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Are the CMIP5 GCMs able to simulate atmospheric blocking situations over Europe ?

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1. Blocking situations

A blocking situation is defined as a sequence of at least 7 days grouped in classes favourable to heat waves and droughts (≈ anticyclonic classes)

2. Classification

Euclidian distance (DIST)

 pattern and height

Pearson rank correlation (RANK)

 pattern only

Because of the geopotential height increase due to global warming,

height becomes too dominant for DIST-Z500 for future projections, and this affects the results by

grouping to much days in the

anticyclonic classes. To avoid this artefact, the mean Z500 increase is removed for the future experiments.

Are the CMIP5 GCMs able to simulate

atmospheric blocking situations over Europe ?

Belleflamme A., Fettweis X., Erpicum M.

Laboratory of Climatology and Topoclimatology, Department of Geography, University of Liège, Belgium,

A.Belleflamme@ulg.ac.be (Ph. D. Student) Abstract

Heat waves and droughts, which occur during anticyclonic blocking situations, are projected to become more frequent and longer during summer over Europe due to climate change. Since GCMs have difficulties to reproduce the observed atmospheric circulation, it is interesting to study their ability to reproduce anticyclonic blockings.

We use different automatic circulation type classifications to individualise the circulation types over Western Europe for summer (JJA) over 1976-2005 by using the ERA-40 reanalysis. The circulation types are then projected onto the GCM datasets.

For recent climate (Historical - 1976-2005), the number of blocking situations is very different from one GCM to another and is strongly influenced by the frequency biases of the anticyclonic types. For future climate (RCP4.5 and RCP8.5 - 2011-2040,2041-2070 and 2071-2100), no systematic and significant changes are found.

3. Results

The number of blocking situations is very dependant from the frequency biases.

For future, no systematic change (for all classifications and the two future experiments) is found when considering only the pattern.

When counting the number of days instead of the number of blockings, the results are the same. So, there are not important biases in the

persistence of blocking situations (e. g. a too low number of blockings is not due to longer blockings).

4. Conclusion

The number of blocking situations is highly dependant on the frequency bias of the anticyclonic classes.

For future, there are no systematic and significant changes in the

number of blocking situations.

But, the projected increase of heat waves and droughts might be due to temperature increase, soil drying

out…

Sum of the frequencies of the classes

favourable to heat waves and droughts.

DIST-Z500 classification of the ERA-40 reanalysis over

1976-2005 for summer (JJA). The mean situation of each class is

shown by the black isohypses (in meters). The colours indicate the class anomaly with regard to the JJA mean. The class

frequencies are shown in brackets.

Number of anticyclonic blocking situations over 30 years.

References

-Fettweis X., Mabille G., Erpicum M., Nicolay S., Van den Broeke M., 2011 : The 1958-2009 Greenland ice sheet surface melt and the mid-tropospheric

atmospheric circulation. Climate Dynamics, doi: 10.1007/s00382-010-0772-8. -Huth R., 2000 : A circulation classification scheme applicable in GCM studies,

Theoretical and Applied Climatology, 67, 1-18.

-Meehl G., Tebaldi C., 2004 : More intense, more frequent, and longer lasting heat waves in the 21st century. Science, 305, 994-997.

-Philipp A., Bartholy J., Beck C., Erpicum M., Esteban P., Fettweis X., Huth R., James P., Jourdain S., Kreienkamp F., Krennert T., Lykoudis S., Michalides S., Pianko K., Post P., Rassilla Alvarez D., Schiemann R., Spekat A., Tymvios F. S., 2010 : COST733CAT - a database of weather and circulation type

classifications. Physics and Chemistry of the Earth, 35(9-12), 360-373.

-Vautard R., Yiou P., 2009 : Control of recent European surface climate change by atmospheric flow. Geophysical Research Letters, 36, doi:

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