2000 years of Drought Variability in the Central United States

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trusion of the Atlantic anticyclonic subtropical gyre (associated with the Bermuda high) into the interior United States (Tang and Reiter 1984; Helfand and Schubert 1995; Higgins et al. 1997). Another source of summer precipitation is mesoscale convective com- plexes (MCCs), which can contribute between 30% and 70% of the total warm season precipitation over much of the Great Plains (Fritsch et al. 1986). Less consistently, synoptic -scale upper -level disturbances also contribute summertime moisture (Helfand and Schubert 1995; Mock 1996). In spring, the mixing of cold air masses from the Arctic with warm, moist air tropical masses from the Gulf of Mexico causes an in- crease in precipitation (Bryson 1966). During this sea- son, meridional troughs and cutoff lows in midlatitude frontal systems draw moisture from the Gulf of Mexico into the western Great Plains (Hirschboeck 1991). Fall is a relatively dry season as Pacific air domi- nates most of the region (Bryson 1966; Mock 1996). Drought in the Great Plains can occur during any season, but since late spring and summer are the sea- sons when most of the precipi- tation falls, these are the most r- important drought seasons. In general, Great Plains drought is characterized by a semiperma- nent mid- to upper- tropospheric anticyclone over the plains, sustained by anticyclones in both the eastern central Pacific and eastern central Atlantic and accompanied by intervening troughs (Namias 1955, 1983) that can persist throughout the summer. Under this configura- tion, the jet stream is diverted to the north and the plains anticy- clone blocks moisture from the Gulf (Borchert 1950). The Great Plains region is commonly not homogeneous with respect to drought because of the spatially variable influence of the circu- lation features related to sea- sonal precipitation (Karl and FIG. 12. Spatial di Koscielny 1982). Figure 12 illus- drought years (1988, trates this by showing the spatial in the Great Plains, l distribution of PDSI values for years mapped. This s points in 1934, in 19 three different twentieth- century whereas in 1988, dro drought years and accompany- the northern Great P ing PDSI time series for three NOAA/NESDIS we different regions. The position of the semipermanent ridge of high pressure appears to be particularly im- portant. At times when the ridge is displaced east of its usual position over the west - central United States; Gulf of Mexico moisture is unable to penetrate into the central United States (Oglesby 1991), but there appear to be varying degrees of displacement. The 1950s drought was most severe in the southern Great Plains, suggesting a complete failure of Gulf of Mexico moisture to enter the central United States (Borchert 1971). In contrast, the 1988 drought was characterized by an inverted U shape, in which drought was largely restricted to the northern Plains as well as the west coast and southeastern United States, while Gulf moisture was able to find a way into the south - central United States (Oglesby 1991) (Fig. 12). Once a drought- inducing circulation pattern is set up, dry conditions can be perpetuated or ampli- fied by persistent recurrent subsidence leading to heat waves, clear skies, and soil moisture deficits (Chamey 1975; Namias 1983; Oglesby and Erickson 1989). r Bulletin of the American Meteorological Society 19,14 1456 1988 19m - ;a:o ' � a >� stribution of observed PDSI values for three severe twentieth- century 1956, 1934) (left) and time series of observed PDSI for three grid points 900 -94. (right). Gray vertical bars in the time series mark the drought et of maps shows that although PDSI values are low for all three grid 56 drought was more'severe in the central and southern Great Plains, ught is only reflected in the Montana time series, and on the map, across lains [Karl et al. 1990; Guttman 1991; Cook et al. J996; see.also the b site (URL given in text)]. 2707