European Summer Outlook 2020
So this is tricky.
April has been a unusually warm and sunny across the UK and large parts of Western Europe have experienced a warmer than average month. The record of warm Aprils and subsequent summers over the last two decades are not great, tending to result in wetter and cooler conditions, particularly for Northern European regions, including the UK. Remember 2007, a record breaking deluge ? That was a near mirror image of current observed warmth.
So that’s not a great portent, and there is some logic to warm Aprils preceding a wet summer. Warmer Aprils tend to be driven by a delayed and stronger breakdown of the polar vortex and the atmosphere typically lining up for a strong La Nina like tendency. Intriguingly, this spring we have evidence of neither of these two factors, so the warm April – wet summer relationship may not hold true.
So what are they potential drivers facing us as we head into the summer ?
Firstly, a word about long range or ‘seasonal’ weather forecasts and how they are put together.
Over the course of seasonal and sub-seasonal periods there are around eight dominant ‘weather makers’ or key drivers. These are related to the strength of influence from facets of the the Northern Hemisphere’s oceans (which tend to be long lived and highly persistent) and its atmosphere. On occasions, the Southern Hemisphere can also get into the act and indirectly influence the Northern Hemisphere’s drivers.
There will inevitably be some short term fluctuations in weather patterns, typically on a week to three week time scale related to shifts in the location of tropical thunderstorm activity, but on balance, identifying which of the drivers is dominating or where actively interfering or suppressing other drivers is a key skill which turns the balance of probabilities in favour of the forecaster. Even allowing for weekly variation, identifying a lead driver or where drivers are working in concert gives the forecaster a greater chance of success at the monthly range.
Polar air pressure trends tend to be long lived with ramifications for summer patterns across the mid latitudes. Whilst the positive phase of the Indian Ocean Dipole has receded, the latest temperature plots show abnormally high sea temperatures across the Tropics, which will likely continue to have an influence on the European Summer.
Other factors such as the arrangement of warm and cold pools of ocean across the North Atlantic, the North Pacific and continued low levels of solar activity are considered to be significant enough to consider as key drivers for the recurring weather patterns which will shape the summer.
The winter and spring has been dominated by persistently low air pressures over the Polar region. As a measure of this, the Arctic Oscillation (the pressure differential across the Arctic and mid latitudes) has recorded the largest average positive value for the January to March period in the 70 year data set, something that will almost certainly be cemented by April’s value making it comfortably the most positive January – April period ever recorded. The next nearest comparable years were 1990 and 1989.
In figure 4, note how the shift to positive winter time Arctic Oscillation (AO) values occurred during 1990. This year’s extreme positive value may mark a further step change to subsequent strongly positive AO winter values.
The trend for summer time AO values is less coherent, and recent summers have included quite negative AO values despite winter positive values. The best and most insightful analysis of the polar profile is to look at the breakdown of the stratospheric polar vortex and possible long term trends.
As of mid to late April, the breakdown of the polar vortex is occurring, at about the time climatology suggests this is most likely to occur. The strength of the breakdown appears to be quite weak, and anomalous positive (westerly) winds and lower than normal air temperatures have dominated the middle parts of the polar stratosphere throughout the second half of the winter.
The trend observed since January has been for a stronger than normal polar vortex (westerly winds, colder air) to develop in the mid and lower stratosphere, which has steadily moved down through the atmosphere. This has driven the extremely low surface pressures over the Arctic which have manifested in unusually sustained westerly winds throughout the mid latitudes, effectively bottling up the cold air over the Arctic.
Polar atmospheric clues are extremely useful indicators. They reflect long term trends which can span up to six months hinting at how low or high surface pressures over the Arctic will be in the following summer.
The most comparable years for this observed profile are 1990 and 1997.
These give us an idea as to how the polar atmosphere might behave this summer, particularly 1990 which shares a number of similar other drivers to this year.
The observed Arctic Oscillation values for the June through August period for 1990 and 1997 were between -0.82 and +0.3, and broadly weakly negative (more so in 1997). What these comparisons show is that there is no signal for a strong breakdown of the stratospheric polar vortex, and no strong blocking (high pressure signal) over the Arctic subsequently conferred during the summer. They do not however completely rule this scenario out.
Spatial tropical sea warmth
Heat content across the Tropics is well above average, and probably the warmest since records began. Warmer than normal waters extend across large parts of the Equatorial and Tropical Atlantic, central and western Pacific and much of the Indian Ocean.
Warmer tropical waters tend to promote westerly wind transports and enhanced sub-tropical ridges, similar to the response typically associated with an El Nino. A key feature of these summers are twin oceanic basin lower pressure signals, and subsequent ridges to the east of these (think Euro high pressure).
Summers with a warm tropical ocean coverage tend to be moderate to start with an unsettled June but switching to warmer and drier conditions for July and August.
There is however an important caveat here.
The warm must remain in place well into the first part of the summer. Several years have started with a warm tropical ocean coverage, only for this to reverse dramatically by the start of the summer. These were associated with a transition to La Nina events and marked by a concerted attempt by the atmosphere to scrub westerlies from the global system and replace them with a net easterly budget.
The impact of this La Nina signal on the European Summer is strong, signalling a very sustained and unsettled Northern and Western Europe and hotter South-eastern Europe (think 1998, 20007 and 2010). Clearly, the track of any transition to a La Nina is a very big deal to the forecast.
El Nino / La Nina
This is one of the most significant aspects of the forecast discussion. Currently, sea temperatures across the central part of the Equatorial Pacific Ocean where El Nino / La Nina is measured is slightly above normal, close to the threshold for a weak El Nino.
Atmospheric indicators are slightly lower than this oceanic signal and more inclined towards near neutral conditions. What they don’t advertise at this stage is the atmosphere running as an outrider to subsequent La Nina development. However, lurking beneath the surface of the Equatorial Pacific is a pool of colder than normal water.
Notice in figure 7 how the cold pool of water in the Pacific has intensified over the last month and begun to move towards the surface.
Depending on atmospheric conditions above the ocean, the cold pool would take several months to influence the surface, and for these conditions it would take three weeks before global weather patterns are affected.
It is not unreasonable that a La Nina could develop this summer although there are many things that need to fall into place for this to occur, not least a degree of atmospheric coordination. Additionally, it is not just the development of La Nina that is uncertain, but the scale of any La Nina. A weak La Nina developing would mirror the conditions of the memorable summer of 1995.
Model forecasts are widely scattered on projections with some predicting a La Nina of moderate to strong intensity developing by the autumn, whilst others persist with near neutral conditions.
On balance, the lack of any coherent atmospheric forcing towards a La Nina event and consensus of model forecasts seems to suggest that near neutral conditions will extend into the early part of the summer, possibly becoming more tilted towards La Nina during August. If we take that on trust, then analogous years suggest a warm, dry summer for northern Europe and wetter southern Europe.
The sun regularly goes through an eleven year cycle of activity. Currently we at the low point of the cycle, with sustained periods without sun spots. The relationship with solar activity and summer weather patterns is weak although low solar conditions do seem to be associated with a higher than normal propensity for wetter conditions across southern Europe and drier conditions further north and west.
North Atlantic sea temperatures
The arrangement of anomalously warm or cold pools of water in the North Atlantic can have an influence on summer weather patterns across Europe, particularly when these align with other drivers for either higher or lower air pressures in the basin.
Currently, the North Atlantic basin is characterised by extensive slightly cooler than normal waters extending from nearly off the Newfoundland Coast to the west of Ireland, as well as warmer than normal waters extending across the southern part of the basin.
Analysis of this pattern suggests that lower than normal surface pressures will be experienced over the central / eastern part of the North Atlantic and corresponding higher pressures over Greenland and the southern half of the North Atlantic.
Are the drivers constructively or destructively interfering with each other, and what type of pattern are they suggesting will occur more often the average ?
All of the considered drivers depict a tendency for lower than normal air pressures centred in the central parts of the North Atlantic, and a corresponding higher than normal pressure trend for Greenland. Where they differ is the strength of the ‘blocking’ signal or higher than normal pressure over Greenland and its downstream impact on Europe, more especially during July and August.
There are two patterns (neutral El Nino / La Nina and low solar conditions) which seem to suggest a meandering jet stream where heat plumes would push further northward, and a second group (North Atlantic sea temperatures and warm tropics) which suggest a slightly flatter circulation with more emphasis on higher pressure further south and east over Europe and much of Northern Europe under the influence of the jet stream.
There is good agreement amongst the considered drivers for lower surface than normal pressures to dominate a line of latitude between 40 and 50 degrees north in the middle part of the North Atlantic. There is also agreement that June is more likely to be on the unsettled side for much of Europe, possibly Greece and Turkey seeing early warmth.
During July there seems to be something of a pivot point. Either the unsettled pattern continues and intensifies, which would deliver wetter and cooler conditions across the entire summer for much of North-west Europe (and conversely warmer and drier further south and east), or, the switch occurs for pressure patterns to become more inverted across Europe within a more meridional and weaker jet stream. This latter pattern will become progressively warmer and drier for large parts of Northern Europe, wetter further south.
The key to unlocking which way this pattern goes is the extent to which any negative tendency in the atmosphere develops during May and June, in effect, how strongly the signal becomes for a La Nina like pattern to develop.
Scenario 1: the response to any La Nina signal remains muted and weak; angular momentum and the Multi-variant El Nino Southern Oscillation Index (MEI) remain in neutral values throughout May and the summer (threshold values -0.5 > <+0.5). Outcome: increasingly dry and warm weather for much of Continental Europe and the UK (more especially the south and east) with an increasingly more unsettled pattern for southern Europe. June expected to be more unsettled for North-west Europe, potentially extending for a time into July.
Scenario 2: the response to La Nina signal is more coherent and coupled throughout the ocean-atmosphere system; the MEI drops month on month through negative values and reaches <0.5 by June with angular momentum consistent reaching -1 to -2 SD values; within this the Quasi Biennial Oscillation (QBO) intensifies to strongly negative values. Outcome: progressively unsettled and cool for North-west Europe, ironically warmer for the early part of June. Progressively warmer and drier for Southern European States.
At present, the forecast would be hedged towards scenario 1, so the expectation would be for a near average temperature regime for North-west Europe during June with average to slightly above average rainfall. Warmer and drier than normal start to the summer for Southern Europe, most especially Greece and Turkey.
During July, temperatures are expected to increase to above normal and drier conditions for large parts of central and northern parts of Europe, wetter conditions developing at times, potentially from unstable plumes. Increased risk of high impact weather events for both Southern Europe (flash flooding) and Northern and Central Europe (sustained elevated temperatures, localised flash flooding).
August continues to the risks of high impact weather events, with elevated temperatures across North-western and Western Europe, cooler and more unsettled conditions further east with increased risk of unsettled conditions across South-eastern Europe.
This translates to above normal temperatures for large parts of North-west, Western and Central Europe and drier than normal conditions with reduced wind energy potential and increased solar potential. For Southern Europe, rainfall events may help to reduce the risk of drought conditions (although note the emphasis here of a switch around if scenario 2 more favoured).
Just how much above average temperatures and drier conditions for the UK will characterise the summer overall is still open to some doubt but I would hedge right now to somewhere close to average for rainfall and above average for temperatures, particularly so further south and east within a ‘Euro High’ type scenario, bolstered by periodic ridging from the Azores.
By way of example, and adopting of a scale between 1 (summer 2007) and 10 (summer 1995), I would hedge this summer at around 6/10 to 8/10 for the UK.
What could go wrong ?
This revolves around the degree to which any La Nina signal is embraced by the atmosphere, or indeed to what degree the atmosphere begins to couple and force the Pacific towards a strengthening La Nina. If there is a concerted drop in momentum budgets (both stratospherically via the QBO and in the troposphere), then this would shift the forecast significantly towards scenario 2.
The key indices to watch: MEI at: https://psl.noaa.gov/enso/mei/ and angular momentum at: https://atlas.niu.edu/analysis/aam/MR-latest-30days.png
I will aim to provide an update of this forecast prior to the end of May when a more definitive steer can be made.
Forecaster: Stewart Rampling
Copy of the forecast available as a pdf upon request firstname.lastname@example.org