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| Titel |
The relation of extreme North Atlantic blocking frequencies, cold and dry spells in ERA-40 in winter |
| VerfasserIn |
C. C. Raible, T. Buehler, T. F. Stocker |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023534
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| Zusammenfassung |
| One of the most prominent features of mid-latitude atmospheric variability is blocking.
Blocking events are anticyclones with an equivalent barotropic signature and persistent in
time. The longer than synoptic day-to-day time scales has led to intrinsic interest during the
last decades aiming to expand the predictability beyond classical numerical weather
prediction.
The study aims to present the extreme behavior of blocking and to investigate relations to
other extreme events, like cold and dry spells. To assess the behavior of blocking events we
developed a new event-based method. The method identifies 500-hPa geopotential height
maxima and tracks these relative maxima with a next-neighborhood search in time. This new
method agrees with the reference grid-point based method in the deduced climatological
pattern of blocking frequency.
Applying the method to ERA-40 data in winter for the Atlantic-European region we
found a trend towards a reduction of blocking episodes. The mean surface temperature and
precipitation shows a clear response: winters with an increased number of blocking events are
associated with negative temperature anomalies over central to eastern Europe and dryer
conditions, whereas southern Europe experiences warmer and wetter conditions during such
episodes. Using extreme value statistics, we show evidence that cold spells and to
some extent dry spells are strongly associated with extremes in blocking frequency
over central Europe. We also showed that cold spells need time to establish during
blocking events, thus return periods of cold spells are longer than those for blocking
events. |
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