After 1 1/2 Years, La Nina is Finally Ending!

The La Nina climate pattern is finally ending, and that spells changes to weather in the Northern Hemisphere. What is La Nina? La Nina refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific. Typically, La Nina events occur every 3 to 5 years or so, but on occasion can occur over successive years - like the one we've been experiencing for the last year and a half!  So, now, what's next? Here's the update from NOAA (The National Oceanic and Atmospheric Administration, who houses the National Weather Service and National Climatic Data Center, and more):

(Caption: Sea surface temperatures in the tropical Pacific Ocean from early January through early March 2023 compared to the long-term average. East of the International Dateline (180˚), waters remained cooler than average, a sign of La Niña. Graphic by Climate.gov, based on data from NOAA’s Environmental Visualization Lab. Description of historical baseline period. Credit: NOAA)

March 2023 ENSO update: no more La Niña!

BY EMILY BECKER, NOAA PUBLISHED MARCH 9, 2023

La Niña—the cool phase of the El Niño-Southern Oscillation (ENSO) climate pattern—has left the building! After a year and half of non-stop La Niña, the tropical Pacific ocean-atmosphere system has transitioned to neutral, allowing NOAA to issue its “Final La Niña Advisory.” What can we expect for ENSO through the summer and into next fall and winter? I’ll get to that!

First, though, let’s bid La Niña adieu. The most recent weekly measurement of the sea surface temperature in the Niño-3.4 region (our primary monitoring region for La Niña and El Niño) was a mere -0.2°Celsius (-0.4˚ Fahrenheit) compared to the long-term average.  Also, the latest monthly value from a NOAA dataset was -0.4°C (-0.7˚ F) for this same region. The threshold for La Niña is cooler than -0.5 °Celsius, so we can say the ocean surface has moved away from La Niña.

Sea surface temperatures in the tropical Pacific Ocean from early January through early March 2023 compared to the long-term average. East of the International Dateline (180˚), waters remained cooler than average, a sign of La Niña.  

The atmosphere is a little more complicated, as it tends to be! 

We still see some indications of a La Niña-like strengthened Walker circulation, with more rain and clouds than average over Indonesia, less over the central Pacific, and enhanced trade winds. 

WHY WILL LA NINA DIMINISH?  Without the cooler sea surface characteristic of La Niña, it’s likely this pattern will diminish in the coming weeks.

(Image: the Walker Circulation during a La Nina event. Credit: NOAA) 

WHAT IS THE WALKER CIRCULATION?  It is an atmospheric system of air flow in the equatorial Pacific Ocean. The trade winds across the tropical Pacific flow from east to west: air rises above the warm waters of the western Pacific, flows eastward at high altitudes, and descends over the eastern Pacific.

(Image: A second way we describe the air pressure anomalies over the tropical Pacific is the Equatorial Southern Oscillation Index (EQSOI). The EQSOI is based on pressure differences between two regions located on the equator (Figure 3). The SOI is monitored because it has a very long record available, stretching back to the 19th century; the EQSOI depends on satellite observations, which means it is a shorter record, but it gives a better picture of what’s happening right along the equator.) 

ANOTHER FACTOR INDICATING WEAKENING OF LA NINA - The EQSOI 

One measure of the atmospheric component of ENSO is the Equatorial Southern Oscillation Index (EQSOI), which measures the relationship between surface air pressure in the western and eastern Pacific. When this index is positive, it indicates that the Walker circulation is amped up. In February, after nearly a year at or above 1.0, the EQSOI was just 0.1 This tells us that at least one element of the atmospheric La Niña response has weakened.

WHAT ABOUT NEUTRAL CONDITIONS?  We’re often going on and on here at the NOAA ENSO Blog about how ENSO is a seasonal phenomenon, meaning the ocean and atmosphere criteria must be met for several consecutive months in order to qualify as La Niña or El Niño. The same is not true for neutral conditions, however. Once the tropical Pacific ocean-atmosphere system is showing signs of decoupling, such as a monthly Niño-3.4 index value warmer than -0.5 °C, we can say that neutral conditions have likely arrived.

FOR MORE of the article: https://www.climate.gov/news-features/blogs/march-2023-enso-update-no-more-la-ni%C3%B1a