September La Niña Update: It’s Q & A

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13 Sep 2022 2:00 AM
[Sea surface temperatures around the equator in the central and eastern Pacific were mostly cooler than average (blue) in August 2022. A few warm pockets (orange) dotted the far eastern Pacific. NOAA Climate map from our Data Snapshots collection.] [From NOAA Climate written by Emily Becker]   Ocean and atmospheric conditions tell us that La Niña—the cool phase of the El Niño-Southern Oscillation (ENSO) climate pattern—currently reigns in the tropical Pacific. It’s looking very likely that the long-predicted third consecutive La Niña winter will happen, with a 91% chance of La Niña through September–November and an 80% chance through the early winter (November–January).

91%! That’s very high. Why so confident?

The first reason is that La Niña is already clearly in force in the tropical Pacific. The August sea surface temperature in the Niño-3.4 region, our primary location for ENSO monitoring, was about 1.0 °C (1.8 °F) cooler than the long-term average, according to ERSSTv5, our favorite dataset for sea surface temperature. (“Long-term” is currently 1991–2020.) This is substantially cooler than the La Niña threshold of 0.5 °C (0.9 °F) below average. [Sea surface temperatures in the tropical Pacific Ocean from mid-June through early September 2022 compared to the long-term average. East of the International Dateline (180˚), waters remained cooler than average, a sign of La Niña. Graphic by, based on data from NOAA’s Environmental Visualization Lab. Description of historical baseline period here.] La Niña’s characteristic tropical atmospheric response—more rain and clouds over Indonesia, less over the central Pacific, and stronger-than-average winds both aloft and near the surface—was also clearly active in August. Taken together, the oceanic and atmospheric conditions tell us that La Niña is solidly in place. Once active, La Niña conditions are reinforced by feedback processes between the ocean and atmosphere. Read more about those feedbacks here. [During La Niña, the surface trade winds (small gray arrows) blow more strongly than normal, piling up warm surface water in the western Pacific and drawing up deep, cool water in the eastern Pacific.  Over the warm waters, the air becomes more buoyant (upward thick arrow), and rainfall increases. Over the cool waters, it is less buoyant, favoring subsidence (downward gray arrow) and less rainfall. These feedbacks between ocean and atmosphere strengthen the La Niña event. schematic by Emily Eng and inspired by NOAA PMEL.]

What else is providing confidence in the forecast?

There is a substantial amount of cooler-than-average water under the surface of the eastern-central tropical Pacific. This subsurface water will provide a source of cooler water to the surface over the next couple of months. Also, the computer climate model consensus predicts that La Niña will continue into the winter.

How long will La Niña last?

While there’s high agreement through the winter, there is a lot of uncertainty about how long this La Niña will last and when we will see a transition to neutral conditions. Current forecaster consensus gives La Niña the edge through January–March (54%), with a 56% chance of neutral for the February–April period. [NOAA Climate Prediction Center forecast for each of the three possible ENSO categories for the next 8 overlapping 3-month seasons. Blue bars show the chances of La Niña, gray bars the chances for neutral, and red bars the chances for El Niño. Graph by Michelle L'Heureux.]

When have previous La Niñas transitioned to neutral?

There are 24 La Niña winters in our historical record, which dates back to 1950. Of those, only one (2016–17) changed to neutral in December–February. Four transitioned to neutral in January–March, one (2000–01) by February–April, two by March–May, and 16 in April–June or later. Especially when you’re slicing and dicing a relatively short record, it’s tough to find truly analogous events. For example, this will be only the third La Niña three-peat on record, and the first not to follow a strong El Niño event. [Three-year history of sea surface temperatures in the Niño-3.4 region of the tropical Pacific for the 8 existing double-dip La Niña events (gray lines) and the current event (purple line). Of all the previous 7 events, 2 went on to La Niña in their third year (below the blue dashed line), 2 went on to be at or near El Niño levels (above the red dashed line) and three were neutral. Graph is based on monthly Niño-3.4 index data from CPC using ERSSTv5. Created by Michelle L’Heureux.] All this is to say that past La Niñas aren’t providing much guidance on how long we can expect this event to last. The current forecaster estimate, which favors an earlier than typical transition to neutral, is based on computer model guidance.

Remind me why I should care about La Niña…?

I admit, as scientists, we sometimes get wrapped up in how interesting the inner workings of El Niño and La Niña are! But ENSO has some serious practical applications. In a nutshell, La Niña and El Niño affect global atmospheric circulation patterns in (somewhat) predictable patterns, altering jet streams and storm tracks around the world and influencing temperature, rain/snow, and tropical cyclone seasons. Since we can predict ENSO months in advance, we can get an early picture of potential upcoming climate patterns. Of course, nothing is guaranteed with weather and climate—ENSO merely “tilts the odds” toward certain patterns. For more on how ENSO affects climate patterns, as well as why it's so difficult to make specific predictions, check out Michelle’s post here.

Can I have some examples of how La Niña can affect North American weather?

Yes! Here’s a map, followed by a list of some specifics. [During La Niña, the Pacific jet stream often meanders high into the North Pacific. Southern and interior Alaska and the Pacific Northwest tend to be cooler and wetter than average, and the southern tier of U.S. states—from California to the Carolinas—tends to be warmer and drier than average. Farther north, the Ohio and Upper Mississippi River Valleys may be wetter than usual. image.]

What about global impacts?

  [Temperature and precipitation patterns that are typical of La Niña during (top) Northern Hemisphere winters and (bottom) summers. Map by NOAA, based on originals from the Climate Prediction Center. Larger images and maps for El Niño are available in this post.] Edited for WeatherNation by Mace Michaels
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