El Niño/La Niña
Written November 2010
Every place in the world has a typical climate, a predictable seasonal pattern of rainfall and temperatures. Weather is highly variable day-to-day and deals with only short term timelines (a cold, rainy day during a hot, dry summer) and small geographical areas, but over time, the “average” weather paints a picture of an area’s climate.
Like weather, climate can also vary. Some years may be wetter than others, some cooler. The strongest cause of year-to-year climate variations around the world is called the El Niño/Southern Oscillation (ENSO) phenomenon.
ENSO refers to an increase or decrease of sea surface temperatures along the Equator in the Pacific Ocean. This temperature change in one part of the globe can lead to a change in weather patterns across its whole surface. This is because the earth has a conveyer belt-like system of ocean and wind currents that move heat and moisture across its surface. Adding or decreasing heat in one part of the conveyer belt can change the amount of heat and moisture carried to different areas or change the pattern of flow entirely. The heating and cooling during ENSO phases affect tropical storm and rainfall patterns, which change seasonal weather patterns throughout the world.
When the temperatures of the ocean’s surface in the Pacific are normal along the Equator this is a Neutral phase of ENSO. During the Neutral phase, the trade winds that blow east to west along the Equator blow normally. When temperatures are increased, it is called an El Niño phase; when decreased, La Niña.
During El Niño years, the surface of the equatorial Pacific Ocean becomes warmer. The warm seas cause the air to be warmer as well. The warm air pulls in and holds more moisture, which increases the amount of rainfall and thunderstorms. El Niño also affects the trade winds by causing them to decrease or even reverse.
La Niña years are characterized by lower than normal sea surface temperatures in the equatorial Pacific. This strengthens the trade winds, which brings up more cold water from the ocean floor (“upwellings” that occur on the west coasts of North and South America). The cool water also cools and dries the air, decreasing cloud cover and rainfall in the area.
In the Southeast
ENSO phases can have substantial influence over the Southeast’s climate and weather. The strongest effects of ENSO occur in the winter months between October and April. El Niño often causes a wetter and cooler than normal season, while La Niña will cause a warmer and drier season (although La Niña phases have been observed to start off with a period of cooler than average weather).
The National Weather Service has predicted that a current La Niña phase (as of October 2010) will last into the spring of 2011 in the Northern Hemisphere. Sea surface temperatures in the Pacific are currently 1.4 degrees Celsius cooler than normal; a strong La Niña (temperatures averaging 1.5 degrees Celsius cooler than normal) is predicted by November to January of this winter.
For more in-depth information on El Niño and La Niña, global atmospheric and oceanic interactions, and climate variability and weather patterns, visit NOAA’s National Climactic Data Center and Climate Prediction Center.
Adapted and excerpted from:
Clyde Fraisse, Joel Paz, and Charles Brown, Using Seasonal Climate Variability Forecasts: Crop Yield Risk (CIR1498), UF/IFAS Agricultural and Biological Engineering Department (01/2007).
NOAA, “The ENSO Cycle,” NOAA Climate Prediction Center (12/2005).
NOAA, “Frequently Asked Question About El Niño and La Niña,” National Weather Service Climate Prediction Center (12/2005).