2000 years of Drought Variability in the Central United States

Home Browse Search

decadal -scale precipitation variability is the —20 yr solar —lunar cycle. This cycle has been investigated for years by many researchers (e.g., Mitchell et al. 1979; Stockton et al. 1983; Currie 1981,1984a,b; Cook et al. 1997) and is a feature that is increasingly being dis- cussed as a control on drought occurrence in the west- ern United States, although the physical link between these cycles, atmospheric circulation, and solar —lunar variability has not yet been determined. A number of proxy records reflect oscillations at the wavelength of these cycles, including tree -ring width chronologies for the western United States (Cook et al. 1997), sa- linity in North Dakota lake sediments (Laird et al. 1998), and varve thickness records in western Min- nesota (Anderson 1992). At longer timescales, low - frequency variability in ocean SSTs and ocean — atmosphere interactions is a likely source of persistent Great Plains drought con- ditions in the past. Research has shown changes in the conditions in oceans, such as occurred recently in the North Pacific Ocean, are manifested in long -term cli- mate and proxy records that suggest low - frequency variations have occurred in both Pacific and Atlantic Oceans (e.g., Michaelsen 1989; Duplessey et al. 1992; Rasmussen et al. 1995; Jennings and Weiner 1996; Keigwin 1996). Tree -ring chronologies from the southwestern United States that are sensitive to varia- tions in ENSO reveal a tendency toward low - frequency variations in ENSO events on century scales (Michaelsen 1989). It is known that ENSO events are linked to precipitation in the Great Plains on an event basis (Trenberth et al. 1988; Kiladis and Diaz 1989; Palmer and Brankovi6 1989; Bunkers et al. 1996; Phillips et al. 1996; Ting and Wang 1997), and Rasmussen et al. (1995) suggest that variations in ENSO intensity at the century timescale may broadly correspond to a modulation of interdecadal variations in drought in the Great Plains, with more severe drought epochs (i.e., 1930s- 1950s) coinciding with intervals of low ENSO variability. At present, it is not known whether decadal- to century-scale ENSO vari- ability is due to internal variability or external mecha- nisms, or a combination of both. Currently, there are no good long centuries -long records of North Pacific variability. Variations in the base state of the Atlantic Ocean may be an important influence on Great Plains precipi- tation if these variations change the position of the Bermuda high /Atlantic gyre or affect in other ways the ability of Gulf of Mexico moisture to penetrate into the interior United States. For example, Forman et al. Bulletin of the American Meteorological Society (1995) suggested that dune reactivation about 1000 years before present was due to a small easterly shift of the Bermuda high from its usual position in com- bination with a slight eastward shift of a western-cen- tral U.S. ridge aloft, a set of conditions that leads to drought today. There are several sources of proxy data in the North Atlantic Ocean that suggest low -fre- quency changes in the conditions of this ocean have occurred. A 1300 -yr -long record of changes in the East Greenland Current from sediment cores on the coast of eastern Greenland shows a cold interval beginning around A.D. 1270 and peaking around 1370 (Jennings and Weiner 1996), which roughly coincides with the western U.S. drought of the late thirteenth century. However, another cold period in this North Atlantic record spanned the mid — sixteenth century to the early twentieth century, a period not notable for drought in the Great Plains (in fact, the early part of this period was characterized by a lack of drought). In the Sar- gasso Sea, century-scale variations in SSTs are re- flected in 010 changes in planktonic foraminifera from marine sediments ( Keigwin 1996). Temperatures yielded from this record indicate oscillations from a minimum in A.D. 250 -450, to a maximum in A.D. 950- 1050, to another minimum in A.D. 1500 -1650. All three of these periods correspond to periods of Great Plains drought. Although periods of major Great Plains drought appear to correspond to extremes in the SST record of either sign, perhaps an Atlantic — drought link is related to periods of anomalous conditions or peri- ods of significant change in SST. It is also likely that the effects of anomalous conditions in the Atlantic on Great Plains drought may interact with the impacts of conditions in the Pacific in ways that may enhance or diminish drought conditions. 5. Droughts of the future A review of the available paleoclimatic data indi- cates that twentieth - century droughts do not represent the full range of potential drought variability given a climate like that of today. In assessing the possible magnitude of future drought, it is necessary to consider this full range of drought. It is possible that the condi- tions that lead to severe droughts, such as those of the late sixteenth century, could recur in the future, lead- ing to a natural disaster of a dimension unprecedented in the twentieth century. Two factors may further com- pound the susceptibility of the Great Plains to drought in the future: 1) increased vulnerability due to human 2709