| 22 Jul 2025
North Atlantic Oscillation (NAO) in the Paleoclimate Modelling Intercomparison Project (PMIP)
Abstract. The North Atlantic Oscillation (NAO) is one of the main modes of climate variability and the dominant mode of large-scale atmospheric variability in the North Atlantic basin and has large impacts on the European climate, whose future behaviour remains uncertain. Here we assess the NAO response in past and future climates by looking at a comprehensive set of coupled model simulations performed by the Paleoclimate Model Intercomparison Project (PMIP) and the Coupled Model Intercomparison Project (CMIP) for four experiments: the mid-Holocene (6 ka; midHolocene), the Last Glacial Maximum (21 ka; lgm), the last interglacial (127 ka; lig127k) and an idealised future warming scenario with abrupt quadrupled CO2. Although there are various setups across experiments, the midHolocene and lig127k are mainly characterised by altered orbital configurations, inducing variations in the seasonal cycle, and the lgm and abrupt4xCO2 are mainly characterised by various GHG forcing that induces great global temperature change. Our results show that the NAO is sensitive to GHG-forcing-induced temperature changes but not the orbital configurations. NAO weakens in response to cooling and strengthens to warming. The associated teleconnections change consistently with the theory and are sensitive to the change in NAO amplitude. The two orbital experiments do not show a clear change in associated temperature and precipitation. The weakened NAO in the lgm is associated with a cooler and drier northern Europe, while the enhanced NAO in the abrupt4xCO2 causes a warmer and wetter northern Europe as compared to the piControl. No clear relationship is found in the ENSO-NAO teleconnection.
I wrote some comments and saved them inermediately, but they were all lost in the system. Sorry about that, but there were no major issues. Some typoes and the language could be improved. I like the way they maed a synthesis analysis based on different CMIP expeiments and made the most out of available data. One issue to look at is the use of common EOFs for model evaluation and for comparing spatio-temporal covariance structures in the different results (see e.g. Tim Barnet's paper from 1999 https://doi.org/10.1175/1520-0442(1999)012%3C0511:CONSAT%3E2.0.CO;2 and a more recent review from Nature Climate Change http://www.nature.com/nclimate/journal/v7/n10/full/nclimate3393.html).Â
The analysis is very nice, and it would be interesting to know next whether there is a clearer NAO signal in the number of wet days (or wet-day frequency) than total precipitation. The total preciptation is the product between the number of days, the wet-day frequency and the wet-day mean precipitation. Ther ehave been some indications that the wet-day frequency is more strongly affected by circulation whereas the intensity is influenced by other factors.Â
Make Fig. 1Â & 2 (also 3?) bigger - fill the whole space.
Figs 4 & 5 are difficult to interpret. Why does piControl dots vary?
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