Strengthening La Niña likely to continue through start of 2021

Weather Blog

New data from NOAA shows not only are La Niña conditions currently present, there are indications the pattern is strengthening and likely to continue through at least winter 2020-21.

What is La Niña?

La Niña and its counterpart, El Niño, are part of a large ocean-atmosphere climate interaction known as the El Niño Southern Oscillation (ENSO) cycle. The ENSO cycle is defined by fluctuations in temperature in the Equatorial Pacific and a correlating shift in atmospheric patterns (specifically, the strength of the Trade Winds). ENSO typically occurs every two to seven years, lasting for various durations (most last 9-12 months, but some have been observed for years).

La Niña is often referred to as the ‘cold phase’ of ENSO, as it indicates a negative deviation—or colder-than-normal—sea surface temperatures between the International Date Line and ~120° West. This happens when stronger than normal Trade Winds push warmer surface waters west… allowing colder water to fill its place (termed upwelling). El Niño is indicated by the opposite—a positive deviation—or warmer-than-normal sea surface temperatures along the Equatorial Pacific due to weaker Trade Winds.

During an El Niño, widespread thunderstorm activity along the Equatorial Pacific allows the Pacific Jet Stream to extend and persistent further south, amplifying the storm track. This amplified storm track allows the opportunity for more potent and frequent storm systems (brought in by the jet stream) to impact the southern U.S. This typically results in cooler and wetter than normal conditions along the Gulf Coast with drier and warmer weather further north.

Favorable conditions in an El Niño winter

Colder water off the coast of Mexico does the opposite. It’s less conducive for widespread thunderstorm activity, meaning the Polar Jet Stream and potent winter storms trend further north. This typically results in warmer than normal winter temperatures and drier than normal conditions in the southern U.S.

Favorable conditions in a La Niña winter

Fall 2020 – La Niña returns

This year’s equatorial Pacific waters started to trend cooler mid-May—with the official onset of La Niña conditions observed this fall.

IN-DEPTH: The Climate Prediction Center considers La Niña conditions to be present when three-consecutive months of sea surface temperatures in the equatorial Pacific are -0.5°C below average. An El Niño is considered to occur when sea surface temperatures are +0.5°C above average for three-consecutive months. During those same three months, associated atmospheric conditions must be observed.

ENSO forecast – Winter 2020/2021 & Spring 2021

The Climate Prediction Center’s ENSO forecast indicates a high likelihood of La Niña continuing in the months to come.

-85% chance of La Niña continuing through Northern Hemisphere winter (DEC/JAN/FEB)
-60% chance of La Niña continuing through Northern Hemisphere spring (FEB/MAR/APR)

FIGURE: model predictions of ENSO showing SST anomalies observed in the fall (SON – SEPT/OCT/NOV) likely to continue through at least spring (FMA – FEB/MAR/APR)

For more information, visit NOAA’s El Nino & La Nina information webpage here.

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