October 2022 #LaNiña Update: Snack Size – NOAA #ENSO

Click the link to read the article on the NOAA website (Emily Becker):

For what appears to be 247^e months in a row, La Niña is still in charge in the tropical Pacific. It’s actually only been about a year with La Niña continuing, as it went on hiatus in the summer of 2021 and redeveloped in October 2021, but that seems longer! There is a 75% chance that La Niña will be present this winter (December-February); forecasters favor a transition to neutral in February-April 2023.

3 Musketeers

Call it what you will – triple dip salad, three peats, three beans – we’re facing the third La Niña winter in a row. This is the third time in our historic ENSO (El Niño-Southern Oscillation, Entire El Niño and La Niña System) record, dating back to 1950, that we have had three La Niña winters in a row. That’s a lot of threes! The other stretches were 1973–1976 and 1998–2001.

As I mentioned, La Niña conditions took a vacation last summer, but the Niño-3.4 index has been negative since mid-2020. The Niño-3.4 index, our main measure for ENSO, measures the difference between the current and long-term mean sea surface temperature in a specific region of the tropical Pacific, where the long-term is currently 1991-2020. According to ERSSTv5, our favorite sea surface temperature dataset, the Niño-3.4 index is slightly more negative at -1.1°C in September 2022. That’s about on par with 1999 for the 6^e the most negative Niño-3.4 index ever recorded for the month of September.

Kit-Kat

Forecasters are very confident that La Niña will continue until the end of the year: the probability of La Niña from October to December is 95%. I went into detail about the sources behind the high level of confidence last month, and they remain the same this month. First, there is this Niño-3.4 index, significantly exceeding the La Niña threshold by less than -0.5°C.

In addition, the La Niña atmospheric response is clearly locked in, exemplified by stronger than average near-surface winds along the equatorial Pacific Ocean (trade winds), less rainfall than average over the central tropical Pacific and more rain over Indonesia. All of these factors illustrate improved walker circulation. One of the ways we measure the Walker Circulation is the Southern Oscillation Index (SOI), which relates the surface atmospheric pressure above Darwin, Australia, to the pressure above Tahiti.

September 2022 was the 4^e the strongest September SOI since 1950. Another measure is the Southern Equatorial Oscillation Index, measuring the surface pressure relationship between the eastern and western equatorial Pacific. By this measure, September 2022 tied for 10^e strongest September since 1949. Not as impressive, but still a solid indication of Walker’s amplified circulation.

A third confidence factor is that there is still a substantial amount of cooler than average water beneath the surface of the tropical Pacific. Our records for subsurface ocean temperature date back to 1979, and September 2022 is tied for 8^e the coolest basement in September. Not a staggering record or anything, but enough to further bolster confidence in the forecast. Yet more confidence comes from predictions from computer models, almost all of which predict that La Niña will persist through the Northern Hemisphere winter.

Skittles

We spend so much time and energy studying La Niña and El Niño because they affect global atmospheric circulation, changing weather patterns in somewhat predictable ways. Check out the second half of last month’s article for an overview of the potential effects of La Niña on North American and global weather and climate.

There are many different things that go into a seasonal forecast, but the two biggies are ENSO and recent trends, i.e. the trend in temperature and rain/snow over the past 10 or 15 years. Tom has described how recent trends work, so take a look at this article for more details.

Obviously, when you combine the characteristic La Niña temperature pattern with recent trends, you end up with a warmer than average pattern from November to January over almost all of the contiguous United States (you can see maps with La Niña and separate trends here.) Additionally, the southern plains tend to be drier than average, with more rain and snow falling in the northwest. (Separate maps here.) The Climate Prediction Center’s updated outlook for November-January and the coming winter will be released next Thursday.