Flash droughts present a new challenge
for subseasonal-to-seasonal prediction
Item Type Article
Authors Pendergrass, Angeline G.; Meehl, Gerald A.; Pulwarty, Roger;
Hobbins, Mike; Hoell, Andrew; AghaKouchak, Amir; Bonfils,
Céline J. W.; Gallant, Ailie J. E.; Hoerling, Martin; Hoffmann,
David; Kaatz, Laurna; Lehner, Flavio; Llewellyn, Dagmar; Mote,
Philip; Neale, Richard B.; Overpeck, Jonathan T.; Sheffield,
Amanda; Stahl, Kerstin; Svoboda, Mark; Wheeler, Matthew C.;
Wood, Andrew W.; Woodhouse, Connie A.
Citation Pendergrass, A.G., Meehl, G.A., Pulwarty, R. et al. Flash droughts
present a new challenge for subseasonal-to-seasonal prediction.
Nat. Clim. Chang. 10, 191–199 (2020). https://doi.org/10.1038/
s41558-020-0709-0
DOI 10.1038/s41558-020-0709-0
Publisher NATURE PUBLISHING GROUP
Journal NATURE CLIMATE CHANGE
Rights This is a U.S. government work and not under copyright
protection in the U.S.; foreign copyright protection may apply
2020.
Download date 10/08/2022 05:41:21
Item License https://creativecommons.org/publicdomain/mark/1.0/
Version Final published version
PersPective
https://doi.org/10.1038/s41558-020-0709-0
1
National Center for Atmospheric Research, Boulder, CO, USA.
2
NOAA/Earth System Research Laboratory/Physical Sciences Division, Boulder, CO, USA.
3
Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA.
4
Department of Civil and Environmental
Engineering, and Department of Earth System Science, University of California, Irvine, CA, USA.
5
Program for Climate Model Diagnosis and
Intercomparison, Lawrence Livermore National Laboratory, Livermore, CA, USA.
6
School of Earth, Atmosphere and Environment, Monash University,
Clayton, Victoria, Australia.
7
ARC Centre of Excellence for Climate Extremes, Monash University, Clayton, Victoria, Australia.
8
Denver Water, Denver, CO,
USA.
9
US Department of Interior, Bureau of Reclamation, Albuquerque, NM, USA.
10
Oregon Climate Change Research Institute, and Graduate School,
Oregon State University, Corvallis, OR, USA.
11
School for Environment and Sustainability, University of Michigan, Ann Arbor, MI, USA.
12
NOAA/NIDIS,
Scripps Institution of Oceanography, La Jolla, CA, USA.
13
University of Freiburg, Freiburg, Germany.
14
National Drought Mitigation Center, University
of Nebraska–Lincoln, Lincoln, NE, USA.
15
Australian Bureau of Meteorology, Melbourne, Victoria, Australia.
16
School of Geography and Development,
University of Arizona, Tucson, AZ, USA.
✉
e-mail: apgrass@ucar.edu
D
rought is perhaps the most complex and least understood of
all “weather and climate extremes”
1
. Droughts can span tim-
escales from a few weeks to decades, and spatial scales from
a few kilometres to entire regions. Their impacts usually develop
slowly, are often indirect and can linger for long after the end of the
drought itself. The drought risk, therefore, is often underestimated
and continues to remain a ‘hidden’ hazard
2
. A comprehensive over-
view of traditional drought characteristics, processes, mechanisms
and impacts is provided in ref.
3
.
In a future warmer climate, droughts are likely to increase in dura
-
tion and intensity in many regions of the world
4,5
. A better under-
standing of drought phenomena, especially of the physical processes
leading to drought, their propagation through the hydrological cycle,
and the societal and environmental vulnerability to drought and its
wide-ranging impacts is more important than ever. The key chal
-
lenge is to move from a reactive society, responding to impacts, to a
proactive society that is resilient and adapted to drought risk—that
is, adopts proactive risk management strategies
3,6
.
Droughts whose impacts arise in part from their long duration,
such as the Dust Bowl and the 2011–2015 California drought, have
formed strong imagery in the United States, and ‘megadroughts’
lasting more than 20 years have also been documented in tree-ring
records. Much research has been conducted on aspects of drought
that play out over multiple years, but more recently attention has
been drawn to the rapid development of some drought events, in
the space of a few weeks: ‘flash droughts’, a specific definition for
which we will provide below. These events, distinguished by their
sudden onset and rapid intensification, can have severe impacts
7,8
.
Flash droughts develop on the subseasonal-to-seasonal (S2S) times
-
cale (weeks to months) and present a new challenge for prediction
efforts on that timescale, which are currently surging in interest
9
.
One flash drought that brought attention to the phenomenon
occurred in the US Midwest in 2012
8,10
(Fig. 1). The areal extent of
abnormally dry conditions expanded from 30% of the continental
United States (CONUS) in May 2012 to over 60% by August. This
event had considerable impacts on agriculture and water-borne
transportation in the region. Although other rapidly developing
droughts had been identified before
11
, the widespread impacts of
the 2012 event caught the attention of the US public and leadership.
Flash drought is not confined to the United States
12
. For exam-
ple, processes that can produce flash droughts are foci of research
in China
13,14
. In southern Queensland, Australia, a flash drought in
early 2018 de-vegetated the landscape and drove livestock numbers
to their lowest level in a century, a significant impact for agriculture
15
.
A drought monitoring and early-warning system is the founda
-
tion of effective, proactive drought policy because it enables notice
of potential and impending drought conditions. It identifies climate
and water-resource trends, and detects the emergence or probability
of occurrence and the likely severity of droughts and their impacts.
Reliable information must be communicated in a timely manner
Flash droughts present a new challenge for
subseasonal-to-seasonal prediction
Angeline G. Pendergrass
1
✉
, Gerald A. Meehl
1
, Roger Pulwarty
2
, Mike Hobbins
2,3
,
Andrew Hoell
2
, Amir AghaKouchak
4
, Céline J. W. Bonfils
5
, Ailie J. E. Gallant
6,7
,
Martin Hoerling
2
, David Hoffmann
6,7
, Laurna Kaatz
8
, Flavio Lehner
1
, Dagmar Llewellyn
9
,
Philip Mote
10
, Richard B. Neale
1
, Jonathan T. Overpeck
11
, Amanda Sheffield
12
,
Kerstin Stahl
13
, Mark Svoboda
14
, Matthew C. Wheeler
15
, Andrew W. Wood
1
and
Connie A. Woodhouse
16
Flash droughts are a recently recognized type of extreme event distinguished by sudden onset and rapid intensification of
drought conditions with severe impacts. They unfold on subseasonal-to-seasonal timescales (weeks to months), presenting a
new challenge for the surge of interest in improving subseasonal-to-seasonal prediction. Here we discuss existing prediction
capability for flash droughts and what is needed to establish their predictability. We place them in the context of synoptic to
centennial phenomena, consider how they could be incorporated into early warning systems and risk management, and propose
two definitions. The growing awareness that flash droughts involve particular processes and severe impacts, and probably a
climate change dimension, makes them a compelling frontier for research, monitoring and prediction.
NATURE CLIMATE CHANGE | VOL 10 | MARCH 2020 | 191–199 | www.nature.com/natureclimatechange
191
PersPective
NaTure ClimaTe CHaNge
to water and land managers, policy makers and the public through
appropriate communication channels to trigger actions documented
in a drought plan, which is particularly critical for flash droughts.
That information, if used effectively, can form the basis for reducing
vulnerability and improving mitigation and response capacities of
people and systems at risk.
In this Perspective, we build on a recent review of flash droughts
8
and discuss the observational and predictive skill of key processes
with an eye towards impact assessment and early warning of flash
drought. We highlight the current understanding of the physical
processes that can drive flash droughts, the existing capabilities of
predicting them, and what is needed to make progress to establish
the predictability and effective early warning of flash droughts on
S2S timescales. Following earlier suggestions of possible definitions
for flash droughts
8,16
, we propose, for consideration by the com-
munity, two quantitative definitions for flash drought that can be
used for applications related to operations, analysis of observations,
model simulations of present and future climate, and assessing S2S
initialized model predictions.
Physical processes that produce flash drought
To illustrate the physical processes involved with producing a
flash drought, we consider another recent flash drought, in the US
northern Great Plains in 2017. This event shows some recurring
flash-drought characteristics, including precipitation deficit and
above-average temperatures preceding or coinciding with a rapid
soil-moisture decline (Fig. 2). Precipitation deficits began before
April, when precipitation would climatologically increase. Soil
moisture was nonetheless high in April, but continued precipita
-
tion deficits throughout the month eroded it slowly at first, before a
rapid decline in May.
We can examine the physical processes driving the moisture bal
-
ance of the land surface to understand mechanisms that can lead to
rapid drought intensification
17,18
. Moisture flux into the surface is
driven by precipitation. Like other aspects of drought, precipitation
deficit often plays an important role
19
. Moisture flux away from the
surface—evapotranspiration (ET)—can also play an important role
in flash drought, driving feedbacks between the land and atmosphere.
An important concept is the demand for moisture from the atmo
-
sphere—evaporative demand, which is the amount of evaporation
that would occur given an unlimited supply of moisture. Evaporative
demand can be thought of as the ‘thirst’ of the atmosphere. It
both drives and responds to ET. Starting from a state with suffi
-
cient soil moisture (energy-limited conditions; Fig. 3), evaporative
demand and evaporation vary together—when evaporative demand
increases, evaporation follows. With enough evaporation and no
replenishment, surface moisture eventually becomes insufficient to
supply further water for evaporation; water becomes the limiting fac
-
tor. Under water-limited conditions, further increases in evaporation
can no longer continue, and evaporation decreases. If the same fac
-
tors that had been driving increases in evaporative demand persist,
then evaporative demand will diverge from evaporation. Meanwhile,
sensible heat flux increases instead of evaporation, which increases
near-surface air temperature and vapour pressure deficit, and thus
also evaporative demand—an amplifying feedback
20–22
.
Although much of the focus on flash droughts has been in humid
regions, flash droughts and their impacts are also a concern in semi-
arid and arid regions where evaporative demand usually exceeds
evapotranspiration (locations that start on the right side of Fig. 3;
see ‘Impacts-based early warning’ below). Starting from a dry, mois
-
ture-limited state, flash droughts in arid regions can be driven by
precipitation deficits, and amplification of warm air temperatures
by sensible heat flux feedbacks is also of concern.
The local moisture imbalance during flash drought is condi
-
tioned by large-scale atmospheric circulation. The large-scale circu-
lation can modify the frequency and intensity of precipitation, and
it can increase evaporative demand by reducing cloud cover (which
increases incoming solar radiation at the surface), increasing wind
speeds and/or increasing temperatures
16,23,24
. In the midlatitudes
in summer, this can involve a persistent ‘blocking’ pattern, with a
strong quasi-stationary ridge of positive geopotential height anom
-
alies and associated anomalously high surface pressure
16
.
Large-scale atmospheric circulation associated with flash
droughts can vary from one event to the next and between different
regions. While moisture-bearing storms were largely absent dur
-
ing the 2012 US Midwest flash drought, the atmospheric circula-
tion during the event varied from one month to the next
23
. For the
Drought development in the Midwest, 2012
3 July 5 June 1 May
D0 Abnormally dry
D1 Moderate drought
D2 Severe drought
D3 Extreme drought
D4 Exceptional drought
ED0 70–80
ED1 80–90
ED2 90–95
ED3 95–98
ED4 98+
High Plains DM
12
June
26
June
1
June
3
July
100%
50%
0
i
d
c
2-week EDDI US Drought Monitor
Drought developing
across region
Flash drought (including ED3,
ED4) in MO, AR, KS, IL
Persistent intense drought
in region
D0 in IL, IN, TN; no drought in
MO, AR, OK, NE
Drought expands in region, but
not in intensity
D3 edges into region
7 August
Persistent intense drought in
region, ED4 area decreasing
D3, D4 emerge over much of
region 2 months after EDDI
US Drought Monitor
intensity categories
EDDI categories and
percentile bounds
h
g
fb
ea
Fig. 1 | Evolution of a flash drought across the US Midwest in 2012.
a–d, Evaporative drought (ED) categories based on 2-week Evaporative
Demand Drought Index (EDDI) at 5-week intervals during the drought
onset. e–h, US Drought Monitor (USDM). i, Percent of High Plains region
in USDM categories from 1 June to 3 July 2012. Adapted with permission
from: e–h, ref.
85
; i, US National Drought Mitigation Center.
NATURE CLIMATE CHANGE | VOL 10 | MARCH 2020 | 191–199 | www.nature.com/natureclimatechange
192
PersPective
NaTure ClimaTe CHaNge
southern US Great Plains, the atmospheric circulation associated
with rapid declines in soil moisture in conjunction with precipita
-
tion deficits can be different from that associated with such declines
in conjunction with heat waves
24
.
Flash droughts may be triggered or exacerbated by compound
extreme events—extremes of multiple factors that occur simultane
-
ously
25
. A classic example would be an extreme deficit of precipi-
tation coinciding with a heat wave, such as occurred in southern
Queensland in January 2018
15
. If these are superimposed on more
slowly evolving factors, such as a building soil-moisture deficit,
rapid onset or intensification of drought conditions can result.
Vegetation type can also influence flash drought through its
mediating role in transpiration. Trees become moisture-stressed
over the course of long-term drought, whereas crops and pasture
can be moisture-stressed much more quickly and might be more
sensitive to moisture in the upper soil layer.
The challenge of drought for S2S prediction
Compared with slowly evolving droughts, the relatively fast devel-
opment timescale of flash droughts requires different approaches to
monitoring and prediction. Many drought monitoring and predic
-
tion products are updated at monthly or at most weekly timescales.
Given a flash drought’s onset timescale of only a few weeks, these
are not sufficient. Instead, products that update daily are required.
This provides an opportunity to leverage synoptic weather forecasts
in combination with seasonal forecasting efforts that have recently
become available at shorter timescales, such as the SubX system
26
.
Prediction efforts focused on flash drought are currently in their
infancy. One key challenge is skilfully forecasting precipitation defi
-
cit on the S2S timescale. However, for a successful flash-drought
prediction, more is needed than just a forecast of deficient precipi
-
tation. Prediction skill is also required for other potential ingredi-
ents of rapid drought onset and intensification: high temperatures,
low humidity, strong winds and excess insolation. In the 2012 US
Midwest event, high temperatures and precipitation deficits may
have been driven by a blocking high, whereas the substantial soil-
moisture deficit may have been due to anomalous seasonal circula
-
tion associated with La Niña
27
. For other flash-drought events and
locations, different processes and phenomena probably contribute
to or affect development, such as land–atmospheric interaction, the
Madden–Julian Oscillation, the Southern and Northern Annular
Modes, and the Indian Ocean Dipole. Each of these has been argued
to provide or influence predictability of surface-climate variables on
timescales relevant for flash drought
28–31
, and so these processes and
phenomena are fundamental to the prospects of S2S prediction
32
.
Global coupled prediction systems show some S2S skill for pre
-
cipitation
33–35
and temperature
35–38
. Seasonal forecasts of evaporative
demand are more skilful than for precipitation over the CONUS
39
,
and at least as skilful globally
40
, although skill for extreme condi-
tions on subseasonal timescales, which may be more relevant for
flash drought, has not been established. Predictions are only as
accurate as the models that make them. In the case of global cli
-
mate models, which are the primary tool for S2S prediction systems,
there are significant biases.
Soli-moisture percentileMax. temperature (ºC)
2017 Drought evolution over eastern Montana
100
80
60
40
20
0
1 April 1 May 1 June 1 July 1 Aug 1 Sep 1 Oct 31 Oct
Precipitation (mm)
8.0
6.0
4.0
2.0
0.0
1 April 1 May 1 June 1 July 1 Aug 1 Sep 1 Oct 31 Oct
1 April 1 May 1 June 1 July 1 Aug 1 Sep 1 Oct 31 Oct
a d
b
c
May–July 2017 precipitation rank
Driest
33
55 Wettest
Rank relative to 1895–2017
40
30
20
10
0
Date in 2017
Fig. 2 | US Northern Great Plains flash drought in May 2017. a, Soil-moisture percentile (top 1 m) from University of Washington simulation of the variable
infiltration capacity (VIC) model
86
, forced by an estimate of the time-varying meteorology
87,88
. b, c, Precipitation (b) and daily maximum temperature (c)
in 2017 depicted as departures from the long-term climatology (solid black lines) from a collection of Global Historical Climatology Network (GHCN-D)
stations. d, Rank of accumulated May–July precipitation relative to the 1895–2017 climatology. The time series are data averaged over eastern Montana
(demarcated by the dotted line in the right panel). Adapted with permission from ref.
73
.
NATURE CLIMATE CHANGE | VOL 10 | MARCH 2020 | 191–199 | www.nature.com/natureclimatechange
193
