricane activity. Over the Atlantic basin, the anomalous upper-level ridge is associated with weaker upper- and lower- level winds, both of which reduce the vertical wind shear and increased hurricane activity. La Niña also favors increased Atlantic hurricane activity by decreasing the amount of sinking motion and decreasing the atmospheric stability.