Monday, October 7, 2013

New paper finds models unable to reproduce natural atmospheric oscillations which control climate

A paper under open review for Climate of the Past reconstructs the climate over the past 6,000 years and finds climate models were not able to reproduce the changes in European climate during during the mid-Holocene Climate Optimum, which was warmer than present-day climate. In particular, the authors find the models are unable to reproduce the atmospheric circulations of the Arctic Oscillation [AO] and the North Atlantic Oscillation [NAO] of key importance to European & global climate, and in fact, simulated changes opposite to those found in the climate proxy reconstruction. 

The authors find "The poor representation of [natural] changes in Mid-Holocene atmospheric circulation in models is consistent with similar model deficiencies found [during the present climate], and may be important in understanding why Europe has recently been warming faster than predicted"

The authors determine the primary failing of climate model simulations is an over-reliance and exaggeration of radiative forcing of climate, instead of an emphasis on natural atmospheric and oceanic oscillations and circulatory patterns. The primary obsession of the IPCC is also an over-simplification, exaggeration, and over-reliance that climate is controlled by radiative forcing forcing alone.

Note: the primary influence of the solar variation appears to be via influence of natural atmospheric and oceanic oscillations and circulatory patterns, rather than direct radiative forcing.


Many of the recent and historical periods of climatic warming in Europe have been explained by changes in atmospheric circulation. For instance, much of the warming that occurred in the late 20th Century in Europe has been attributed to the increased number of winters with a high index AO/NAO (Hurrell, 1995; Visbeck et al., 2001). High index AO/NAO conditions are also thought to have occurred during the Medieval 15 Climate Anomaly, providing a dynamic explanation for the winter warmth experienced  over Europe at this time (Trouet et al., 2009). Similarly in summer, the increased occurrence of heat waves in recent years has been shown to be the result of anomalous atmospheric circulation associated with blocking anticyclones (Kysely and Huth, 2006). This pattern has also been shown to underlie summer warming on longer timescales 20 in the late Holocene (Della-Marta et al., 2007; Trouet et al., 2012; Yiou et al., 2012). 

Changes in atmospheric circulation have a significant influence on European climate (Sepp and Jaagus, 2002; van Ulden and van Oldenborgh, 2006; Hoy et al., 2013), but many climate models have difficulty reproducing this aspect of modern climate (van Ulden and van Oldenborgh, 2006; Woollings, 2010; Kjellstrom et al., 2011; Brands et 25 al., 2013). The warming in Europe during the mid-Holocene simulated in climate models differs fundamentally from that shown in the data, and indicates a high sensitivity in models to the effects of the amplified seasonal insolation cycle experienced at this time, showing greater warming (cooling) in summer (winter) in response to increased (decreased) summer (winter) insolation. Our reconstructed climate in contrast shows a greater warming in winter than in summer at the European scale, and a spatial pattern of anomalies that is more consistent with changes in atmospheric circulation rather than simple direct radiative forcing by insolation. This suggests a greater role for atmospheric dynamics in explaining interglacial warming, and a challenge to conventional ideas about the simple role of Northern Hemisphere high latitude summer insolation in driving interglacial climates. It could also lend support to alternative orbital forcing’s such as the winter latitudinal insolation gradient that has an identical orbital signature to summer insolation, and which could influence the atmospheric circulation through 10 its control of the latitudinal temperature gradient (Davis and Brewer, 2009).

Our data-model comparison highlights significant differences between the reconstruction and the model simulation. We explain these differences in terms of atmospheric circulation, which appears strongly imprinted on the reconstructed climate, but subsumed in the model in favour of a simple direct radiative response to the change in the amplitude of the seasonal insolation cycle. We suggest that the MH climate of Europe was characterised by a strong zonal circulation in winter consistent with a positive or high index AO/NAO teleconnection. This brought milder wetter conditions into Northern Europe and cooler drier conditions to many parts of Southern Europe. In summer, we suggest that the zonal circulation was weak, and that anti-cyclonic blocking occurred close to Scandinavia, comparable with a positive or high index SCAND teleconnection. This caused a more meridional circulation, which brought clear skies and dry and warm conditions to Northern Europe, but relatively cooler and somewhat wetter conditions to many parts of Southern Europe. Both of these seasonal changes 5 in atmospheric circulation have been suggested by previous authors, and particularly in the case of the winter AO/NAO, are supported by a large number of studies based on many different proxies.

The poor representation of changes in Mid-Holocene atmospheric circulation in models is consistent with similar model deficiencies found on contemporary timescales, and may be important in understanding why Europe has recently been warming faster than predicted (van Oldenborgh et al., 2009). It also suggests that the atmospheric circulation may be more important in driving interglacial warming than previously considered based on model experiments that appear too sensitive to direct insolation forcing. Future work will extend this MH reconstruction to the complete Holocene, and investigate this problem by comparing this climate record with transient Holocene model simulations.

Clim. Past Discuss., 9, 5569-5592, 2013

The influence of atmospheric circulation on the mid-Holocene climate of Europe: a data-model comparison

A. Mauri1, B. A. S. Davis1,*, P. M. Collins1, and J. O. Kaplan1,*
1ARVE Group, Institute of Environmental Engineering, Ecole Polytechnique Fédérale de Lausanne, Switzerland
*now at: ARVE Group, Institute of Environmental Science, University of Geneva, Switzerland

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 (positive SCAND). We find little agreement between the reconstructed anomalies and those from a climate model simulation, which as with most model simulations shows a much greater sensitivity to local radiative forcing from 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 climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.

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