Relasjoner mellom optimale pådriv for endret hyppighet av COWL

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. iii