Abstract
Abstract. The atmospheric circulation is a key area of uncertainty in climate model simulations of future climate change, especially in mid-latitude regions such as Europe where atmospheric dynamics have a significant role in climate variability. It has been proposed that the mid-Holocene was characterized in Europe by a stronger westerly circulation in winter comparable with a more positive AO/NAO, and a weaker westerly circulation in summer caused by anti-cyclonic blocking near Scandinavia. Model simulations indicate at best only a weakly positive AO/NAO, whilst changes in summer atmospheric circulation have not been widely investigated. Here we use a new pollen-based reconstruction of European mid-Holocene climate to investigate the role of atmospheric circulation in explaining the spatial pattern of seasonal temperature and precipitation anomalies. We find that the footprint of the anomalies is entirely consistent with those from modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive AO/NAO) and a weak westerly circulation in summer associated with anti-cyclonic blocking (positive SCAND). We find little agreement between the reconstructed anomalies and those from 14 GCMs that performed mid-Holocene experiments as part of the PMIP3/CMIP5 project, which show a much greater sensitivity to top-of-the-atmosphere changes in solar insolation. Our findings are consistent with data–model comparisons on contemporary timescales that indicate that models underestimate the role of atmospheric circulation in recent climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.
Highlights
Global Climate Models (GCMs) are essential tools for investigating future climate change but their ability to simulate future climate remains uncertain, especially at the regional scale (Hawkins and Sutton, 2009; Deser et al, 2012)
A common feature of climate model simulations for the MH is a relatively uniform summer warming across the Northern Hemisphere extra-tropics (Braconnot et al, 2004, 2007b, Harrison, 2013). We find this warming in the PMIP3/CMIP5 ensemble mean in Europe (Fig. 4) and as a robust feature in almost all of the 14 GCMs which all show little if any summer cooling (Fig. 5)
We have presented a new seasonal gridded temperature and precipitation reconstruction for Europe during the MH, based on a fossil pollen and modern pollen data set that is greatly improved in both size and quality compared with previous studies. This climate reconstruction has been compared with the latest PMIP3/CMIP5 GCM simulations and with the modern winter AO/NAO and summer Scandinavian pattern (SCAND) that represent potential analogues of modes of climate variability connected with atmospheric circulation during the MH
Summary
Global Climate Models (GCMs) are essential tools for investigating future climate change but their ability to simulate future climate remains uncertain, especially at the regional scale (Hawkins and Sutton, 2009; Deser et al, 2012). One way to reduce model uncertainty is to perform GCM simulations of past climates, which can be independently evaluated using palaeoclimate archives. The midHolocene (MH, 6000 years BP) has become a standard benchmark time period for this kind of data–model comparison, being sufficiently distant in the past to be substantially different from the present but close enough that the model boundary conditions are well known and actual climate conditions can be reconstructed in some detail. The MH is a period rich in palaeoecological records (Wanner et al, 2008), providing the spatial coverage suitable for continental-scale data–model comparison that is comparable with the grid-box resolution of climate models (Davis et al, 2003)
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