Modeling Hydroclimate during the Medieval Climate Anomly

We have used a coral‐based tropical Pacific SST field for the period 1320–1462 A.D. to force an ensemble of AGCM simulations. The coral based SSTs are characterized by a La Niña‐like basic state in the tropical Pacific with variability on inter‐annual to inter‐decadal timescales. The model response to these conditions is similar to that seen in the observational record and model studies of modern persistent drought. La Niña‐like SSTs in the tropical Pacific force a poleward shift in the northern hemisphere transient eddies leading to increased subsidence and drying in the northern subtropics and midlatitudes as far east as the European continent. Increased precipitation in the northern tropics, with the exception of West Africa, is associated with a strengthening of the monsoons in Asia and Central America. The hydrological changes simulated by the POGA‐1L model for the medieval period are in agreement with paleo‐proxy records from this period for much of the Western Hemisphere, northern Eurasia, and the northern tropics. The regions where the model‐data intercomparison fails (e.g. tropical West Africa, Scotland) are regions that are influenced by Atlantic SSTs and the North Atlantic Oscillation (NAO). Seager et al. [2007] suggest that positive phases of the AMO and NAO likely played important roles in explaining the hydroclimate records for many such regions of the globe that are only influenced by the tropical Pacific. As such, tropical Pacific SST anomalies were an important forcing agent of Medieval hydroclimate anomalies but cannot fully explain the reconstructed pattern.

Reference:

Burgman, R., R. Seager, A. Clement, and C. Herweijer (2010), Role of tropical
Pacific SSTs in global medieval hydroclimate: A modelingstudy, Geophys. Res.
Lett., 37, L06705, doi:10.1029/2009GL042239.

Long term mean differences of vertically integrated soil wetness (shaded, (mm3H2O)/(mm3soil)) and SST (contour, 0.10 C) between POGA‐1L (1320–1462 A.D.) and GOGA simulation (1856–2005 A.D.) overlaid with records of medieval hydroclimate and SST. SST records are plotted as red (warm) or blue (cool) dots relative to modern observations. Green and brown dots indicate proxy records of wet or dry medieval hydroclimate. The numbers cross‐reference the records to the references in the text and Table S1 (see auxiliary material). The medieval period is taken to be, approximately, from 800 A.D. to 1500 (adapted from Seager et al. [2007]).