North American Droughts of the Last Millennium from a Gridded Network of Tree-Ring Data

Abstract

Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time.The gridded drought reconstructions reveal the existence of successive "megadroughts," unprecedented in persistence (20-40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today's North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium A.D., with a climate varying as today's, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest.Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool "La Niña-like" conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval megadroughts were forced by protracted La Niña-like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic "footprint" of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Niña-like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.

The global footprint of persistent extra‐tropical drought in the instrumental era

“…In the latter study, the authors reveal a pattern of climate anomalies that largely matches the global hydroclimatic regime accompanying modern day North American drought. In-phase with the Medieval megadroughts of the American west, drought conditions occur in southern South America, central east Africa and much of Europe, while tropical land regions including the tropical Americas and the African Sahel are wet (Herweijer et al, 2007). In each case-present day and Medieval-a protracted La Niña-like tropical Pacific is associated with this pattern.…”

“…On the heels of this study, several relevant studies by the authors have further investigated the global pattern of low-frequecy drought variability. Seager (2007) examine the causes and global context of 'turn-of-the-century' drought, while Herweijer et al (2007) look at the global hydroclimatic OfootprintÕ of the North American Medieval 'megadroughts'. In the latter study, the authors reveal a pattern of climate anomalies that largely matches the global hydroclimatic regime accompanying modern day North American drought.…”

Simulating multiyear drought events in North America with the HadCM3 climate model

“…These records suggest that droughts of greater magnitude and duration than those seen in the twentieth century have occurred during the last 1000 years as a result of natural variability (Woodhouse and Overpeck, 1998;Cook et al, 2004;Herweijer et al, 2007). In addition, it is suggested that anthropogenic climate change is contributing towards a transition to a more arid climate, particularly in the south-western United States .…”

“…Persistent droughts in the United States have severe economic, social and environmental impacts, costing $6-8 billion annually (Herweijer et al, 2007). Detailed instrumental records have enabled us to identify a number of major multiyear droughts in the last 150 years (Woodhouse and Overpeck, 1998).…”

“…Seager et al, 2005;Herweijer et al, 2007;Seager et al, 2007). GCMs are potentially very useful in assessing the possible mechanisms of drought and changes in these mechanisms over long periods of time.…”