I am going to present a thesis/theory/analyse, of the effect that LaNina has on the storm seasons in Central Europe.

It was originally written in April 2011, but updated in December, when the latest ENSO response reanalysis charts from NOAA were avaliable.

Written by: Marko Korošec and Andrej Flis (me).

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If we look at April 2011, there were a lot of convective storms already across the western and central Europe. The weather pattern in Europe was being ˝run˝ by the negative ENSO 3.4 anomaly in the equatorial Pacific, the LaNina.

LaNina in mature stage:


LaNina in April:


Here we can see the jet stream position in the LaNina and the ElNino (opposite of LaNina).


With the LaNina, the jet stream in Europe is positioned more towards the north. This means that the path of the cyclones is altered, and so the entire weather pattern can change.



So, much of Europe gets a lot of warmth coming from the south because of the blocking present over the western and central Europe, because the cyclones are moving more towards the north, and so ridges can expand a lot easier. This also means more storms with the throughs coming behind in waves.





In 2008, we had a very similar situation.



CFS temperature anomaly forecast:



The severe weather season was very active in 2008, across much of Europe. One of the main reasons was the LaNina, which in 2008, disappeared in the summer.



This is a ESWD map of storm reports, for the 2008 storm season (01.04.08 to 31.08.08). If compared to other years, in 2008, there were a lot more hail events reported, associated with severe thunderstorms. We have to realize that there were more events, which were not reported! Especially over Western Europe.



This year we have a very similar pattern building up with the LaNina lasting well into the spring and maybe early summer.





La Nina can knock the jet stream off course in the summer and so causing fluctuations in jet stream and polar front, and all the wild weather associated with it in the summer. Especially when like now in April, the temperatures across much of Europe are above normal and there are higher temperature contrasts with passing cold fronts and upper level lows.

Let’s look at some global maps for different parameter anomalies, caused mostly by the LaNina. The maps were updated a week ago, so are quite new, and agree with this idea a bit more then previously thought. :)

If we look at the sea level pressure, we can see large negative anomaly. Especially over Alpine region, Central and East Europe. This means lower pressures than normal in average. It can also be associated with more throughs or weaker than normal high pressure systems (anticyclones), which can mean more convection in the spring/summer months with the diurnal heating.



There is also an effect on the precipitation rate. We see an increase of the precipitation rate in the central and east Europe. This would mean more total precipitation in average.



The temperature anomaly is a bit negative. This is expected, because with a more southerly polar front and more cold front passings, the overall average temperature can fall. As observed, I noticed a tendency for lower temperatures in June and July, in 08 and 11. And troughs do bring colder than normal air where they pass, and after they pass. Weather fronts, MCSs and cyclones, can also cover the sky for longer periods and so preventing the normal temperature rise during the day.



If we look at the 700mb omega, we can see a negative anomaly in Central and East Europe.. Omega is used to describe where there are rising and sinking motions in the atmosphere. Areas with negative numbers indicate areas of rising motion in the atmosphere (which may lead to clouds and precipitation). With cold fronts, and storms in general, cyclones, there is allot of rising motion, which could cause the negative omega response, if they happen in a above average numbers.



Another interesting feature is the 200mb zonal wind. it is neutral over most of Europe, meaning no changes, but, it is slightly above average over Atlantic, which could mean a tendency for a more zonal flow over Atlantic and into western Europe. This will be good for comparison with surface winds.



Lets have a look at surface zonal and meridional winds. It is interesting, that Central Europe is on the line of positive to negative surface zonal anomaly. If we than look at surface meridional flow, it is generally normal over alpine region, with negative value over N central Europe. If the overall upper zonal flow is normal (average), but the surface flow is normal to negative, and meridional is normal, more S to SW surface winds would be expected over the region. And with Normal zonal Flow over Europe and stronger than normal zonal over Atlantic, this could mean better shear situations in average.





A sign of a stronger jet and deeper throughs, can also be found in the 500mb anomaly. Overall anomaly is negative, which could correspond with more troughs than normal, or more dynamic (southerly) Polar front than normal.



Final thought: As we have seen the effects of the LaNina on Europe parameters, we can say that more active than normal storm season can be expected. By active, I mean the number of storms or storm days. It is logical to assume, that stronger jet, cold fronts, and higher temperatures, are factors that can also affect the severe storms intensity and number.

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Now some of my personal thoughts:

If I look back, I can say that this summer was pretty much a typical LaNina summer. There were however, some differences. Mainly because the last LaNina was untypical or "Super LaNina" in December/January.
So this summer can be compared more with the summers with Double year La Nina`s (1999, 1974), which tend to have a bit more intense effect on the atmospheric circulation than a usual one year LaNina. There were also some features present with this LaNina that are not really common. There is also a small lag effect present between LaNina and its effect in Europe. It can vary from 1-3 months. There are also other factors that can affect storm season alongside with the LaNina, like PDO, AMO, MJO,... and are effected by the LaNina. So there is a synergy between these factors, with each having its own effect on the other.

Here is a chart of ESWD reports from 1.4.2011 to 31.8.2011. I have excluded Dust devils and Heavy rainfall.
It was quite an intense and a very dynamic storm season across much of the Central Europe.



We also know, that the Omega blocking was present in the most of Fall, with some troughs in October.
This is a comparison between 2010 (left) and 2011 (right) ENSO 3 region SST anomaly. The bold line represents 0°C. There is a clear difference in strength and it is obvious that there is less dynamics this year, and a more steady pattern with ENSO. But that cannot be said for last year. If you look at the last years La Nina, you can see by the line, that there was more "action" and more dynamics with the easterlies and Hadley cell in equatorial pacific.



Even if you look at SOI (Southern Oscillation Index), which is directly connected to ENSO, you can see it was quite in record values last year and in the beginning of this year. Currently, we are in a more lower and normal values, expected with this weaker La Nina.



Now I want to point out, that each La Nina event, can have different effect on the overall patterns. The fall of 2011 was very similar to the fall of 1967.

This is a comparison of the ONI index between 1967 and 2011.


If I point out just two dates of 1967 and 2011 as a comparison of the fall pattern.




and this case.





These are just two examples of a similar pattern with a similar ENSO / SOI setup. It is important to point out, that intense large scale events like SSW (Sudden Stratospheric Warming), can change the overall response of the ENSO or other oscillations.

I also have to add, that the fluctuations of the jet stream caused by the LaNina, can have different effects in each month.

Here you can also find a great study on the ENSO effect on Europe, in the Fall-Spring period. :)
http://www.giub.unibe.ch/klimet/docs/cli...imann_et_al.pdf

Enough for now. Thank you for reading.

Best regards. :)

22.10.2011 - ENSO update

ENSO - Updates & Theory

27.12.2011

- BIG Thesis update, with latest reanalysis graphics from NOAA / ESRL-PSD
- A few additional info on the latest La Nina events.

Worth reading. ;)

Best regards.