Solving the winter conundrum

It’s that time of year again when the energy sector, hedge funds and weather enthusiasts are deluged with long range weather forecasts, the key question – what will the winter hold ?

I’ve been given dispensation to release some of my thinking around the forecast which has been available to clients for some time. For those who might find all this a bit technical, I’ve provided a quick takeaway summary at the end of this blog.

El Nino / La Nina

When trying to evaluate this year’s winter forecast, the most obvious but somewhat unhelpful aspect is that there is no coherent La Nina or El Nino steer. Surface temperatures across the central part of the Equatorial Pacific are near or just above normal and we have what could be described as ‘warm – neutral’ ENSO conditions.

Crucially, all other atmospheric markers also mirror this status: the Southern Oscillation Index (SOI) is hovering within the slightly negative banding; the Multi Variant El Nino Southern Oscillation Index (MEI) has been in the range of +0.2 and +0.4 (below El Nino threshold of +0.5) for the last seven months ; and angular momentum has recently arrived at near neutral values following a late summer dip into La Nina type values.

Figure 1: El Nino values since August
Figure 2: Latest SOI values
Figure 3: Angular momentum values

So we genuinely have both the oceanic and atmospheric El Nino markers in signal suggesting slightly warm of normal neutral conditions for the Equatorial Pacific. That should leave the door open for another driver to take centre stage.

The Quasi Biennial Oscillation

In this instance, at particularly for a winter forecast, the go to would have been the Quasi Biennial Oscillation or QBO. However, here’s another conundrum to add the forecast dilemma. Latest values for this index indicate a weak westerly phase which will most likely decay during December and go into a very weak and transitional phase during the heart of the winter.

Figure 4: QBO evolution, similar years and forecast values

Whilst I would not want to use neutral El Nino conditions as a basis for forecasting, transitional QBOs are still of some interest as the upper atmosphere is imprinted with ascending and descending long lead wave patterns. The closest matches in evolution from west to east phases are the winters of 1995/96, 1978/79, 2002/03 and 1969/70. This suggests a cold signal across large parts of Europe, more especially Eastern Europe.

Figure 5: Surface temperature anomalies for transitional QBO winters, west phase to east phase

The relative state of the solar cycle and levels of solar activity (both in terms of sunspots and geomagnetic activity) is also a key consideration to put along side the QBO. East phases of the QBO generally predispose a weaker than normal stratospheric polar vortex which is associated with cold outbreaks across Eurasia whilst west phases signal the opposite. This signal is enhanced under low solar conditions.

We are now at the probable low point in this solar cycle and solar sunspot activity is currently very low and geomagnetic activity generally low. This makes the west to east phase transition a difficult call.

Figure 6: Solar activity levels since 1996

The winter of 1995/96 is the closest match for solar conditions and QBO phase although both 1969/70 and 1978/79 also have some similarities for the QBO transition and for other drivers. Whilst a small sample, this factor suggests that the transitional west to east QBO phase is no inhibitor to a weakened stratospheric polar vortex.

Analysis of tropical forcing (see below) and the low solar / QBO transition suggests that the stratospheric polar vortex will also be displaced this winter more towards the Siberian sector. This would allow for a more meridional flow across the North Atlantic and weaker jet stream although such winters tend to focus the cold outbreaks more towards central Siberia rather than further west towards Western Russia and Scandinavia. Such a scenario characterised the 2002/03 winter.

Tropical forcing

Since September there has been a particularly discernible pattern in the Tropical Indian and Pacific Oceans. Convective activity has been enhanced in the Western Indian Ocean, Western and central parts of the Pacific, and across Africa.

Figure 7: Pattern of tropical rainfall this autumn

This particular pattern is related to a near record positive value of the Indian Ocean Dipole (IOD). This represents the differential in the surface temperature across the Indian Ocean which features anomalously warm waters to the west and cooler waters to the east. Subsurface temperatures in these regions across this gradient vary up to 14C suggesting that the positive phase of the IOD will persist throughout the winter.

Analysis of velocity potentials across the tropics during the autumn suggests that this driver has had substantive influence across the Tropics and beyond. The wet autumn across parts of North West Europe may in part have been influenced by this.

Figure 8: Velocity potential anomalies for this autumn (top) and those associated with a positive phase of the Indian Ocean Dipole (bottom)

In determining what a strongly positive phase of the Indian Ocean Dipole during the winter might mean, caution is advised. Strongly positive IOD events tend to occur in conjunction with some but not all strong El Nino events, so not all positive IOD events will carry the same signal (as the strong El Nino signals would largely contaminate and obscure the IOD signal).

Analysis of positive IOD events not under El Nino or La Nina conditions suggest that the polar vortex will be weaker than normal and pressure lower over the North Atlantic.

Figure 9: Mean sea level pressure anomalies for positive phase IOD events December through February

With tropical forcing likely to be centred in the West Indian Ocean and central part of the Pacific, this will allow for bursts of anomalous westerly winds. One of these westerly wind bursts took place during September, which in turn helped to rewarm the central part of the Equatorial Pacific.

Figure 10: Sea surface temperature anomalies (left) and low level wind anomalies (right) for the Equatorial Pacific from May through present

Further bursts will maintain slightly warmer than normal sea temperatures to the central part of the Pacific, in turn, helping to reinforce the pattern of tropical forcing – twin convective centres in the West Indian Ocean and the central part of the Pacific as well as across Africa. This pattern of tropical forcing leads to higher than normal air pressures to the north of Scandinavia and notable cold signal for Eastern Europe.

Figure 11: Sea level pressure anomalies (top) and temperature anomalies (above) for tropical forcing pattern expected

Sea surface temperatures across the Indian and Pacific Oceans

More generally across the Pacific and Indian Ocean Basins, the profile of sea temperatures is likely to exert some influence on downstream weather patterns given the absence of any coherent El Nino signature.

Figure 12: Current sea surface temperature anomalies across the Indian and Pacific Ocean Basins (top) and those for similar years (above)

Based on analysis where a similar pattern of warm and cold pools of ocean were observed, there is a suggestion that a more westerly and milder winter would be the outcome for Europe, the focus for colder weather shifted further east into Russia. Confidence in this signal would be low as the composite analogue reflects a strongish El Nino signature, which would trend milder for the European Winter.

What about the models ?

The principal long lead models are strikingly consistent in depicting a mild and potentially wet winter for much of Europe.

Figure 13: CFS forecast for 700hPa pressure (top) and temperature (above) anomalies for winter

Analysis of how the models handled the autumn suggests that they were incorrect over much of the North Pacific and Eurasian sectors, which tempers any enthusiasm in trusting the current model output. Both the CFS and UK MET models were culpable in this respect.

Figure 14: Comparison of CFS forecast for September – November 700 hPa pressure anomalies (top) and observed (above)

Putting it all together….

In the absence of any coherent El Nino signal, the lead driver is expected to be the pattern of tropical forcing attached to an unusually strong Indian Ocean Dipole event and broadly favourable environment for stronger than normal westerly (weaker easterly) winds across the Western and Central Tropical Pacific. This is expected to allow for notable influence from intraseasonal tropical wave development and enhancement periodically through the Western Hemisphere (Madden Julien Oscillation phases 7-8-1). Angular momentum is expected to exhibit a weak positive trend, which would allow for rapid extension and retraction of the Asian and Pacific Jet Streams and associated mountain torques. This will favour some intraseasonal variability and also impact the polar stratosphere through favourable upward wave activity.

This is expected to lead to a displaced stratospheric polar vortex and broadly weaker than normal tropospheric polar vortex with main cold displacements towards Siberia with a secondary cold displacement over North-east Canada.

Although the low solar signal and transitional QBO appear to suggest a cold pattern for Europe, this signal is only likely to discernible in low frequency across the forecast period and weak in magnitude. Net influence is therefore considered to be weak but trending for slightly colder than normal conditions, more particularly through Eastern Europe and Scandinavia.

The composite analogue drawn from these considered drivers with weighting for tropical forcing has good similarity for that observed for November giving confidence going forward (allowing for a pronounced negative Arctic Oscillation index observed for the autumn which may well have been a facet of the solar minimum).

Figure 15: Observed 500 hPa pressure anomaly pattern for November so far (top) and that for the composite analogue (above)

Projected forward, the trend across the forecast period is for slightly higher than normal air pressure to the north of Scandinavia and troughing over Europe as the jet stream is displaced southward of its normal position by virtue of a negative Arctic Oscillation. The North Atlantic Oscillation is forecast to be weakly negative. The most pronounced high pressure anomaly or blocking over the Polar region is expected to be located towards Alaska during January although there will be a signal for weak positive height or pressure anomalies centred over the Iceland to Svalbard region.

This pattern can hold extended periods of cold air across Europe. However, with the stratospheric polar vortex expected to be displaced rather than split, the focus for cold air is likely to be within the Siberian sector. The composite analogue is coherent in suggesting the core of any cold signal across Europe to be more favoured for Northern and Eastern Europe.

Figure 16: Expected pattern of colder than normal conditions for Europe December – February

There is likely to be a high degree of intraseasonal variability due to timing of tropical wave development. The pattern for December is likely to feature a disrupted jet stream flow across the North-eastern Atlantic allowing for cold pool development within Scandinavia. This will allow for potential snow events to northern parts of Europe, particularly with elevation. Low lying regions are likely to experience rain, and one of the key aspects of the forecast period is for a continuation of wetter than normal conditions for much of North-west Europe. Temperatures overall considered to be near average or slightly above for the UK, above average for France and near average for Germany and Scandinavia.

January is expected to be very cyclonic with an extensive trough across much of Europe bringing wetter than normal conditions and average to above average temperatures.

February holds a consistent signal for enhanced blocking over the polar region and a more coherent cold signal across Europe associated with a negative phase of the Arctic and North Atlantic Oscillations. Temperatures across Eastern and North Eastern Europe considered likely to be well below normal with below normal temperatures extending further west to France and the UK.

Overall, temperatures are forecast to be average to slightly above for the UK, above average for France, near average for Germany and average to below average for Scandinavia. Iberia and South-eastern Europe considered to be warmer than average.

Wetter than normal conditions are anticipated across much of Europe with the exception of Scandinavia whilst above normal surface winds are expected to provide favourable conditions for wind generation. The expected troughing signal and temperatures are likely to provide favourable conditions for snowfall in the Alpine and Nordic resorts.

Overall confidence in the forecast is low due to a lack of any coherent El Nino signal and particularly in respect of the potential for the Arctic Oscillation to be more negative than anticipated due to low solar activity and the transition of the QBO. If this does occur, the pattern will broadly be the same but with more extensive cold.

The key take outs…

Across much of Europe the winter is expected to be unsettled with a continuation of above normal rainfall for the UK and Northern France.

Temperatures will be broadly slightly above average overall, but probably colder than normal across Scandinavia and Eastern Europe. December likely to feel cold and raw across much of North-west Europe, with the threat of extensive colder than normal weather likely to be focused in February.

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