|by Richard Seager, Celine Herweijer and Ed Cook|
|Lamont-Doherty Earth Observatory of Columbia University|
|Top, the percent of the area of the American West experiencing moderate to severe drought at any one time as reconstructed from tree ring records over the last millennium. The time series has been filtered to emphasize variations on timescales of many decades to centuries. The lower panel shows a blowup for the last century emphasizing that the recent drought was not historically exceptional. The figure is taken from Cook et al. (2004, Science). During Medieval times serious drought affected large areas of the West. Following that there was a long period of more moderate drought (corresponding to the Little Ice Age) and, since then there appears to have been a return to a more drought stricken climate.|
|Tree ring records of modern droughts. The spatial distribution of tree ring summer Palmer Drought Severity Index (PDSI) is at left and the PDSI averaged over the West is at right.|
|Summer drought patterns from tree rings for the period 1000-2003 AD as estimated from Rotated Empirical Orthogonal Analysis. The fraction of the total variance explained by each REOF is indicated. According to Preisendorfer's Rule N (bottom right) these three patterns are physically distinct.|
|The SST (left) and SLP (right) patterns associated with the first three REOF patterns of drought evaluated on the 1856-2003 period. Pattern 1 if a typical decadal La-Niña-like pattern. Pattern 2 could be the summer North Atlantic Oscillation. The third drought pattern does not seem to be related to any clear mode of SST variation. These results support our modeling work of the 1856 to current period in that tropical Pacific influence dominates but that there may be a secondary Atlantic influence too.|
|Tree ring records of some medieval droughts. Spatial distribution is at left and the time history at right.|
|Histograms of annual summer tree ring derived PDSI for (top) the medieval period (or medieval climate anomaly (MCA)), (middle) the Little Ice Age and (bottom) the modern, post 1856, period.|
- The similarity of the spatial patterns suggests that the physical processes that caused the modern droughts also caused the medieval megadroughts.
- The global atmosphere ocean conditions that currently cause modern droughts for a few years at a time were the prevailing ocean climate during the medieval period.
- Despite the shift in the mean tropical ocean climate ENSO variability continued as now but oscillating about a colder mean state.
|Proxy evidence for medieval hydroclimate. Brown indicates a proxy indicator of dry conditions and green an indicator of wet conditions. The pattern resembles that of the global hydroclimate associated with modern day North American droughts.|