Some methods for the detection of links between climatic trends and changes in atmospheric circulation

Monika Cahynovácahynova@ufa. cas.cz

Institute of Atmospheric Physics& Faculty of Science, Charles UniversityPrague, Czech Republic

COST733 WG4 meeting, Ioannina, 9-10 May 2008

Basic problems of analyses dealing with circulation classifications

huge number of classification methods and catalogues – results are not comparableCT is a discrete variable, not a continuumdifferent number of CTs in each classificationseasonal occurrence of CTs – some types do not occur throughout all the seasonsfrequency of CTs – not suitable for monthly analyses (some CTs are too rare)

Some methods using CTs in studies on climatic variability and change

(1) identifying typical (average) weather conditions of CTs on a seasonal or monthly basis (e.g. extreme temperatures, precipitation, snowfall,…)

→ understanding the synoptic causes of observed weather patterns on a local to regional scale (e.g. Twardosz and Niedźwiedź, 2001; Fragoso and Tildes Gomes, in press; Goodess and Jones, 2002)

Some methods using CTs in studies on climatic variability and change

(2) simple or multiple regression of CTs frequencies with monthly (seasonal) means of climatic variable (or teleconnection pattern)

e.g. Trigo and DaCamara, 2000; Goodess and Jones, 2002; Sepp, 2005; Lorenzo et al., in press

Some methods using CTs in studies on climatic variability and change

(3) CT as a “predictor”: construction of a new daily time series of a specific climatic element, whose value on each day is equal to the long-term average (monthly or seasonal) within the CT present on that day

→ new, “hypothetical” or “reconstructed” daily time seriescorrelation of monthly means from the “new” series with observed data – depends on the within-type variability of a climatic element (Bárdossy and Caspary, 1990)“skill” of the new daily series to represent the observed data (Buishand and Brandsma, 1997)

Some methods using CTs in studies on climatic variability and change

analysis of trends in this new time series, comparison with observed climatic trends → what proportion of the observed trends is caused by circulation changes? (e.g. Huth, 2001)all these methods assume stationarity of the connection between CTs and local weather (do not account for within-type trends)

(4) assessment of within-type trends, which can sometimes be quite different from the observed overall changes (e.g. Huth, 1999)

Some methods using CTs in studies on climatic variability and change

(5) decomposition of frequency-related and within-type related climatic changes between subsequent 31-year moving time slices, separately for months, since 1780 (Beck et al., 2007)

(6) "conditional downscaling" separately for every circulation type, on a daily basis (Enke and Spekat, 1997; Huth et al., in press)

References

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References

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Thank you for your attention.