European spring 2012: Warm? Why?

Spring forecast Europe: hot spring of 2012??
Temperature probability chart showing an ensemble forecast of climate models, by Columbia University’s International Research Institute for Climate and Society, IRI. This image is an average for the months of March, April and May 2012 – the used models have high uncertainty beyond March though. This image is easy to misinterpret, as it only shows probability for a temperature anomaly direction – not expected absolute temperature anomalies.

Tomorrow is the first of March, the meteorological start of the boreal spring. For West Europe the start of the season will not bring anything spectacular. With a small high pressure system enjoying a comfortable yet temporary position above the relatively cold waters of the North Sea, the first spring day will bring a quiet and dry weather type, with little wind and rather mild temperatures. If you live in France, you have a good chance of seeing the sun; elsewhere in West Europe there are more clouds and it will be mostly gray. But what is next?

‘The 30-degree April of 2012’

Several media in Belgium, France and the Netherlands have speculated on the approaching season. Their news reports state West Europe will be in for a hot spring, with especially ‘strongly above average temperatures’ in April.

First of all, these articles are all based on the same faulty interpretation of the seasonal outlook of IRI, the multi-model probability forecast presented by the International Research Institute for Climate and Society, which is hosted by Columbia University, in cooperation with NOAA.

That chart, as shown above, indeed is all red, suggesting March-April-May will be hot as hell. But that’s not what the numbers say. These suggest an up to 60 percent chance of experiencing an statistically relevant positive temperature anomaly.

That is not quite the same as suggesting temperatures will be 50 or 60 percent [counting from zero, we suppose?] above average. These media reports state maximum temperatures in April therefore could/will reach 30 degrees Celsius – far from the average values for the month.

How can Europe get a warm spring?

In spring the dominant factor for high temperatures is different from that in winter. A European winter is warm when westerlies are dominant, because in that season Atlantic air is milder than continental air.

March is an intermediate month, but in April and May the amount of sunshine becomes a dominant temperature elevator [next to of course wind direction (especially in spring southern (tropical) winds are the warmest)]. That is because after March 21 the days are longer than the nights, which means clear skies create more warming by the Sun than they radiate heat away to the stars (the reason why high pressure systems bring frosty nights in winter and early spring).

That means when (West) Europe has a spring which is dominated by high pressure systems (directly overhead) first of all precipitation is low and solar radiation is high. In other words you have ‘a dry and sunny spring,’ the weather type most people favour when winter has come to an end.

In March such high pressure blockades generally tend to bring around normal temperatures, whereas progression of the spring season with ongoing low cloud cover weather situations would increase the chance of above average temperature anomalies, first in April – and, under formation of actual drought, also in May.

This is the scenario that occurred in West Europe last year. The spring season of 2011 was exceptionally sunny and dry – and especially in April and May temperatures rose to values high above the climatic averages.

Spring 2012 versus spring 2011 – what is different, what is same?

The dry, sunny and therefore warm spring of 2011 was partially caused by a delayed La Niña effect. The route is still poorly understood (some have suggested long-distance atmospheric wave trains started by SE Asian SSTs), but empirical evidence shows after a La Niña winter Atlantic depression activity tends to be just a bit smaller than for instance during an El Niño – therefore in Europe La Niña springs tend to be a bit drier.

El Niño rainy spring
Opposite reasoning: measured effects of El Niño on European spring. El Niño on average leads to higher depression activity and therefore higher precipitation during European spring. As El Niños influence climatic average it is legitimate to suggest the La Niña connection is the statistical opposite: during La Niña springs in Europe one can expect less rain. Image source: Royal Netherlands Meteorological Institute KNMI. This does not exclude other climatic factors from creating a different weather scenario in any given year or season – as explained below.

Of course Atlantic depression activity is also directly influenced by Atlantic sea surface temperature anomalies. In the situation of last year these showed an indirect link with East Pacific La Niña cold water, which through the Western Hemisphere Warm Pool (WHWP) can also lead to below average temperatures in the Caribbean and Gulf of Mexico. This in turn translates to below average water temperatures in the Gulf Stream – and that (depending on course of the Gulf Stream and other Atlantic SST anomalies) may influence Atlantic air pressure distribution.

At this point it is worthwhile to compare last year’s end February SST charts to the current ones (by NOAA) – and see the one La Niña year can have a notably different sea temperature distribution from the other:

La Niña SST spring 2011
Global SST anomaly distribution chart as observed by NOAA at the onset of the boreal spring of 2011. Note the clear La Niña pattern in the Pacific, with a slight El Niño disturbance in Niño area 1/2 off the Peruvian coast. A band of negative SST anomalies also stretches from the Gulf of Mexico along the Atlantic coastal waters of the US and SE Canada narrowing down towards the British Isles and the North Sea. Towards western Europe this cold water anomaly seems small, but its importance is relative, as both to the north and south SST anomalies were clearly positive.

La Niña SST spring 2012
Global SST anomaly distribution chart as observed by NOAA at the onset of the boreal spring of 2012. There are many similarities with the 2011 situation, but also a couple of important differences. First, as in 2011, the boreal spring of 2012 starts with a La Niña SST distribution in the Pacific Ocean. The forecast transition to an El Niño by mid-spring or summer is already visibly underway. Another clear difference is formed by the temperature distribution in the Atlantic Ocean. There is a patched band of cold anomalies from Newfoundland towards the British Isles. But most of the western North Atlantic is too warm (representing a winter with less cold winds blowing off the North American continent). What could be of far greater significance though is a tropical ‘La Niña mirror’ in the mid-Atlantic, around West Africa. (When the forecast Pacific El Niño scenario sets in and Atlantic tradewinds pick up, this pattern will likely be reinforced.) The cold water anomaly now stretches around Morocco towards the Mediterranean and encompasses part of the Iberian Peninsula. And that creates a very comfortable alternative position for the Azores high, which was forced further to the north during the 2011 spring – and that could be what will determine the big difference between the two spring seasons –  as suggested below.

Temperature and precipitation forecast

Of special interest is the seasonal forecast of the NCEP model. The temperature charts show a similar direction as the IRI image on the top of this story, but differentiate that for the three spring months. March would be close to average and in West Europe April and May could be up to one degree warmer than normal. So now you know why IRI ‘thinks’ the entire spring will be warm – two months have more weight than one. Unfortunate thing is only the March forecast shows high reliability, largely passing the NCEP model’s own skill test.

So when it comes to temperature – based on model forecasts – we can only conclude the following:

  • March is likely to have around average temperatures
  • There is an inclination towards above average temperatures later on

The NCEP precipitation chart tells a more interesting story though. Again, the model only passes (and quite poorly) its own skill test for the month of March, so here especially April and May should be ignored.

What is remarkable though is the strong negative precipitation anomaly for parts of Portugal, Spain, southern France and northern Italy. This corresponds well with the current SST charts that would favour transition scenarios with strong presence of the Azores high over the Iberian Peninsula and the south of France.

If this scenario would happen there are strong precipitation consequences (for March only) for the whole of West Europe:

  • Dry and sunny in Southwest Europe
  • Mixed scenario for Northwest Europe: northflank of high pressure system would leave remaining Atlantic influence and cloud cover. High pressure ridges would bring dry, sometimes sunny weather, alternating with rainy days influenced by (weak) weather fronts of depressions to northwest and north. On average perhaps a bit drier than normal, depending on northward progression of Azores high.

spring weather Europe warm
European spring temperature forecast according to NOAA’s NCEP model. All this really says is March will be close to average. There is a clear trend towards relatively high temperatures later on in spring, but there the reliability of the model ends.

spring weather Europe dry rain
Thus far the biggest weather news: lots of sunshine for Spain, Portugal and South France. Perhaps also a tad drier for other regions in West Europe. But especially this precipitation forecast only has value for March.

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