Abstract
This study examines the spatial structures of the North Atlantic Oscillation (NAO) and Pacific–North American (PNA) pattern across five reanalyses and investigates their temporal changes. Using winter monthly mean anomalies of 500-hPa geopotential heights from ensemble mean fields and individual ensemble members, we apply a moving principal component analysis (PCA) to 40-yr periods. Our findings reveal that the modes can vary significantly even with slight changes in the analyzed period. We observe that the lack of constraint over the North Pacific leads to different PNA patterns in individual ensemble members, resulting in a smoothed and weaker PNA in the ensemble mean. Therefore, we recommend using ensemble members instead of the ensemble mean before 1900 when analyzing low-frequency variability in 20CRv2 or 20CRv3. Despite differences in data assimilation, period length, and models among reanalyses, trends in explained variance and spatial changes of both PNA and NAO are similar. We also identified a decrease in the similarity of the NAO pattern around 1962–2001 across all datasets, linked to a sudden southward shift in the mode, indicating a change in variability in the reanalyses, independent of the data source or the model. The temporal changes, including long-term trends, likely have a fundamental basis, although sudden changes from period to period are rather artifacts from the PCA.
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