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1962 JOURNAL OF CLIMATE VOLUME 22

Global and Continental Drought in the Second Half of the Twentieth Century: Severity–Area–Duration Analysis and Temporal Variability of Large-Scale Events

J. SHEFFIELD Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey

K. M. ANDREADIS Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington

E. F. WOOD Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey

D. P. LETTENMAIER Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington

(Manuscript received 10 July 2008, in final form 15 October 2008)

ABSTRACT

Using observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected of moisture, the authors identified 296 large-scale drought events (greater than 500 000 km2 and longer than 3 months) globally for 1950–2000. The drought events were subjected to a severity–area–duration (SAD) analysis to identify and characterize the most severe events for each and globally at various durations and spatial extents. An analysis of the variation of large-scale drought with SSTs revealed connections at interannual and possibly decadal time scales. Three metrics of large-scale drought (global average soil moisture, contiguous area in drought, and number of drought events shorter than 2 years) are shown to covary with ENSO SST anomalies. At longer time scales, the number of 12-month and longer duration droughts follows the smoothed variation in Pacific and Atlantic SSTs. Globally, the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity. This physically based and probabilistic approach confirms well-known droughts, such as the 1980s in the of , but also reveals many severe droughts (e.g., at high latitudes and early in the time period) that have received relatively little attention in the scientific and popular literature.

1. Introduction mosphere and feedbacks with the and land sur- face (e.g., Jiang et al. 2006; McCabe and Palecki 2006). Drought is a naturally occurring climate phenomenon These are modulated by external forcings such as vari- that impacts and environmental activity glob- ations in solar input and atmospheric composition, ei- ally. It is among the costliest and most widespread of ther natural or anthropogenic. natural disasters (Wilhite 2000). One of the reasons for The scientific body of into the processes that this is the usually large spatial extent of droughts and cause drought to develop and persist is growing rapidly. their lengthy duration, sometimes reaching continental This research has highlighted a number of factors that scales and lasting for many years. Drought is generally may potentially impact drought occurrence including driven by extremes in the natural variation of climate, large-scale atmospheric mechanisms that are associated which are forced by the internal interactions of the at- with modes of climate variability and surface tem- perature (SST) anomalies (Schubert et al. 2004; Seager Corresponding author address: Justin Sheffield, Department of Civil and Environmental Engineering, Princeton University, et al. 2005), and evidence that land–atmosphere feed- Princeton, NJ 08544. backs play a role in their persistence (Long et al. 2000; E-mail: [email protected] Wang and Eltahir 2000).

DOI: 10.1175/2008JCLI2722.1

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However, much of this research is based on coupled because it provides an aggregate estimate of available land–atmosphere– models, and may be model spe- water from the balance of precipitation, evaporation, cific. Part of the reason that the research has favored and runoff fluxes. It also reflects the delays in the hy- model-based approaches, and for our general lack of drologic system caused by infiltration, drainage, snow understanding of the mechanisms that control drought accumulation and melt, and the impacts of variation and development and persistence, is the dearth of detailed anomalies in the meteorological drivers (such as changes observational data of the occurrence and variability of in storm frequency and intensity). In drought termi- droughts over large time and space scales. Alterna- nology it most closely represents agricultural drought tively, land surface models forced by surface climate through its control on transpiration and thus plant observations (which generally are more available than growth, but reflects meteorological and hydrological the relevant terrestrial hydrologic variables) can pro- drought because of the intimate links through the land- vide spatially and temporally consistent derived fields of surface water balance. variables that are not observed directly (Mitchell et al. We calculate an index of drought as the deficit of soil 2004; Sheffield and Wood 2007; Wang et al. 2009). They moisture relative to its seasonal climatology at a loca- can also form the basis for seasonal hydrologic predic- tion (Sheffield et al. 2004a). This allows us to compare tion (Wood and Lettenmaier 2006) and thus drought the occurrence of drought between different locations forecasting (Luo and Wood 2007). in a consistent and meaningful way. Daily soil moisture The Palmer drought severity index (PDSI) (Palmer data output from the VIC model are averaged to a monthly 1965) has generally been the tool of choice for observation- time step and transformed to percentiles. A drought is based indices of drought, and has been used by Cook then defined conceptually as a sequence of spatially et al. (1999) and Dai et al. (2004b), among others, for contiguous deficits below the 20th percentile, which drought reconstruction. However, the PDSI has notable represents relatively rare conditions. This value has deficiencies, including its inability to represent the ef- been used in a previous application to the fects of snow and the absence of a sound probabilistic (A05) and is in line with drought thresholds used by the interpretation for the resulting index values. An alter- U.S. Drought Monitor (National Drought Mitigation native to the PDSI is the use of land surface models that Center 2003). In semiarid and arid regions, it should be simulate the detailed processes of water and energy noted that absolute soil moisture values are generally transfer at the ’s surface (see e.g., Sheffield et al. low and drought can be considered to be the norm in 2004a; Andreadis et al. 2005, hereafter A05). terms of available water. The 20th percentile threshold This study follows this approach and builds on pre- therefore represents very dry values in these regions, and vious work into the occurrence and trends in drought our definition of drought should be regarded as con- during the twentieth century over the United States ceptual rather than physical. We summarize below the (Sheffield et al. 2004a; A05; Andreadis and Lettenmaier hydrologic simulation and the methods for drought 2006) and globally (Sheffield and Wood 2007, 2008). We identification and analysis. For more details, the reader is analyze derived soil moisture from simulations of the referred to Sheffield and Wood (2007) and A05. terrestrial hydrologic cycle using the Variable Infiltra- a. Variable Infiltration Capacity global hydrologic tion Capacity (VIC) model to quantify the global oc- simulation currence of drought for 1950–2000. Using the severity– area–duration (SAD) analysis methods of A05 we The VIC model (Liang et al. 1994; Cherkauer et al. identify major drought events and explore their spa- 2002) simulates the land-surface water and energy bal- tiotemporal characteristics and relationships with large- ances and has been used extensively for modeling scale climate variability. studies at regional (Maurer et al. 2002; Su et al. 2006) to global scales (Nijssen et al. 2001; Sheffield et al. 2004b) and regional climate change impact assessment (Hamlet 2. Datasets and methods and Lettenmaier 1999; Hayhoe et al. 2007). The simu- Soil moisture fields are taken from observation-driven lations were for 1950–2000 at 1.08 resolution over simulations of the terrestrial hydrologic cycle using the global land areas except and . VIC model [although the VIC model is our choice ow- They were driven by a hybrid observation–reanalysis ing to its heritage as outlined above, Wang et al. (2009) meteorological dataset (Sheffield et al. 2006) that com- show that comparable results can be achieved using bines gridded observations of precipitation, tempera- alternative land surface models and, in any event, the ture, and radiation with data from the National Centers methods described here are not specific to the VIC for Environmental Prediction–National Center for Atmo- model]. Soil moisture is a useful indicator of drought spheric Research (NCEP–NCAR) reanalysis (Kalnay et al.

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1996) to produce a high-resolution and bias-corrected corresponding to the storm centers of the DAD analysis forcing dataset. The simulations are described in detail (WMO 1969). The block neighborhood of the center is in Sheffield and Wood (2007) and have been validated then searched for the next largest severity and that pixel against available terrestrial hydrologic observations is taken with the center to form an intermediate drought by J. Sheffield and E. F. Wood (2008, unpublished area. This process is continued until the maximum manuscript). spatial extent of the drought event is reached, with the severity and extent recorded in area increments of 20 b. Clustering algorithm for drought identification model pixels (;200 000 km2). This is repeated for all We identified drought events in time and space using overlapping time intervals (equal to a preselected du- the clustering algorithm of A05. At each time step, ration) of the particular event. The maximum severity individual drought clusters are identified as spatially for each area increment is then selected to form the contiguous areas that are allowed to merge or break SAD curve for that event, thus providing a way to es- up through time. A minimum cluster area threshold of timate an absolute drought magnitude without being 500 000 km2 was used, which is higher than the one used constrained to an individual basin or area. After calcu- in A05 (25 000 km2). Initial experiments with 25 000 and lating the SAD relationships for all identified events, we 100 000 km2 thresholds showed that droughts could formed the envelope SAD curve that represents the shrink to a few pixels and traverse through maximum bound, or envelope, of severities from the set tenuous spatial connectivities, thus persisting, incor- of all droughts that occurred in the region. These curves rectly, for multiple years (see section 3b). The 500 000 allow the identification of the most severe events for km2 threshold ensured that events only persisted if there each area increment and duration. was a reasonable number of adjoining pixels in drought. Although such long-term (decadal) drought events have been identified in the literature and are the subject of 3. Results intensive research, especially for [see Cook a. Global and continental drought statistics et al. (2007) for a summary of recent work], these are generally based on deviations in precipitation below the Using the data and methods in this study, there have mean. In this paper we focus on contiguous regions of been 296 droughts greater than 500 000 km2 globally dry soil moisture identified using a systematic clustering from 1950 to 2000. Table 1 summarizes the occurrence method and a relatively low threshold (20th percentile of drought globally and for each continent including the of soil moisture). The longest duration drought found longest and most spatially extensive events. The longest using this method was about 4 years, thus precluding duration drought was 49 months (4 yr) in from such decadal droughts from our analysis. 1984 to 1988, closely followed by the 1950–53 North American drought (44 months). The most spatially ex- c. Severity–area–duration analysis tensive was the African drought of the early 1980s, which A05 developed the SAD analysis technique, based on reached its peak extent in April 1983 when it covered the widely used depth–area–duration (DAD) analysis over 11 million square kilometers. (Grebner and Roesch 1997), to simultaneously evaluate Table 2 lists the top five drought events for each the severity and spatial extent of droughts for different continent ranked by duration and maximum extent. We durations. Severity (S) is defined as also show time series of drought in Fig. 1 for the and the continents in terms of the areally averaged soil åP moisture percentile, area in drought, and contiguous S 5 1 2 , t area in drought from the cluster analysis. The mean soil moisture percentile over the full time period is 50% by where P is the monthly percentile of soil moisture, sum- construct, and the mean area in drought is 20%. The med over duration t (months). Average drought sever- mean contiguous area in drought will be less than or ities are calculated for successively larger areas, from a equal to 20% (a value of 20% can only occur if all pixels minimum of about 100 000 km2 to the maximum spatial experience drought at the same time, which at large extent of each event. The process used to calculate these scales is unlikely). The variability of the time series for average severities for each predefined duration (3, 6, 12, and Asia are damped because of averaging 18, 24, 36, and 48 months) starts with calculating and over very large spatial scales. The smaller continents ranking the severities for each pixel that ‘‘experienced’’ ( and ) show much higher variability, the drought event in question. The model pixels with the also a result of climate variability in those regions. The maximum severity are taken as the ‘‘drought centers,’’ droughts of the early 1970s and mid-1980s are clearly

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TABLE 1. Summary of large-scale drought occurrence for the six continents. For the last column, the extent as a percentage of total area and the date when the maximum spatial extent was attained are given in brackets. Oceania is defined as , , New , and the Pacific Islands.

Number of Number of Number of Longest duration Maximum spatial droughts droughts # 6 months droughts $ 12 months (months) extent (km2) Africa 44 28 4 19 (1982–84) 11 346 000 (40.0%, April 1983) Asia 86 37 22 49 (1984–88) 8 193 000 (18.5%, October 1997) Europe 40 24 4 20 (1959–61) 4 264 000 (42.8%, June 1950) North America 57 34 8 44 (1950–53) 8 231 000 (39.3%, March 1956) Oceania 24 17 1 12 (1951–52) 6 525 000 (80.2%, February 1965) 45 37 4 16 (1958–59) 9 038 000 (51.2%, October 1963) visible in the African time series, and the early 1990s 1974–77 Asian drought were much more extensive in also stands out in terms of spatial area in drought. In the Eurasian analysis as they spanned into Europe and Europe, high variability in the 1950s is associated with ranked a little differently in the SAD analysis. multiple periods of large drought extent that are not The example maps in Figs. 3 and 4 are identified by repeated until the mid-1970s. In North America, the dry calculating a combined metric of severity and area as the spell of the 1950s shows up clearly as a prolonged se- average severity of the pixels in a drought multiplied by quence of large drought extent reaching 36% of the the drought area for a particular month. The droughts are total continental area. Other notable dry periods are then ranked by this metric and the top three months are around 1976–77 and the late 1990s. For Oceania, there plotted. Note that these events are identified based on a are several events with high percentage extent in drought

(e.g., 82% in 1965) and also the almost complete lack of TABLE 2. List of top five ranked drought events for each con- contiguous drought in the mid-1970s. Drought extent is tinent in terms of duration and maximum spatial extent. The du- also very low (, 20%) from 1995 to the end of the time ration (months) and extent (millions km2) are given in brackets series, which is in contrast to the subsequent extended after the time period of each event. drought conditions between 2003 and 2007 (Australian Region Duration (months) Spatial extent (106 km2) Bureau of Meteorology 2008). South America is char- Africa 1982–84 (19) 1982–84 (11.3) acterized by extended dry periods in the early 1950s, 1973–74 (14) 1990–91 (8.4) 1960s, and 1990s. 1991–92 (14) 1984–85 (7.9) 1985–86 (14) 1991–92 (6.9) b. Severity–area–duration analysis of continental 1994–95 (11) 1985–86 (6.7) drought Asia 1984–88 (49) 1997–98 (8.2) 1974–77 (37) 1999/2000 (7.5) SAD envelope curves that represent the maximum 1978–80 (27) 1972–73 (5.0) severity for each combination of duration (3, 12, 24, 1981–83 (24) 1964–65 (4.9) and 48 months) and spatial extent are shown in Fig. 2. 1950–52 (23) 1982 (4.7) Spatial snapshots of some of the identified major events Europe 1959–61 (20) 1950 (4.3) are plotted in Figs. 3 and 4 for each continent. The 1976–77 (12) 1953–54 (4.0) 1975–76 (12) 1975–76 (4.0) analysis was carried out separately for each continent 1951–52 (12) 1959 (3.9) and the results are discussed for each continent next. 1995–96 (11) 1951–52 (3.8) Droughts that span more than one continent are North America 1950–53 (44) 1954–57 (8.2) therefore treated separately, which may bias the anal- 1954–57 (39) 1952–53 (7.6) ysis toward smaller spatial scale events. This is likely 1999–2000 (20) 1976–77 (6.3) 1958–59 (13) 1988/89 (6.0) only problematic for droughts that cover parts of Eu- 1976–77 (13) 1953–54 (5.7) rope and Asia that have a substantial common land Oceania 1951–52 (12) 1965 (6.5) border. To see the impact of separating Europe and 1977 (11) 1961–62 (5.1) Asia, we ran the SAD analysis for a combined Eurasian 1965 (11) 1957 (5.0) landmass. This showed only small differences in the 1961–62 (11) 1951–52 (4.8) 1985 (9) 1986 (4.7) total area in drought, but in terms of individual drought South America 1958–59 (16) 1963–64 (9.0) events and their characteristics the differences were 1982–83 (15) 1961 (6.5) more significant. The Eurasian analysis identified 116 1963–64 (14) 1968 (6.3) events compared to the combined total of 126 events 1965–66 (12) 1951 (5.1) in Europe (40) and Asia (86). Droughts such as the 1991–92 (11) 1997–98 (5.0)

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FIG. 1. Monthly time series of area-averaged soil moisture percentile, percentage area in drought, and percentage contiguous area in drought. A drought is defined for soil moisture below the 20th percentile. metric calculated for an individual month and, therefore, these events covered high latitudes of and may not show up on the SAD curves as these are based (although they joined temporarily with U.S. on metrics calculated over three or more months. droughts during their lifetime) where freezing temper- atures prolong the anomalously low soil moisture values 1) NORTH AMERICA through the winter (Sheffield and Wood 2007). The The longest droughts identified for North America most spatially extensive droughts were the 1954–57 are in the 1950s (Cook et al. 1999; Stahle and Cleaveland ( and much of Canada), the 1952/ 1988; Palmer 1965; Karl and Quayle 1981), with only the 53 (most of the United States and southern Canada), 1999–2000 drought (Cook et al. 2004) of comparable and the 1976–77 events ( and cen- duration (see Table 2). It should be noted that some of tral Canada) (Cook et al. 1999; Keyantash and Dracup

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04/1954-06/1957 04/1963-05/1964 01/1950-08/1953 02/1958-05/1959 07/1976-07/1977 10/1991-08/1992 05/1999-12/2000 08/1982-10/1983 04/1963-05/1964 02/1968-09/1968 08/1965-07/1966

12/1982-06/1984 05/1973-06/1974 07/1984-05/1985 09/1991-10/1992

10/1953-07/1954 05/1975-04/1976 10/1959-05/1961 01/1950-07/1950 05/1951-04/1952

06/1984-06/1988 11/1951-10/1952 12/1999-12/2000 01/1965-11/1965 04/1974-04/1977 07/1997-06/1998 07/1978-09/1980 11/1950-09/1952

FIG. 2. Continental SAD envelope curves for durations of 3, 6, 12, and 24 months. Each curve represents the maximum bound, or envelope, of severities from the set of all droughts that occurred over the continent. They are generated by selecting the drought with the maximum severity for each incremental drought area. The curves may be made up of data from several different drought events, as some events are more severe for small areas while others are more severe for large areas. For example, for Asia at 24-month duration, the curve is derived from the 1978–80, 1984–88, and 1974–77 droughts that are the most severe depending on the area under consideration.

2004). The three largest severity-extent months (Octo- started in September 1952, quickly covered most of the ber 1952, January 1997, and June 1988) are typified by central and by the autumn, extensive coverage over the central United States and and then migrated northward before dissipating in southern Canada (see Fig. 3). The 1952–53 drought the Canadian the following spring. The 1976–77

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FIG. 3. Severity and maximum spatial extent of selected major drought events for North America, Europe, and Asia. The events are extracted by calculating the mean severity of all grid cells in the drought multiplied by the area and searching for the maximum value. event that spanned the United States is interesting be- and the most spatially extensive events were mainly in the cause it was at its peak during the winter and so had rel- 1950–60s (see Table 2). The three largest severity-extent atively little impact nationally, although regionally it was months all occurred in the 1960s: October 1963, June the drought of record in California and the Columbia 1968, and September 1961 (Fig. 4). All three droughts River basin. The 1954–57 drought dominates the SAD mostly covered the , with the October envelope curves (Fig. 2) for all durations, with the ex- 1963 drought extending northeastwards and the June ception of shorter-term drought, whereas the 1976–77 1968 event stretching down into southern Brazil and droughtwastheworstdroughtforareasuptoabout7 Paraguay. The SAD envelope curves (Fig. 2) indicate million square kilometers. Note that, despite the 1988 that the 1963/64 drought (Marengo et al. 2008) was the drought (Janowiak 1988; Namias 1991; Ropelewski 1988; worst drought for durations up to 12 months, especially Trenberth et al. 1988) being considered the worst U.S. for larger spatial extents. This drought initiated in east- drought on record in economic terms (Trenberth and ern Brazil in April 1963 and extended to its maximum Branstator 1992), it does not appear on the envelope curve, extent and severity in October before moving northeast although it does rank highly in terms of our combined and dissipating in southern Amazonia the following metric of maximum monthly severity and extent (see spring. Droughts that occurred in 1958–59, 1991–92, and Fig. 3). This is because other drought events were more 1982–83 (Marengo et al. 1998; Foley et al. 2002) (all of severe for all combinations of duration and extent, yet which extended over Peru, Colombia, Venezuela, and their actual impacts may have been lower because of northern Brazil) contribute to the curves but only for their location and/or timing. The high economic impacts extents less than about 3 million square kilometers. of the 1988 drought were a result of its location over the 3) EUROPE northern and eastern Great , regions of intensive , during the spring and summer. The longest droughts that were identified in Europe include events during 1959–61, 1976–77, and 1975–76. 2) SOUTH AMERICA These 1970s droughts were focused in western and east- In South America the longest droughts identified oc- ern Europe, respectively, with the former initiating in curred during 1958–59, 1982–83, and 1963–64 (14 months) and moving into and the

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FIG. 4. Severity and maximum spatial extent of selected major drought events for South America, Africa, and Oceania (New Zealand, Papua , and the Pacific Islands are excluded as no droughts greater than 500 000 km2 occurred in these regions).

Baltic states and the latter stretching from the Baltic to is the most severe for 12-month durations except for the Caspian . The months with highest severity- relatively small areas for which the 1959–61 event was extent values (Fig. 3) are all in the 1950s (Briffa et al. the most severe. 1994; Lloyd-Hughes and Saunders 2002; van der Schrier 4) AFRICA et al. 2006). The 1953–54 drought covered most of and west into and Germany, African droughts are dominated by events during the while the two early 1950s droughts (1950 and 1951–52) mid-1970s, 1980s (Hulme 1992; L’Hoˆ te et al. 2002, Oba reached their peak in European , the former et al. 2001; Dai et al. 2004a; Tarhule and Lamb 2003), more southerly than the latter. The 1953–54 drought and early 1990s (Rouault and Richard 2005) (Table 2). dominates the 3-month SAD envelope with the 1950 Note that the 1982–84, 1984–85, and 1985–86 events drought (Briffa et al. 1994) being the worst for areas could be considered to be the same drought in terms greater than about 4 million km2. The 1975–76 drought of their spatial and temporal proximity, but the chosen

Unauthenticated | Downloaded 10/06/21 05:11 PM UTC 1970 JOURNAL OF CLIMATE VOLUME 22 thresholds for the cluster algorithm meant that they were 2006; Meshcherskaya and Blazhevich 1997; Patel et al. treated separately. In terms of spatial extent, the larg- 2007; Zhou et al. 2005) but also at regional scales est droughts were in the 1980s and 1990s. The 1982–84 (Barlow et al. 2002; Min et al. 2003). In this study, the drought began in in December 1982 longest drought identified (1984–88) persisted over and joined with a drought that started in central before migrating southeast to northern to reach its peak extent in April 1983 (Fig. 4). It then and back again. The 1974–77 event was centered migrated northwest until it settled across the Sahel over Kazakhstan during 1974–76 and moved into the region at the end of 1983, where it persisted (as the Urals region of Russia during 1976–77. This event was 1984–85 and 1985–86 droughts) until September 1986. physically connected to the concurrent eastern Euro- The mid-1970s (1973–74) and early 1990s (1990–91) pean drought, but is considered separately here in this events similarly affected the Sahel region. The subse- continental analysis. Other long duration droughts are quent droughts (1991–92 and 1994–95) affected mainly the 1978–80 (central Siberia), 1981–83 (Siberia), and 1950– southern Africa, with the former initiating over Ethio- 52(Iran–Turkmenistan–Kazakhstanregion)events.Sim- pia and then migrating southward before stalling over ilar to the high-latitude droughts in North America, the southern Africa for 10 months. The latter covered south- Siberian events are prolonged by freezing temperatures ern Africa, centered over Botswana, but with much over the winter. The 1997–98 drought was the most smaller spatial extent. The SAD envelope curves (Fig. 2) spatially extensive (Fig. 3), covering more than 8 million show that the 1982–84 drought was the most severe square kilometers from eastern China to in for large extents and short durations. The 1984–85 and October 1997, which was also the peak month in terms 1973–74 droughts were generally the worst for smaller of severity times area. However this event only lasted 12 extents. months. Dry conditions in central-southwest Asia dur- ing 1999–2000 are highlighted in the map for May 2000, 5) OCEANIA but note that this drought actually continued beyond the In Oceania, the most spatially extensive events were period of this study (Barlow et al. 2002). The early 1950s two of the three longest droughts (1951–52, 1961–62, drought (1950–52) was most severe and extensive in 1965) (Plummer et al. 1999; White et al. 2003). The 1965 May 1951 when it was centered on Kazakhstan. Not drought (Mpelasoka et al. 2007; Australian Bureau of surprisingly, the SAD envelope curve for 48-month dur- Meteorology 2008) affected the majority of Australia ations is completely derived from the 4 yr 1984–88 Si- (82%) at its peak (Fig. 4), sparing only the extreme berian drought (Fig. 2). Note how the peak severity was north and west, while the earlier 1961 drought initiated reached at about 2 million square kilometers and not in the interior before migrating westward. The 1957 at the smallest spatial extent as would be expected, event (Fig. 4) was very intense (high severity but only most likely because this drought consisted of multiple 3-month duration) and spanned the entire continent. subdroughts. Some of the longer-duration droughts were more re- c. SAD curves for the top five continental events gional, such as the 1977 drought, which persisted for 11 months over , and the 9-month 1982– Further insight into the severity of the individual 83 drought (Nicholls 2004; Mpelasoka et al. 2007) that drought events can be gathered from Fig. 5, which coincided with the centers of agriculture and population shows 3- and 12-month SAD curves for the top five in the east and southeast. Interestingly, despite its large droughts in each continent (chosen by first ranking by economic impacts, the 1982–83 drought does not appear duration and then by maximum extent). All droughts on the SAD envelope curves (Fig. 2), which are domi- have approximately the same severity at the lowest nated by the 1965 (3-month duration) and 1951–52 spatial extent, which at 3-month duration is close to the (12-month duration) events, somewhat expected from maximum possible value. This is because the spatially the maps of drought shown. For small areas, the 1982– extensive and long duration droughts considered here 83 drought is comparable to these other events (al- are likely to contain several cells that are close to though always numerically less severe) but with in- maximum severity, and these are identified by the SAD creasing area, its severity decreases far more rapidly, algorithm and used to construct the curves. With in- likely due to its more localized coverage over eastern creasing spatial extent, and therefore increasing spatial parts of Australia and shorter duration (9 months). variability in soil moisture and severity, the curves tend to diverge with, in some cases, one particular drought 6) ASIA clearly more severe (e.g., 1963–64 in South America). Droughts in Asia have been studied by many authors, Elsewhere, two or more droughts are similarly severe often on a national basis (Gruza et al. 1999; Jiang et al. for a large range of spatial extents (e.g., the 1950–52 and

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1982-84 1991-92 1985-86 1973-74 1984-85

1984-88 1974-77 1978-80 1981-83 1950-52

1959-61 1975-76 1951-52 1976-77 1995-96

1950-53 1954-57 1999-00 1976-77 1958-59

1951-52 1965-65 1961-62 1977-77 1972-73

1958-59 1982-83 1963-64 1965-66 1991-92

FIG. 5. SAD curves for the six continents for (left) 3-month duration and (right) 12-month duration. Curves are shown for the top five drought events on each continent ranked by duration and then by maximum spatial extent. Note that some of the drought events that make up the SAD envelope curves in Fig. 2 may not appear on this figure. For Oceania, only the 1951–52 event persists for 12 months or more.

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1984–88 Asian droughts for 3- and 12-month duration). usually shows a local severity minimum at 3–4 (1) mil- In Africa, several droughts are of comparable severity lion square kilometers for 3 (12) month duration. for 3-month durations, but the 1973–74 and 1982–84 d. Global SAD envelope curves droughts clearly dominate at 12-month duration for small and large extents, respectively. The 1973–74 drought In Fig. 6, we merge the SAD envelope curves for the did not reach extents beyond about 11 million square world and identify the most severe droughts globally kilometers, so the 1982–84 drought stands out on its own over the second half of the twentieth century. The droughts for very large spatial extents. Note the shallow gradients are listed according to the number of points that they of the African curves, which indicate that the rate of contribute to the SAD envelope curves and are domi- decrease in severity with increasing area is lower in nated by events in Asia and North America. The Africa than in other continents, with the exception of 1974–77 Asian drought that covered much of Kazakhstan a small number of events in North (e.g., 1954–57) and and western Russia contributes to most of the 12- and South America (1963–64). 24-month curves. The rest of the 24-month curve comes In Asia the 1984–88 and 1950–52 droughts are in- from the 1954–57 and 1950–53 North American droughts. separable in terms of severity at 3-month durations, and The 1982–84 African drought contributes to the re- only at relatively large spatial extents are any of the mainder of the 12-month curve for large spatial extents other droughts comparable. For 12-month duration, the and the 1978–80 Asian drought completes the curve 1984–88 event is dominant, but other droughts come for extents less than about 2 million km2. The 3-month close at low and high extents. Note that the 1997–98 and curve is made up of contributions from seven different 1999–2000 events that make up much of the 3-month events that do not appear on the 12- and 24-month curve in Fig. 2 are not among the top five events in terms curves plus the 1982–84 African drought, the most no- of duration and extent and so do not appear here. table of which are the 1999–2000 Asian (central and Similarly, the European 3-month curve is mostly de- southwest) and the 1982–84 African events. Only one rived from the 1953–54 and 1950 droughts, but these are drought from each of Europe (1953–54), South America not among the top five events under the definition here. (1963–64), and Oceania (1951–52) contribute to the At 12-month duration, the 1975–76 event is by far the curves and for only a limited number of points. worst European drought at extents larger than about 2 million square kilometers. In North America, 3-month 4. Global variability in large-scale drought and duration severities are similar for all events at low teleconnections extents. At extents beyond 3 million square kilome- ters, the 1976–77 and 1954–57 droughts dominate. At Much work has been carried out on the covariability 12-month duration the SAD envelope curve in Fig. 2 is of climate with terrestrial states and fluxes and their made up solely from data from the 1954–57 drought, extremes, including drought. Rajagopalan et al. (2000) and this is confirmed in Fig. 4. However, at smaller found relatively robust relationships in the southwest- spatial extents the 1950–53 and 1976–77 droughts are ern United States between PDSI and several climate comparably severe, and at very large extents the 1999– indices, although these varied in time. McCabe et al. 2000 drought is very close. It would be interesting to see (2004) looked at the association of the Pacific decadal how this drought would compare if the analysis ex- oscillation (PDO) and Atlantic multidecadal oscillation tended beyond 2000. (AMO) with decadal variability in North American The top five droughts in Oceania show similar be- PDSI. Globally, Dai et al. (2004b) found the PDSI havior in terms of the slope of severity versus spatial to be coupled to ENSO variability. Jiang et al. (2006) extent at 3-month duration. The 1965 drought domi- looked at proxy indicators for the River for nates the envelope curve (Fig. 2) but is only slightly 1868–2003 and found it to be related to ENSO. McCabe more severe than other events at low spatial extents. and Palecki (2006) explored covariability between PDSI Beyond about 4 million square kilometers it is clearly and global SSTs on decadal to multidecadal time scales the worst 3-month duration event. For 12-month dura- and revealed strong relationships and a dominance of tion, the 1951–52 drought is the only one that persists the PDO and AMO in explaining most of the variabil- for 12 months and so provides all points on the curve, ity. Sheffield and Wood (2008) explored the variability although the 1977, 1965, and 1961–62 droughts lasted of the VIC soil moisture dataset used here and showed it for 11 months. In South America, the 1963–64 drought was mainly related to ENSO with the NAO and the is easily the worst at either duration, except for below AMO possibly playing secondary roles. We extend this 4 million square kilometers where nearly all other droughts to look specifically at the occurrence of major drought are of similar severity. Note the 1965–66 drought that un- events and large-scale climatic variability.

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04/1974-04/1977 Asia 04/1984-06/1988 Asia 04/1954-06/1957 N. America 12/1982-06/1984 Africa 12/1999-12/2000 Asia 11/1950-09/1952 Asia 01/1950-08/1953 N. America 04/1963-05/1964 S. America 07/1978-09/1980 Asia 07/1976-07/1977 N. America 07/1997-06/1998 Asia 11/1951-10/1952 Oceania 10/1953-06/1954 Europe

FIG. 6. As for Fig. 2 but for all global land areas, excluding Greenland and Antarctica.

a. Variability of global large-scale drought of events and a change in the temporal variation. In fact, occurrence for these very large droughts, the series shows a decadal variation with 5–10 years of activity interspersed with Figure 7 shows the monthly time series of the number a few years of no drought events. The drop in the of large-scale global droughts as defined by the cluster number of droughts greater than 1 000 000 and 5 000 000 analysis. The number of droughts is counted separately km2 is an interesting result that likely reflects the geo- in each month such that a single drought event will graphic or climatic factors that control the size of drought contribute to several months within the time series (the events. variability of droughts of different durations is explored in the next section). Also shown is the number of b. Links with large-scale climate variability droughts filtered by their maximum spatial extent, where the data for each line is calculated for incre- Given the influence of ENSO on global climate and mental thresholds of spatial extent, to see the how soil moisture, it is tempting to compare the time series the temporal variation changes for increasingly larger of global drought numbers with an index of ENSO. In drought events. The mean number of global droughts Fig. 8 we compare the number of droughts with the time . 500 000 km2 occurring in any month is about 4.5 (or series of the Nin˜ o-3.4 SSTs (an indicator of ENSO 55 yr21) with a standard deviation of 1.6. This time se- variability). As ENSO episodes (warm El Nin˜ o or cold ries is quite variable and indicates several periods of La Nin˜ a) and their impacts persist on time scales of increased global drought activity: the mid-1950s, 1960s, several months to a year or two, the drought time series late 1980s to early 1990s, and late 1990s. The mid-1970s is calculated here for only those drought events of to mid-1980s are characterized by the lowest number 24-month duration or less (Fig. 8, top panel). Correla- of droughts, apart from a short burst of activity around tion is weak (r 5 0.36 for the 13-month filtered data) but 1976–77. The year with most drought months is 1992 the year-to-year variation is somewhat similar, except- (74). Table 3 shows the top and bottom five ranked ing around the early 1950s, 1979, and 1988 when the two years in terms of the number of months with drought time series diverge briefly. This similarity is consistent extent . 500 000 km2. with reported covariabilities between ENSO and soil When filtered by spatial extent, the number of droughts moisture indices (modeled soil moisture or PDSI) between 500 000 and 1 000 000 km2 shows similar mentioned before, although here the relationship with magnitude and variability. For droughts greater than the number of contiguous large-scale dry extremes of 5 000 000 km2 there is a large reduction in the number soil moisture is somewhat less distinct. At smaller scales,

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FIG. 7. Monthly time series of the number of global droughts for different areal extent thresholds. The data are smoothed using a 13-month running mean. An individual drought is defined as a con- tiguous region of soil moisture deficit below the 20th percentile that will likely persist for multiple months (its extent and severity will change through time) and so it will contribute to several months within the time series. the correlations are likely to be stronger in regions region (58N–58S, 1208–1708W). The figure shows results known to be influenced by ENSO, such as Oceania and for two sets of ENSO years: 1) all ENSO years, defined , as suggested by the high correlations as years with five consecutive overlapping anomalies found by Sheffield and Wood (2008) between ENSO greater than 60.58C and 2) strong ENSO years, simi- and soil moisture in these regions. larly defined but for 61.08C. Statistical significance of Links between ENSO and large-scale drought may the composite index was determined using the hyper- also be revealed using a nonlinear, categorical-type re- geometric distribution by representing the index’s de- lationship rather than a linear correlative one. Figure 9 parture from zero as a binomial variable with values shows composites of three characteristics of drought ‘‘success’’ and ‘‘failure’’ (Dracup and Kahya 1994). A (soil moisture, contiguous area in drought, and the num- ‘‘success’’ is defined as the occurrence of an index value ber of droughts) over a 36-month window that straddles above (below) zero. The hypergeometric distribution the year of an El Nin˜ o or La Nin˜ a event, helping to show was then used to determine the cumulative probability the potentially lagged response of the land surface. The soil moisture data are averaged over all terrestrial land regions of the simulation. The contiguous area is de- TABLE 3. Top and bottom ranked global drought years based on rived from the cluster analysis and was shown previ- the number of months with drought extent . 500 000 km2. ously as a time series in Fig. 1. The number of droughts is again filtered for droughts of duration less than 24 Top ranked years months. The three characteristics are first standardized 1992 1957 1963 1965 1987 to z scores and then averaged for each month in the 36- No. of droughts 74 73 71 70 67 month window over all El Nin˜ o or La Nin˜ a years. ENSO Bottom ranked years years are taken from the Climate Prediction Center 1975 1962 1968 ˜ (CPC) oceanic Nino index (ONI), which is the 3-month 1984 1986 1993 1978 1980 running mean of extended reconstructed SST version 3 (ERSST.v3) dataset SST anomalies over the Nin˜ o-3.4 No. of droughts 38 40 41 43 44

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FIG. 8. (top) Monthly time series (smoothed using a 13-month running mean filter) of number of droughts of duration less than or equal to 24 months ( line) and Nin˜ o-3.4 SST anomalies (blue line). (bottom) Monthly time series (smoothed using a 121-month running mean filter) of number of droughts of duration longer than 12 months (black line) and the AMO (red line) and PDO indices (blue line). Note that the number of long-term droughts at the start and end of the time series may be undersampled because of events that started before the simulation start date and ended after the simulation end date, respectively. that at least m successes (the number of successes oc- larger El Nin˜o index values being statistically significant. curring in La Nin˜ a or El Nin˜ o years) are obtained in n This is consistent with the known impacts of ENSO on trials (the number of La Nin˜ a or El Nin˜ o years) from a global terrestrial climate and highlights that large-scale finite population of size N (the total number of years) drought occurrence in particular is impacted and that El containing k successes (the total number of successes). Nin˜ o events may act to increase global drought. The soil A cumulative probability value less than 0.05 (95% moisture response is much shorter because the global confidence level) indicates that the index value is average value reflects changes in all parts of the world, significant. including those that are essentially unaffected by ENSO There is a clear tendency for all drought characteristics variability, and may thus act to cancel out the signal to be more pronounced in the second half of the ENSO derived from ENSO-affected regions. year and the beginning of the following year. During El The bottom panel of Fig. 8 shows the time series of Nin˜o (La Nin˜a) years, soil moisture is more likely to be long-term drought events (duration . 12 months; the drier (wetter) globally with a contemporaneous increase number of droughts longer than 24 months were too few (decrease) in the number of large-scale drought events. to discern their temporal variation). We compare this The signal is weaker for all ENSO years (the composite with the AMO and PDO, which are the main modes of index stays below 0.5) but is much more robust for strong decadal climate variability in the North Atlantic and ENSO years. The response for contiguous area is not as Pacific, respectively. All time series in Fig. 8 (lower strong, especially for all ENSO years, with the La Nin˜a panel) are smoothed using a 121-month (10 yr) running signal being particularly weak. The peak composite index mean filter. Correlations of the smoothed and filtered for all characteristics falls around the end of the ENSO number of droughts with the smoothed climate indices year (September–November), and the anomalies per- are0.62fortheAMOand0.57forthePDO.Asthe sist to about May of the following year with only the smoothed time series are serially correlated, we determined

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FIG. 9. Composite average soil moisture, contiguous area in drought, and the number of large-area droughts occurring globally for a 36-month window centered on positive (El Nin˜ o, red bars) and negative (La Nin˜ a, blue bars) anomalies of ENSO. Pink bars indicate the overlap of the positive and negative phase composite values when they are of the same sign. Results are shown for (left) all ENSO years, defined as years with five consecutive overlapping anomalies greater than 60.58C, and for (right) strong ENSO years, similarly defined but for 61.08C. the statistical significance of the correlations using Monte correlations of each smoothed pair was then used Carlo simulation (McCabe and Palecki 2006). A set of to determine whether the observed correlation was 10 000 pairs of time series (one for the number of droughts statistically significant for a two-tailed test at the 95% and one for the climate oscillation, AMO or PDO) were level. The results show that the neither correlation is generated that possessed the mean, variability, and lag- significant at this level, although the AMO correlation is 1 correlation of the original data. The distribution of the significant at the 90% level.

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5. Discussion and conclusions algorithm does allow for droughts to traverse conti- nents, either as a single cluster or from the merging of a. Some caveats two or more subdroughts (such as the 1982–86 African Before summarizing the main results, we give some drought). As subdroughts that are well separated in caveats concerning the simulated soil moisture data, space are likely driven by different forcing mechanisms including the forcing data, and the assumptions that (e.g., SST patterns), this makes it somewhat difficult to impact their accuracy. We also highlight some of the interpret them when they are considered as a single uncertainties introduced by the clustering methodolo- event. The fact that the movement of the drought gies and the associated chosen thresholds. First, the clusters is smooth and continuous does, however, hint at soil moisture fields are taken from a single hydrologic some common underlying mechanisms. model. Although we consider this to be currently our b. Consistency with other studies best estimate of soil moisture variability globally, the results may change if we used a different hydrologic Many other studies have analyzed large-scale drought, model. Comparisons of drought for the United States either as individual events or from a regional climato- for four land surface models forced by the same mete- logical perspective, and their impacts are generally well orology (Wang et al. 2009) show that, although major documented. In the following we qualitatively compare droughts are identified by all models, they can develop our results with some of these studies to gain insight into differently in terms of severity. This is, in part, due to their robustness and consistency. intermodel differences in soil depth and water-holding This study follows on from the continental U.S. anal- capacities that impact soil moisture retention and per- ysis of A05, and we begin by looking at how well the sistence. There are also uncertainties and intermodel dif- results of that study are replicated here, given the - ferences in the representation of vegetative controls on vious similarities but also the differences in their ap- soil moisture. proaches. Both studies use VIC simulated soil moisture Second, there are uncertainties in the meteorological and the same cluster analysis for identifying droughts forcings that were used to drive the simulation. The and their SAD characteristics. However, they differ in a forcings are derived from observationally based gridded several important ways, such as the spatial scale (0.58 datasets with daily variability from reanalysis, as de- continental United States versus 1.08 global for A05 and scribed in Sheffield et al. (2006). Although this is argu- this study respectively), the temporal period (1920–2003 ably the best estimate we have currently of continuous versus 1950–2000), the meteorological forcings (gauge- and consistent fields of meteorological forcings, there based precipitation and temperature versus bias-corrected are nevertheless unknown biases and spurious trends reanalysis) and the minimum cluster threshold. in the data, caused by instrument errors and/or defi- Excluding the 1930s Dust Bowl event, which we do ciencies in the methodologies, and these will impact the not simulate, A05 found the 1950s drought (1950–57) to depiction of drought. This is especially likely in regions be the most severe for large areas and long durations. of sparse instrumentation, such as Africa, where, for Although our chosen cluster threshold splits this drought example, the lack of gauges may have a large im- into two events (1950–53 and 1954–57), we also find that pact on the accuracy of simulated drought development. these are the most severe for large extents and dura- Third, as mentioned in Sheffield and Wood (2007, tions. Both studies identify the 1976–77 drought that 2008), the simulation makes a number of assumptions covered much of the United States and southern about land use change and anthropogenic influences. Canada (1975–79 in A05) as one of the most severe for Specifically, we assume that land cover and land use short durations. A05 also find that the 1998–2003 event does not change, although it varies spatially and sea- is less severe but comparable to the 1950s (and 1930s) sonally, and that irrigation and water withdrawals do droughts, especially for smaller spatial extents. We also not play a major role in large-scale soil moisture vari- find this to be true for the equivalent 1999–2000 drought ability and, thus, drought development. identified here, although the end of the simulation in Finally, the focus of the paper is on the identification 2000 hinders proper assessment of this event. For both and analysis of large-scale contiguous drought events studies, the important 1988 drought (1988–89 in this that may give different results to, say, an areally aver- paper, 1987–93 in A05) is considered a severe event but aged metric of drought, in terms of the number of events does not contribute to the SAD envelope curves. or the ranking of wet and dry years (this is noted for For Europe, Lloyd-Hughes and Saunders (2002) Europe in section 5b below). This is also dependent on and van der Schrier et al. (2006) used PDSI and 3- and the specific clustering method and related thresholds we 12-month standardized precipitation index (SPI) and have employed. It should be noted that the clustering found the 1950s and 1990s to be the most drought prone.

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This is certainly reflected for the 1950s in the time series In Oceania (Australia), the 1982–83 drought is con- (Fig. 1) and the SAD envelope curves (Fig. 2) in which sidered to be one of the most severe and damaging four out of the five drought events that form the Eu- events of the second half of the twentieth century ropean curves are from this period. Furthermore, the mainly because it was intense (short and severe) causing three events with the highest monthly severity-extent over 3 billion Australian dollars ($AUS) in damages index are from the 1950s (Fig. 3). Although the 1995–96 (Australian Bureau of Meteorology 2008). It does not drought ranks highly in terms of severity at 3-month appear on the 3-month SAD curve, however, and is only duration, our analysis shows that the 1990s is actually sixth on the list of most spatially extensive and lengthy the decade with the least events (6), with the 1980s hav- events and so does not appear in Fig. 5, which is limited ing the most (11). At the monthly time scale, these two to the top five events. However, its severity is compa- previous studies use essentially the same precipitation rable to these events for 3-month durations, especially and temperature as the present study, yet differences in for relatively small spatial extents (not shown). The the modeling and analysis approaches can give quite Australian Bureau of Meteorology also highlights the different results in terms of identifying periods of drought. 1965 drought as particularly severe (Australian Bureau This is especially likely when comparing spatially aver- of Meteorology 2008) with an estimated 300–500 million aged metrics [in the case of Lloyd-Hughes and Saunders $AUS in agricultural losses (Mpelasoka et al. 2007), and (2002) and van der Schrier et al. (2006)] and spatially this forms most of the 3-month SAD envelope curve. contiguous metrics as used here. There are several references to drought in the Ama- There has been considerable study of drought con- zon in the literature, particularly those related to ENSO ditions in the Sahel (e.g., Hulme 1992; L’Hoˆ te et al. variability (e.g., the 1982–83 drought identified here as 2002) because of the devastating impacts, and it is no one of the longest events; Marengo et al. 1998). How- surprise that the SAD curves are dominated by events ever, the drought of 1963–64 has received little attention that primarily affected this region (1973–74, 1982–84, (Marengo et al. 2008) despite being identified here as and 1984–85). Elsewhere in Africa, Roualt and Richard having the largest peak extent and the most severe up to (2005) identify the 1991–92 drought as the most spatially 12-month duration. extensive event in southern Africa in terms of 6-month c. Summary and conclusions SPI, which is identified here as the most severe drought outside of the Sahel-centered events. We have described the spatiotemporal characteristics For Asia, the identified high-latitude droughts (e.g., of large-scale drought events globally over the second 1984–88, 1978–80, 1981–83) are not well documented half of the twentieth century. Drought is defined in in the literature, given the sparse population and low terms of anomalies of soil moisture derived from an economic activity in these regions, although indirect observationally and reanalysis-driven simulation of the evidence from, for example, remotely sensed fire ac- terrestrial water cycle. Cluster analysis is used to iden- tivity across the boreal (e.g., Balzter et al. 2007) is tify large-scale drought events (. 500 000 km2) as spa- consistent with large-scale dry conditions during the tially connected regions of soil moisture below the 20th 1980s. We found the drought events in central/south- percentile. We have analyzed these events in terms of west Asia (1950–52 and 1999–2000) to be very severe, their severity, area, and duration and identified the se- especially at 3- and 12-month durations, and the latter is verest droughts for each continent and globally for consistent with Barlow et al. (2002), who show this pe- different spatial extents and durations. riod to be unusual in terms of precipitation. The 1974–77 Using this dataset and methodology, there have been central Asian/ drought was the most 296 major droughts (. 500 000 km2 and lasting longer severe globally for 12- and 24-month durations at mod- than 3 months) globally from 1950–2000. Half of these erate extents (see Fig. 6). Meshcherskaya and Blazhevich events lasted for 6 months or less and half were smaller (1997), in their assessment of dry and wet years in the than 3 million square kilometers. The longest drought Former Soviet Union (FSU), identified 1976 as the sec- event lasted for 49 months from 1984 to 1988 in Siberia. ond driest year in the European part of the FSU (1981 The most spatially extensive was in Africa during 1982– was the driest), although this was in terms of the spatial 84, which covered over 11 million square kilometers in extent of low values of a spring–summer moisture index. April 1983. On average there were about 4.5 unique The 1997–98 (from eastern China to central Asia) drought droughts occurring globally in any month (or 55 yr21), is also reported by Zhou et al. (2005), who looked at and this varies considerably from year to year. The mid- PDSI over China from 1951 to 2003 and this event is 1950s showed the highest drought activity and the mid- typical of the reported increase in drought in northern 1970s to mid-1980s the lowest activity. The number of China and its persistence since 1997. very large droughts (. 5 000 000 km2) is significantly

Unauthenticated | Downloaded 10/06/21 05:11 PM UTC 15 APRIL 2009 S H E F F I E L D E T A L . 1979 lower than the total number of droughts, partly a result comparison of drought events across regions and through of the size and shape of continental landmasses that time. It is therefore well suited to evaluation not only of limit the maximum size of a drought, but may also be changes in the preinstrumental relative to the observa- related to the length scales of climate anomalies as tional period but also to evaluation of the implications of driven by planetary waves (Herweijer and Seager 2008). future climate for drought. The global number of large-scale droughts of short duration (24 months or less) shows a weak relationship Acknowledgments. This work was supported by the with ENSO variability in terms of correlations and U.S. Department of Energy (Grant DE-FG02-04ER63873 composite analysis. There is a tendency for more (less) to the University of Washington and NA08OAR4310579 short-term drought events during El Nin˜ o (La Nin˜ a) to Princeton University) and by the National Oceanic and episodes. The number of droughts tends to maximize or Atmospheric Administration (Grant NA07OAR4310458 minimize (depending on the ENSO phase) toward the to the University of Washington). We thank the three end of the ENSO year with considerably stronger anonymous reviewers for their helpful comments. composite values for stronger ENSO years as defined by larger SST anomalies. The smoothed number of longer- REFERENCES duration droughts (. 1 yr) varies on decadal time scales Andreadis, K. M., and D. P. Lettenmaier, 2006: Trends in 20th somewhat in line with the smoothed variation in the century drought over the continental United States. Geophys. AMO and PDO, although only the correlation with the Res. Lett., 33, L10403, doi:10.1029/2006GL025711. AMO is significant at the 90% level. 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