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ENSO 2023-24

sryanbruen

To get the basics out of the way for those that don't know. ENSO (El Niño-Southern Oscillation) is an index reflecting the state of the sea surface temperatures (SSTs) and the atmospheric setup in the central Pacific Ocean which can have lasting effects on global weather patterns. El Niño is the warm phase whilst La Niña is the cool phase. Any link to European weather is weak and tenuous - strongest in winter when ENSO events usually peak and drivers in general are more active whilst weakest in summer when the opposite is the case. Generally speaking, it is said that El Niño has a tendency for +NAO earlier on in winter and La Niña has a tendency for -NAO early on.

Since the summer of 2020, we had been in a prolonged La Niña phase - likely the most significant since the mid-70s. Historically, La Niña on a long time-scale has coincided with periods of blocked atmospheric patterns in our part of the world and northward displacement of the Azores high. Whether this is directly linked to ENSO can only be theorised and not concrete, and could be down to other drivers instead whilst it just happens to coincide with La Niña. The La Niña and little change to shake up the atmosphere in my opinion is likely much of the reason why we've had such a prolonged warm period since mid-2021 with the main exception of course being December 2022 which was aided by an active MJO (usually not the case in a La Niña) and a weak stratospheric polar vortex.

Through the course of early 2023, the La Niña has finally unravelled and there has been significant warming in the equatorial Pacific. In fact, the eastern part over towards Peru has seen exceptional warming that is only comparable to sea surface temperature anomalies seen in 1997 and 2015. These years might seem familiar as they were the most recent super El Niño episodes. The central portion of the Pacific (ENSO region 3.4) where ENSO events are designated if they occur, meanwhile has only just gone positive for the first time since 2020.

Unanimously, the model output is very confident for a strong El Niño to develop through summer and into the autumn of 2023. Given the current east central Pacific warming, this does not surprise me whilst the central Pacific has been much slower in warming up which of course is deeper waters and deeper waters do take longer. I think a strong El Niño is personally unlikely at this point in time though because going from La Niña straight to El Niño is relatively unusual in of itself, never mind considering the strength. Normally ENSO neutral happens after La Niña, 2023 will not be going this way. The Pacific Decadal Oscillation (PDO) is also not in a position conducive to aiding an El Niño taking off but that's not to say it can't happen - the 1972 strong Niño happened with a similar PDO state. A borderline weak to moderate El Niño is what's most likely in my opinion, and an eastern based one too rather than modoki (central based).

What implications does an El Niño have for summer 2023 and perhaps autumn/winter 2023-24 too? Well, as I already said, any correlation or link is weak and tenuous at best. Historically, summers that coincide with developments of strong El Niño events tend to be poor for warmth/dry weather but the sample size is not large - there have been 8 strong El Niño events since 1950. Weak to moderate events meanwhile have often coincided with warm but fairly unstable summers - a fairly pronounced Atlantic trough and a continental ridge that might extend to Ireland from the east. These kinds of summers can be dry for some but very wet for others, particularly if you find oneself under attack from constant threat of thundery downpours.

Meanwhile for autumn/winter, El Niño would probably mean a wet and mild period. Wet Novembers are a regular feature during El Niño though obviously it's a wet month climatologically regardless and is rarely dry. If a strong Niño were to occur, the early winter would probably be very mild and zonal going by history whilst the risk of colder patterns increases later on into the spring but even then likely to be very zonal. Weak to moderate El Niños are much harder to decipher with plenty mixed results.

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sryanbruen

Here's the overdue update on what's going on with ENSO.

The following below are the latest weekly ENSO region anomalies. The central Pacific is currently in weak El Niño territory with an anomaly of +0.9C whilst the east Pacific, regions 1+2, are extremely warm with an anomaly of nearly positive 3 degrees Celsius. That is super Niño territory but for these events to be designated in terms of strength, it's based on region 3.4 which is in weak as already mentioned. The SST anomaly timeseries show that region 3.4 had a significant rise through the final week of May but has since flatlined rather than seeing any further increase. Regions 1+2 have been seeing a steady increase in the past month. It's likely certain now that autumn and winter 2023-24 will be El Niño and given we are only in July, it will probably be a moderate El Niño event.

On the globe, we are already seeing the effects of this with daily global records being set including the warmest day on record since at least 1951 (likely much longer) set three times this week. June 2023 was probably the warmest on record. With El Niño set to continue and of course combined with the background warming, expect further global warm records to fall.

The Southern Oscillation Index (SOI) reflects the atmospheric state over the south Pacific Ocean. Negative SOI reflects Nino whilst positive reflects Nina. As shown by the graph below, May 2023 had a very negative SOI, the most negative SOI month since April 2016 (the year of the super Niño, 2015-16). However, it was much more neutral in June. This is likely the reason why the central Pacific seen an increase in temperature during May but flatlined during June as ENSO events require the atmosphere to couple up with the ocean for them to take off. Recent days this neutral or slightly positive SOI has continued but things can change dramatically. If the SOI does go back significantly negative, we should see a further strengthening of El Niño in the central Pacific. For the moment, that is not happening.

Now for the relevance back home. We seen during the course of June after the opening period, the pattern I described that was likely under El Nino did indeed happen. That was a significant Atlantic trough with high pressure to the east that could extend to Ireland with the threat of thundery downpours. In fact, June 2023 was likely one of our most thundery months of the past 35 years including 8 consecutive days of thunder across the country. This idea was aided by the MJO which had been active during late May and past composites showed the signal. Now in late June, things completely changed. The MJO activity died a death, the atmosphere was more reflective of neutral or even slightly Nina-esque and amplification in the Atlantic allowed pressure to build northward into Greenland. Initially, it was looking like the NAO would actually go positive and this in turn would have likely brought summery conditions back to Ireland through mid-July but none of that has happened.

As it remains the case at the moment, we see ourselves in more of a Nina-esque situation with Atlantic ridging going northward and a trough through northwest Europe. This is set to continue through to at least the third week of July. The Azores ridge is in more of a classic position compared to early June when it was very displaced allowing the North Atlantic to have a marine heatwave. To be lifted from this trough led pattern, we rely on the Pacific to get going by the second half of July which is simply a matter of watch this space.

Billcarson

I just hope this winter isn't a complete mild and wet fest. I hope for a central based event to give us some hope.

sryanbruen

https://www.boards.ie/discussion/comment/120818706#Comment_120818706

I'll talk about winter more once summer is out of the way but indeed we don't want a strong event to enhance the polar vortex. What could also be a threat is a positive IOD is expected this autumn based on current forecasts - we know what happened in autumn 2019 and subsequently the winter after that..

The CFSv2 seems to be (as expected) reducing its super Niño forecast, now it's close to borderline moderate to strong. ECM remains very stubborn in predicting a strong El Niño event however.

sryanbruen

No clearer consensus from the latest models (850mb wind anomaly charts provided by World Climate Service) on what is expected with El Niño. As mentioned in the prior post, we have seen El Niño flatline in recent weeks - this is especially the case in the central Pacific. This is due to the atmosphere being at odds with the ocean.

The ECM shows strong signs of the atmosphere reversing back to a Niño reflective set up with convection (indicated by the orange colours below with dates on the left and longitude along the bottom) to increase across the central and eastern portion of the Pacific. The CFS does not show this until the very end of its run through the third week of August keeping the convection over towards Indonesia. The GFS is inbetween the two, leaning towards the ECM solution as we get into August.

If the ECM solution were to verify, we would likely see a significant strengthening of the weak El Niño to pretty mature moderate territory.

sryanbruen

The latest weekly returns show:

Niño 1+2 +3.4C

Niño 3.0 +1.6C

Niño 3.4 +1.1C

Niño 4.0 +0.7C

The warmth in the eastern tropical Pacific is extreme and way way into super Niño territory whilst the central Pacific has just recently reached moderate threshold in a slow fashion. I'd expect this to continue given modelling forecasts of weak trade winds in the tropics and should the MJO become active again. This will strengthen the El Niño to borderline moderate or strong I suspect.

The CFSv2 shows an interesting progression towards a strong central based El Niño in the coming months with a significant cooling in the eastern Pacific from its extreme point right now which is literally off the scale. It has been hinting at this for a few months now but only getting around to mentioning it. The only years I can think of maybe similar to this combined with an easterly QBO were 1965-66 and 1972-73 but that was a very different time and I don't see them being relevant at this moment for 2023 so yet another year of unique things maybe..

sryanbruen

The latest weekly returns:

Niño 1+2 +3.0C

Niño 3.0 +1.7C

Niño 3.4 +1.2C

Niño 4.0 +0.8C

There has been a slight westward progression from Peru of the warmth as regions 1+2 have cooled somewhat but still well into super Niño territory. Other than that, there has been little change with not much strengthening in the central Pacific which remains in moderate strength.

I would anticipate further strengthening of the El Niño in central regions. The long range modelling continues in favour of a moderate to strong El Niño for winter 2023-24 with some going to super Niño territory but the consensus is for a peak of around +1.7C.

sryanbruen

I'm not a fan of Ryan Maue but he posted an interesting post on Twitter (X pffff).

The multivariate ENSO index (MEI), which I have not really touched on before as to avoid complexity, is still only +0.3 to present as of the latest data from NOAA. The MEI takes into account numerous variables including but not limited to the SOI and the sea surface temperatures.

So in spite of well above average SSTs and a fairly negative SOI recently, something in the atmosphere is still holding El Niño back from developing into a big event. According to some, I cannot confirm whether this is true or not and remember every year is unique with no ENSO event being the same, this is sort of similar to what happened in 2009. However, in terms of SSTs, the 2009 event didn't take off until the summer whilst clearly 2023 has had a head start. It took until the autumn in 2009 to reach where the current El Niño stands the past month.

I will need to look at this more in-depth but perhaps even more of a puzzle to figure out the strength of El Niño this winter and it won't be straightforward at all.

https://twitter.com/RyanMaue/status/1689090873823207424?s=20

sryanbruen

I think more than enough time has now passed to give an update on the ENSO situation in the Pacific. We continue to be in quite unknown times.

Of course, we're well and truly into official El Niño territory in terms of sea surface temperatures. The central 3.4 region is +1.5 whilst regions 1+2 are +2.3. The latter regions have cooled down 0.7C since the last update I gave here in early August but is still well into very strong territory. Modelling continues to show this cooling for the next few months. 3.4 has seen a slight warming to borderline moderate/strong territory. The timeseries below illustrates the situation well in terms of SST anomalies for each of the ENSO regions including region 4 which is over towards Indonesia and region 3 which is inbetween 3.4 and 1+2.

Regions 1+2 have had a steady decrease since peaking back in the summer whilst other regions have mostly been trending upward though there was a slight cooling in 3.4 and 3 through September interestingly enough. So this El Niño is still very very strongly focused on the east Pacific and is far from a modoki event.

The SOI favours El Niño well into negative threshold - you can see the slight positive period through the summer when El Niño stalled and did not grow in strength that I mention in old posts. Now the SOI or atmospheric setup in the Pacific is no longer a hindrance to the El Niño, at least at this current point in time.

CFSv2 favours a strong El Niño developing through to January, would make sense if the above were to continue along with an active MJO.

So this sounds pretty straightforward to a strong El Niño for 2023-24, what's the unknown? I refer you to this latest sea surface temperature anomaly map from yesterday Sunday. There is one very odd thing.

The northeast Pacific and Chile coasts are at odds with the central Pacific. They are cooler than average and the PDO is negative. Normally with El Niño, we see these also going warmer than average. It's like the remnants of the triple dip La Niña are still there, despite September 2023 being historically warm and smashing all kinds of warm records.

The recent ECM seasonal was also strange with showing north Pacific ridging and a lack of low pressure - again this is more what you'd expect during La Niña. Of course we also have the combination of likely strong El Niño and easterly QBO which rarely happens. I haven't seen a year as quirky or weird as this in my years observing the Pacific.