Case I.1
Long-term trends in the frequency and persistence of atmospheric circulation
types in European regions (Monika Cahynová) 1
Introduction
Circulation changes are a crucial part of the climate system that can both reflect and affect local
climatic trends. Here we study the changes in the seasonal occurrence and persistence of circulation
types in European regions in many objective (automated) and subjective classifications from the
COST733 database in the ERA-40 period (9/1957-8/2002). This comparative approach allows us to
distinguish real circulation changes from those caused by manual synoptic classification, such as the
mid-1980s enhancement of persistence in the Hess-Brezowsky catalogue that has been previously
attributed to a systematic change of the circulation itself (Werner et al., 2000, Recent climate change
in the North Atlantic/European sector. Int. J. Climatol.).
Data & Methods
Objective and subjective circulation classifications from the COST733 cat. 1.2 were considered for two
all the domains. Seasonal circulation trends in the period 1957-2002 were estimated using linear leastsquares regression applied to the seasonal occurrence of days with a specific circulation type (CT),
and to the average seasonal persistence (lifetime) of CTs. The common t-test was performed to
evaluate the statistical significance of trends.
Because of the uneven relative frequency of CTs, we have further calculated the proportion of days
within those CTs that bear a significant trend in their seasonal occurrence (at the 95% level).Trends in
the persistence of synoptic situations were estimated both for the individual CTs and for all the CTs
combined.
Results
In the objective catalogues the proportion of days occupied by CTs with trends in seasonal frequency
significant at the 95% level is mostly very low except for Central and Eastern Europe domains in winter
and the domain Mediterranean in winter and summer (see Figure Error! No text of specified style in
document.-1). Generally, the magnitude of trends is the highest in winter. For Mediterranean domain
the significant trends of frequency usually occur in the prevailing (most common) CTs in summer,
whereas in other seasons and regions there is no such preference.
The subjective catalogues contain a substantial proportion of CTs with significant trends in frequency
in all seasons, also the trend magnitudes are larger. Whether these long-term trends in the subjective
catalogues reflect real climatic changes or result from manual data evaluation is still an open question.
Seasonal trends in the persistence of all CTs combined are mostly insignificant in the objective
catalogues, therefore the recently reported enhancement of persistence in the subjective HessBrezowsky in the mid-1980s (see Figure Error! No text of specified style in document.-2) is rather
an inhomogeneity than a real change of the circulation regime. Individual CTs in the objective
catalogues bear both positive and negative trends (their numbers being comparable), whereas the
1
More information can be found in Cahynová M., Huth R. (2009): Enhanced lifetime of atmospheric circulation types over Europe: fact or
fiction? Tellus 61A: 407-416.
Cahynová M., Huth R.: Circulation vs. climatic changes over the Czech Republic: A comprehensive study based on the
COST733 database of atmospheric circulation classifications. Phys. Chem. Earth, doi:10.1016/j.pce.2009.11.002.
Hess-Brezowsky CTs underwent notable positive trends in persistence. The subjective Hungarian
PECZELY catalogue shows a decrease in persistence in 1985/86, coinciding with the opposite shift to
higher values in the Hess-Brezowsky.
Figure Error! No text of specified style in document.-1 Seasonal percentage of days contained in
circulation types with significant linear trend in frequency (at the 95% level). Average of all
objective scalable classifications.
8
7
days
6
5
4
3
2
1
1955
1965
HBGWL
1975
OGWL-3d+
1985
OGWL
1995
2005
PECZELY
Figure Error! No text of specified style in document.-2. Average annual. persistence of all
circulation types combined. HBGWL = Hess-Brezowsky, OGWL-3d+ = objectivized Hess-B. with
a minimum 3-day duration of types.
Summary & Conclusions
Seasonal linear trends in the frequency and persistence of CTs were studied in classifications from
COST733 cat. 1.2, in all the spatial domains, in the period 1957-2002. The total number of individual
CTs was >55,000. Because of the different relative frequency of individual CTs, we have calculated
the percentage of days occupied by CTs that have a significant trend in seasonal frequency. This
proportion is mostly very low except for Central and Eastern Europe in winter and the Mediterranean in
winter and summer, which suggests that apart from the winter intensification of the NAO, very few
“systematic” changes in circulation took place over Europe in the study period.
The sudden enhancement of persistence (lifetime) of CTs in the subjective Hess-Brezowsky catalogue
in 1985/86 was not found in any of the objective catalogues. The overall persistence of synoptic
situations has not changed in any season according to the objective catalogues.
Recommendations
No recommendations regarding the objective catalogues, since this analysis does not aim at
comparing them according to their performance. The subjective catalogues HBGWL (HBGWT) and
PECZELY most probably contain inhomogeneities that are reflected by sudden shifts in the
persistence of CTs. We therefore recommend a cautious use of these catalogues, even though they
may outperform the objective ones in their better representation of local weather and climate.