UPDATED 5 SEPT, 2024

Re-Analysis Connecting Last Winter-Summer-Winter Ahead Based on NINO Transitions & Delayed Response

In order to improve long range seasonal forecasts, we must look back at what’s happened and learn from mistakes.

When analysing last winter, we transitioned from La Nina (3 years in a row) to El Nino. SST’s in the Pacific warmed quite quickly during summer and autumn but the atmosphere didn’t respond. We had a hurricane season much more reflective of La Nina and that may have proved vital to what happened through autumn and early winter for the mid to high latitudes.

The active Atlantic hurricane season (despite quickly developing El Nino within the ocean) had a lot of ‘fish storms’ or systems which headed out of the tropics into the mid latitudes. I believe this aided early season blocking and even an early onset to winter in December. The polar vortex was close to average at 10mb but weaker down at 50mb through Sep and Oct, this may have helped provide quite a blocky, -AO/NAO late in autumn and December as well as through the first half to January.

Frequent stratospheric warmings occurred during the 2023-24 winter and while they MAY have played a role in the somewhat cold first half to winter, the MJO I believe played one of, if not the biggest role along with busy late tropical season. Like clockwork, when the MJO rolled through phases 4 and 5, we turned mild but when rotating through 7, 8 and 1 as it did in September and October, we had colder spells. By watching the MJO and atmospheric response, a ‘repeating pattern’ had set up and believed December would be cold for NW Europe as the MJO was in a cycle of warm then cold phases, possibly enhanced by the tropics.

Many forget, despite the overall winter being mild last year, it was a colder than average first half. I believe the combo of tropics, MJO and to a lesser extent the weak PV and constant warming attempting to put pressure on the PV, was behind the cold.

What changed through January? EL NINO and the slow but eventual coupling from ocean to atmosphere. In my opinion, the MJO and SSW’s proved ineffective once the atmosphere locked on to the warm east Pacific and this overwhelmed the entire northern hemisphere pattern, super charging the polar jet over Pacific then downstream across N America, Atlantic into Europe.

I had a small theory as to why we observed so many SSW’s (2 of which revered the zonal winds). Perhaps the record warmth over Asia and waves of Siberia air sweeping south, forcing the heat to punch from troposphere to stratosphere. A sort of ripple effect within the atmosphere?

I did state my concern of a late response of El Nino in my winter forecast. This hunch proved correct.

With regard to spring, we observed two major stratospheric warmings with a wind reversal at 10mb, this likely helped bring the highly blocky pattern back late spring lasting into early summer. Remember the AO/NAO went negative.

Regarding the summer that’s just finished, based on my own observation, the -AO/NAO which commenced in May and lasted through the first half of June ended and a somewhat ‘return of El Nino’ may have occurred. If we look at the SOI in particular, in July it actually dropped further into an El Nino (not La Nina) base state. We observed stronger westerlies (also quite flat) throughout much of the hemisphere which then continued through August. If we look at the hurricane season so far, it started fast then died off through the second half of July through August as the westerlies strengthened but also SHEAR increased through the deep tropics. This is more of an El Nino signature, is it a coincidence that this all occurred at a time the SOI fell deeper into El Nino territory? That’s likely WHY La Nina somewhat stalled in it’s development.

It’s also just speculation but I find it also interesting that there was a very slow atmospheric response this time last year as La Nina transitioned to El Nino. While ocean reflected El Nino, the atmosphere reflected La Nina, hence the likely reason behind the unusually busy Atlantic hurricane season in an El Nino.

So, what does all the above suggest going forward?

The summer just gone had a very POSITIVE AO/NAO but that may be changing as we go deeper into September. I refer back to la nina, check out the cooling we’ve seen in just the last couple of weeks.

The easterly trade winds have strengthened and waters are responding BUT we are now seeing the SOI head quite quickly into positive territory. That means the atmosphere is beginning to respond to the cooling east/central equatorial Pacific. Essentially La Nina ‘steals momentum’ from the westerlies while El Nino speeds them up.

Early winter ahead?

My point is that the tropics are slowly showing signs of life with potential recurves away from the tropics into the mid latitudes. Like last September we have started HOT but the AO/NAO is flipping negative, the slowing of the jet appears to also be allowing warmer air trapped within the subtropics to move towards the pole and then in turn helps draw colder air south. The modelling is all over colder air drifting south over eastern North America and W Europe mid Sept.

The MJO ‘may’ drift towards the same phases which aided short lived cold spells last autumn and the combo of slowing westerlies, phases 7, 8 or 1 and tropical storms or hurricanes lifting north… repeating patterns this autumn into early winter??

Latest Teleconnections (early Sept)

• Developing La Nina (following a summer lull, it’s happening)

Cooling waters within the east and central equatorial Pacific stalled during the past summer while the atmosphere remained in more of an El Nino rather than La Nina base state.

However, easterly trade winds strengthened during the 2nd half of August as this hovmueller chart nicely shows. Blues show easterly, reds westerly.

The response can be seen on the ocean surface.

Part of the reason behind the strengthening of the easterlies and ramping up of La Nina can be attributed to large scale rising motion north of Australia and sinking over the east Pacific. This MJO phase 4-5 is kind of giving the atmosphere a kick and almost mimicking a La Nina type Walker Circulation in itself with winds at the surface blowing stronger from the high (sinking) over the east Pacific to low (rising) over the west Pacific, helping the SOI climb towards a La Nina state.

 

SOI on the Rise

The SOI (southern oscillation index) is now going from negative (El Nino) to positive (La Nina), this measure of air pressure between Darwin and Tahiti shows the atmosphere beginning to respond.

The new run of the ECMWF for OND shows a stronger La Nina developing compared to previous runs.

Note the ECMWF then slightly weakens the cool pool and shifts it further towards the central basin DJF, making it a more modoki type.

Latest multi-model forecast

The below graphic was produced by meteorologist Matthew Euler capturing developing La Nina years during specifically the July through September period. I’ve added red dots representing a mild to warm winter and purple when winters were cold following the Nina development.

(winter 22-23 had some decent cold early on)

La Niña events have been observed for hundreds of years, and occurred on a regular basis during the early parts of both the 17th and 19th centuries.^[64] Since the start of the 20th century, La Niña events have occurred during the following years:^[65]

Recent years when La Niña Modoki events occurred include 1973–1974, 1975–1976, 1983–1984, 1988–1989, 1998–1999, 2000–2001, 2008–2009, 2010–2011, and 2016–2017.

The effects of both El Nino and La Nina are very much INDIRECT due to the distance we are from the source. However, it’s pretty well understood that STRONG El Nino and La Nina events (Strong warm/cool waters up against the South American coast) tend to bring WARMER winters to both Eastern North America and Western Europe but weaker and particularly central Pacific or Modoki events can help bring colder conditions.

El Nino’s tend to lean towards a colder 2nd half to winter while La Nina, the 1st half.

We shall continue to monitor the development of La Nina but in the ocean and atmosphere and update it’s progress in the 2nd update!

• Coolest Summer since 2015 & Coolest CET For 1st Half Since 2012

First half to summer…

Credit: Neil Kaye

Summer was coolest since 2015 for UK but hottest on record for Balkans, much of E and SE Europe.

Summers above and below average for UK.

Cool NW, Hot SE Europe Summer Aided By Positive AO/NAO & Record Strong In August??

Tweet by World Climate Service

Wow – the August NAO (MSLP EOF) index was the most positive on record for any calendar month, and by far the most positive for August. (Data back to 1900 via reanalysis.) This was very well predicted by the seasonal models, being linked (I believe) to the March SSW event.

Good verification in the models from negative in June to strong positive in August.

WAS THE ABOVE A BYPRODUCT OF THE SOI REFLECTING EL NINO AND FALLING IN JULY??

• Latest North Atlantic & Pacific SST’s

Thanks to stronger than normal westerlies during the 2nd half to summer, we’ve seen a notable cooling of the North Atlantic from just off the south of Greenland to UK while anomalous warmth remains just to the south within the subtropics extending up into the Baffin Straits.

The Pacific we have notable warmth extending from Japan to North America with strong cooling up into the Bering Sea, likely due to lower than normal pressure during the summer.

Change in the last 12 months

These SST’s will continue to be monitored through autumn.

• West QBO

What is the QBO? It’s a mean zonal wind blowing within the stratosphere over the equator. What’s the significance of these winds? They periodically flip from westerly to easterly. A west blowing QBO tends to provide an added boost to the polar jet which marks the dividing line between tropical air and arctic air. The polar jet separates warm air from the tropics with the tropospheric polar vortex.

When the QBO blows easterly, it ‘can’ but not always, have frictional effects of the polar jet, causing it and the polar vortex to become weaker allowing warmth north and cold south (blocking pattern). It has to be said, we’ve had warm winters with a strong jet and PV in an easterly phase of the QBO and cold winters with weak jet/polar vortex in westerly QBO’s.

This winter will be westerly.

Past east/west QBO’s.

• Quiet Tropical Atlantic Following Fast Start

I believe the extremely busy mid to late Atlantic hurricane season 2023 (given El Nino) and the amount of northward tracks boosted Greenland/N Atlantic blocking and helped drive a cold start to the 2023-24 winter.

This year as of Sept 5, quietest mid August to mid-September in over 50 years.

With MJO ‘expected’ to head east eventually along with other atmospheric factors, the Atlantic is likely to liven up later Sept, Oct and possibly extending into the season’s final month, November.

The busy 2nd half to last season combined with MJO, slow response of developing El Nino and weak polar vortex likely helped bring the brief cold spells last autumn and cold December/1st half to January before the Nino took over.

• Solar Cycle 25 Maximum

We’re in the midst of solar maximum. August just witnessed the most sunspots in a single month since 2001.

Northern Polar Vortex Is Back but will it be Weaker?

As cold builds over the pole, the cold pool and westerly winds strengthen.

Models suggest a weaker vortex through October…

Weaker winds circling the PV mean warmth from outwith can more easily disrupt the vortex, forcing displacements, splits and collapse later down the road. It’s easily for arctic air to push towards the mid latitudes while a strong vortex typically keeps the mid latitudes milder with ocean air flooding the continents.

There is of course an upper level (stratosphere) and a low level (troposphere) polar vortex and the lower level vortex strength is measured nicely when looking at the Arctic Oscillation.

A positive AO suggests a strong PV while a negative is weak with cold pushing south away from the pole.

Next week sees the AO going negative. The result is cold drifting south into North America and Europe.

With the suggestion of a weaker than normal polar vortex during October, this suggests more -AO/NAO episodes and greater chance of colder spells

The combo of a Southern SSW, solar max and west QBO have been known to increase chances of a Northern SSW the following winter.

Recent solar max years 

Cycle 21: Dec 1979

Cycle 22: Nov 1989

Cycle 23: Nov 2001

Cycle 24: Apr 2014

Cycle 25: 2024**

Close matches between Solar max & developing La Nina’s (Jul-Sep)

1988 (year before solar max)

2000 (year before solar max)

Solar max 2014 occurred in between the dev Nina years of 2011 & 2016.

Combo of solar max + developing La Nina + west QBO years

La Niña     QBO     Solar Max or Near     Cool UK summer

1950
1951
1955
1956
1962
1971
1974
1976
1989            E                         MAX
1999            W
2000           W                       NR MAX                *
2008           W
2011             W                                                        *
2012            W-E                                                    *
2021             W
2022            W

2024            W                        MAX                       *

From Severe Weather Europe.

Quite the opposite to the newest ECMWF seasonal

CANSIPS Precip/500mb anomaly Nov-Jan

Quite the phase 4-5 MJO look with both ECM and CANSIPS!

2000-01 kind of stands out to me with la nina, west QBO and near solar max but SST’s entirely different. This was how it turned out…

2011-12

2012-13

The southern SSW is worth considering