2025-2026 ENSO

[40/70 Benchmark]

Weak, East-Based La Niña Impact on Extra Tropical Pacific

 The expectation for both a weaker and east-based La Niña influence during winter 2025-2026 remains consistent with earlier analysis this summer. Note that the behavior of the subsurface during the month of August was anticipated quite well last month, and continues to set the stage for an eastern biased event centered around 120 degrees longitude.

 

Historically speaking, in east-based La Niña winter seasons there is both a high latitude blocking signature and a prevalent poleward Aleutian ridge displaced to the northwest, off of the west coast of the CONUS relative to the central-based and Modoki La Niña composites. 

 

 

These are shared features with the weak La Nina composite, which is similar due to the fact that the majority of east-based events are also weaker cool ENSO episodes .

 

Thus it is not surprising that the composite of the three primary El Niño Modoki Index analogs of 1995, 2017 and 2021 were all centered in the vicinity of 120 West longitude, and 2/3 were weak events; with the 2021-2022 La Nina representing the lone moderate season.

 

 

Nor is it a shock that the pronounced Aleutian ridging positioned off of the coast and protruding poleward in conjunction with some degree of NAO blocking is also very evident.

 

 

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Not much has changed in the September update:

The persistence of this regime is generally supported by the consensus of guidance, which is perhaps slightly more supportive of an eastward leaning basin-wide event by winter, as opposed to a bonafide east-base event. This is perceived to be a rather trivial discrepancy, especially given the modest strength of the ENSO event.

 

 

The newest El Niño Modoki Index (EMI) guidance from JAMSTEC has also remained fairly consisted in it's depiction of an EMI value of around -.20 to -.30 during the boreal winter season.

 

 

This range is similar to the EMI analogs of 1995-1996, 2017-2018 and 2021-2022, which is supported by the similarity in both the placement (near 140 longitude) and the intensity of the respective subsurface cold pool analogs relative to 2025.

 

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Now, one could argue that the character of La Nina isn't important since it's so weak, which is true.....then you would  have to default to this:

Solar Max/E QBO (2025-2026): The Holton-Tan relationship applies early on given that PV disruptions are likely in December, before the relationship reverses and they are less likely later in the season. The final warming is of course the exception to this rule, while it is usually more relevant for spring, it can and does sometimes occur early enough to have an impact for the major population centers in terms of late season cold and/or snowfall.

Below are a couple of annotations to aide in the simplification and visualization of these relationships.

 

Interestingly enough, although statistically speaking early and late season disruptions are favored, there is a an outlier set of major PV disruptions noted in the solar max/E QBO quadrant of the chart above. This also becomes a theme when comparing the solar QBO analog composites. 

Here is a list of cool ENSO/ solar analogs for the winter 2025-2026 season.

 

Here is a DM composite of these seasons.

 

 

There are obvious similarities between the Solar composite (right) and the QBO composite (left) given that they both feature high latitude blocking and poleward Aleutian ridging.

 

 

The primary difference is that the QBO analog composite is biased more towards early and late season NAO blocking, whereas the solar composite focuses the NAO blocking more in the early portion of the season.

 

 

The discrepancy is primarily with respect to late season owed to the aforementioned outlier set of mid winter PV disruptions, which is evident in the list of SSW analogs compiled from the solar and QBO composites, respectively.

January 17, 1971: PV Split

December 4, 1981: PV Displacement

February 11, 2001: PV Split

January 2, 2002: PV Displacement

February 22, 2008: PV Displacement

January 7, 2013: PV Split

February 14, 2018: PV Split

March 20, 2025: PV Displacement

The majority of the SSW events are split between the mid winter months of January and February with three events per month. There is also an outlier event in December, 1981 and March 2025.  The primary analog events of January 17, 1971 and February 11, 2001 were both accompanied by PV splits and are the only two members to appear in both of the QBO and solar analog composites, which lends more credence to the mid winter outlier scenario that contains 6/8 the analogs. Thus the favored timeframe for a SSW and subsequent split of the polar vortex is during the January 14 to February 14 timeframe, which favors late season NAO blocking to commence roughly between February 8 through March 6. 

[DarkSharkWX]

4 hours ago, snowman19 said:

We disagree on poleward ridging. -QBO/-ENSO supports a destabilized tropical tropopause, on equator forcing/convection and a flat Aleutian ridge. Not saying no poleward ridging at all but as much as we saw last winter? Nope IMO

while this is true, the current ssta configuration in the tropics is favorable for MJO propagation with a warm pool in the west pacific that's backed by the subsurface along -QBO and an east based nina, so we should still be able to get a decent amount of poleward ridging episodes 

-QBO and solar max should cancel each other out but in general could be more open to -NAO/-AO episodes early on in winter(HT relationship for (+QBO/solarmax) says weak PV start, strong PV mid-late winter)

I do think that we see more southeast ridging this winter in general though

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8 minutes ago, DarkSharkWX said:

while this is true, the current ssta configuration in the tropics is favorable for MJO propagation with a warm pool in the west pacific that's backed by the subsurface along -QBO and an east based nina, so we should still be able to get a decent amount of poleward ridging episodes 

-QBO and solar max should cancel each other out but in general could be more open to -NAO/-AO episodes early on in winter(HT relationship for (+QBO/solarmax) says weak PV start, strong PV mid-late winter)

I do think that we see more southeast ridging this winter in general though

Yes, intuitively this is correct.....but that mid season SSW outlier group matches this year pretty well with respect to the solar and exact QBO. This is why I hedge towards the mid season SSW....will I be shocked if it doesn't work out?

Nope-

I do agree with a PV surge in early January after a rather tepid start, which likely concides with a stratospheric reflection event and subsequent +NAO/ poleward Aleutian ridging ala 2001, 2002, 2014, 2018.

Probably a pretty warm stretch early in January.

[40/70 Benchmark]

Here is a review of the QBO/Solar relationship for review/reference:

The aforementioned Holton-Tan relationship, which dictates that an easterly QBO is correlated with both a greater incidence of, and an earlier development of SSW, becomes more complicated when considering the solar cycle. In some cases, when the QBO and solar cycle are in conflict, they can either over ride one another or even cancel each other out (Gray et al 2004). Here is a list of combinations and the theorized relationships for clarity.

Solar min/W QBO: This is entirely consistent with the Holton-Tan relationship in that the westerly QBO essentially "wins". The PV is likely to be stronger than average until the final warming in spring.

Solar max/W QBO: In this case, the Holton-Tan relationship is applicable early on in the season, as major warmings are unlikely. However, the Holton-Tan relationship reverses mid winter and the latter portion of the season from February onward is susceptible to warmings and major PV disruption. Winter 2024-2025 was a textbook example of this.

Solar Min/E QBO: There are no inconsistencies here, as the entire season is more prone to polar vortex disruptions and SSW as per the Holton-Tan relationship.

Solar Max/E QBO (2025-2026): The Holton-Tan relationship applies early on given that PV disruptions are likely in December, before the relationship reverses and they are less likely later in the season. The final warming is of course the exception to this rule, while it is usually more relevant for spring, it can and does sometimes occur early enough to have an impact for the major population centers in terms of late season cold and/or snowfall.

[DarkSharkWX]

36 minutes ago, 40/70 Benchmark said:

Yes, intuitively this is correct.....but that mid season SSW outlier group matches this year pretty well with respect to the solar and exact QBO. This is why I hedge towards the mid season SSW....will I be shocked if it doesn't work out?

Nope-

I do agree with a PV surge in early January after a rather tepid start, which likely concides with a stratospheric reflection event and subsequent +NAO/ poleward Aleutian ridging ala 2001, 2002, 2014, 2018.

Probably a pretty warm stretch early in January.

early in January? wouldn't that be later on in mid-late january

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4 minutes ago, DarkSharkWX said:

early in January? wouldn't that be later on in mid-late january

Well, the warmth usually preceedes reflection events due to a Pacific trough regime, so if you have one mid month, the warmth would be earlier...maybe 2nd week? But really no need to split hairs at this range.

[40/70 Benchmark]

Stratospheric Reflection events, which are triggered by Pacific trough regimes, trigger a transition to AK ridging and colder patterns across the CONUS. They are more common in +QBO seasons (30/44 since 1980), which is maybe what snowman was getting at....but I am going against the grain a bit in feeling as though we get one due to the prevlence of them in my analogs.

Usually very warm a few days prior to a few days after the start, but it begins turning colder quickly....so I guess you could say mid January warmth. Early to mid-January, anyway.

[PhiEaglesfan712]

9 minutes ago, DarkSharkWX said:

early in January? wouldn't that be later on in mid-late january

With this being a -ENSO and strong -IOD, there is a much higher than normal potential for a very warm January. That's what happened the last 2 times we had that combination, and it continued into February.

 

December is probably the best chance for snow and cold this winter, although 16-17 was saved by a snowstorm in mid-March. But it seems like the winters are ending earlier post-2020, and you can't count on a snowstorm anymore. Heck, snow seems hard to come by since the end of the mid-2010s el nino.