Fig. 7. Differences in December-March temperature
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Support for global climate reorganization during the "Medieval Climate Anomaly"
Climate Dynamics
DOI: 10.1007/s00382-010-0914-z
N.E. Graham1, C.M. Ammann2, D. Fleitmann3,
K.M. Cobb4, and J. Luterbacher5
1 Hydrologic Research Center, San Diego, CA, USA and Scripps Institution of Oceanography, La Jolla, CA, USA
2 National Center for Atmospheric Research, Boulder, CO, USA
3 Institute of Geological Sciences, University of Bern, Bern, Switzerland
4 Oeschger Centre for Climatic Change Research, University of Bern, Bern, Switzerland
5 Georgia Institute of Technology, Atlanta, GA, USA
6 Justus-Liebig-University, Giessen, Germany
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ABSTRACT:
Widely distributed proxy records indicate that the Medieval Climate Anomaly
(MCA; ~900-1350 AD) was characterized by coherent shifts in large-scale
Northern Hemisphere atmospheric circulation patterns. Although cooler
sea surface temperatures in the central and eastern equatorial Pacific
can explain some aspects of medieval circulation changes, they are not
sufficient to account for other notable features, including widespread
aridity through the Eurasian sub-tropics, stronger winter westerlies
across the North Atlantic and Western Europe, and shifts in monsoon
rainfall patterns across Africa and South Asia. We present results from
a full-physics coupled climate model showing that a slight warming of
the tropical Indian and western Pacific Oceans relative to the other
tropical ocean basins can induce a broad range of the medieval circulation
and climate changes indicated by proxy data, including many of those not
explained by a cooler tropical Pacific alone. Important aspects of the
results resemble those from previous simulations examining the climatic
response to the rapid Indian Ocean warming during the late twentieth
century, and to results from climate warming simulations - especially
in indicating an expansion of the Northern Hemisphere Hadley circulation.
Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST)
change responsible for producing the proxy-model similarity in our results
agrees well with MCA-LIA SST differences obtained in a recent proxy-based
climate field reconstruction. Though much remains unclear, our results
indicate that the MCA was characterized by an enhanced zonal Indo-Pacific
SST gradient with resulting changes in Northern Hemisphere tropical and
extra-tropical circulation patterns and hydroclimate regimes, linkages
that may explain the coherent regional climate shifts indicated by proxy
records from across the planet. The findings provide new perspectives
on the nature and possible causes of the MCA - a remarkable, yet
incompletely understood episode of Late Holocene climatic change.
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