Friday 26 August 2005

P2
1050-1230 hours

459
The medieval climate anomaly - recent centenial climate change in the Pacific
Graham, Nicholas¹, Hughes, Malcolm², Ammann, Caspar³, Hoerling, Martin⁴, Xu, Taiyi⁴, Cobb, Kim⁵
1 Hydrologic Research Center and Scripps Institution Of Oceanography, San Diego, CA, USA
2 LTRR, University of Arizona, Tucson, AZ, USA
3 NCAR, Boulder, CO USA
4 NOAA Climate Diagnostics Center, Boulder, CO USA
5 Georgia Institute of Technology, Atlanta, GA, USA
Author email: ngraham@hrc-lab.org
Newly developed syntheses of proxy records and model results are converging on a portrayal of the Medieval Climate Anomaly (MCA; often referred to as the Medieval Warm Period) as a relatively recent and surprisingly extreme instance of centennial-scale climate change. Intriguingly, this evolving research indicates that substantial changes in tropical Pacific sea surface temperatures (SSTs) played a central role in producing the substantial MCA climate anomalies inferred from proxy records, particularly at locations around the Pacific Basin. The rapidity and magnitude of this Medieval climate excursion, and the fact that amplification apparently involved sensitive components of the climate system, makes the MCA relevant from the perspective of the potential impacts of future climate change. Western North America is one region where proxy evidence for MCA climate change is especially coherent, and suggests that the MCA was much drier than the modern climate and punctuated by severe drought. Intriguingly, marine proxy records indicate that MCA sea surface temperatures (SSTs) were much cooler than in the modern record along the coast of central California and in the central and eastern equatorial Pacific, and much warmer than present in the far western equatorial Pacific. The idea that changes in background tropical Pacific SSTs played a role in producing MCA climate changes seen elsewhere (as inferred from proxy records) is supported by the tropical - extra-tropical teleconnection patterns seen in the modern record. To further examine this hypothesis, we have performed proxy-guided experiments with atmospheric general circulation models (AGCMs) using tropical Pacific SST changes suggested by the proxy records as prescribed boundary forcing for the AGCMs. The results are consistent with the hypothesized "tropical forcing" mechanism for the MCA-LIA climate transition in western North America and elsewhere. These results will be presented and discussed, along with those from ongoing sensitivity experiments.

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