Early‐to-late winter 20th century North Atlantic multidecadal atmospheric variability in observations, CMIP5 and CMIP6

The strong multi-decadal variability in North Atlantic (NA) winter atmospheric circulation is poorly understood and appears too weak in climate models. Recent research has shown peak atmospheric multi-decadal variability over the NA in late winter, particularly March, linked to Atlantic multi-decadal variability (AMV) of the ocean. Here a range of NA atmospheric circulation indices are assessed to provide a comprehensive picture of early-to-late winter low-frequency variability and its representation in the latest generation of climate models (Coupled Model Intercomparison Project Phase 6 (CMIP6)). As found for CMIP5, CMIP6 models exhibit too-weak multi-decadal NA atmospheric variability compared to reanalysis data over the period 1862-2005. Consistent with previous research, the eastern part of the NA westerly jet (U700NA) exhibits peak low-frequency variability in March. However, for NA-wide jet speed and the NAO, low-frequency variability and model-reanalysis discrepancies are strongest in January and February, associated with too-weak NA ocean-atmosphere linkages.

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