2000 years of Drought Variability in the Central United States

• Description: Report on droughts from the past 2000 years that were analyzed using paloeoclimatic records ‎(tree rings, archeological remains, etc.)‎.
• State: CO
• Date: 12/12/1998
• Author: Woodhouse, Connie; Overpeck, Jonathan
• Title: 2000 years of Drought Variability in the Central United States
• Water Supply Pro - Doc Type: Publication

What sets up these drought- producing atmospheric
circulation patterns, and what drives the variability that
leads to the spatial distribution of drought in the Great
Plains? There is strong evidence that the state of the
oceans, both Pacific and Atlantic, can lead to drought
conditions in the Great Plains, directly or indirectly,
by inducing perturbations in patterns of atmospheric
circulation and the transport of moisture (Trenberth
et al. 1988; Palmer and Brankovi61989; Trenberth and
Guillemot 1996; Ting and Wang 1997). The position
of the ridge of high pressure over the plains has been
found to be associated with the strength and position
of the surface Bermuda high (also called the Atlantic
subtropical high) over the Atlantic, which is also
linked to the strength of the LLJ (Helfand and Schubert
1995). When this Bermuda high is in a position far-
ther east and north than normal, moist air flows around
the high and into the eastern coast, while the Great
Plains remains dry. In its usual position, farther south
and west, the moist Gulf of Mexico air moves around
the high and into the central United States (Forman
et al. 1995). The position of this high is likely related
to sea surface temperatures (SSTs) in the Gulf of
Mexico (Oglesby 1991).
Conditions in the Pacific also influence Great
Plains drought- associated circulation patterns. A
number of studies have linked conditions in both the
equatorial Pacific [El Nino — Southern Oscillation
(ENSO)- related conditions] and the northern Pacific
to spring and summer precipitation variability in the
Great Plains (Trenberth et al. 1988; Kiladis and Diaz
1989; Palmer and Brankovic 1989; Bunkers et al.
1996; Phillips et al. 1996; Ting and Wang 1997). SSTs
in the equatorial Pacific appear to have more influence
on summer drought conditions in the northern Great
Plains, whereas northern Pacific SSTs are more
closely linked to conditions in the central and south-
ern Great Plains (Ting and Wang 1997). The two
modes of SST (i.e., the patterns of covariance between
SST and precipitation in the two regions) operate in-
dependently for the most part and can compound or
cancel out one another's impacts on precipitation in
the Great Plains. -For instance, in 1988, SSTs from
both areas contributed to drought conditions, whereas
in 1987, modes were in opposition, and precipitation
was near normal throughout the Great Plains. The in-
terplay between conditions in the tropical and north-
ern Pacific have been linked to decadal- scale.PDSI
variability in areas of the United States that include
the Great Plains (Cole and Cook 1998). Although
Pacific SSTs are not directly linked to the transport
of Pacific moisture into the Great Plains, they appar-
ently cause changes in circulation patterns, which, in
turn, influence the transport of Gulf of Mexico mois-
ture into the region and the position of the jet stream
(Ting and Wang 1997). The position of the jet is as-
sociated with the locations of surface fronts and cy-
clogenesis (Barry and Chorley 1987).
Persistence of drought- producing conditions is a
key feature of drought (Namias 1983). The causes of
persistent drought- producing conditions on the
timescales of months to a season are fairly well un-
derstood, but the causes of droughts with durations of
years (e.g., 1930s) to decades or centuries (i.e.,
paleodroughts) are not well understood. Twentieth -
century droughts have occurred on subdecadal
timescales and persistence related to these droughts
has been attributed to anomalous circulation patterns
supported by low soil moisture, strong surface heat-
ing, and large -scale subsidence (Namias 1983). These
synoptic -scale to mesoscale patterns are also main-
tained by variability in modes of seasonally related
large -scale atmospheric circulation (Diaz 1986;
Barnston and Livezey 1987; Diaz 1991).
A principal difference between major droughts of
the twentieth century and major droughts of the more
distant past is the duration, which is on the order of
seasons to years compared to decades to centuries.
What caused persistence of drought conditions on
these timescales? A number of mechanisms may be
influencing persistence on decadal timescales. One
possible cause of long -term persistence may be related
to persistent anomalous boundary conditions influ-
enced by low- frequency variations in the thermal char-
acteristics of oceans (Namias 1983). There is evidence
that variations in large -scale patterns of atmospheric
circulation and atmosphere —ocean interactions that
impact regional precipitation occur on the order of
decades to centuries. A recent example of decadal-
scale variation is the change in conditions in the North
Pacific atmosphere and ocean beginning in the mid -
1970s, which impacted climate conditions across the
United States (Miller et al. 1994; Trenberth and
Hurrell 1994). In the Atlantic Ocean, decadal -scale
variations in the Northern Atlantic oscillation (NAO)
have been detected and linked to climatic conditions
in Europe and the Mediterranean (Hurrell 1995).
Other less well investigated twentieth - century decadal
shifts in atmospheric circulation patterns have been
characterized by changes in zonal versus meridional
.circulation over North America (Dzerdzeevskii 1969;
Granger 1984). Another possibly important source of
2708 Vol. 79, No. 12, December 1998