Evidence for global climate reorganization during medieval times

Figure 1: Schematic diagram of MCA vs LIA climate shifts as seen in a range of globally distributed proxy records

A range of in-situ proxy records indicate that tropical eastern and central Pacific sea surface temperatures (SSTs) were relatively cool during the Medieval Climate Anomaly (MCA, ca. 900-1350 AD; e.g., Cobb et al., 2003; Rein et al., 2004; Conroy et al., 2008). This idea is supported by hydroclimate changes indicated by proxy records from extra-tropical western North and South America (e.g., Swetnam et al., 1993; Stine et al., 1994; Jenny et al., 2002; Cook et al., 2004; see Fig. 1). At the same time, indications of a distinct MCA appear in proxy records distributed around the planet, many far removed from the Pacific sector (also Fig. 1). Many of the climate shifts inferred from these latter records are inconsistent in strength or character of the changes expected on the basis of a cooler tropical Pacific alone (at least as judged from observations and model results) suggesting an important role for SST changes in other tropical oceans (Seager et al., 2007; Graham et al., 2007; Graham et al., 2010). Among such “inconsistent” shifts are indications of a more “positive NAO”-like circulation pattern during boreal winter, with impacts on European climate (e.g., Lamb, 1965; Mangini et al., 2005; Trouet et al., 2009), North Atlantic SST and sea ice (e.g., Keigwin, 1996; Jensen et al., 2004; Sicre et al., 2008; Wanamaker et al., 2008; Massé et al., 2009), aridity in equatorial Africa (Verschuren et al., 2004; Russell et al., 2007; Shanahan et al., 2009) and in parts of southwest Asia (Hassan, 1981; von Rad et al., 1999; Fleitmann et al., 2003), and increased monsoon rainfall in parts of south-central and eastern Asia (Sinha et al., 2007; Tan et al., 2008; Zhang et al., 2008; Buckley et al., 2010).

Figure 2: A) Changes in December-March precipitation (fraction of control simulation climatology; color) and sea level pressure (difference with control simulation climatology; hPa, contours) for a simulation with increased tropical Indian and western Pacific SST; precipitation changes are shown only where the differences exceed the 95% confidence level. B) As in (A), but for December-March temperature (°C, color; values are SST over ocean and 2-meter temperature over land). Colors are shown only where temperature differences exceed the 95% confidence level. Lined contour interval is 0.5°C between 30°N and 30°S and 1°C elsewhere. Global average temperature change has been subtracted.

Model simulations of the climate and circulation changes ensuing from the late 20th century warming of the Indian Ocean (e.g., Bader and Latif, 2003; Hurrell et al., 2004; Hoerling et al., 2004; Bader and Latif, 2005) show some of the “inconsistent” features noted above. To explore the possibility that similar warming may have occurred during the MCA, simulations were performed with a full-physics global coupled model (NCAR CCSM) in which tropical Indian, or tropical Indian and western Pacific, SSTs were increased over the range ~0.2-0.8°C. The simulated global climate and circulation shifts for boreal winter (Figs. 1 and 2; see Graham et al. (2010) for the corresponding results for boreal summer) show many of the climate changes inferred from global proxy records for the MCA, including many of those not well explained by a cooler tropical Pacific alone. These include a systematically stronger NAO during boreal winter, with associated changes in North Atlantic SSTs and sea ice, and European/North African precipitation. The simulated changes also include cooling and reduced rainfall in the equatorial eastern Pacific (boreal winter), seasonal aridity in equatorial and northeast Africa and into southwest Asia, transitioning towards relatively more moist conditions proceeding east across the Indian subcontinent, southeast Asia and into parts of China.

Overall, the findings support the general pattern of tropical SST changes seen in a recent statistical reconstruction (Mann et al., 2009), with a stronger zonal SST gradient between the Indo-Pacific Warm Pool and the eastern/central tropical Pacific during medieval times, relative to subsequent centuries.

acknowledgements

N.E.G. was supported by funding from grants NA06OAR4310120 and NA08OAR4310732 from the US National Oceanographic and Atmospheric Administration (NOAA) CCDD program. N.E.G. is also grateful to PAGES program and the Oeschger Centre for Climate Research at the University of Bern for their support of a visit to the PAGES office and Oeschger Centre in 2008 during which the research reported here was advanced.

references

Cobb, K.M., Charles, C.D., Cheng, H. and Edwards, R.L., 2003: El Niño / Southern Oscillation and tropical Pacific climate during the last millennium, Nature 424: 271-276.

Hurrell, J.W., Hoerling, M.P., Phillips, A.S. and Xu, T., 2004: Twentieth Century North Atlantic Climate Change. Part I: Assessing Determinism, Climate Dynamics 23: 371-389.

Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G. and Ni, F., 2009: Global signatures and dynamical origins of the “Little Ice Age” and “Medieval Climate Anomaly”, Science 362: 1256-1260.

Stine, S., 1994: Extreme and persistent drought in California and Patagonia during Medieval time, Nature 369: 546-549.

Trouet, V., Esper, J., Graham, N.E., Baker, A., Frank, D. and Scourse, J., 2009: Centuries-long positive North Atlantic Oscillation mode dominated Medieval Warm Period, Science 324: 78-80.

For full references please consult: http://pastglobalchanges.org/products/newsletters/ref2011_1.html