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After an unusually warm and  early spring a severe cold-spell hit central Europe in late April 2016. Parts of Austria and in particular southern Styria were especially affected.

“Sow in April – Within a few hours a spring landscape turned into a winter landscape” (“Schnee im April – Binnen weniger Stunden verwandelte sich am Mittwoch die Frühlings- in eine Winterlandschaft”), a local newspaper headed on the 27th of April 2016.

The European Commissions JRC MARS Bulletin – Crop monitoring in Europe reported large damages on crops, orchards, and vineyards in Austria, Slovenia, Slovakia, and Croatia

The cold-spell even got its own article in German Wikipedia.

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Figure 1: Daily mean temperatures from April 10th to May 10th 2016 at the University of Graz meteorological station. (c) by ZAMG. Printed with permission.

Fig. 1 shows daily mean temperature measured at the University of Graz.  Up until the 23rd of April temperatures were above the reference period (1981 to 2010) mean and for several day even scratched at the all-time (i.e., since measurements began in 1894) record. But at the end of April temperatures plunged down just shy of the all-timer cold record at the 28th of April.

What had been going on?
I have already written about atmospheric blocking and its possible implications for temperature extremes. Fig. 2 shows the high pressure system of a block, which was located west of the British Isles in late April 2016 and significantly contributed to the cold spell which hit Europe at that time.

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Figure 2: Surface pressure on April 23rd 12:00UTC. (c) by ZAMG. Printed with permission.

The spring season has so far received little attention in blocking-research. Most studies focus on blocking in winter “and summer leading to high temperature anomalies in absolute terms. However, especially spring temperature extremes are of special relevance because vegetation during this season is particularly vulnerable to abnormal temperatures. Late spring frost can severely harm or even destroy fresh leaves, subsequently requiring considerable additional resource use by plants” (Brunner et al. 2017). In agriculture this often results in partial or even total crop failure.

Fig. 3 shows temperature at about 1.5km altitude for the 25th of April 2016. The wavelike structure in the atmosphere triggered by the stationary block west of the British Isles is clearly visible in the temperature field. The black arrows indicate the movement of air masses and point to the origin of the cold air which affected Europe.

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Figure 3: Temperature at the 850hPa level (about 1.500m) on April 25th 2016. Based on data from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim).

We recently published a study in the Journal of Climate focusing on the impacts of atmospheric blocking in European spring from 1979 to 2014 (Brunner et al. 2017). “Our results show statistically significant correlations of blocking frequency and the occurrence of cold spells and warm spells throughout the spring season, with sensitivity to the location of the block. We found blocking in winter and early spring to be stronger connected to cold conditions while blocking in late spring and summer is stronger connected to warm conditions” (Brunner et al. 2017). Our study particularly focuses on this transition in spring by monthly resolving the blocking and cold/warm spell link.

The location of the block is found essential for its impact on European extreme temperatures. Blocking west of the British Isles (like the block in April 2016) and over northern Scandinavia is clearly connected with cold spells in southern Europe and therefore exhibits a strong remote effect (in areas outside of the blocked-region itself). Blocking over central Europe and southern Scandinavia is associated with warm spells in northern Europe (i.e., has a strong direct effect in the same region where the block is located).

The blocking-frequency during cold spells in southern Europe is up to four times higher than the climatological frequency in spring (Fig. 4). This indicates a strong link between blocking and cold conditions in this region. Highest blocking-frequencies are found west of the British Isles, in the exact region where also the block in April 2016 was located.

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Figure 4: Blocking-frequency in European spring (1979-2014) during cold spells in southern Europe (gray box). Regions with statistical significance at the 95% level are marked with a plus-sign. The black contour-lines show the 1979-2014 climatology. Adapted from Brunner et al. (2017).

Fig. 5 shows the distribution of (relative) cold spell occurrence with a block present anywhere in the Euro-Atlantic blocking region (30°W to 45°E and 45°N to 72.5°N). In central and southern Europe a distinctly increased occurrence rate of cold spells during blocking is obvious. This hints at the crucial role blocking plays in European cold spells in spring.

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Figure 5: Fraction of cold spells with a blocking occurring at the same time anywhere in the Euro-Atlantic blocking region. Regions with less cold spells during blocking than normal are shaded in blue, regions with more cold spells are shaded in orange. Regions with statistical significance at the 95% level are marked with a dot. Adapted from Brunner et al. (2017).

Our study is the first to explicitly focus on blocking in the spring season. It highlights the evolution of the blocking and cold/warm spell link throughout spring. We find a twofold effect of blocking in spring where it can lead to both, extended cold and warm spells in Europe. Especially with global warming leading to a shift towards earlier plant green-up spring cold spells can have an even higher impact. A more detailed investigation of underlying dynamics and the processes connecting blocking and temperature extremes in spring promises exciting research also in the future. In the light of high-impact events like the cold-spell in April 2016 this research also shows high potential of societal and economic relevance.

References:

Brunner, L., G. C. Hegerl, A. K. Steiner (2017): Connecting atmospheric blocking to European temperature extremes in spring, J. Climate, doi:10.1175/JCLI-D-16-0518.1.

I acknowledge the Austrian Zentralanstalt für Meteorologie and Geodynamik (ZAMG) for providing Figures 1 & 2.

Creative Commons License
If not stated otherwise the text and all pictures/figures are by Lukas Brunner and licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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