How relevant are frequency changes of weather regimes for understanding climate change signals in precipitation in the North Atlantic-European sector?

• Main Authors: Fischer, L., Bresch, D., Büeler, D., Grams, C., Röthlisberger, M., Wernli, H.
• Format: Conference Object
• Language: English
• Published: 2023
• Subjects: North Atlantic
• Online Access: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5020998

Various methods have been developed to characterize the large-scale atmospheric circulation, e.g., by classifying the flow into so-called weather regimes or circulation types. We introduce a novel conceptual framework to quantify how relevant frequency changes of such weather regimes are for understanding climate change signals in precipitation. For every regime, a spatially varying parameter γ corresponds to the ratio of the contribution from regime frequency changes to the climate change signal of precipitation to the contribution of regime intensity changes. Conceptual considerations show that γ is (i) proportional to the relative change of the regime frequency, (ii) proportional to the regime-specific anomaly of precipitation, and (iii) inversely proportional to the climate change effect on regime intensity. The combination of these independent and competing factors makes the study of γ interesting and insightful. As a specific example application of this framework, we consider a 7-category weather regime classification in the North Atlantic-European sector and large ensemble simulations with the CESM1 climate model under the RCP8.5 emission scenario for the periods 1990-1999 and 2091-2100. Considering γ for surface precipitation P in this simulation setup reveals, as the main results, that γ values are typically less than 0.1 and therefore, to first order, frequency changes of regimes are not relevant for explaining climate change signals in P, and that the main reason for the generally low values of γ are the comparatively weak regime frequency changes and the limited skill of the regime classification in stratifying precipitation in particular over continental Europe.