2024-2025 La Nina

[40/70 Benchmark]

@Stormchaserchuck1Here in excerpt from my ENSO discussion within the Outlook publication last fall....worked out very well for the most part...was on the weaker end of my expectation envelop and it didn't end up obtaining official designation, as suggested, but I am in agreement that it acted as a borderline weak/moderate event.

The primary ENSO analog of 2008 was very predicative in terms of the late developing nature, progression westward, and even the ultimate intensity per RONI and MEI.

 

ENSO 2024-2025

Primary ENSO Analogs: 2021-2022, 2020-2021, 2011-2012, 2008-2009

Intensity & Structure Discussion

The October update of ENSO guidance from the International Research Institute reflected a significant reduction in the forecasted peak intensity of La Niña with a -.6 NDJ peak. 

 

 

 

While this is in better agreement with the -.4 DJF peak suggested by statistical guidance, the remaining difference still implies uncertainty with respect to whether an official La Niña will be declared. This is especially true given the paltry ASO ONI of -0.2 in conjunction with the fact that the weekly reading in region 3.4 was down from -0.5 on 10/23 to -0.2 as of 10/31. The reason for this is attributable to the behavior of the subsurface in conjunction with the zonal wind pattern during latter portion of the month of October.

The trend in the subsurface since mid October has been for the cold pool to shift east and even intensify.

 

 

 

However, during this same period of time, the trade winds have also shifted to the east and completely abated and even reversed to westerlies over region 3.4 

 

 

This has triggered a warming over region 3.4 with a burst of cooling over the eastern flank of ENSO. Note the sea surface temperature anomaly profile as of September 17th with the max anomalies focused in region 3.4

 

As compared to the max anomalies having relocated to region 3 on November 4th.

 

 

 

These changes have cooled the eastern flank of region 1.2 from -0.1C on October 16th to -0.5 on October 30, while both region 4 and 3.4 warmed from 0.1 to 0.2 and from -0.3 to -0.2, respectively. Although the trade winds are anticipated to re establish over the western flank in region 4, the forecast moving forward is precarious. There is uncertainty as to whether or not the occasional flirtation of the eastern extent of the trades with the western periphery of region 3.4 will be able to colocate with the western periphery of the subsurface cold pool to foster the development of an official La Niña event per ONI. The dynamical guidance suite, as stated, remains at odds with the statistical guidance. The former, albeit weaker, insists on the development of La Niña, and the latter has been steadfast on the ENSO neutral scenario.

 

 

This recent trend for the trade winds to be relegated to the east is already reversing, as they have to die died off and redevelop over region 4, which has halted cooling for the entire eastern half and likely even induce some warmer fluctuations. In fact, as of the November 6ht update, the region 1.2 has already warmed back up to -0.2 from -0.5 on the 30th and region 3.4 is back down to -0.3. Despite this, the notable absence of a subsurface cold pool beneath region 4 will prevent this event from becoming a full-fledged Modoki event, and it should instead remain central-based in nature. 

 

 

Guidance affirms this central based assessment, but like most hybrid events, there will be variation and mixed elements of each.

 

 

The convective forcing that has been redolent of the mixed type La Nina pattern should remain as such in the aggregate seasonal mean, however, expect it to assume that of a Modoki during warmer times, and that of an east-based event during the anticipated period of deviation. 

 

 

Note the positive precipitation anomaly beneath this area of maximum convective forcing  on model guidance.

 

The El Nino Modoki Index (EMI) forecast for the Japanese Meteorological Society confirms.

EMI analogs include  2008, 2010, 2011, 2020 and 2021 and the EMI forecast is between -.5 and -.7

Overall, while it may still appear somewhat muddled as to whether or not La Nina will officially come to fruition due to its recent stagnation coupled with the discrepancy between statistical and dynamical guidance, analog data offers support to scenario depicted by dynamical intensity guidance.

 

Analog Data Confirms that Official La Nina Designation is Likely

Eastern Mass Weather has identified five ONI analogs, three of which peaked as cool ENSO neutral and two with a weak La Niña designation. The three ENSO cool neutral seasons are as follows:

 

1996: -0.4 ASO ONI, -0.5 peak ONI, 10/30 3.4 weekly reading of -0.1, 12/25 weekly peak of -0.8

 

2001: -0.2 ASO ONI, -0.3 peak ONI, 10/30 3.4 weekly reading of -0.5, 11/21 weekly peak of -0.7

 

2013: -0.3 ASO ONI, -0.5 peak ONI, 10/30 3.4 weekly reading of -0.1, 2/5 weekly peak of -1.3

 

And the two weak La Niña analogs:

 

2005: -0.1 ASO ONI, -0.9 peak ONI, 11/2 3.4 weekly reading of -0.4, 11/18 weekly peak of -1.1

 

2008: -0.2 ASO ONI, -0.8 peak ONI, 10/29 3.4 weekly reading of -0.1, 12/22 weekly peak of -1.0

 

Here is the subsurface and zonal wind juxtaposition for later October-early November 2024 for comparison with the rest of the dataset, as measured against the appropriate climate period.

 

2024:

 

 

Weak La Niña members:

 

2005:

 

 

2008:

 

 

Cool ENSO Neutral Members:

 

The glaring discrepancy between the ENSO neutral portion of the dataset and 2024 is that the sample from 1996 is completed devoid of any trades and has a very weak subsurface cold pool.

 

1996:

 

 

The years 2001 and 2013 have neither a cold pool nor any semblance of trade winds, which is why neither of these years featured the development of an official La Niña per ONI.

2001:

 

 

2013:

 

 

The recent descent of the Indian Ocean Dipole (IDO) into negative territory represents a nod towards the La Niña analogs is thus also supportive of a marginal cool ENSO event crossing the threshold into official La Niña designation as an added measure of confidence.

 

 

 

 

Given the congruence of 2024 with the weak La Niña composite of 2005 and 2008 in conjunction with continued support from the dynamical ENSO guidance, one last concerted cooling of region 3.4 between mid November and mid December should be sufficient to ultimately achieve a weak La Niña designation, with a peak DJF ONI of between -0.6 and -0.8. This range is slightly more aggressive than suggested by the current consensus of guidance out of deference to the ongoing severe cold phase of the Pacific and its tendency to constructively interfere with and reinforce episodes of cool ENSO. This is reflected well by the Multi ENSO Variate Index (MEI) and Relative Ocean Nino Index (RONI).

Relative Ocean Nino & Multivariate ENSO Index Expected to be Indicative of Moderate La Niña Peak

In addition to the absolute intensity analogs (peak ONI) of 2005 and 2008, the strong 1973 La Nina event will be considered as a relative ENSO analog, due to the fact that it may inform the degree of additional development from this point forward considering that this year also featured a robust El Nino transitioning into a La Nina during a deeply negative PDO regime. This is precisely when the RONI is more critical than the ONI for assessing the overall intensity of ENSO. The reason why that is the case is because the RONI attempts to remove the trend due to global warming in an effort to produce a measure that is of more direct relevance to changes in tropical convection driven by SST anomalies, rather than the general warming around the globe. This in conjunction with a lack of appreciation for the Maritime forces competing with ENSO were the primary sources of confusion with respect to the inaccurate diagnosis of last season's El Niño. When used in conjunction with the MEI, which combines both oceanic and atmospheric variables into a single index as an overall assessment of ENSO, a more accurate measure of the true strength of the interface between land and sea that is represented by ENSO can be made. The ASO RONI values for the aforementioned three seasons and the subsequent peaks are as follows:

 

1973: -.1.54 ASO RONI and -1.95 peak.

2005: -.02 ASO RONI and -.93 peak.

2008: -.27 ASO RONI and -.89 peak.

 

 

What is apparent when examining the data is that the residual, baseline cool ENSO influence that is being reflected by the RONI is enhancing the current La Nina, as opposed to when it was competing and in conflict with El Niño last year. These analogs support a peak RONI range of -1.2 to -1.4, which is of course a significant, moderate cool ENSO event. An analysis of the ground truth can be completed for the atmosphere to confirm what an exhaustive analysis of the data has suggested.

The evolution of the Walker Cycle dictates that it is the upwelling of cooler subsurface waters via the enhancement of easterly trade winds that drives the development of La Niña, as touched upon previously. This is evident by the robust trade winds that are evident when viewing a composite of strong La Niña events.

 

The contrast between this composite and that of weak La Niña events is clear.

 

When viewing the May-August zonal wind anomalies for 2024, it is obvious that the meager strength of the trades winds thus far in the mean spanning the last several months has inhibited the development of La Niña. This corresponds with the earlier analysis citing the collapse of the trades in conjunction with some nuances concerning the subsurface cold pool as the reason for the recent stall in the development of La Niña.  

However, there are a multitude of indicators that are suggestive of a cool ENSO event that is appreciably better coupled with the atmosphere than the meager ONI value would imply.

 

 

 

This configuration is clearly more representative of the a stronger La Niña, such as the 1973-1974 event, rather than the weaker 2005 analog.

 

 

This represents a splendid illustration of why having the North Pacific in phase with La Niña, as it is now during the current deeply -PDO, results in a RONI that is stronger than the ONI and very likely a cool ENSO event that is well coupled with the atmosphere. Last season, this was not the case with El Niño, as it usually is not when the PDO is so deeply negative. Having climate change enhance the West Pacific warm pool that is a staple of an already record powerful Pacific cold phase only accentuates this phenomenon further. This is clearly evinced when comparing October composites of Pacific pressure anomalies during weak and strong La Niña events to October 2024.

Normally in a healthy La Niña event, the abundance of convection fostered by warmer waters over the western Pacific MC result in low pressure anomalies, which result in sinking air across the eastern portion of the basin and higher pressure. 

 

 

Note how diffuse this dipole is in the weak la Nina May-October composite.

 

 

The dipole is comparatively much more pronounced in the strong La Niña composite.

 

However, its even more well defined in 2024, despite the paltry ONI reflection to date.

 

 

Prior to this recent intensification of the pressure dipole, its initial slow response over the summer was at least partially attributable to the influence of the Southern Oscillation Index, which had been lethargic to respond to the burgeoning cool ENSO event given the magnitude of the strong El Niño event that preceded it. This resulted in a lag due to the extensive recovery necessary for the atmosphere to switch phases and couple with the ocean. However, during the middle to latter portion of summer, and then again in October, the SOI finally began to elevate and the MEI tumbled more precipitously, which is now fueling a faster intensification of the cool ENSO walker cycle.

 

The notable lack of a Pacific pressure dipole during the early portion of the summer season was also present during the relative ENSO analog season of 1973, which like this event, was transitioning from a strong Pacific cold phase El Niño to La Niña.

 

Now here is the May-August pressure pattern prior to strong La Nina events.

 

 

Note that the pressure dipole during the summer of 2024 was still rather disjointed in comparison to the strong La Niña composite.

 

However, as of the last update for the JA bi-monthly period, the MEI had already descended to -0.7.

 

 

The MEI calculation has unfortunately not been updated since last summer due to the immense flooding in the aftermath of Hurricane Helene, however, it is safe to expect given the above data and recent trends that a peak MEI value between -1 to -1.2 is to be expected.