Abstract
Forcing perturbations, which are optimal in producing atmospheric respons
patterns resembling the Cold-Ocean-Warm-Land (COWL) flow regime over
4 days, have been calculated with the atmospheric Integrated Forecast System
(IFS) model from the European Centre for Medium-Range Weather
Forecast (ECMF). When studying the control integration, the sensitivity
of the forcing perturbations for the upper and lower 5% sensitive periods
were analysed. The most sensitive states were found to have different flow
characteristics (-NAO and - PNA) from the least sensitive (-AO). The dominant
flow patterns of the most and least sensitive periods, were found to be
robust throughout the total time period, while some changes occur over the
PNA area. The NAO and PNA patterns, which the composite consists of,
in the most sensitive periods, seem to be independent. There are indications
that the NAO index has a stronger influence on the prominent patterns in
the least sensitive periods. In case studies, a more steady development of
flow patterns is obersved in the least sensitive cases rather than the most
sensitive. When a comparison of the control integration and the perturbed
integration was made, a weaker development of a strong blocking system and
the development of a negative NAO pattern was obtained.
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