ADIOS LA NIÑA: KEY PATTERN RELAXES AND MAY SHAKE UP WEATHER AROUND THE WORLD ~ The Washington Post

The pattern, known to enhance Atlantic hurricane seasons, has returned to neutral levels

A look at sea surface temperature anomalies around the world. (Tropical Tidbits) 

By  Matthew CappucciMay 13, 2021 at 3:12 p.m. MDTAdd to list

The National Oceanic and Atmospheric Administration announced on Thursday that a key weather feature that affects global temperature and precipitation has shifted into a “neutral,” or average, state. La Niña, one of the factors behind last year’s extremely active Atlantic hurricane season and a contributor to below-average rainfall in the South and Southwest, has faded away.

That means we’re currently in a middle ground between El Niño and La Niña. The former describes an anomalous warming of waters in the eastern tropical Pacific, while La Niña reflects a cooling of the waters there.

The change in sea surface temperatures triggers a chain reaction of events that can affect weather both nearby and half a world away.

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Atmospheric scientists abbreviate the mechanism as ENSO, or the El Niño Southern Oscillation. Each phase of El Niño can last for about three to seven years. Shifts in ENSO can affect agriculture, fishing and way of life, especially in developing countries.

NOAA anticipates that ENSO neutral conditions will last through at least the summer of 2021, but there’s a chance shades of La Niña could return during the fall or winter.

What the end of La Niña means

The demise of La Niña could have implications for the Atlantic hurricane season.

Last year, La Niña first became prominent from July to September, and coincided with a sharp uptick in Atlantic hurricane activity. Cooler water in the eastern tropical Pacific caused air overhead to sink, fostering greater rising motion, or lift, in the Atlantic.

That proved instrumental in kindling marginal weather disturbances that might otherwise not have developed, contributing to the record-shattering 30 named storms that formed during the 2020 Atlantic hurricane season.

2020 tied a record for the most major hurricanes in the Atlantic, after further review

The forecast for the upcoming hurricane season, taking into account the fading of La Niña, is somewhat tempered.

In his seasonal hurricane outlook issued in April, Colorado State hurricane researcher Phil Klotzbach took into consideration the expected reeling back of La Niña, calling for somewhat above-average activity, but not an exceptionally busy season like last year.

Hurricane Laura approaches landfall with considerable lightning in and around the eyewall during late August 2020. La Niña conditions are known to exacerbate Atlantic hurricane season. (Cooperative Institute for Meteorological Satellite Studies) 

The shift away from La Niña has additional implications on weather in the United States, but they may not manifest themselves for months.

In California, any drought relief associated with the end of La Niña won’t come during the dry season that runs well into next fall.

Moreover, while ENSO neutral conditions might favor more precipitation by next rainy season if they last that long, they’re often also associated with above-average temperatures. Hotter temperatures sap the ground of moisture and dry out vegetation. Considering the state’s ongoing exceptional drought, there’s little to reduce the threat of a particularly bad fire season.

Elsewhere across the Lower 48 states, ENSO neutral patterns typically have little effect during the summer. That means predicting the summer will depend on other features.

The National Weather Service’s Climate Prediction Center is anticipating warmer than average temperatures nearly everywhere in the Lower 48 and Alaska, especially concentrated in the Four Corners region and the West. Dry weather is likely in the West, while the Eastern Seaboard may pick up some extra rainfall. 

Summer temperature forecast from the National Weather Service. 

If ENSO neutral conditions persist into winter, they could have more discernible effects, including by steering an active subtropical jet stream into Mexico and across the Southeast. That often brings rainy weather to the South and Southeast, while spilling cold air down from Canada over the Northeast and Great Lakes as the polar jet stream dips south.

Elsewhere around the globe, La Niña favored a wet and warm pattern in eastern Australia, which is expected to ease with more ENSO neutral conditions. South Asia could see a respite from cool and wet summer monsoonal conditions exacerbated by La Niña.

Across northern Chile, coastal Peru and Ecuador, La Niña yields cooler-than-average conditions, while El Niño brings a torch of warmth. More temperature conditions can be anticipated with ENSO neutral conditions.

It wasn’t always a textbook La Niña

The current, now-faded La Niña reached its peak intensity during the fall and early winter of 2020. By early 2021, its influence was dwindling. Some remarked that the upper-air pattern over the Lower 48, particularly during February, didn’t look like that of a traditional La Niña.

During a textbook La Niña, the jet stream — a river of swiftly moving air in the upper atmosphere — usually dives south from the Pacific Northwest to the central Plains, recurving northward over the Appalachians. That means areas to its north, including the Upper Midwest, Northern Tier and Alaska, are frigid and frosty. Mild air is more favored in the South and Southeast.

A typical wintertime La Niña pattern over the Lower 48. (NOAA/Climate.gov) 

But February was dominated by a series of crashing cold fronts that plunged all the way down to the Gulf and into Mexico. That brought the coldest air since 1989 to Texas and battered the South with a series of high-impact snow and ice storms, knocking out power to more than 4 million in the Lone Star State.

“There are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa),” wrote NOAA.

Global temperature implications

The presence of La Niña and cooler-than-normal ocean waters in the tropical Pacific have helped somewhat lower the average global temperature in recent months. NOAA announced that this April was the ninth warmest on record while April 2020 ranked as the second warmest. 2021, overall, is less hot than several other recent years.

But, with La Niña relaxing, the Earth’s average temperature may gradually tick up in the coming months.

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