La NiƱa is likely to hang around through the spring, with a transition to neutral favored for the MayāJuly period. Hop in, and weāll cruise through some updates on current conditions and the recent past!
On the road again
The NovemberāJanuary average Oceanic NiƱo Index, that is, the three-month-average sea surface temperature anomaly in the NiƱo-3.4 region of the tropical Pacific, was -1.0 Ā°C. Anomaly means the difference from the long-term average; long-term is currently 1991ā2020. This marks our fifth three-month period in a row with an Oceanic NiƱo Index that exceeds the La NiƱa threshold of -0.5 Ā°C. Passing this mile marker means this La NiƱa has persisted long enough to be awarded a bold blue color in our historical table. Congratulations, La NiƱa 2021ā22, already the second La NiƱa of this young decade.
Will there be a third? We still donāt have a very clear picture of that. Right now, thereās a 77% chance that La NiƱa will last through the spring (MarchāMay), largely based on computer model forecasts and bolstered by a recent uptick in the trade winds. Neutral is most likely for summer (JuneāAugust), with a 57% chance. By fall (SeptemberāNovember), neutral still has the edge, but forecasters canāt currently give any category a strong chance.
Every day is a winding road
ENSO (El NiƱo/Southern Oscillation, the entire El NiƱo/La NiƱa system) is a seasonal forecasterās best friend because it changes atmospheric circulation in (somewhat) predictable ways, allowing us to get an early picture of how the average seasonal climate might turn out. For example, during La NiƱa, the Pacific jet stream tends to be retracted to the west, and high pressure often forms south of Alaska. These effects tend to lead to a colder Northwest/warmer Southeast pattern over North America, along with more rain and snow than average in the Pacific Northwest and Ohio/Tennessee valleys, and drier conditions across the southern tier of states.
Iām sure you noticed all the qualifying words in the previous paragraph. āSomewhat,ā āmight,ā ātends,ā etc. Believe me, I wish we could make stronger statements and more confident predictions! But I try to remember that itās pretty amazing that we can get any idea of what the average weather might be like months into the future, given our complex and wildly chaotic Earth system. La NiƱa doesnāt guarantee warmer weather in the southeast or dry conditions in southern Californiaāfar from itābut it makes those conditions more likely overall.
Thereās more that goes into the seasonal forecast than ENSO, of course, like trends, or other climate patterns. Check out Mike Halpertās post on the winter outlook for an overview of the seasonal outlook process, and a set of maps that illustrate just how much outcomes can vary from one La NiƱa event to another. That said, ENSO has a big imprint on the seasonal forecast. You can see the Climate Prediction Centerās outlook for NovemberāJanuary 2021ā22 here.
The NovemberāJanuary average temperature ended up looking a lot like what weād expect during La NiƱa: colder than average through Canada, warmer over most of the US. Not exactly the same, but reasonably similar.
Seasonal climate averages matterāfor example, your heating bill is going to reflect if the winter was warmer or colder than average overall. However, sometimes itās hard to see the forest for the trees, like when you have a December thatās 6Ā° F warmer than average, followed by a hair-pin turn into a January thatās 2Ā° F colder than average. (Hello, Annapolis area!) More on this just a mile down the road.
The precipitation map for NovemberāJanuary also looks a fair bit like the typical La NiƱa impacts map for this season. Lots of rain and snow in the Pacific Northwest, substantially drier than average through the south-central and southeastern states.
Shut up and drive
If we break December and January out individually, we can see some big changes between the two, especially in temperature, and rain/snow in California. For simplicity, I left out November here. You can toggle between the various months for precipitation and temperature in the IRI Maproom.
Whatās behind these big swings? Itās tough to do a full attribution in the time I have to write my monthly ENSO Blog post, but we do have some thoughts about a culprit. The Pacific-North American pattern (aka the PNA) is a major atmospheric circulation pattern that has a big impact on North American weather. The PNAās positive phase is primarily characterized by below-average air pressure over the North Pacific and above-average pressure over northwestern North America.
The negative phase of the PNA is the opposite: higher pressure south of Alaska, lower pressure over Canada. Be sure to check out Michelleās post on the PNA, featuring one of our more excellent titles; the PNA Index can be viewed here. During La NiƱa, the PNA tends to be in its negative phase (that higher pressure over the North Pacific should sound familiar from earlier in this post). However, the PNA can change quickly, so, like the weather, the relationship to ENSO is weaker on a month-to-month basis.
December 2021 featured the strongest negative winter monthly PNA pattern on record (1950āpresent). Then, in January 2022, the PNA moved into a positive phase, making the largest jump on record from one December to January. Why did the PNA flip? That is a topic for another day. The PNA can be affected by other climate patterns, but, as Michelle says in her earlier post, āa large chunk of the PNA is internally driven.ā This means that apparently random, chaotic behavior, aka internal variability, often determines the state of the PNA.
The PNA is forecasted to move back into a more La NiƱa-consistent negative phase in mid-late February, so this La NiƱa is not done with us yet. With Nature behind the wheel, weāre all just along for the ride. However, thatās not going to stop us from trying to figure out where weāre going, and how we got where we are! See you next month.
This post first appeared on the climate.gov ENSO Blog and was written by Emily Becker.