What is El Niño & La Niña?
The El Niño – Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of
waters in the central and eastern tropical Pacific Ocean and the patterns of sea level pressure, lower- and upper-level
winds, and tropical rainfall across the Pacific basin.
When Does This Occur?
On periods ranging from about two to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal. This irregular oscillation between warm and cool patterns, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.
ENSO Phases
ENSO-neutral: Normally, strong trade winds blow from the east along the equator, pushing warm water into the western Pacific Ocean.
El Niño conditions occur when abnormally warm waters accumulate in tropical latitudes of the central and eastern Pacific Ocean associated with a weakening of the low-level easterly winds. Consequently, tropical rains that usually fall over Indonesia shift eastward.
La Niña conditions occur when cooler-than-average waters accumulate in the central and eastern tropical Pacific, associated with a strengthening of the low-level easterly winds over the central tropical Pacific. Heavy rainfall occurs over Indonesia and Malaysia.
The Thermocline is a layer of water in which there is an abrupt change in temperature separating the warmer surface water from the colder deep water.
ENSO Impacts
Although the strongest signal of ENSO impacts is in the areas closest to the equatorial Pacific, El Niño and La Niña are such powerful forces that they can shift seasonal temperature and precipitation patterns around the globe. These shifts, known as teleconnections, occur via the effects of tropical sea-surface temperatures on the upper atmosphere. When different parts of the tropical ocean warm and cool and the pressure gradients shift, the atmospheric wind patterns also shift to alter precipitation patterns. Because each El Niño and La Niña event has unique characteristics of timing, intensity and specific pattern changes, such shifts are never exactly the same during every El Niño and La Niña event. In addition, the atmospheric effects due to changes in sea-surface temperatures are responsible for only part of the regional climate observed; chaotic fluctuations within the atmosphere and sea surface temperatures in other areas of the globe also influence the weather and climate we experience. Because of this, anticipated ENSO impacts in seasonal forecasts are treated probabilistically and not with absolute certainty
Source : https://www.weather.gov
When Does This Occur?
On periods ranging from about two to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal. This irregular oscillation between warm and cool patterns, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.
ENSO Phases
ENSO-neutral: Normally, strong trade winds blow from the east along the equator, pushing warm water into the western Pacific Ocean.
El Niño conditions occur when abnormally warm waters accumulate in tropical latitudes of the central and eastern Pacific Ocean associated with a weakening of the low-level easterly winds. Consequently, tropical rains that usually fall over Indonesia shift eastward.
La Niña conditions occur when cooler-than-average waters accumulate in the central and eastern tropical Pacific, associated with a strengthening of the low-level easterly winds over the central tropical Pacific. Heavy rainfall occurs over Indonesia and Malaysia.
The Thermocline is a layer of water in which there is an abrupt change in temperature separating the warmer surface water from the colder deep water.
ENSO Impacts
Although the strongest signal of ENSO impacts is in the areas closest to the equatorial Pacific, El Niño and La Niña are such powerful forces that they can shift seasonal temperature and precipitation patterns around the globe. These shifts, known as teleconnections, occur via the effects of tropical sea-surface temperatures on the upper atmosphere. When different parts of the tropical ocean warm and cool and the pressure gradients shift, the atmospheric wind patterns also shift to alter precipitation patterns. Because each El Niño and La Niña event has unique characteristics of timing, intensity and specific pattern changes, such shifts are never exactly the same during every El Niño and La Niña event. In addition, the atmospheric effects due to changes in sea-surface temperatures are responsible for only part of the regional climate observed; chaotic fluctuations within the atmosphere and sea surface temperatures in other areas of the globe also influence the weather and climate we experience. Because of this, anticipated ENSO impacts in seasonal forecasts are treated probabilistically and not with absolute certainty
Source : https://www.weather.gov
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