Friday, November 13, 2015

El Niño, part I: The basics

With words like "Godzilla" and "strongest on record" being thrown around in relation to this winter's pending El Niño, you might be wondering just what El Niño is and what effects it will have on Mid-South weather. This is the first of a two-part blog series answering those exact questions.

In this installment, we'll answer the question "What is El Niño?" by putting the semi-technical terminology and atmospheric explanations on a shelf where you can reach it. In part 2, we'll look more closely at what the effects of El Niño conditions usually are globally and in our little corner of the world.

History of El Niño

El Niño traces its roots to a warming of the Pacific Ocean along the Ecuador/Peru coast in December, when the fishing season typically ends. El Niño literally means "The Little Boy" or "Christ child" in Spanish because the warming typically coincided with the Christmas season. Despite some annual warming, it was discovered that every few years the warming was more pronounced and coincided with heavy rainfall in typically dry coastal regions of Ecuador and Peru. Mid-20th century scientists discovered that the warming wasn't confined to the far eastern Atlantic, it actually spread across the eastern into the central Pacific near the Equator.

Link to the atmosphere

Further study in the 20th Century resulted in a discovery that the warming of the ocean water was also coupled with an atmospheric pressure oscillation called the Southern Oscillation. The oscillation of pressure results in changes in trade wind patterns (a prevailing pattern of surface winds blowing from the east found in the tropics) and rainfall distribution.

In "normal" conditions which are not El Niño or La Niña (commonly called "La Nada"), trade winds across the Pacific blow towards the west, causing warmer water near the surface of the ocean to be pushed westward. The temperature difference between cooler water to the east and warmer water in the west results in rising air over the western Pacific. This air evacuating the low levels of the atmosphere lowers the air pressure near the surface. In combination with warmer water that produces more evaporation in the same area, more clouds form and abundant rainfall occurs in the western Pacific. At the same time, in the east Pacific (off the coast of South America), atmospheric pressure is higher due to sinking air and rainfall is minimized.

Normal ("La Nada") conditions are shown above in a graphic from the Australian Bureau of Meteorology. Easterly trade wind pushes warmer water westward, resulting in lower pressure and increased precipitation over the west Pacific and cooler, drier conditions in the eastern Pacific.

In El Niño conditions, the trade wind weakens (or even reverses course in the strongest El Niños) and the warmer water shifts to the central and eastern Pacific, rather than the west Pacific. Rainfall patterns also shift, as rain tends to fall over warmer water for the reasons described above. Thus the normally-dry western coast of South America becomes wet and rainfall slacks off in the western Pacific. The shift in atmospheric heat, as a result of the displacement of warmer Pacific waters, disrupts atmospheric circulations globally, causing changes in weather and climate in places far removed from the Pacific Ocean, including across the United States.

El Niño conditions are shown above in another graphic from the Australian BOM. Easterly trade wind weakens, or even reverses, resulting in warmer water in the central and eastern Pacific and increased precipitation in the same regions. The western Pacific becomes drier with higher average pressure readings.

The ENSO Cycle

The complete name for El Niño is "El Niño/Southern Oscillation," or ENSO. The complete cycle from La Niña (the reverse condition of El Niño) to El Niño make up the "ENSO Cycle." El Niño episodes tend to occur every 2-7 years and typically last about 9-12 months. Much like a human being with a fever, there are varying degrees of El Niño and La Niña conditions, some relatively weak and others quite strong. It would follow that influences of ENSO can also vary widely, including the timing, duration, and intensity of the conditions.

How is ENSO measured? (this is a little more technical...)

The typical measurement that the National Oceanic and Atmospheric Administration (NOAA) uses to officially declare that El Niño conditions are occurring is based on sea surface temperatures (SST) in key areas of the equatorial Pacific, more specifically sea surface temperature anomalies (departure from normal). NOAA's Climate Prediction Center defines El Niño conditions as existing when three conditions are met:

  • a one-month positive SST anomaly of 0.5°C or greater is observed in the Niño-3.4 region of the Pacific Ocean (see map below),
  • an expectation that the 3-month Oceanic Niño Index (ONI, defined below) threshold will be met, and
  • an atmospheric response typically associated with El Niño is observed over the equatorial Pacific

The Niño regions along the equatorial Pacific are shown above. Niño 3.4, which is the key region for declaration of an El Niño pattern, encompasses the central portions of the Niño 3 (in red) and Niño 4 (in yellow) regions.

The ONI is a running three-month average of SST anomalies for the key Niño 3.4 region. "Events" (El Niño or La Niña) are defined as five consecutive overlapping 3-month periods at or above the +0.5°C anomaly for warm (El Niño) events and at or below the -0.5°C anomaly for cold (La Niña) events. In other words, El Niño is not officially declared until five consecutive three-month averages are at least 0.5°C above normal, which happens seven months into an El Niño event, since running averages are used.

For example, ONI values over the past year (shown below) featured three consecutive three-month periods that met the 0.5°C threshold (from OND '14 through DJF '15), but fell short of official El Niño status when the JFM '15 anomaly was just below the 0.5°C threshold. However, since FMA '15, the Pacific has produced five consecutive ONI values at or above the 0.5°C threshold (in red below), including the most recent 1.7°C value. Thus, El Niño officially began in February 2015, but was not declared until the June-Aug. '15 average was calculated in early September.

Oceanic Niño Index (ONI) values since winter 2014. The past 7 three-month periods have equaled or exceeded 0.5°C. When 5 of these periods reach 0.5°C consecutively, an El Niño is declared by NOAA.
According to NOAA, El Niño conditions are expected to peak this winter and then decline heading into next spring and beyond, thus El Niño will be a major driver of weather and climate throughout the upcoming winter.

For more information on ENSO, see NOAA's Climate Prediction Center website, NOAA's El Niño Portal, or this page from the Pacific Marine Environment Laboratory.

In the second part of this blog series, we will look more closely at 1) the effects of El Niño both globally and locally, 2) Mid-South weather in historical El Niños, and 3) what we expect from the weather this coming winter! Will it mean a colder or wetter winter? Find out here!

Erik Proseus
MWN Meteorologist

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