We’re deep into what has been a particularly wet autumn and naturally thoughts are beginning to turn to winter. Will winter bring more of this unpleasant, unsettled weather or will we see a shift to something more… festive?
In this blog I’m going to discuss some of the drivers we have going into winter, what these could mean and then give a conclusion of the most probable pattern through the early winter period, December. I’m not going to touch too much upon mid-late winter as what happens through the early season may determine the outcome of the latter season.
*Look out for the blue bolded text, I’ve linked to various different sites with information on that specific topic should you wish to read more in-depth explanations on what we’re discussing here.
ENSO – El Nino
El Nino is characterised by warm sea surface temperatures that stretch across the equatorial Pacific. Whilst the direct impact of El Nino on the UK weather is very minimal compared with other parts of the world, the indirect impact of changing global weather conditions can give a clue on what we can expect. For example, El Nino winters often see cold periods more likely later in winter vs earlier, the opposite is true for La Nina. It’s important to get the context of all other variables at play however, ENSO conditions cannot be used in isolation for long term seasonal forecasting in the UK.
Going into this winter we have a well established El Nino following a three year triple dip La Nina.
The indirect impact of El Nino is a rise in Atmospheric Angular Momentum (AAM). I’ll go into this more later however a positive AAM is the signature of increased westerlies within the atmosphere which whilst on the face of it may sound counter intuitive, generally leads to a more perturbed jet stream, those looking for blocking/cold patterns want to see a +AAM, but it’s not quite that simple.
Indian Ocean Diapole (IOD)
Similar to ENSO above, the IOD is a measure of sea surface temperatures across the Indian Ocean. This year, we have a very positive IOD, in fact, it’s the second highest positive on record beaten only by 2019.
The IOD has peaked with forecasts suggesting a decline as winter progresses. A positive IOD correlates to a stronger Atlantic jet stream with unsettled and mild conditions across NW Europe, the winter of 2019 was a textbook example of this. Unlike in 2019 however, the IOD this winter will be declining rather than peaking so whilst it may still suggest a more mobile pattern is likely, at least initially, the influence may begin to wane as winter progresses. The positive IOD may also serve to hamper MJO progression by weakening convection. I’ll discuss the MJO next.
Madden-Julian Oscillation (MJO)
The MJO is an often predictable and cyclical eastwards movement of tropical convection, beginning in the west Indian Ocean before transferring eastwards across the Pacific. Where this convection resides impacts the jet stream in that area which in turn impacts the jet down stream of this, eventually reaching the Atlantic and thus the UK.
During a positive IOD, this convection is often weaker compared with a negative IOD. We’ve seen this in the last two months with the MJO failing to emerge in anything resembling an amplified state and as a result, mid-longer range forecasting has been particularly tricky with models often over-estimating the MJO cycle ahead of time.
The next MJO passage eastwards is predicted to occur during December, it’s worth noting that the below image is based on the CFS and thus is just model data which is subject to change. We’ll have to monitor how this forecasts progresses in the coming weeks.
With this in mind, an emerging MJO into phases 7-8-1 through December could indicate the potential for blocked and thus colder weather patterns to emerge for the UK and NW Europe late December/early January however given what we discussed above with the IOD hampering this convection, it’s possible this will be weaker similar to the last few cycles and could therefore have limited impact. Timing is also another issue, the above is only a model prediction and thus timing of this emergence is subject to change.
It’s also very important to note that the MJO is just one variable of the broader global windflow budget and should never be used in isolation. It should always be viewed in the broader context of the Global Synoptic Diagnostic Model (GSDM). When the tropics & subtropics are singing in harmony that gives greater confidence to the forecast. I’ll attempt to discuss Atmospheric Angular Momentum (AAM) next.
Atmospheric Angular Momentum (AAM)
The AAM forms part of the GSDM, a diagnostic model conceived by the brilliant late Ed Berry. The idea is to measure global wind flow budgets and use this to diagnose and predict medium-long range weather patterns. Sound complicated? It is. I’ll try and keep this as simple as possible.
The relative atmospheric angular momentum (AAM) about the Earth’s axis is a convenient global index of the intensity of the zonal circulation that can be monitored for climate purposes. It is a function of the zonal winds integrated through the depth of the atmosphere.Salstein et al. 1993
Broadly speaking, a +AAM is a result of increased westerly inertia within the global atmospheric circulation, most common during El Nino years thanks to weaker easterly trade winds across the Pacific which increases the overall westerly circulation within the atmosphere. The opposite is true during La Nina, stronger easterly trade winds across the Pacific serve to scrub the atmosphere of westerly momentum and thus, a -AAM is dominant.
There are several other mechanisms besides ENSO conditions which can trigger a +AAM or -AAM however I wont go into those here. I may do a future blog discussing this more specifically in real time should an event unfold during the winter.
Technical bit out of the way, broadly speaking a +AAM leads to a more perturbed jet stream and thus a jet stream more susceptible to bucking and amplifying, leading to large blocking areas of high pressure. The MJO spoken about above plays a vital role in this, as does the Global Wind Oscillation (GWO) but I wont go into that here.
The AAM has been in a negative state for a while now, despite El Nino which means we’re seeing atmospheric conditions more similar to La Nina, there’s been an ocean/atmospheric disconnect. A recent westerly wind burst (WWB) in the Pacific has allowed this to rise into positive territory and due to that we’re going to see a mid Atlantic ridge forming next week (22nd-26th November). If the MJO wave had been more amplified (discussed above) it’s likely this mid Atlantic ridge would have amplified into Greenland producing a blocked and potentially cold pattern but due to the low amplitude MJO signal, this high pressure is most likely to remain a transient mid-latitude block and thus, whilst it may turn cooler it wont be sustained.
That recent westerly wind burst (WWB) is now waning and being replaced by easterlies again, this resurgence of easterly winds will scrub the atmosphere of that westerly momentum and we’ll see the AAM tendency return to negative values once again.
Sea Surface Temperatures (SST’s)
Atlantic SST’s have been at unprecedented heights for much of this summer and autumn, indeed smashing records. Given this, it’s difficult to know just what impact this will have during the winter, certainly, it’s contributed to the wettest autumn on record for the UK with four named storms already this season.
The latest SST’s show what could be an emergence of a warm – cold – warm tripole, this pattern is linked with a negative North Atlantic Oscillation (NAO) during winter which can lead to colder weather across the UK and NW Europe more broadly.
Stratosphere: El Nino combined with a -QBO can lead to Sudden Stratospheric Warming which can increase the likelihood of cold weather outbreaks into the mid latitudes. This needs monitoring through the winter, as does the overall strength of the stratospheric polar vortex which has been record breaking strong recently. Dr Simon Lee’s recent blog post about the stratosphere & Sudden Stratospheric warming is well worth a read for further context.
Conclusion + Outlook
Now, for the exciting part! The round up and conclusion of the above variables, I am absolutely certain you read every single word and didn’t immediately skip to this part (I see you!)
December looks to see a continuation of unsettled weather as the AAM declines and essentially “resets” the pattern back to what we’ve been seeing through much of Autumn. Further spells of unsettled or very unsettled weather seem likely, particularly during the first half of the month.
Strong westerly winds developing in the stratosphere look to descend and ‘couple’ with the troposphere, further amplifying this unsettled signal, I’d not be surprised to see further named storms during December as a result.
If we see the next wave of MJO activity emerge as forecast, we could see a change later in December and into early January with blocking patterns emerging, potentially bringing the risk of cold/snowy patterns. This however is dependent on a coherent and amplified MJO signal and a return to +AAM tendency thanks to further westerly wind bursts in the Pacific, both of which need monitoring in the coming weeks. Assuming these two variables come together then the second half of winter will see an increased likelihood of cold weather patterns. A declining +IOD should allow for an increasingly robust MJO signal as winter progresses.
The first half of winter is most likely to be milder and wetter than average with frequent low pressure systems pushing in across the UK. It’s possible we could see transient colder periods as these lows clear bringing in colder air from the north. The greatest risk of impacts from cold weather look to be through the middle and latter half of winter where various different variables could come together to bring an increase in blocking areas of high pressure.
Potentially winter could extend into early spring given the above variables.
Overall though, I expect winter as a whole to be around or above the long term seasonal average. We will revisit the second half of winter around mid-December to see how things have progressed.
Thanks for reading!