NOVEMBER 1997-&OLPDWH
DAI ET AL.
2943
Surface Observed Global Land Precipitation Variations during 1900–88
AIGUO DAI*
Department of Applied Physics, Columbia University, New York, New York
INEZ Y. FUNG
School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada, and
NASA/Goddard Institute for Space Studies, New York, New York
ANTHONY D. DEL GENIO
NASA/Goddard Institute for Space Studies, New York, New York
(Manuscript received 9 July 1996, in fina form 11 October 1996)
ABSTRACT
The authors have analyzed global station data and created a gridded dataset of monthly precipitation for the
period of 1900–88. Statistical analyses suggest that discontinuities associated with instrumental errors are large
for many high-latitude station records, although they are unlikely to be significan for the majority of the stations.
The firs leading EOF in global precipitation field is an ENSO-related pattern, concentrating mostly in the low
latitudes. The second leading EOF depicts a linear increasing trend (;2.4 mm decade21) in global precipitation
field during the period of 1900–88. Consistent with the zonal precipitation trends identifie in previous analyses,
the EOF trend is seen as a long-term increase mostly in North America, mid- to high-latitude Eurasia, Argentina,
and Australia. The spatial patterns of the trend EOF and the rate of increase are generally consistent with those
of the precipitation changes in increasing CO2 GCM experiments.
The North Atlantic oscillation (NAO) accounts for ;10% of December–February precipitation variance over
North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is
above normal in northern (.508N) Europe, the eastern United States, northern Africa, and the Mediterranean,
while below-normal precipitation occurs in southern Europe, eastern Canada, and western Greenland.
Wet and dry months of one standard deviation occur at probabilities close to those of a normal distribution
in midlatitudes. In the subtropics, the mean interval between two extreme events is longer. The monthly wet
and dry events seldom (probability , 5%) last longer than 2 months. ENSO is the single largest cause of global
extreme precipitation events. Consistent with the upward trend in global precipitation, globally, the averaged
mean interval between two dry months increased by ;28% from 1900–44 to 1945–88. The percentage of wet
areas over the United States has more than doubled (from ;12% to .24%) since the 1970s, while the percentage
of dry areas has decreased by a similar amount since the 1940s. Severe droughts and flood comparable to the
1988 drought and 1993 floo in the Midwest have occurred 2–9 times in each of several other regions of the
world during this century.
1. Introduction
Station rain gauge records have been used to estimate
the monthly climatology (e.g., Jaeger 1983; Shea 1986;
Legates and Willmott 1990) and large-scale variations
of precipitation during this century (Bradley et al. 1987;
Diaz et al. 1989; Vinnikov et al. 1990). These largescale analyses of precipitation changes have revealed
* Current affiliation National Center for Atmospheric Research,
Boulder, Colorado.
Corresponding author address: Dr. Aiguo Dai, NCAR, P.O. Box
3000, Boulder, CO 80307.
E-mail: [email protected]
q 1997 American Meteorological Society
that during the last 4–5 decades precipitation has increased over northern midlatitudes and most Southern
Hemisphere land areas, but has decreased in northern
low-latitude land areas. However, these long-term precipitation trends have been interpreted cautiously due
to the possible inhomogeneity and the limited spatial
coverage of the precipitation station data (Følland et al.
1990, 1992).
It is well known that terrestrial rain gauge records
contain various instrumental errors (Sevruk 1982; Legates and Willmott 1990; Groisman et al. 1991; Groisman
and Legates 1994). The errors may be grouped into two
types: undercatch and inhomogeneity. The undercatch
error, which results from wind blowing, wall wetting,
evaporation, and splashing, varies from less than 5% in
the Tropics to more than 50% near the poles in winter
VOLUME 12
JOURNAL OF CLIMATE
AUGUST 1999
Effects of Clouds, Soil Moisture, Precipitation, and Water Vapor on Diurnal
Temperature Range
AIGUO DAI
AND
KEVIN E. TRENBERTH
National Center for Atmospheric Research,* Boulder, Colorado
THOMAS R. KARL
NOAA/National Climatic Data Center, Asheville, North Carolina
(Manuscript received 9 July 1998, in fina form 9 September 1998)
ABSTRACT
The diurnal range of surface air temperature (DTR) has decreased worldwide during the last 4–5 decades and
changes in cloud cover are often cited as one of the likely causes. To determine how clouds and moisture affect
DTR physically on daily bases, the authors analyze the 30-min averaged data of surface meteorological variables
and energy fluxe from the the First International Satellite Land Surface Climatology Project Field Experiment
and the synoptic weather reports of 1980–1991 from about 6500 stations worldwide. The statistical relationships
are also examined more thoroughly in the historical monthly records of DTR, cloud cover, precipitation, and
streamflo of this century.
It is found that clouds, combined with secondary damping effects from soil moisture and precipitation, can
reduce DTR by 25%–50% compared with clear-sky days over most land areas; while atmospheric water vapor
increases both nighttime and daytime temperatures and has small effects on DTR. Clouds, which largely determine
the geographic patterns of DTR, greatly reduce DTR by sharply decreasing surface solar radiation while soil
moisture decreases DTR by increasing daytime surface evaporative cooling. Clouds with low bases are most
efficien in reducing the daytime maximum temperature and DTR mainly because they are very effective in
reflectin the sunlight, while middle and high clouds have only moderate damping effects on DTR. The DTR
reduction by clouds is largest in warm and dry seasons such as autumn over northern midlatitudes when latent
heat release is limited by the soil moisture content. The net effects of clouds on the nighttime minimum
temperature is small except in the winter high latitudes where the greenhouse warming effect of clouds exceeds
their solar cooling effect.
The historical records of DTR of the twentieth century covary inversely with cloud cover and precipitation
on interannual to multidecadal timescales over the United States, Australia, midlatitude Canada, and former
U.S.S.R., and up to 80% of the DTR variance can be explained by the cloud and precipitation records. Given
the strong damping effect of clouds on the daytime maximum temperature and DTR, the well-established
worldwide asymmetric trends of the daytime and nighttime temperatures and the DTR decreases during the last
4–5 decades are consistent with the reported increasing trends in cloud cover and precipitation over many land
areas and support the notion that the hydrologic cycle has intensified
1. Introduction
The diurnal range of surface air temperature (DTR)
has decreased since the 1950s worldwide (especially
over the Northern Hemisphere land areas) mainly but
not always due to an increase in nighttime minimum
temperature (Tmin ) that exceeds the increase in daytime
maximum temperature (Tmax ) (Karl et al. 1993; Horton
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: Dr. A. Dai, NCAR, P.O. Box 3000,
Boulder, CO 80307.
E-mail: [email protected]
q 1999 American Meteorological Society
1995; Houghton et al. 1996; Easterling et al. 1997).
Coincident increases in total cloud cover have been
found in a number of locations (Henderson-Sellers
1992; Dessens and Bücher 1995; Kaas and Frich 1995;
Jones 1995) and are often cited as a likely cause for the
observed DTR decrease (Kukla and Karl 1993; Karl et
al. 1993; Karl et al. 1996; Dai et al. 1997a). Annual
and seasonal DTR are strongly correlated with cloud
cover over the contiguous United States with the highest
correlation in autumn (Plantico and Karl 1990; Karl et
al. 1993). A jump in cloud cover was found to concur
with a large decrease in DTR over the former U.S.S.R.
(Karl et al. 1996). Strong correlation between annual
DTR and cloud cover/precipitation also exists over other
continents where data are available (Dai et al. 1997a).
Clouds can reduce Tmax by reflectin the sunlight and
2451
15 FEBRUARY 2001-&OLPDWH
485
DAI ET AL.
Climates of the Twentieth and Twenty-First Centuries Simulated by the
NCAR Climate System Model
AIGUO DAI, T. M. L. WIGLEY, B. A. BOVILLE, J. T. KIEHL,
AND
L. E. BUJA
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 3 November 1999, in fina form 24 March 2000)
ABSTRACT
The Climate System Model, a coupled global climate model without ‘‘flu adjustments’’ recently developed
at the National Center for Atmospheric Research, was used to simulate the twentieth-century climate using
historical greenhouse gas and sulfate aerosol forcing. This simulation was extended through the twenty-firs
century under two newly developed scenarios, a business-as-usual case (ACACIA-BAU, CO 2 ø 710 ppmv in
2100) and a CO 2 stabilization case (STA550, CO 2 ø 540 ppmv in 2100). Here we compare the simulated and
observed twentieth-century climate, and then describe the simulated climates for the twenty-firs century. The
model simulates the spatial and temporal variations of the twentieth-century climate reasonably well. These
include the rapid rise in global and zonal mean surface temperatures since the late 1970s, the precipitation
increases over northern mid- and high-latitude land areas, ENSO-induced precipitation anomalies, and Pole–
midlatitude oscillations (such as the North Atlantic oscillation) in sea level pressure fields The model has a
cold bias (28–68C) in surface air temperature over land, overestimates of cloudiness (by 10%–30%) over land,
and underestimates of marine stratus clouds to the west of North and South America and Africa.
The projected global surface warming from the 1990s to the 2090s is ;1.98C under the BAU scenario and
;1.58C under the STA550 scenario. In both cases, the midstratosphere cools due to the increase in CO 2 , whereas
the lower stratosphere warms in response to recovery of the ozone layer. As in other coupled models, the surface
warming is largest at winter high latitudes ($5.08C from the 1990s to the 2090s) and smallest (;1.08C) over
the southern oceans, and is larger over land areas than ocean areas. Globally averaged precipitation increases
by ;3.5% (3.0%) from the 1990s to the 2090s in the BAU (STA550) case. In the BAU case, large precipitation
increases (up to 50%) occur over northern mid- and high latitudes and over India and the Arabian Peninsula.
Marked differences occur between the BAU and STA550 regional precipitation changes resulting from interdecadal variability. Surface evaporation increases at all latitudes except for 608–908S. Water vapor from increased
tropical evaporation is transported into mid- and high latitudes and returned to the surface through increased
precipitation there. Changes in soil moisture content are small (within 63%). Total cloud cover changes little,
although there is an upward shift of midlevel clouds. Surface diurnal temperature range decreases by about 0.28–
0.58C over most land areas. The 2–8-day synoptic storm activity decreases (by up to 10%) at low latitudes and
over midlatitude oceans, but increases over Eurasia and Canada. The cores of subtropical jets move slightly upand equatorward. Associated with reduced latitudinal temperature gradients over mid- and high latitudes, the
wintertime Ferrel cell weakens (by 10%–15%). The Hadley circulation also weakens (by ;10%), partly due to
the upward shift of cloudiness that produces enhanced warming in the upper troposphere.
1. Introduction
The earth’s climate system (the atmosphere, ocean,
biosphere, etc.) has been altered by human activities
(e.g., fossil fuel combustion and deforestation) in many
ways. One of the consequences is a sharp rise in atmospheric concentrations of greenhouse gases (CO 2 ,
CH 4 , etc.) and aerosols since the late-nineteenth century
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: Aiguo Dai, National Center for
Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.
E-mail: [email protected]
q 2001 American Meteorological Society
(Watson et al. 1990; Schimel et al. 1996). A large number of climate model experiments predict substantial
climate changes (including surface warming) in response to increased concentrations of CO 2 and other
greenhouse gases (e.g., Mitchell et al. 1990; Kattenberg
et al. 1996). Because the atmospheric loading of greenhouse gases is likely to rise in the twenty-firs century
as a result of the expected growth in the world economy
and population, meaningful predictions of climate
changes in the next century have many practical applications (e.g., for impact assessments and to guide future
emission controls).
Reliable predictions of future climates are, however,
still a big challenge for climate researchers. They require
a) comprehensive models of the earth’s climate system
that include all of its major components and the inter-
1112
JOURNAL OF CLIMATE 2001
VOLUME 14
Global Precipitation and Thunderstorm Frequencies.
Part II: Diurnal Variations
AIGUO DAI
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 24 January 2000, in fina form 8 May 2000)
ABSTRACT
Three-hourly present weather reports from ;15 000 stations around the globe and from the Comprehensive
Ocean–Atmosphere Data Set from 1975 to 1997 were analyzed for diurnal variations in the frequency of
occurrence for various types of precipitation (drizzle, nondrizzle, showery, nonshowery, and snow) and thunderstorms. Significan diurnal variations with amplitudes exceeding 20% of the daily mean are found over much
of the globe, especially over land areas and during summer. Drizzle and nonshowery precipitation occur most
frequently in the morning around 0600 local solar time (LST) over most land areas and from midnight to 0400
LST over many oceanic areas. Showery precipitation and thunderstorms occur much more frequently in the late
afternoon than other times over most land areas in all seasons, with a diurnal amplitude exceeding 50% of the
daily mean frequencies. Over the North Pacific the North Atlantic, and many other oceanic areas adjacent to
continents, showery precipitation is most frequent in the morning around 0600 LST, which is out of phase with
land areas. Over the tropical and southern oceans, showery precipitation tends to peak from midnight to 0400
LST. Maritime thunderstorms occur most frequently around midnight. It is suggested that the diurnal variations
in atmospheric relative humidity contribute to the morning maximum in the frequency of occurrence for drizzle
and nonshowery precipitation, especially over land areas. Solar heating on the ground produces a late-afternoon
maximum of convective available potential energy in the atmosphere that favors late-afternoon moist convection
and showery precipitation over land areas during summer. This strong continental diurnal cycle induces a diurnal
cycle of opposite phase in low-level convergence over large nearby oceanic areas that favors a morning maximum
of maritime showery precipitation. Larger low-level convergence induced by pressure tides and higher relative
humidity at night than at other times may contribute to the nighttime maximum of maritime showery and
nonshowery precipitation over remote oceans far away from continents.
1. Introduction
Documentation of the diurnal variability of precipitation over various regions has been a topic of hundreds
of published articles. Studies before 1975 have been
summarized by Wallace (1975). Dai et al. (1999a) summarized more recent analyses for the United States (e.g.,
Schwartz and Bosart 1979; Balling 1985; Easterling and
Robinson 1985; Englehart and Douglas 1985; Riley et
al. 1987; Landin and Bosart 1989; Tucker 1993; Higgins
et al. 1996). There are also more recent studies for the
coastal and island regions in eastern Asia (Hu and Hong
1989; Oki and Musiake 1994; Lim and Kwon 1998),
tropical America (Kousky 1980), East Africa (Majugu
1986), and the Sahel (Shinoda et al. 1999). The results
of these studies, which are based on station records, can
be summarized as follows: 1) summer precipitation and
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: A. Dai, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.
E-mail: [email protected]
thunderstorms over inland regions tend to be more frequent during the afternoons while the maximum rainfall
is at night or during early morning over the coastal and
small-island areas. 2) There are, however, exceptions to
this general tendency. For example, summer precipitation is most frequent from middle night to early morning
over the central United States and around 1400–1600
local solar time (LST) over the entire Florida Peninsula
(Wallace 1975; Dai et al. 1999a). 3) During the other
seasons, precipitation has a much weaker diurnal cycle
than in summer, with a morning maximum in winter
over many land areas (Dai et al. 1999a). 4) Precipitation
intensity generally has much smaller diurnal variations
than precipitation frequency (Oki and Musiake 1994;
Dai et al. 1999a). 5) Although a single large peak is a
dominant feature in most station rain gauge records,
there is a secondary peak or a weak semidiurnal (12-h)
cycle of rainfall at many tropical (peak around 0300
LST) and midlatitude (peak around 0600 LST) stations
(Hamilton 1981a,b; Oki and Musiake 1994).
Over the open oceans, in situ observations of precipitation are sparse. Kraus (1963) analyzed weather reports from nine fixe weather-ships in the northern (308–
660
JOURNAL OF HYDROMETEOROLOGY
2002
VOLUME 3
Estimates of Freshwater Discharge from Continents: Latitudinal and
Seasonal Variations
AIGUO DAI
AND
KEVIN E. TRENBERTH
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 22 March 2002, in fina form 12 July 2002)
ABSTRACT
Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 18
resolution using several methods. The most accurate estimate is based on streamflo data from the world’s
largest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios of
runoff and drainage area between the unmonitored and monitored regions. Simulations using a river transport
model (RTM) forced by a runoff fiel were used to derive the river mouth outflo from the farthest downstream
gauge records. Separate estimates are also made using RTM simulations forced by three different runoff fields
1) based on observed streamflo and a water balance model, and from estimates of precipitation P minus
evaporation E computed as residuals from the atmospheric moisture budget using atmospheric reanalyses from
2) the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR)
and 3) the European Centre for Medium-Range Weather Forecasts (ECMWF). Compared with previous estimates,
improvements are made in extending observed discharge downstream to the river mouth, in accounting for the
unmonitored streamflo , in discharging runoff at correct locations, and in providing an annual cycle of continental
discharge. The use of river mouth outflo increases the global continental discharge by ;19% compared with
unadjusted streamflo from the farthest downstream stations. The river-based estimate of global continental
discharge presented here is 37 288 6 662 km 3 yr 21 , which is ;7.6% of global P or 35% of terrestrial P. While
this number is comparable to earlier estimates, its partitioning into individual oceans and its latitudinal distribution
differ from earlier studies. The peak discharges into the Arctic, the Pacific and global oceans occur in June,
versus May for the Atlantic and August for the Indian Oceans. Snow accumulation and melt are shown to have
large effects on the annual cycle of discharge into all ocean basins except for the Indian Ocean and the Mediterranean and Black Seas. The discharge and its latitudinal distribution implied by the observation-based runoff
and the ECMWF reanalysis-based P–E agree well with the river-based estimates, whereas the discharge implied
by the NCEP–NCAR reanalysis-based P–E has a negative bias.
1. Introduction
In a steady state, precipitation P exceeds evaporation
E (or evapotranspiration) over land and the residual water runs off and results in a continental freshwater discharge into the oceans. Within the oceans, surface net
freshwater fluxe into the atmosphere are balanced by
this discharge from land along with transports within
the oceans. The excess of E over P over the oceans
results in atmospheric moisture that is transported onto
land and precipitated out, thereby completing the land–
ocean water cycle. While E and P vary spatially, the
return of terrestrial runoff into the oceans is mostly concentrated at the mouths of the world’s major rivers, thus
providing significan freshwater inflo locally and
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: A. Dai, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307.
E-mail: [email protected]
q 2002 American Meteorological Society
thereby forcing the oceans regionally through changes
in density (Carton 1991; Nakamura 1996).
To study the freshwater budgets within the oceans,
therefore, estimates of continental freshwater discharge
into the ocean basins at each latitude are needed. Baumgartner and Reichel (1975, BR75 hereafter) derived
global maps of annual runoff and made estimates of
annual freshwater discharge largely based on streamflo
data from the early 1960s analyzed by Marcinek (1964).
Despite various limitations of the BR75 discharge data
(e.g., rather limited station coverage, areal integration
over 58 latitude zones, no seasonal values), these estimates are still widely used in evaluations of ocean and
climate models (Pardaens et al. 2002) and in estimating
oceanic freshwater transport (Wijffels et al. 1992; Wijffels 2001), mainly because there have been few updated
global estimates of continental discharge. The primary
purpose of this study is to remedy this situation.
Correct simulations of the world river system and its
routing of terrestrial runoff into the oceans has also
become increasingly important in global climate system
DECEMBER 2004
J. Hydrometeorology
1117
DAI ET AL.
A Global Dataset of Palmer Drought Severity Index for 1870–2002: Relationship with
Soil Moisture and Effects of Surface Warming
AIGUO DAI, KEVIN E. TRENBERTH,
AND
TAOTAO QIAN
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 24 February 2004, in fina form 26 May 2004)
ABSTRACT
A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical
precipitation and temperature data for global land areas on a 2.58 grid. Over Illinois, Mongolia, and parts of
China and the former Soviet Union, where soil moisture data are available, the PDSI is significantl correlated
(r 5 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The
strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt
plays an important role. Basin-averaged annual PDSI covary closely (r 5 0.6 to 0.8) with streamflo for seven
of world’s largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy
of both surface moisture conditions and streamflo . An empirical orthogonal function (EOF) analysis of the
PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Niño–
Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The
global very dry areas, define as PDSI , 23.0, have more than doubled since the 1970s, with a large jump in
the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to
surface warming, while global very wet areas (PDSI . 13.0) declined slightly during the 1980s. Together, the
global land areas in either very dry or very wet conditions have increased from ;20% to 38% since 1972, with
surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the
increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures
and increased drying.
1. Introduction
Droughts and flood are extreme climate events that
percentage-wise are likely to change more rapidly than
the mean climate (Trenberth et al. 2003). Because they
are among the world’s costliest natural disasters and
affect a very large number of people each year (Wilhite
2000), it is important to monitor them and understand
and perhaps predict their variability. The potential for
large increases in these extreme climate events under
global warming is of particular concern (Trenberth et
al. 2004). However, the precise quantificatio of
droughts and wet spells is difficul because there are
many different definition for these extreme events (e.g.,
meteorological, hydrological, and agricultural droughts;
see Wilhite 2000 and Keyantash and Dracup 2002) and
the criteria for determining the start and end of a drought
or wet spell also vary. Furthermore, historical records
of direct measurements of the dryness and wetness of
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: A. Dai, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.
E-mail: [email protected]
q 2004 American Meteorological Society
the ground, such as soil moisture content (Robock et al.
2000), are sparse. In order to monitor droughts and wet
spells and to study their variability, numerous specialized indices have been devised using readily available
data such as precipitation and temperature (Heim 2000;
Keyantash and Dracup 2002).
The Palmer Drought Severity Index (PDSI) is the
most prominent index of meteorological drought used
in the United States (Heim 2002). The PDSI was created
by Palmer (1965) with the intent to measure the cumulative departure (relative to local mean conditions)
in atmospheric moisture supply and demand at the surface. It incorporates antecedent precipitation, moisture
supply, and moisture demand [based on the classic work
of Thornthwaite (1948)] into a hydrological accounting
system. Palmer used a two-layer bucket-type model for
soil moisture computations and made certain assumptions relating to fiel water-holding capacity and transfer
of moisture to and from the layers based on limited data
from the central United States (Palmer 1965; Heim
2002). The Palmer model also computes, as an intermediate term in the computation of the PDSI, the Palmer
moisture anomaly index (Z index), which is a measure
of surface moisture anomaly for the current month without the consideration of the antecedent conditions that
930
JOURNAL OF CLIMATE
VOLUME 17
The Diurnal Cycle and Its Depiction in the Community Climate System Model
AIGUO DAI
AND
KEVIN E. TRENBERTH
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 29 May 2003, in fina form 26 August 2003)
ABSTRACT
To evaluate the performance of version 2 of the Community Climate System Model (CCSM2) in simulating
the diurnal cycle and to diagnose the deficiencie in underlying model physics, 10 years of 3-hourly data from
a CCSM2 control run are analyzed for global and large-scale features of diurnal variations in surface air
temperature, surface pressure, upper-air winds, cloud amount, and precipitation. The model-simulated diurnal
variations are compared with available observations, most of which were derived from 3-hourly synoptic reports
and some new results are reported for surface air temperatures. The CCSM2 reproduces most of the large-scale
tidal variations in surface pressure and upper-air winds, although it overestimates the diurnal pressure tide by
20%–50% over low-latitude land and underestimates it over most oceans, the Rockies, and other midlatitude
land areas. The CCSM2 captures the diurnal amplitude (18–68C) and phase [peak at 1400–1600 local solar time
(LST)] of surface air temperature over land, but over ocean the amplitude is too small (#0.28C). The CCSM2
overestimates the mean total cloud amount by 10%–20% of the sky from ;158S to 158N during both December–
January–February (DJF) and June–July–August (JJA) and over northern mid- and high-latitude land areas in
DJF, whereas it underestimates the cloud amount by 10%–30% in the subtropics and parts of the midlatitudes.
Over the marine stratocumulus regions west to the continents, the diagnostic cloud scheme in the CCSM2
underestimates the mean stratocumulus amount by 10%–30% and does not simulate the observed large diurnal
variations (;3%–10%) in the marine stratocumulus clouds even when driven by observational data. In the
CCSM2, warm-season daytime moist convection over land starts prematurely around 0800 LST, about 4 hours
too early compared with observations, and plateaus from 1100 to 1800 LST, in contrast to a sharp peak around
1600–1700 LST in observations. The premature initiation of convection prevents convective available potential
energy (CAPE) from accumulating in the morning and early afternoon and intense convection from occurring
in the mid to late afternoon. As a result of the extended duration of daytime convection over land, the CCSM2
rains too frequently at reduced intensity despite the fairly realistic patterns of rainy days with precipitation .1
mm day 21 . Furthermore, the convective versus nonconvective precipitation ratio is too high in the model as
deep convection removes atmospheric moisture prematurely. The simulated diurnal cycle of precipitation is too
weak over the oceans, especially for convective precipitation. These results suggest that substantial improvements
are desirable in the CCSM2 in simulating cloud amount, initiation of warm-season deep convection over land,
and in the diurnal cycle in sea surface temperatures.
1. Introduction
Solar heating near the surface and in the atmosphere
generates strong diurnal (i.e., 24 h) and subdiurnal (e.g.,
12-h semidiurnal) oscillations in surface and atmospheric temperature, pressure, and wind fields These regular
oscillations, which are often referred to as atmospheric
tides (Chapman and Lindzen 1970; Dai and Wang 1999),
are among the most pronounced signals of earth’s climate and weather. Considerable diurnal variations, modulated by synoptic weather events, are also found in
moist convection and cloudiness (e.g., Hendon and
* The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
Corresponding author address: Aiguo Dai, NCAR, P.O. Box 3000,
Boulder, CO 80307.
E-mail: [email protected]
q 2004 American Meteorological Society
Woodberry 1993; Rosendaal et al. 1995; Bergman and
Salby 1996; Sui et al. 1997; Garreaud and Wallace 1997;
Yang and Slingo 2001), precipitation (e.g., Wallace
1975; Oki and Musiake 1994; Janowiak et al. 1994;
Chang et al. 1995; Dai et al. 1999a; Dai 2001b; Sorooshian et al. 2002; Nesbitt and Zipser 2003), and atmospheric water vapor (Dai et al. 2002). These diurnal
variations affect surface and atmospheric fluxe of energy (Bergman and Salby 1997), water (Trenberth et al.
2003), and momentum (Dai and Deser 1999). For example, because surface solar heating typically has a
sharp midday peak, precipitation that occurs during the
day evaporates from the surface more rapidly than that
at night. This changes the fraction of rainfall involved
in runoff versus evaporation and the partitioning of surface energy into latent and sensible heat fluxes Climate
models without a diurnal cycle cannot simulate these
nonlinear processes, resulting in degraded model simulations (Wilson and Mitchell 1986).
3270
JOURNAL OF CLIMATE
VOLUME 18
Atlantic Thermohaline Circulation in a Coupled General Circulation Model: Unforced
Variations versus Forced Changes
AIGUO DAI, A. HU, G. A. MEEHL, W. M. WASHINGTON,
AND
W. G. STRAND
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 30 September 2004, in final form 10 March 2005)
ABSTRACT
A 1200-yr unforced control run and future climate change simulations using the Parallel Climate Model
(PCM), a coupled atmosphere–ocean–land–sea ice global model with no flux adjustments and relatively
high resolution (⬃2.8° for the atmosphere and 2/3° for the oceans) are analyzed for changes in Atlantic
Ocean circulations. For the forced simulations, historical greenhouse gas and sulfate forcing of the twentieth
century and projected forcing for the next two centuries are used. The Atlantic thermohaline circulation
(THC) shows large multidecadal (15–40 yr) variations with mean-peak amplitudes of 1.5–3.0 Sv (1 Sv ⬅ 106
m3 s⫺1) and a sharp peak of power around a 24-yr period in the control run. Associated with the THC
oscillations, there are large variations in North Atlantic Ocean heat transport, sea surface temperature
(SST) and salinity (SSS), sea ice fraction, and net surface water and energy fluxes, which all lag the
variations in THC strength by 2–3 yr. However, the net effect of the SST and SSS variations on upper-ocean
density in the midlatitude North Atlantic leads the THC variations by about 6 yr, which results in the 24-yr
period. The simulated SST and sea ice spatial patterns associated with the THC oscillations resemble those
in observed SST and sea ice concentrations that are associated with the North Atlantic Oscillation (NAO).
The results suggest a dominant role of the advective mechanism and strong coupling between the THC and
the NAO, whose index also shows a sharp peak around the 24-yr time scale in the control run. In the forced
simulations, the THC weakens by ⬃12% in the twenty-first century and continues to weaken by an
additional ⬃10% in the twenty-second century if CO2 keeps rising, but the THC stabilizes if CO2 levels off.
The THC weakening results from stabilizing temperature increases that are larger in the upper and northern
Atlantic Ocean than in the deep and southern parts of the basin. In both the control and forced simulations,
as the THC gains (loses) strength and depth, the separated Gulf Stream (GS) moves southward (northward)
while the subpolar gyre centered at the Labrador Sea contracts from (expands to) the east with the North
Atlantic Current (NAC) being shifted westward (eastward). These horizontal circulation changes, which are
dynamically linked to the THC changes, induce large temperature and salinity variations around the GS and
NAC paths.
1. Introduction
Atlantic Ocean circulations, in particular the thermohaline circulation (THC), have large impacts on regional and global climate (Rahmstorf 2002). Because of
this, the natural variability of the THC and its potential
response to anthropogenic forcing have attracted considerable attention (e.g., Broecker 1987, 1997; Rahm-
* The National Center for Atmospheric Research is sponsored
by the National Science Foundation.
Corresponding author address: Aiguo Dai, NCAR, P.O. Box
3000, Boulder, CO 80307.
E-mail: [email protected]
© 2005 American Meteorological Society
JCLI3481
storf 1999). Paleoceanographic records suggest that the
THC exhibited different modes during the last glacial
cycle and its mode changes have been linked to abrupt
climate changes over the North Atlantic region (Clark
et al. 2002). Early ocean model studies suggest that the
THC may have multiple equilibrium states (Stommel
1961; Bryan 1986). More recent studies show that the
THC may exhibit a hysteresis behavior in which it can
have two distinct states for a given value of the control
variable (e.g., heat or freshwater flux; Stocker and
Wright 1991; Mikolajewicz and Maier-Reimer 1994;
Rahmstorf 1995), which is a common feature of nonlinear physical systems. A number of model studies on
low-frequency THC variability have focused on the
relative role of thermal and haline forcing in driving the
variation (e.g., Marotzke and Willebrand 1991; Weaver
1 AUGUST 2006
J. Climate
DAI
3589
Recent Climatology, Variability, and Trends in Global Surface Humidity
AIGUO DAI
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 20 July 2005, in final form 22 November 2005)
ABSTRACT
In situ observations of surface air and dewpoint temperatures and air pressure from over 15 000 weather
stations and from ships are used to calculate surface specific (q) and relative (RH) humidity over the globe
(60°S–75°N) from December 1975 to spring 2005. Seasonal and interannual variations and linear trends are
analyzed in relation to observed surface temperature (T) changes and simulated changes by a coupled
climate model [namely the Parallel Climate Model (PCM)] with realistic forcing. It is found that spatial
patterns of long-term mean q are largely controlled by climatological surface temperature, with the largest
q of 17–19 g kg⫺1 in the Tropics and large seasonal variations over northern mid- and high-latitude land.
Surface RH has relatively small spatial and interannual variations, with a mean value of 75%–80% over
most oceans in all seasons and 70%–80% over most land areas except for deserts and high terrain, where
RH is 30%–60%. Nighttime mean RH is 2%–15% higher than daytime RH over most land areas because
of large diurnal temperature variations. The leading EOFs in both q and RH depict long-term trends,
while the second EOF of q is related to the El Niño–Southern Oscillation (ENSO). During 1976–2004,
global changes in surface RH are small (within 0.6% for absolute values), although decreasing trends of
⫺0.11% ⬃ ⫺0.22% decade⫺1 for global oceans are statistically significant. Large RH increases (0.5%–2.0%
decade⫺1) occurred over the central and eastern United States, India, and western China, resulting from
large q increases coupled with moderate warming and increases in low clouds over these regions during
1976–2004. Statistically very significant increasing trends are found in global and Northern Hemispheric q
and T. From 1976 to 2004, annual q (T ) increased by 0.06 g kg⫺1 (0.16°C) decade⫺1 globally and 0.08 g kg⫺1
(0.20°C) decade⫺1 in the Northern Hemisphere, while the Southern Hemispheric q trend is positive but
statistically insignificant. Over land, the q and T trends are larger at night than during the day. The largest
percentage increases in surface q (⬃1.5% to 6.0% decade⫺1) occurred over Eurasia where large warming
(⬃0.2° to 0.7°C decade⫺1) was observed. The q and T trends are found in all seasons over much of Eurasia
(largest in boreal winter) and the Atlantic Ocean. Significant correlation between annual q and T is found
over most oceans (r ⫽ 0.6–0.9) and most of Eurasia (r ⫽ 0.4–0.8), whereas it is insignificant over subtropical
land areas. RH–T correlation is weak over most of the globe but is negative over many arid areas. The q–T
anomaly relationship is approximately linear so that surface q over the globe, global land, and ocean
increases by ⬃4.9%, 4.3%, and 5.7% per 1°C warming, respectively, values that are close to those suggested
by the Clausius–Clapeyron equation with a constant RH. The recent q and T trends and the q–T relationship are broadly captured by the PCM; however, the model overestimates volcanic cooling and the trends
in the Southern Hemisphere.
1. Introduction
Atmospheric water vapor provides the single largest
greenhouse effect on the earth’s climate. All climate
* The National Center for Atmospheric Research is sponsored
by the National Science Foundation.
Corresponding author address: A. Dai, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.
E-mail: [email protected]
© 2006 American Meteorological Society
JCLI3816
models predict increased atmospheric content of water
vapor with small changes in relative humidity as the
global-mean surface temperature rises in response to
increased CO2 and other greenhouse gases (Cubasch et
al. 2001; Dai et al. 2001). The increased water vapor
provides the single largest positive feedback on surface
temperature (Hansen et al. 1984) and is the main cause
of increased precipitation at mid- and high latitudes in
the model simulations. It is therefore vital to monitor
changes in atmospheric water vapor content not only
for detecting global warming but also for validating the
large water vapor feedback seen in climate models.
15 SEPTEMBER 2006
J. Climate
DAI
4605
Precipitation Characteristics in Eighteen Coupled Climate Models
AIGUO DAI
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 9 June 2005, in final form 21 December 2005)
ABSTRACT
Monthly and 3-hourly precipitation data from twentieth-century climate simulations by the newest generation of 18 coupled climate system models are analyzed and compared with available observations. The
characteristics examined include the mean spatial patterns, intraseasonal-to-interannual and ENSO-related
variability, convective versus stratiform precipitation ratio, precipitation frequency and intensity for different precipitation categories, and diurnal cycle. Although most models reproduce the observed broad patterns of precipitation amount and year-to-year variability, models without flux corrections still show an
unrealistic double-ITCZ pattern over the tropical Pacific, whereas the flux-corrected models, especially the
Meteorological Research Institute (MRI) Coupled Global Climate Model (CGCM; version 2.3.2a), produce
realistic rainfall patterns at low latitudes. As in previous generations of coupled models, the rainfall double
ITCZs are related to westward expansion of the cold tongue of sea surface temperature (SST) that is
observed only over the equatorial eastern Pacific but extends to the central Pacific in the models. The
partitioning of the total variance of precipitation among intraseasonal, seasonal, and longer time scales is
generally reproduced by the models, except over the western Pacific where the models fail to capture the
large intraseasonal variations. Most models produce too much convective (over 95% of total precipitation)
and too little stratiform precipitation over most of the low latitudes, in contrast to 45%–65% in convective
form in the Tropical Rainfall Measuring Mission (TRMM) satellite observations. The biases in the convective versus stratiform precipitation ratio are linked to the unrealistically strong coupling of tropical
convection to local SST, which results in a positive correlation between the standard deviation of Niño-3.4
SST and the local convective-to-total precipitation ratio among the models. The models reproduce the
percentage of the contribution (to total precipitation) and frequency for moderate precipitation (10–20 mm
day⫺1), but underestimate the contribution and frequency for heavy (⬎20 mm day⫺1) and overestimate
them for light (⬍10 mm day⫺1) precipitation. The newest generation of coupled models still rains too
frequently, mostly within the 1–10 mm day⫺1 category. Precipitation intensity over the storm tracks around
the eastern coasts of Asia and North America is comparable to that in the ITCZ (10–12 mm day⫺1) in the
TRMM data, but it is much weaker in the models. The diurnal analysis suggests that warm-season convection still starts too early in these new models and occurs too frequently at reduced intensity in some of the
models. The results show that considerable improvements in precipitation simulations are still desirable for
the latest generation of the world’s coupled climate models.
1. Introduction
Precipitation is one of the most important climate
variables. The largest impact of future climate changes
on the society will likely come from changes in precipitation patterns and variability. However, it is still a big
* The National Center for Atmospheric Research is sponsored
by the National Science Foundation.
Corresponding author address: A. Dai, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.
E-mail: [email protected]
© 2006 American Meteorological Society
JCLI3884
challenge for coupled global climate models (CGCMs)
to realistically simulate the regional patterns, temporal
variations, and correct combination of frequency and
intensity of precipitation (McAvaney et al. 2001; Covey
et al. 2003; Trenberth et al. 2003; Meehl et al. 2005).
The difficulty arises from the complexity of precipitation processes in the atmosphere that include cloud microphysics, cumulus convection, planetary boundary
layer processes, large-scale circulations, and many others. Errors in simulated precipitation fields often indicate deficiencies in the representation of these physical
processes in the model. It is therefore important to analyze precipitation for model evaluation and development.
J. Climate
15 MAY 2009
DAI ET AL.
2773
Changes in Continental Freshwater Discharge from 1948 to 2004
AIGUO DAI, TAOTAO QIAN, AND KEVIN E. TRENBERTH
National Center for Atmospheric Research,* Boulder, Colorado
JOHN D. MILLIMAN
School of Marine Science, College of William and Mary, Gloucester Point, Virginia
(Manuscript received 16 April 2008, in final form 17 November 2008)
ABSTRACT
A new dataset of historical monthly streamflow at the farthest downstream stations for the world’s 925 largest
ocean-reaching rivers has been created for community use. Available new gauge records are added to a network
of gauges that covers ;80 3 106 km2 or ;80% of global ocean-draining land areas and accounts for about 73%
of global total runoff. For most of the large rivers, the record for 1948–2004 is fairly complete. Data gaps in the
records are filled through linear regression using streamflow simulated by a land surface model [Community
Land Model, version 3 (CLM3)] forced with observed precipitation and other atmospheric forcings that are
significantly (and often strongly) correlated with the observed streamflow for most rivers. Compared with
previous studies, the new dataset has improved homogeneity and enables more reliable assessments of decadal
and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio
over drainage areas with and without gauge records is used to estimate the contribution from the areas not
monitored by the gauges in deriving the total discharge into the global oceans.
Results reveal large variations in yearly streamflow for most of the world’s large rivers and for continental
discharge, but only about one-third of the top 200 rivers (including the Congo, Mississippi, Yenisey, Paraná,
Ganges, Columbia, Uruguay, and Niger) show statistically significant trends during 1948–2004, with the rivers
having downward trends (45) outnumbering those with upward trends (19). The interannual variations are
correlated with the El Niño–Southern Oscillation (ENSO) events for discharge into the Atlantic, Pacific, Indian,
and global ocean as a whole. For ocean basins other than the Arctic, and for the global ocean as a whole, the
discharge data show small or downward trends, which are statistically significant for the Pacific (29.4 km3 yr21).
Precipitation is a major driver for the discharge trends and large interannual-to-decadal variations. Comparisons with the CLM3 simulation suggest that direct human influence on annual streamflow is likely small
compared with climatic forcing during 1948–2004 for most of the world’s major rivers. For the Arctic drainage
areas, upward trends in streamflow are not accompanied by increasing precipitation, especially over Siberia,
based on available data, although recent surface warming and associated downward trends in snow cover and
soil ice content over the northern high latitudes contribute to increased runoff in these regions. The results are
qualitatively consistent with climate model projections but contradict an earlier report of increasing continental
runoff during the recent decades based on limited records.
1. Introduction
Continental freshwater runoff or discharge is an important part of the global water cycle (Trenberth et al.
* The National Center for Atmospheric Research is sponsored
by the National Science Foundation.
Corresponding author address: Dr. Aiguo Dai, National Center
for Atmospheric Research, P.O. Box 3000, Boulder, CO 803073000.
E-mail: [email protected]
DOI: 10.1175/2008JCLI2592.1
Ó 2009 American Meteorological Society
2007). Precipitation over continents partly comes from
water evaporated from the oceans, and streamflow returns this water back to the seas, thereby maintaining a
long-term balance of freshwater in the oceans. The
discharge from rivers also brings large amounts of particulate and dissolved minerals and nutrients to the
oceans (e.g., Boyer et al. 2006); thus it also plays a key
role in the global biogeochemical cycles. Unlike oceanic
evaporation, continental discharge occurs mainly at the
mouths of the world’s major rivers. Therefore, it provides significant freshwater inflow locally and forces
ocean circulations regionally through changes in density
Advanced Review
WIREs Climate Change
Drought under global warming:
a review
Aiguo Dai∗
This article reviews recent literature on drought of the last millennium, followed
by an update on global aridity changes from 1950 to 2008. Projected future aridity
is presented based on recent studies and our analysis of model simulations.
Dry periods lasting for years to decades have occurred many times during
the last millennium over, for example, North America, West Africa, and East
Asia. These droughts were likely triggered by anomalous tropical sea surface
temperatures (SSTs), with La Niña-like SST anomalies leading to drought in
North America, and El-Niño-like SSTs causing drought in East China. Over Africa,
the southward shift of the warmest SSTs in the Atlantic and warming in the
Indian Ocean are responsible for the recent Sahel droughts. Local feedbacks may
enhance and prolong drought. Global aridity has increased substantially since the
1970s due to recent drying over Africa, southern Europe, East and South Asia,
and eastern Australia. Although El Niño-Southern Oscillation (ENSO), tropical
Atlantic SSTs, and Asian monsoons have played a large role in the recent drying,
recent warming has increased atmospheric moisture demand and likely altered
atmospheric circulation patterns, both contributing to the drying. Climate models
project increased aridity in the 21st century over most of Africa, southern Europe
and the Middle East, most of the Americas, Australia, and Southeast Asia. Regions
like the United States have avoided prolonged droughts during the last 50 years
due to natural climate variations, but might see persistent droughts in the next
20–50 years. Future efforts to predict drought will depend on models’ ability to
predict tropical SSTs.  2010 John Wiley & Sons, Ltd. WIREs Clim Change 2011 2 45–65 DOI:
10.1002/wcc.81
WHAT IS DROUGHT?
D
rought is a recurring extreme climate event
over land characterized by below-normal
precipitation over a period of months to years.
Drought is a temporary dry period, in contrast to
the permanent aridity in arid areas. Drought occurs
over most parts of the world, even in wet and humid
regions. This is because drought is defined as a dry
spell relative to its local normal condition. On the
other hand, arid areas are prone to drought because
their rainfall amount critically depends on a few
rainfall events.1
Drought is often classified into three types2,3 :
(1) Meteorological drought is a period of months to
years with below-normal precipitation. It is often
accompanied with above-normal temperatures, and
∗ Correspondence
to: [email protected]
National Center for Atmospheric Research, Boulder,
CO, USA
DOI: 10.1002/wcc.81
Volume 2, January/February 2011
precedes and causes other types of droughts. Meteorological drought is caused by persistent anomalies
(e.g., high pressure) in large-scale atmospheric circulation patterns, which are often triggered by anomalous
tropical sea surface temperatures (SSTs) or other
remote conditions.4–6 Local feedbacks such as reduced
evaporation and humidity associated with dry soils
and high temperatures often enhance the atmospheric
anomalies.7 (2) Agricultural drought is a period with
dry soils that results from below-average precipitation,
intense but less frequent rain events, or above-normal
evaporation, all of which lead to reduced crop production and plant growth. (3) Hydrological drought
occurs when river streamflow and water storages in
aquifers, lakes, or reservoirs fall below long-term
mean levels. Hydrological drought develops more
slowly because it involves stored water that is depleted
but not replenished. A lack of precipitation often triggers agricultural and hydrological droughts, but other
factors, including more intense but less frequent precipitation, poor water management, and erosion, can
 2010 John Wiley & Sons, L td.
45
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D12115, doi:10.1029/2010JD015541, 2011
Characteristics and trends in various forms of the Palmer Drought
Severity Index during 1900–2008
Aiguo Dai1
Received 21 December 2010; revised 17 March 2011; accepted 29 March 2011; published 29 June 2011.
[1] The Palmer Drought Severity Index (PDSI) has been widely used to study aridity
changes in modern and past climates. Efforts to address its major problems have led to new
variants of the PDSI, such as the self‐calibrating PDSI (sc_PDSI) and PDSI using
improved formulations for potential evapotranspiration (PE), such as the Penman‐Monteith
equation (PE_pm) instead of the Thornthwaite equation (PE_th). Here I compare and
evaluate four forms of the PDSI, namely, the PDSI with PE_th (PDSI_th) and PE_pm
(PDSI_pm) and the sc_PDSI with PE_th (sc_PDSI_th) and PE_pm (sc_PDSI_pm)
calculated using available climate data from 1850 to 2008. Our results confirm previous
findings that the choice of the PE only has small effects on both the PDSI and sc_PDSI for
the 20th century climate, and the self‐calibration reduces the value range slightly and
makes the sc_PDSI more comparable spatially than the original PDSI. However, the
histograms of the sc_PDSI are still non‐Gaussian at many locations, and all four forms of
the PDSI show similar correlations with observed monthly soil moisture (r = 0.4–0.8)
in North America and Eurasia, with historical yearly streamflow data (r = 0.4–0.9) over
most of the world’s largest river basins, and with GRACE (Gravity Recovery and Climate
Experiment) satellite‐observed water storage changes (r = 0.4–0.8) over most land
areas. All the four forms of the PDSI show widespread drying over Africa, East and
South Asia, and other areas from 1950 to 2008, and most of this drying is due to recent
warming. The global percentage of dry areas has increased by about 1.74% (of global
land area) per decade from 1950 to 2008. The use of the Penman‐Monteith PE and
self‐calibrating PDSI only slightly reduces the drying trend seen in the original PDSI.
The percentages of dry and wet areas over the global land area and six select regions are
anticorrelated (r = −0.5 to −0.7), but their long‐term trends during the 20th century do
not cancel each other, with the trend for the dry area often predominating over that for the
wet area, resulting in upward trends during the 20th century for the areas under extreme
(i.e., dry or wet) conditions for the global land as a whole (∼1.27% per decade) and
the United States, western Europe, Australia, Sahel, East Asia, and southern Africa.
The recent drying trends are qualitatively consistent with other analyses and model
predictions, which suggest more severe drying in the coming decades.
Citation: Dai, A. (2011), Characteristics and trends in various forms of the Palmer Drought Severity Index during 1900–2008,
J. Geophys. Res., 116, D12115, doi:10.1029/2010JD015541.
1. Introduction
[2] Drought is a recurring extreme climate event over land
characterized by below‐normal precipitation over a period of
several months to several years or even a few decades. It is
among the most damaging natural disasters, causing tens of
billions of dollars in damage and affecting millions of people
in the world each year [Wilhite, 2000]. To quantify drought
and monitor its development, many drought indices have
been developed and applied [Heim, 2000, 2002; Keyantash
1
National Center for Atmospheric Research, Boulder, Colorado, USA.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JD015541
and Dracup, 2002; Vicente‐Serrano et al., 2010a; Dai,
2011]. Among them, the Palmer Drought Severity Index
(PDSI) is the most prominent index of meteorological
drought used in the United States for drought monitoring and
research [Heim, 2002]. The PDSI and its variants have been
used to quantify long‐term changes in aridity over land in the
20th [Dai et al., 1998, 2004; van der Schrier et al., 2006a,
2006b, 2007; Dai, 2011] and 21st [Burke et al., 2006; Burke
and Brown, 2008; Dai, 2011] century. The PDSI has also
been widely used in tree ring‐based reconstructions of past
droughts in North America and other regions [e.g., Cook et al.,
2004, 2007].
[3] The PDSI was originally developed by Palmer [1965]
with the intent to measure the cumulative departure in sur-
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