Using a nested single-model large ensemble to assess the internal variability of the North Atlantic Oscillation and its climatic implications for central Europe

• Published in: Earth System Dynamics
• Main Authors: Böhnisch, Andrea, Ludwig, Ralf, Leduc, Martin
• Format: Article in Journal/Newspaper
• Language: English
• Published: Ludwig-Maximilians-Universität München 2020
• Subjects: Department für Geographie; ddc:550; North Atlantic; North Atlantic oscillation
• Online Access: https://epub.ub.uni-muenchen.de/90449/1/esd-11-617-2020.pdf; https://epub.ub.uni-muenchen.de/90449/; http://nbn-resolving.de/urn:nbn:de:bvb:19-epub-90449-9; https://doi.org/10.5194/esd-11-617-2020

Central European weather and climate are closely related to atmospheric mass advection triggered by the North Atlantic Oscillation (NAO), which is a relevant index for quantifying internal climate variability on multi-annual timescales. It remains unclear, however, how large-scale circulation variability affects local climate characteristics when downscaled using a regional climate model. In this study, 50 members of a single-model initial-condition large ensemble (LE) of a nested regional climate model are analyzed for a NAO-climate relationship. The overall goal of the study is to assess whether the range of NAO internal variability is represented consistently between the driving global climate model (GCM;the Canadian Earth System Model version 2 - CanESM2) and the nested regional climate model (RCM;the Canadian Regional Climate Model version 5 - CRCM5). Responses of mean surface air temperature and total precipitation to changes in the NAO index value are examined in a central European domain in both CanESM2-LE and CRCM5-LE via Pearson correlation coefficients and the change per unit index change for historical (1981-2010) and future (2070-2099) winters. Results show that statistically robust NAO patterns are found in the CanESM2-LE under current forcing conditions. NAO flow pattern reproductions in the CanESM2-LE trigger responses in the high-resolution CRCM5-LE that are comparable to reanalysis data. NAO-response relationships weaken in the future period, but their intermember spread shows no significant change. The results stress the value of single-model ensembles for the evaluation of internal variability by pointing out the large differences of NAO-response relationships among individual members. They also strengthen the validity of the nested ensemble for further impact modeling using RCM data only, since important large-scale teleconnections present in the driving data propagate properly to the fine-scale dynamics in the RCM.