Mid-Holocene NAO: A PMIP2 model intercomparison

The mid-Holocene ( 6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid-Holocene NAO and mean SLP show small changes compared to pre-industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO-negative phase during the mid-Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid-Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid-Holocene.