2025 ENSO/Winter Speculation

[40/70 Benchmark]

On 9/20/2025 at 11:53 AM, weathafella said:

I think this validates what I said-no longer east based but more basin wide and weak.  Probably will be less of a factor in terms of predictability of the season this year.

No; it invalidates it. The eastern zone of 1.2 began warm and the subsurface cold pool emerged there.

[40/70 Benchmark]

On 9/19/2025 at 1:51 PM, weatherwiz said:

Also keep in mind 1.2 is the most volatile region so when assessing changes in SST structure you must factor in what the contributions are to that.

Yes, I think Jerry is focusing on region 3...while 1.2 is volatile....this marked cooling in that zone isn't a function of volatility. If you have been paying attention, the subsurface cold pool emerged due to sustained trades.

[40/70 Benchmark]

Also, if you look at guidance, this has been expected and well forecast.

 

[40/70 Benchmark]

Now, maybe these trends reverse and guidance will be wrong, but that is not what we are seeing now. It's not becoming less east-based.

[40/70 Benchmark]

Region 1.2 has warmed again, speakaing to the volatility that Paul mentioned, but the subsurface remains supportive of an east-biased event.

[H2Otown_WX]

12 hours ago, 40/70 Benchmark said:

Region 1.2 has warmed again, speakaing to the volatility that Paul mentioned, but the subsurface remains supportive of an east-biased event.

Any chance of a weak west based Modoki Nino? 

[40/70 Benchmark]

 

Guidance Aligning with Eastern Mass Weather Expectation for Weak La Niña

Confidence in ENSO Analogs Growing

September ENSO Update

The updated consensus from the International Research Institute (IRI) for the month of September has finally reflected the significant cooling trend that Eastern Mass Weather has been advertising since the month of July. Indeed, after supporting an ENSO neutral peak that was warmer than the La Niña threshold (-0.5C) in region 3.4 all throughout this past summer, anywhere between -.31C and -.42C, the September suite now matches the preliminary Eastern Mass Weather forecast within a peak range of between -0.5C and -0.7C.

 

When viewing the latest subsurface data, it is easy to see why this latest suite of guidance finally followed the relatively aggressive trend towards cool ENSO that had been anticipated. Note the recent burst of easterly trades during much of the month of August in the annotation below, which triggered the upwelling of the rather extensive subsurface cold pool over the eastern half of the ENSO regions to the surface.

 

 

 

The ramifications of this expected development are two-fold. Firstly, the notion of this particular cool ENSO event meeting the technical definition of La Niña per CPC cannot yet entirely be discounted. While the concept of an official La Niña designation remains dubious considering the next round of trades is expected to be weaker, they had been relatively impressive throughout the summer, and were even prevalent enough to compare favorably to those that normally precede the development of weak La Niña.

 

Additionally, the pressure dipole, which consists of lower pressure across the western Pacific and higher pressures to the east in a developing cool ENSO event, has also been slightly more impressive than the weak La Niña group.

 

 

This is unsurprising considering that the MJJA mean SOI value for 2025 is 2.96, which is actually slightly higher (more La Niña like) than the weak La Nina composite MJJA value of 2.71. What is being observed is a continuation of the predominate trend of the past decade for the atmosphere to be more representative of La Niña than the ONI would imply due to the western Pacific warm pool.

 

Warmer West/Cooler East Pacific SST Anomaly Dipole Characteristic of Cool ENSO

 

  This is why the Relative Ocean Nino Index (RONI), which in a simplistic sense de-trends the general background warming of the tropics that is attributed to climate change from ENSO, is of greater importance. The current tri-monthly JJA RONI value of -.46 is already bordering on La Niña territory, and is likely to finish anywhere between. -0.9 and -1.1 based on analog seasons. The continued descent of both the IOD (left) and the AAM (right) is also reflective of an atmosphere that is entering La Niña mode more aggressively than the ONI would imply, and thus rendering an official designation per ONI, which is still considered unlikely due to the fairly late start of the more concerted cooling, moot. 

 

 

 

The second consequence of the stronger interval of easterly trades triggering the surfacing of the subsurface cold pool in the eastern half of the ENSO region is that it reaffirms the idea that this particular cool ENSO event will remain at least somewhat eastern biased throughout the fall and into boreal winter season. 

Expectation for Eastern-Biased Cool ENSO Remains

The rapid cooling recorded in the Eastern region of 1.2 from 0.8C above average on August 6th to -0.3 below average by August 27 is very evident in the CPC data below.

 

    Date              1.2       3        3.4      4

        

06AUG2025      0.8     -0.1     -0.3     0.0

 13AUG2025      0.6     0.0     -0.3     0.1

 20AUG2025     -0.1     -0.3     -0.4    -0.1

 27AUG2025     -0.3     -0.6     -0.4     -0.2

 03SEP2025     -0.2     -0.4     0.3     -0.2

 10SEP2025     -0.2     -0.5     -0.5     -0.3

 17SEP2025    0.0     -0.4     -0.4    -0.3

 

Note that although this was followed by a slight recovery with the abatement and translation westward of the trades in September, the event currently remains consistent with the east-based composite in which the max anomalies are centered between about 120 and 140 degrees longitude.

Current SST Profile Matches East-Based Composite (Max Anomaly 120-140W)

East-Based La Niña Composite (Max Anomaly 120W)

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.

 

 

One final point of interest in closing is that the only slight change from the EMI August guidance is that it delayed the onset of the eastward translation, presumably owed to the fact that it took until latter August for the subsurface cold pool to surface in region 1.2. This will be a trend to watch moving though the fall in the event that the recent cooling in region 1.2 becomes a trend; however, this particular region is prone to volatility and the subsurface is currently not conducive to any such trend.

 

Now that enough information has been gleaned throughout the summer and into the onset of boreal autumn, a preliminary seasonal composite can be introduced.

Preliminary DJFM Seasonal 500mb/Temperature Anomaly Composite

The operative word here is "preliminary", as this these forecast products will be subject to changes over the course of the next six weeks, prior to the issuance of the final products in November. That being said, without further ado:

 

DJFM 2025-2026 per 1991-2020 Climate Period

 

The above composite is the product of an integration of polar, extra tropical Pacific, ENSO and summer-fall temperature analogs.

One final ENSO update will be issued in latter October before the complete Winter Outlook 2025-2026 is published in November.

 

[dryslot]

The 3 driest summers here in Maine are/were 2025, 1995 and 1968, Winter 68-69 for PWM, 110" of snow, 95-96, 123".

[40/70 Benchmark]

37 minutes ago, dryslot said:

The 3 driest summers here in Maine are/were 2025, 1995 and 1968, Winter 68-69 for PWM, 110" of snow, 95-96, 123".

I don't exactly "hate" the 1970 and 2007 analogs, either. 

[dryslot]

7 minutes ago, 40/70 Benchmark said:

I don't exactly "hate" the 1970 and 2007 analogs, either. 

I don't either, 144" and 138" ha ha.