Spatiotemporal Patterns of Drought/Tropical Cyclone Co-occurrence in the Southeastern USA: Linkages...

Geography Compass 8/8 (2014): 540–559, 10.1111/gec3.12148

Spatiotemporal Patterns of Drought/Tropical CycloneCo-occurrence in the Southeastern USA: Linkages toNorth Atlantic Climate Variability

Jason T. Ortegren1 and Justin T. Maxwell2*1Department of Environmental Studies, University of West Florida2Department of Geography, Indiana University

AbstractDroughts and landfalling tropical cyclones (TCs; tropical depressions, tropical storms, and hurricanes) areimportant features of the hydroclimate of the southeastern USA at seasonal, interannual, and interdecadalscales. The societal impacts and climatological aspects of both droughts and Atlantic TCs have beenwidely addressed in the scientific literature. However, in general, previous research has assessed the twophenomena separately. Recently, the spatiotemporal patterns and hydroclimatic impacts of droughtamelioration by landfalling TCs have been analyzed for the southeastern USA, as well as the large-scaledynamic forcing mechanisms that enhance or suppress drought-TC co-occurrence. At multidecadal timescales, both droughts and TCs in this region vary in association with several leading modes of basin-wideand regional climate variability. These climate modes appear to be coherently linked to a backgroundoceanic–atmospheric pattern that either promotes or suppresses the likelihood of both droughts andTC landfalls. The relative frequency of TC landfalls in drought-stricken areas, the importance of theseevents in the regional moisture budget, and the potential for future changes in the large-scale forcingenvironment raise fundamental questions about possible changes in the hydroclimate of the southeasternUSA, where population growth and rising water demand already place strain on freshwater resources. Inthis article, we provide a review and synthesis of the recent research on variability in drought, landfallingTCs, the characteristics of the space–time association between these two phenomena in the southeasternUSA, and the coherent large-scale oceanic–atmospheric environment that either promotes or suppressestheir co-occurrence. Further, we review Global Climate Model projections related to these factors, andwe identify avenues for future research on this important topic.

Introduction

Droughts and tropical cyclones (TCs; tropical depressions, tropical storms, and hurricanes) areclimatologically important events in many regions of Earth, including the southeastern USA(hereafter “the Southeast”). The negative societal impacts of droughts and landfalling TCs sinceapproximately 1900 AD have been widely reported. For instance, since 1900, droughts in theSoutheast have been responsible for millions of dollars (US) in agricultural and other economiclosses (Federal Emergency Management Agency FEMA, 2008; Manuel, 2008), as well aspolitical confrontations over rights to municipal water supplies (e.g., Manuel, 2008; Maxwelland Soulé, 2009; Maxwell et al., 2012; Morehart et al., 1999; Pederson et al., 2012; Seageret al., 2009). The impacts of drought are not limited to major economic losses; however, aswater shortages also typically cause water quality declines, ecological impacts, increased energyfor cooling during the warm season and other indirect costs (e.g., Hayes et al., 2004;Herweijer et al., 2006). Similarly, landfalling TCs often impose substantial societal costs.Between the early 1950s and 2006, landfalling TCs and the remnants of decaying TCs over landwere responsible for over $370 billion in economic losses (adjusted to 2006 US dollars) and

© 2014 The Author(s)Geography Compass © 2014 John Wiley & Sons Ltd

Droughts and Tropical Cyclones 541

more than 3750 human deaths (Prat and Nelson, 2013). The importance of these events in theSoutheast is ref lected in the growing body of scientific research devoted to the climatologicalaspects of both drought variability and TC variability in the region. Both droughts and TCsin the Southeast have been analyzed using proxy paleoclimatic records (e.g., Miller et al.,2006; Mora et al., 2007; Pederson et al., 2012; Seager et al., 2009), instrumental observations(1895–present; Kam et al., 2013; Maxwell and Soulé, 2009; Maxwell et al., 2012; Maxwellet al. 2013a), and numeric climate model projections of future variability (e.g., Cook et al.,2007; Emanuel et al., 2008; Knutson et al., 2008; Knutson et al., 2010; Wehner et al., 2011).Interestingly, in spite of the relatively large body of research on droughts and TCs in the

Southeast, and in spite of evidence that spatial and temporal variability in both phenomenaappears to be driven by some of the same large-scale oceanic–atmospheric forcing mechanisms,the climatological characteristics of droughts and TCs—and their societal impacts—have usuallybeen studied separately, and few investigators have examined the interactions between terrestrialdrought and landfalling TCs. An early analysis of the co-occurrence of the two phenomenaindicated the importance of rainfall from landfalling TCs in the annual and seasonal moisturebudget of the Southeast (Sugg, 1968). Very recently, the ability of landfalling TCs to abruptlyend or substantially ameliorate existing soil moisture deficits has been documented for varioustime periods and locations around the eastern USA (Kam et al., 2013; Maxwell et al., 2012;Maxwell et al. 2013a). A mean climatology for the instrumental period (1895–2011) ofTC-induced “drought busters” (TCDBs) in the Southeast, the region where droughts andTCs interact most frequently, confirmed the hydroclimatic importance of TC-related rainfallin drought amelioration in the region (Maxwell et al. 2013a). Although TCs contribute, onaverage, less than 15% of annual average rainfall in the Southeast (e.g., Prat and Nelson,2013), the relative regional importance of this contribution warrants further attention. Forexample, there has been a documented increase in heavy rainfall frequency across the USA sincethe mid-20th century (Kunkel et al. 2010), in spite of the lack of a long-term trend in landfallingTCs and no trend in the number of daily heavy rainfall events associated with TCs. In theSoutheast, the positive anomaly in heavy rainfall events during 1994–2008 was solely a resultof anomalous TC contributions (Kunkel et al. 2010). Over the same time period, the ratio ofTC-related heavy rainfall events to the overall national number of events doubled. In spite ofthe spatial and temporal limitations on TC rainfall, TC contributions are associated with overone-third of the overall national anomaly in heavy rainfall events (Kunkel et al. 2010).We thinkthe drought-mitigating impacts of TC-related rainfall in the Southeast represent a substantialcomponent of the regional hydroclimate and deserve specific focus. We limit our discussionto the associations between drought and TCs, rather than the associations between droughtsand all heavy rainfall events.In this article, we provide a review of the scientific literature related specifically to drought,

TCs, and the associations between these two phenomena in the Southeast. For comprehensivereviews of Southeast hydroclimate, we refer the reader to Cook et al. (2007) and Labosier andQuiring (2013). While this review does not comprehensively cover all drought or TC researchfor the Southeast, it does document the broad body of research that informs our understandingof each phenomenon and provides a solid background summary of recent research relevant tothe spatiotemporal intersection of droughts and TCs in the region. Much of the researchemphasizes the importance of oceanic forcing of both drought variability and TC variabilityin the Southeast, particularly at decadal and longer time scales. These oceanic inf luences, inturn, hint at the importance of coupled ocean–atmosphere climate model projections andinterpretations of the potential future changes in the large-scale environment that maymodulatethe observed patterns of drought-TC co-occurrence—in the Southeast and elsewhere—withsignificant implications for the future hydroclimate of affected regions. First, we discuss recent



542 Droughts and Tropical Cyclones

research focused on prehistoric and observed drought variability and causal factors in theSoutheast. Second, we summarize the state of knowledge regarding prehistoric and observedTC variability and large-scale inf luences in the Southeast. Third, we describe the findings fromthe recent studies of landfalling TCs that moderate or eliminate existing droughts. Fourth, wesummarize climate model projections relevant to future drought-TC co-variability. Finally, wepresent our conclusions and suggestions of important avenues for future research.

Drought Variability in the Southeastern USA

Most of the recent research on North American drought variability and the causal mechanismsthat drive persistent drought has focused on the arid and semi-arid interior western USA (“theWest”; e.g., Cook et al., 2007; Herweijer et al., 2007; Hoerling et al., 2012; Schubert et al.,2004; Seager et al., 2003; Seager et al. 2005a; Seager et al. 2005b; Woodhouse et al., 2009).Since the late 19th century, widespread decadal-scale droughts have recurred in the West,raising important questions about whether the observed hydroclimatic variability of the regionfalls within the range of natural variability in previous centuries (Cook et al., 2007; Herweijeret al., 2007; Hoerling et al., 2012; Woodhouse et al., 2009). Paleoclimate reconstructions(primarily using tree-ring data) of drought variability in the West for the last millenniumindicate that the decadal-scale droughts of the 20th century, including the 1930s Dust Bowland the 1950s drought in the southwestern USA, were similar to prehistoric droughts in bothseverity and spatial coverage but were far shorter, on average, than the recurring decades-long“megadroughts” of earlier centuries (~900–1600 AD; e.g., Herweijer et al., 2007; Stahle et al.,2000). However, while multi-year droughts have occurred more frequently in the West thanin the eastern USA since the late 19th century, droughts of relatively short duration(2–3 years) in the Southeast have resulted in substantial agricultural and other economic lossesand placed significant strain on municipal water resources, a problem exacerbated by rapidpopulation growth and rising water demand in recent decades (Manuel, 2008; Maxwell andSoulé, 2009; Pederson et al., 2012; Seager et al., 2009). The societal impacts of these recentdroughts have helped to spur an increased research focus on drought variability in the Southeast.Several centuries-long tree-ring based drought reconstructions are available for the Southeast

(e.g., Pederson et al., 2012; Stahle and Cleaveland, 1992; Stahle et al., 1998). The findings fromthese studies generally corroborate that, much like in the West, droughts of the instrumentalperiod in the Southeast are within the range of natural variability in previous centuries. Themain spatial and intensity characteristics of southeastern US droughts are similar in thepaleoclimatic and modern instrumental records, including the relative infrequency ofdecadal-scale droughts, although there is evidence of multidecadal severe droughts in previouscenturies (e.g., late 17th–early 18th centuries and early 19th century), where no multidecadaldroughts occurred during the instrumental period (Pederson et al., 2012; Seager et al., 2009).The causes of these historic multidecadal droughts in the Southeast are not certain, but theymay have been forced by global-scale SST patterns similar to those that forced the historicmegadroughts of the West (Cook et al., 2009). Additionally, both the paleoclimatic andinstrumental records contain evidence of alternating multidecadal (~30 year) regimes ofabove- and below-average warm-season moisture (Enfield et al., 2001; McCabe et al., 2004;Ortegren et al., 2011; Stahle and Cleaveland, 1992).Whereas major advances have been made in identifying and describing the physical dynamics

behind large-scale oceanic forcing of persistent droughts in the West (e.g., Cook et al., 2007;Schubert et al., 2004), the specific mechanisms behind drought variability in the southeasternUSA remain relatively poorly understood. The inf luence of variability in tropical Pacific seasurface temperatures (SSTs), commonly referred to as El Niño/Southern Oscillation (ENSO)




variability, is the dominant driver of long-term drought in the West (Cook et al., 2007; Cooket al., 2009; Herweijer et al., 2007; Hoerling et al., 2012; Seager et al. 2005a; Seager et al. 2005b;Woodhouse et al., 2009). This ENSO inf luence is modulated at low frequencies by the phase ofthe Pacific Decadal Oscillation (PDO), such that PDO� phases (above-average North PacificSSTs near the west coast of the USA and Canada) are associated with increased droughtfrequency over much of North America, but PDO+ phases are more closely associated withwarm-season drought in the Southeast (e.g., Li, L. et al., 2012; McCabe et al., 2004; Miyasakaand Nakamura, 2005).According to Seager et al. (2009), the linkage between ENSO variability and southeastern

US drought is weak and entirely confined to the winter half year. Further, the amplitude ofthe ENSO association with winter drought in the Southeast is nonstationary over time, witha noteworthy lack of significant association during the period 1922–1950 (Diaz et al., 2001;Seager et al., 2009; van Oldenborgh and Burgers, 2005). Unlike in the western USA, ENSOvariability appears to have little effect on interannual warm-season drought variability in theSoutheast, which may respond to internal atmospheric variability not linked to boundaryconditions such as SST (Seager et al., 2009). However, several investigators have identifiedsignificant oceanic–atmospheric inf luences on low-frequency warm-season drought. Bothobservations (McCabe and Palecki, 2006; McCabe et al., 2004) and modeling studies(Feng et al., 2008; Seager et al., 2008) indicate that North Atlantic SSTs and tropical Pacificforcing are important drivers of North American drought (Woodhouse et al., 2009).Specifically, observed multidecadal periods of above- and below-average summer moisture inthe Southeast are associated with variability in North Atlantic SSTs. The most commonly usedindex of North Atlantic SST variability is the AMO index, which represents the decadallyfiltered area-average North Atlantic SST anomaly after linear detrending (Knight et al.,2006). The AMO exhibits low-frequency, roughly cyclical variation on time scales of50–70 years; (Enfield and Mestas-Nuñez, 1999; Enfield et al., 2001; McCabe and Palecki,2006; McCabe et al., 2004; Ortegren et al., 2011; Ortegren et al., 2014; Sutton and Hodson,2005). The AMO variability appears to be driven by the North Atlantic component of theglobal thermohaline circulation (Dima and Lohmann, 2007).It is noteworthy that some studies have identified no discernible role of the AMO in forcing

climate variability. These studies have questioned whether the AMO represents natural oceanicvariability or ref lects variability superimposed on to a long-termwarming trend and also disputethe interpretation of the cyclicity of the AMO, given the relatively short record (Holland andWebster, 2007; Mann and Emanuel, 2006).In contrast, many studies have identified a significant AMO-climate signal in a wide range of

variables in different regions. For example, the AMO is significantly associated with rainfall andriver data in the eastern USA (Enfield et al., 2001; McCabe and Palecki, 2006; McCabe et al.,2004), soil moisture in the Southeast (Ortegren et al., 2011; Ortegren et al., 2014), agriculturalproductivity in the Southeast (Maxwell et al. 2013b), northeast Brazil rainfall, Sahel rainfall,Indian monsoon rainfall (Lu et al., 2006; Zhang and Delworth, 2006), and Atlantic TCvariability (Knight et al., 2006; Nogueira et al., 2012; Zhang and Delworth, 2006). Coupledocean–atmosphere global climate models using historic observed SSTs (without imposingAMO variability in the model) reproduce a realistic, long-lived (millennial) AMO as internal var-iability (Knight et al., 2006; Zhang and Delworth, 2006). These simulations also give confidencein the physical basis for some of the observed AMO-climate linkages and indicate that the AMOcan cause the observed multidecadal variability in the climate factors listed above (Curtis, 2008;Knight et al., 2006; Zhang and Delworth, 2006). We are persuaded that the bulk of the evidencesupports the existence of the AMO as a natural internal oscillation with a period of approximately70–80 years; and which played a major role in forcing 20th century multidecadal climate




variability in the Atlantic sector (e.g., Zhang and Delworth, 2006). However, we acknowledgethat the question is not fully settled, especially because of the difficulty in drawing inferences froma relatively short record.In general, droughts across much of the USA tend to be longer and more frequent during

warm phases of the AMO (AMO+), although the inf luence appears to bemodulated by ENSOand PDO variability (McCabe et al., 2004). For example, both the 1930s and 1950s droughts inthe American Great Plains and Southwest occurred during an AMO+ phase, which may havecombinedwith cool eastern tropical Pacific SSTs to amplify the persistent aridity of those events(Cook et al., 2007; McCabe et al., 2004). During the 20th century, North American droughtswere most severe under AMO+ and PDO� conditions, but the Southeast tended to be driestwhen both indices were positive (McCabe et al., 2004). The role of PDO variability inmodulating the inf luence of the AMO appears to result from Rossby wave propagation acrossNorth America. During PDO+ periods, an anomalous summer anticyclone (associated with anorthwestward extension of the western ridge of the North Atlantic Subtropical Anticyclone[NASH]) centered over the Southeast inhibits convection and is associated with drier summers(Li, L. et al., 2012). Regardless of the phase of the PDO, AMO+ is significantly linked to warm-season soil moisture and streamf low deficiencies in the Southeast (Enfield et al., 2001; McCabeet al., 2004; Ortegren et al., 2011; Ortegren et al., 2014; Sutton and Hodson, 2005). Examplesof anomalous drought frequency and long-term moisture deficits during AMO+ periodsinclude 1840–1880, mid-1920s–late 1950s, and early 1990s–early 2010s. Cool phases of theAMO associated with above-average summer moisture include mid-1880s–1920s and late1950s–late 1980s (Enfield et al., 2001; McCabe et al., 2004).Alternatively, the multidecadal oscillation between wetter and drier warm seasons during the

last millennium documented by Stahle and Cleaveland (1992) was associated (for the 20thcentury) with North Atlantic sea-level pressure (SLP) variability. In their analysis, Stahle andCleaveland (1992) identified a correlation between warm-season rainfall in the SoutheastAtlantic coast states (Carolinas and Georgia) and the mean seasonal zonal SLP gradient betweenBermuda (30°N, 60°W) and NewOrleans (30°N, 90°W). The standardized time series of theseSLP differences (Bermuda minus New Orleans) has since been called the Bermuda High index(BHI; Katz et al., 2003; Ortegren et al., 2011).The BHI has been interpreted as an index of the summertime mean meridional location of

the western ridge of the surface NASH. Positive values of the BHI indicate a stronger pressuregradient and higher SLP at Bermuda, which is interpreted as an “eastward” location of thewestern ridge, while negative BHI values indicate the opposite (Katz et al., 2003; Ortegrenet al., 2011; Ortegren et al., 2014; Stahle and Cleaveland, 1992). According to theseinterpretations, during periods in which the west f lank of the NASH ridged strongly west overthe Southeast (BHI�), water vapor advection was diverted around the Southeast toward thewestern Gulf Coast, and combined with the general subsidence and divergence associated withthe high surface pressure in the Southeast, the result was typically below-average summerprecipitation (Diem, 2006; Katz et al., 2003; Ortegren et al., 2011; Stahle and Cleaveland,1992). Conversely, when the western f lank of the NASH was located farther east (BHI+),the unstable and humid southerly airf low into the Southeast produced wetter-than-averagesummers.Two separate climatologies of the NASH using 20th century SLP observations documented

pronounced multidecadal variability in its mean central pressure during summer (Davis et al.,1997; Sahsamanoglou, 1990). The distinct “epochs” of mean central pressure in the NASHclosely corresponded with phases of the AMO, with above-average central pressure duringAMO– and below-average central pressure during AMO+ (Davis et al., 1997; McCabeet al., 2004). Similarly, the multidecadal periods of above-average (below-average) summer




moisture reported by Stahle and Cleaveland (1992) coincided with multidecadal regimes ofabove-average (below-average) central pressure and thus with cool phases (warm phases) ofthe AMO.This synchronicity, along with the multidecadal variability in Southeast precipitation (Stahle

and Cleaveland, 1992), prompted a multi-variable analysis of the ocean–atmosphere inf luenceson warm-season drought in the Southeast (Ortegren et al., 2011). At multidecadal time scales,the AMO and the BHI were significantly associated, such that AMO+ typically coincided withperiods in which the mean zonal pressure gradient between Bermuda and New Orleans wasrelatively weak, or even reversed (i.e., higher pressure over New Orleans than Bermuda), andvice versa for AMO–. For the period 1895–2010, both the AMO and the BHI weresignificantly associated with low-frequency warm-season drought variability in the Southeast(Ortegren et al., 2011). This relationship between the BHI and Southeast summer droughtwas interpreted (maybe incorrectly) as an indication of the inf luence of east–west variabilityin the western ridge of the NASH (Ortegren et al., 2011). Even if this interpretation of theBHI is incorrect, the significant inverse relationship between the Bermuda–New OrleansSLP gradient (BHI) and both the AMO and Southeast drought conditions indicates a possibleforcing role of the AMO in (at least) the western domain of the NASH. Thus, the NASHand AMO may ref lect coupled variability, and the NASH may be the mechanism by whichthe observed AMO inf luence on drought in the Southeast is manifested.However, a growing body of evidence indicates that the interpretation of the BHI as a direct

indication of the east–west movement of the western ridge of the NASH is overly simplistic.These findings indicate that Southeast warm-season precipitation variability is associated withmeridional, as well as zonal, changes in the NASH western ridge (e.g., Diem, 2013a; Diem,2013b; Li et al., 2011; Li et al., 2013; Miyasaka and Nakamura, 2005; Wu and Liu, 2003;Wu et al., 2009; Zhou et al., 2009). The Southeast precipitation inf luence of the NASHwestern ridge is greatest during summers when the NASH extends strongly westward.However, during summers of westward extension, the north–south position of the westernridge determines the sign of the relationship. Periods in which the NASHwestern ridge extendsnorthwestward are associated with enhanced moisture advection into the Great Plains andMidwest regions, with strong subsidence and reduced precipitation in the Southeast.Conversely, wet summers in the Southeast are associated with a southwestward extension ofthe NASH western ridge and an anomalous upper-level trough over the USA, providingthe appropriate surface and upper-level support for summertime convective precipitation(Li, L. et al., 2012). Periods in which the western ridge is displaced eastward are associated witha subdued inf luence of the NASH western ridge as well as overall reductions in Southeastprecipitation variability (Li, L. et al., 2012).The primary forcing of the summertime subtropical highs is not SST variability but the

diabatic heating contrast over western continent/eastern ocean margins (Li, W. et al., 2012;Rodwell and Hoskins, 2001; Wu and Liu, 2003; Wu et al., 2009). This contrast sets uplower-level cyclonic vorticity anomalies over the continent and anticyclonic vorticity anomaliesover the adjacent ocean, which induce strong equatorward f low and subsidence along thewestern continental margin in order to satisfy Sverdrup vorticity balance (e.g., Li, W. et al.,2012; Miyasaka and Nakamura, 2005; Wu et al., 2009). The equatorward f low at lower levelsis reinforced by local positive feedback mechanisms including the development of persistentlow-level marine stratus clouds and Ekman pumping, which cause further reductions in SSTsand increased atmospheric stability over the ocean. Sensible heating of the relatively drywestern continent enhances the thermal gradient between land and adjacent ocean,which strengthens the surface anticyclone over the eastern ocean. Over eastern continentsand western oceans, the summertime diabatic heating is dominated by deep convective heating




over land and weak longwave radiative cooling and subsidence over ocean, which is associatedwith poleward f low along the eastern continent, enhanced instability, and uplift (Wu and Liu,2003; Wu et al., 2009).Low-frequency spatiotemporal variability in the NASH is associated with the North Atlantic

Oscillation (NAO), which is the leading atmospheric mode of SLP variability in the Atlantic(Colbert and Soden, 2012; McCabe and Palecki, 2006). The NAO index is calculated as thenormalized difference in SLP between Portugal and Iceland. NAO+ is associated with strongmid-latitude westerlies over the Atlantic and a northeastward displacement of the center ofthe NASH (Figure 1; Table 1). Westward displacement of the NASH center is associated withNAO– (Elsner, 2003; Elsner et al. 2000a; Elsner et al. 2000b). This implies that increasedNASHintensity (linked to NAO+) does not necessarily cause westward migration or expansion of theanticyclone. Instead, the NASH center is often farther east under NAO+, possibly because ofthe associated increase in strength of the mid-latitude westerlies and the negative SST anomaliesin the eastern subtropical Atlantic (e.g., Li, W. et al., 2012).

Fig. 1. Generalized conceptual diagram of the coherent low-frequency large-scale oceanic and atmospheric environmentassociated with (A) increased drought-TC co-occurence and (B) decreased drought-TC interactions induced by correspond-ing shifts in the general flow associated with AMO, NAO, and NASH variability. We suggest cautious interpretation of thesefigures, particularly the spatial generalizations, and we emphasize that this represents only a background environment thateither favors or suppresses drought-TC interactions. This pattern is modulated at the interannual scale by ENSO variabilityand possibly other remote forcings.



Table 1. Summary of oceanic–atmospheric conditions that promoted and/or coincidedwithwarm-season pluvials and droughts in the Southeast in the instrumental climate record (~1895–2012).

Index condition Hydroclimate influence Reference(s)

*AMO+ Increased drought frequency;long-term (interdecadal) moisture deficits

Enfield et al. (2001);McCabe et al. (2004);Sutton and Hodson (2005);Ortegren et al. (2011)

*AMO– Decreased drought frequency;long-term (interdecadal) moisture surpluses

Enfield et al. (2001);McCabe et al. (2004);Sutton and Hodson (2005);Ortegren et al. (2011)

*Strong Bermuda–New OrleansSLP gradient

Increased drought frequency;long-term (interdecadal) moisture deficits(corresponds with AMO+ periods)

Stahle and Cleaveland (1992);Katz et al. (2003); Diem (2006);Ortegren et al. (2011)

*Weak Bermuda–New OrleansSLP gradient

Decreased drought frequency;long-term moisture surpluses(corresponds with AMO– periods)

Katz et al. (2003); Diem (2006);Ortegren et al. (2011)

NASH more intense Corresponds with AMO– and NAO+ Sahsamanoglou (1990);Davis et al. (1997)

NASH less intense Corresponds with AMO+ and NAO– Sahsamanoglou (1990);Davis et al. (1997)

Global mean temperature increase Projected increase in summertimeNASH intensity and spatial coverage

Zhou et al. (2009); Li et al. (2012)

*Statistically significant association with drought variability in the Southeast identified by one or moreinvestigator(s).


Thus, there is ample evidence and theoretical support for the role of the NASH inmodulating drought variability in the Southeast. There also is evidence for a link to theAMO, although such a link may be confined to the western portion of the NASH. Further,the relationship between the AMO and the NAO (North Atlantic SSTs effectively explainsome of the NAO variability) supports the notion of a link between the AMO and theNASH (McCabe and Palecki, 2006). Climate models forced only with Atlantic SSTs havegenerally failed to reproduce observed persistent droughts, in the Southeast and elsewherein North America, emphasizing the modulating effects of teleconnections to tropical SSTsand atmospheric modes of climate variability, among other possible factors (Cook et al.,2007). More than one investigator speculated on the possibility that North Atlantic SSTsand the NASH and NAO variability could ref lect coupled ocean–atmosphere variabilityin the North Atlantic sector (Cook et al., 2007; Elsner and Kocher, 2000) and that furtherwork was needed to identify such linkages.In summary, the larger-scale oceanic–atmospheric inf luences on drought in the Southeast are

seasonally variable. The dominant driver of the moisture balance during winter is ENSOvariability, potentially modulated by the AMO and PDO (McCabe et al., 2004; Seager et al.,2009). The dominant drivers of warm-season drought variability are the AMO and the NASH,which may exhibit quasi-coupled behavior, as implied by their mutual associations with NAOvariability (Figure 1; Table 1). These primary inf luences on warm season drought variability inthe Southeast also exert significant inf luence (along with NAO variability) on TC variability inthe Southeast.




Atlantic Basin Tropical Cyclone Variability

TROPICAL CYCLONE FORMATION AND LANDFALL VARIABILITY

Atlantic Basin TC activity and landfall frequency in the eastern USA exhibit substantial spatialand temporal variability at intra-annual, interannual, and interdecadal time scales (Keim et al.,2004; Keim et al., 2007; Nogueira and Keim, 2010). Patterns of TC formation over the Atlanticare inf luenced by a number of variables that appear to interact in complex ways (Elsner et al.,2006; Goldenberg and Shapiro, 1996; Goldenberg et al., 2001; Shapiro and Goldenberg,1998; Virmani and Weisberg, 2006). TC landfall climatologies constructed for select locationsfrom proxy records indicate multidecadal variability in landfall frequency in the eastern USAduring previous centuries (Elsner et al. 2000a; Elsner et al., 2008; Frappier et al., 2007; Liuand Fearn, 2000; Miller et al., 2006; Mora et al., 2007). The multidecadal oscillation betweenhigher and lower TC landfall frequency has been attributed to large-scale oceanic–atmosphericconditions, specifically the NAO and the AMO (Elsner and Kocher, 2000; Elsner et al. 2000b;Elsner et al. 2000a; Elsner et al., 2008; Miller et al., 2006; Mora et al., 2007).While proxy records provide long, multi-century time series, allowing the examination of

temporal trends in TC frequency prior to modern instrumental record keeping, many proxyrecords suffer from inaccuracies and may only be valid for specific locations (Miller et al.,2006; Mora et al., 2007). Instrumental monitoring of TCs in the North Atlantic is consideredaccurate since air reconnaissance started in 1944 (Vecchi and Knutson, 2008). Models forpredicting TC genesis have improved as satellite and radar technology has become moresophisticated. Improvements in statistical methodologies (Elsner and Schmertmann, 1993)and data availability (Lehmiller et al., 1997) have allowed analyses of the driving forces behindTC formation in different subbasins within the North Atlantic.Large-scale oceanic–atmospheric features have been linked to TC formation variability,

including ENSO (Elsner and Jagger, 2006; Elsner et al., 2006; Goldenberg and Shapiro,1996; Larson et al., 2005), the AMO (Delworth and Mann, 2000; Elsner and Kocher, 2000;Elsner et al., 2006; Goldenberg and Shapiro, 1996), the NAO (Elsner and Kocher, 2000; Elsneret al. 2000a; Elsner et al., 2006), and West African Monsoon (WAM) variability (Goldenbergand Shapiro, 1996; Gray, 1990). Both ENSO and WAM inf luence the interannual variabilityof TC formation as the WAM varies interannually and ENSO variability typically operates on2–7 year; cycles (Cordery and McCall, 2000). While ENSO affects TC formation at theinterannual scale via changes in mid-latitude westerlies and vertical shear over the subtropicalNorth Atlantic, its inf luence is less important in the North Atlantic compared to the AMOor the NAO, each of which appears to inf luence both TC formation frequency and locationas well as TC tracks, particularly at interdecadal time scales (Colbert and Soden, 2012; Elsneret al. 2000a; Elsner et al. 2000b; Keim et al., 2007; Nogueira et al., 2012). AMO+ phases areassociated with increased overall TC activity in the Atlantic Basin (Klotzbach, 2011; Landsea,2007; Pielke and Landsea, 1999), while NAO– phases are associated with greater numbers ofmajor hurricanes (category 3–5; Elsner and Kocher, 2000; Elsner et al. 2000b). The genesislocation of TCs also inf luences variability in TC tracks. TCs that form farther south and west(toward the Caribbean) are more likely to make landfall in Texas and the US Gulf Coast, whileTCs that form farther east and north are more likely to make landfall along the US AtlanticCoast or to recurve northeastward over the Atlantic without making landfall in the USA(Colbert and Soden, 2012).Because the main impacts of drought (and nearly all drought research) are related to terrestrial

moisture deficits, our definition of drought-TC co-occurence requires TC landfall. Althoughinterannual TC landfall frequency is related to the total number of TCs that form (Landsea,




2007), interdecadal spatial variability in TC tracks and landfall location also is inf luenced byvariability in the deep-layer steering f low over the western North Atlantic. For example, theenhanced upper-level westerlies and strong vertical wind shear associated with El Niño events(ENSO+) combine to both disrupt the upper-level organization of developing TCs and torecurve TCs eastward over the Atlantic, resulting in the widely reported pattern of reducedbasin-wide TC landfall frequency during El Niño years (Bove et al., 1998; Colbert and Soden,2012; Kossin et al., 2010; Xie et al., 2005). The phase of the NAO index also modulates thesteering f low in the western North Atlantic, with NAO+ linked to increased landfall frequencyon the US East Coast and NAO– phases linked to increased landfall frequency on the US GulfCoast. Ultimately, the NAO inf luence on TC tracks is manifested via its association with theNASH (Colbert and Soden, 2012; Elsner et al. 2000b; Kossin et al., 2010). Typically, duringNAO+ phases, the NASH is centered farther north and east over the North Atlantic, withsteering winds on the west f lank of the NASH enhancing the likelihood of US East Coastlandfalls, relative to the US Gulf Coast (Elsner, 2003; Elsner et al. 2000a; Elsner et al. 2000b).

TROPICAL CYCLONE RAINFALL VARIABILITY

Regional mean climatologies of TC rainfall are in general agreement that TCs contributeapproximately 7–15% of annual average rainfall in the southeastern USA, with maximumproportional contributions in near-coastal regions (Knight and Davis, 2007; Larson et al.,2005; Nogueira and Keim, 2010; Prat and Nelson, 2013; Shephard et al., 2007). ProportionalTC rainfall contributions to average annual rainfall are lower in inland areas of the eastern USA,decreasing northward and westward from the coastal regions to approximately 2% in westernTexas and the Midwestern states (Prat and Nelson, 2013).Several spatiotemporal trends in TC rainfall in the eastern USA have been identified in the

instrumental record (1895–present). For example, total TC rainfall in the US Gulf Coast wassignificantly greater during 1961–2007 compared to 1931–1960. However, this trend primarilyref lects a substantial increase in inland TC rainfall in the latter period, because TC rainfall incoastal regions of the Southeast actually decreased in the same period (Nogueira and Keim,2011; Nogueira et al., 2012). Interdecadal variability in TC rainfall is associated with theAMO index. For the entire eastern USA, AMO+ phases are associated with increased totalvolume of TC rainfall, increased numbers of landfalling storms, and increased area impactedby TC rainfall (Nogueira and Keim, 2010; Nogueira et al., 2012). The AMO also inf luencesthe spatial variability in TC rainfall in a way that enhances the observed out-of-phaseinterdecadal variability in landfall location between the US East Coast and the US Gulf Coast(e.g., Elsner and Kocher, 2000; Elsner et al. 2000a). AMO+ is linked with increased TC rainfallalong the Gulf Coast from Florida to Texas, and negatively associated with TC rainfall along theAtlantic Coast fromNorth Carolina northward to New England, with insignificant associationsin the mid-Atlantic states (Nogueira et al., 2012). The physical basis for such an AMO inf luenceon TC tracks has yet to be described, and the association may arise indirectly, as a result of therelationships between the AMO, NAO, and NASH. El Niño (La Niña) events are negatively(positively) associated with TC rainfall in Texas (Nogueira et al., 2012; Zhu et al., 2013). Totalrainfall from landfalling TCs exhibits an increasing trend since the mid-20th century, althoughthis trend occurs within a distinct multidecadal oscillation associated with the AMO (Nogueiraand Keim, 2010). Extreme precipitation from TCs also increased significantly since the early1970s (Knight and Davis, 2009).A wide range of factors is associated with TC formation and landfall frequency in the North

Atlantic, as well as spatiotemporal variability in TC landfall location in eastern North America.Large-scale oceanic–atmospheric features such as the AMO, ENSO, and NAO have




documented inf luences on TC formation frequency and genesis location, which in turninf luence landfall frequency and spatial variability.As a result of their inf luences on TC genesis and track preference, the AMO and ENSO are

associated, respectively, with low- and high-frequency spatial variability in TC rainfall insubregions of the eastern USA. At the interannual scale, El Niño events typically reduce thelikelihood of TC landfalls, and thus TC rainfall, in the eastern USA. At interdecadal scales,the AMO inf luence on total number of basin-wide TCs is ref lected in the association betweenAMO+ and increases in total TC rainfall in the eastern US, particularly in the Gulf Coast andFlorida (Nogueira and Keim, 2010; Nogueira et al., 2012). The primary feature associated withthe steering f low that inf luences landfall location preference is the NASH, which isteleconnected to NAO variability and AMO variability (Figure 1).Regional to meso-scale systems such as variability in West African Monsoon rainfall and the

Saharan Air Layer (dry air and dust) correspond with the interannual variability of TC formation(Dunion and Velden, 2004; Goldenberg and Shapiro, 1996). These associations may arise fromdirect physical forcing or as a result of the statistical associations betweenWest AfricanMonsoonrainfall, Saharan dust export intensity, and other factors that inf luence TC variability, includingthe AMO and NAO indices (e.g., Dunion and Velden, 2004; Evan et al., 2006; Moulin et al.,1997). The potential physical mechanisms linking the Saharan Air Layer and African dust exportnear the Atlantic TC main development region (MDR) to TC variability include radiativeenhancement of the climatological surface trade wind inversion, increased easterly wind shearin the middle troposphere, and the injection of dry air into developing TC circulations.Together, these factors inhibit deep convection in the MDR and may explain the negativeassociation between African dust export/Saharan Air Layer intensity and Atlantic basin TCfrequency (e.g., Donnelly and Woodruff, 2007; Dunion and Velden, 2004; Evan et al.,2006). Conversely, the intensity of West African monsoon rainfall is positively associated withAtlantic TC frequency, possibly because of the increased frequency of easterly waves duringwetter periods in the Sahel (Evan et al., 2006). Although much uncertainty surrounds thephysical basis, it is possible that West African monsoon rainfall modulates African dust export.A drier wet season would reduce soil particle cohesion and implies a reduction of dust washoutby rainfall during transport (Evan et al., 2006). This is consistent with the observed linkagesbetween high Sahel rainfall, low African dust intensity, and increased TC activity and impliesa possible forcing role for both the ocean (AMO) and the atmosphere (NAO; Figure 1;Table 2).The statistical and/or physical linkages betweenWest African dust/Saharan air layer intensity

and larger-scale TC-forcing mechanisms, such as the AMO and the NAO, may explain thedocumented associations between West African monsoon variability, African dust/Saharan airvariability, and Atlantic TC activity (e.g., Evan et al., 2006). Specifically, the AMO inf luenceon seasonal meridional migration of the ITCZ is associated with Sahel rainfall variability(Knight et al., 2006; Zhang and Delworth, 2006), and the NAO inf luence on circulationand precipitation in north Africa is associated with dust export intensity over the AtlanticMDR (Moulin et al., 1997). We note here the possibility that the Saharan air layer intensitymay represent a local positive feedback, reinforcing a background environment that oscillatesbetween suppressing and promoting TC activity.The concurrent inf luences of AMO+, including multidecadal regimes of below-average

summer moisture in the Southeast Atlantic coast region (Enfield et al., 2001; McCabe andPalecki, 2006; McCabe et al., 2004; Ortegren et al., 2011; Ortegren et al., 2014) as well asincreased total numbers of TCs in the Atlantic Basin, and increased numbers of TC landfallsin the Southeast USA (Gulf and Atlantic; Landsea, 2007), may appear contradictory. But theincreased probability of TC landfall is not enough to completely offset the increased probability



Table 2. Summary of the oceanic–atmospheric conditions that influenced and/or correspondedwith low-frequency Atlantic Basin tropical cyclone (TC) variability in the instrumental climaterecord (~1895–2012). Against this backdrop of low-frequency variability, interannual Atlantic TCactivity is modulated by ENSO and possibly other remote influences.

Index condition TC activity influence Reference(s)

*AMO+ Greater overall TC activity in Atlantic Basin;greater TC rainfall over eastern USA;greater number of landfalling TCs,especially for Gulf Coast and Florida

Pielke and Landsea (1999);Landsea (2007); Klotzbach (2011);Nogueira et al. (2012)

*AMO– Less overall TC activity in Atlantic Basin;less TC rainfall over eastern USA;fewer landfalling TCs, especially forGulf Coast and Florida

Pielke and Landsea (1999);Landsea (2007); Klotzbach (2011);Nogueira et al. (2012)

*NAO+ Greater TC landfall activity for US East Coast;fewer major (category 3–5) hurricanes;typically coincides with AMO– and eastwarddisplacement of NASH center

Elsner et al. (2000a, b);Elsner and Kocher (2000);Elsner (2003); Kossin et al. (2010);Colbert and Soden (2012)

*NAO– Greater TC landfall activity for US Gulf Coast;greater number of major(category 3–5) hurricanes; typically coincideswith AMO+ and westward displacementof NASH center

Elsner et al. (2000a, b);Elsner and Kocher (2000);Elsner (2003); Kossin et al. (2010);Colbert and Soden (2012)

*El Niño At interannual scale, less TC landfall activityand TC rainfall across eastern USA;

Bove et al. (1998);Xie et al. (2005); Kossin et al. (2010);Colbert and Soden (2012)

*La Niña At interannual scale, greater TC landfallactivity and TC rainfall across eastern USA

Bove et al. (1998);Xie et al. (2005); Kossin et al. (2010);Colbert and Soden (2012)

*NASH displacedwestward

Corresponds with NAO– and AMO+;enhanced by La Niña at interannual scale

Elsner et al. (2000a, b);Elsner, 2003; Kossin et al. (2010);Colbert and Soden (2012)

*NASH displacedeastward

Corresponds with NAO+ and AMO–;enhanced by El Niño at interannual scale

Elsner et al. (2000a, b);Elsner, 2003; Kossin et al. (2010);Colbert and Soden (2012)

ITCZ displacednorthward

Associated with AMO+, NAO–, and withabove-average Sahel rainfall; linked toincreased Atlantic TC formation

Zhang and Delworth (2006);

ITCZ displacedsouthward

Associated with AMO–, NAO+,and with below-average Sahel rainfall;linked to decreased Atlantic TC formation

Zhang and Delworth (2006);

*Sahel drought Associated with NAO+,weak West African Monsoon,increased Saharan dust export;linked to decreased Atlantic TC formation

Moulin et al. (1997);Evan et al. (2006);Donnelly and Woodruff (2007)

*Sahel pluvial Associated with NAO–,strong West African Monsoon,decreased Saharan dust export;linked to increased Atlantic TC formation

Moulin et al. (1997);Evan et al. (2006);Donnelly and Woodruff (2007)

*Statistically significant associationwith TC variability in the Southeast identified by one ormore investigator(s).





of summer moisture deficiency. The fact that rainfall specifically from TCs is above average duringAMO+ does not necessarily imply increased regional Southeast wetness at multidecadal time scales.This is mostly because the TC rainfall inf luence is proportionally small (~7% for the entire easternUSA, 9–11% for coastal regions from TX to ME; Prat and Nelson, 2013) when compared to an-nual average rainfall, and also because the TC rainfall contribution is inherently spatially confined.In summary, while TCDBs are an important component of regional hydroclimate in the

Southeast (~13% of droughts were ended by TCDBs 1895–2011), it is not correct to assumethat periods of increased TC rainfall (AMO+) will be wetter than average (at interannualand multidecadal scales) because of the increased TC activity. Rather, AMO + promoteswidespread regional drought, which is sometimes ameliorated, at the local scale, by TCDBs.

Observed Patterns of Drought-Tropical Cyclone Co-occurrence

Some beneficial aspects of landfalling TCs, including basic evidence of drought ameliorationresulting from TC precipitation, were discussed several decades ago (Sugg, 1968). Recentanalyses have quantified the drought-mitigating impacts of precipitation from landfalling TCsand the spatiotemporal variability of drought-TC co-occurrence during the period ofinstrumental climate observation (Brun and Barros, 2013; Kam et al., 2013; Maxwell et al.,2012; Maxwell et al. 2013a). This increased research focus on drought-TC intersections hasilluminated the frequency with which landfalling TCs have modulated short- and long-termdroughts in the Southeast in the last several decades and has emphasized the relative importanceof large-scale dynamic mechanisms that promote both drought and TC impacts in the region.During 1895–2011, 13% of all TC season ( June–November) droughts—measured as

“moderate” or worse soil moisture deficits using the Palmer Drought Severity Index (Palmer,1965)—in the Southeast coastal states from Texas to North Carolina were abruptly ended byprecipitation from landfalling TCs or the remnants of decaying TCs. In some parts of the region,TCs ended up to 30% of all droughts during the same period (Maxwell et al. 2013a). Both short-term (<3months) and long-term (>3months) droughts were ended by TCs, and drought of“moderate” or worse severity never returned to a given location during the same calendar yearafter being ended by a TC (Maxwell et al. 2013a). While TCs ended droughts in nearly everypart of the study area, the Gulf Coast states and Atlantic Coast states were unequally impacted byTC-related drought relief. In the Atlantic Coast states, TCs ended droughts more frequentlyand farther inland than in the Gulf Coast states, and a significant increase in drought-TCintersections over time also was documented for the Atlantic Coast states that was not significantalong the Gulf Coast. However, the total area alleviated of drought conditions by TCs didexhibit a significant increasing trend over time in the Gulf Coast states that was absent in theAtlantic Coast region (Maxwell et al. 2013a). The significant larger-scale inf luences promotingdrought-TC intersection for the entire Southeast coastal region were a warm Atlantic Oceansurface (AMO+) and weak westerlies (NAO–). For the whole study area, the odds ofdrought-TC intersection were 6.4 times greater under NAO– conditions and three timesgreater under AMO+ conditions (Maxwell et al. 2013a).Drought-TC co-occurrences in the Southeast during the last three decades also have been

examined using simulated daily moisture f lux from a land surface model (Kam et al., 2013).A comparison of modeled soil moisture with observed TC precipitation included and withTC precipitation removed indicated that between 1980 and 2007, landfalling TCs led toreduced drought duration, late drought initiation, early drought demise, and an overall decreasein the spatial extent of droughts (Kam et al., 2013). Similarly, at the watershed scale, TCscontributed important precipitation for soil moisture recharge and stream f low recovery fromdrought conditions during 2002–2011 (Brun and Barros, 2013). Although TC precipitation




exhibited high spatiotemporal variability, it played an important role in regulating warm-seasondrought and was a key agent of regional meteorological drought mitigation at seasonal and interan-nual scales (Brun and Barros, 2013; Kam et al., 2013). These recent studies illustrate that in previousdecades, drought-TC intersections in the Southeast were frequent and represented an importantcomponent of the hydroclimate in a region with growing susceptibility to freshwater shortages.Regarding the background oceanic–atmospheric environment in which drought-TC intersec-

tions occur, a coherent pattern emerges. The low-frequency forcing mechanisms that promotedrought in the Southeast also appear to promote TC landfall. Although some of the physical and/or statistical relationships require further description, the coherent pattern of conditions can be sum-marized as follows. Both droughts andTC landfalls aremore likely in the Southeast during periods ofAMO+. The findings described above indicate that AMO+ is directly or indirectly associatedwith acombination of NAO–, a weakened NASH, warm tropical Atlantic SSTs, reduced tropical AtlanticSLPs, reduced vertical shear in the Atlantic MDR, a general northward shift in the ITCZ associatedwith above-average summermonsoon precipitation in the Sahel and India, and reduced dust exportintensity in northern andwestern Africa (Figure 1; Table 2). At the interannual scale, vertical shear inthe MDR and overall TC activity are strongly modulated by ENSO variability.A reversed, but similarly coherent, pattern is associated with periods of reduced drought and

TC landfall frequency in the Southeast. AMO– is associated with NAO+, a strengthenedNASH, reduced SSTs in the MDR, increased SLPs and vertical shear over the MDR, asouthward displacement of the ITCZ associated with weaker monsoon precipitation in WestAfrica and India, and increased Saharan dust export intensity in northern and western Africa.In this context, the potential future changes in the hydroclimatic impacts of drought-TC

co-occurrence deserve attention. Some important questions remain unanswered. How willthe large-scale mechanisms driving drought and TC variability in the Southeast respond tothe projected positive radiative forcing? Will spatiotemporal patterns of drought-TCintersections be modulated by the projected warming in coming decades? Will the relativeclimatic importance of drought-TC intersections increase or decrease?

Climate Model Projections Related to Drought-Tropical Cyclone Co-variability

Large-scale oceanic–atmospheric conditions inf luence droughts and TCs, as well as theprobability of TC-related drought amelioration (i.e., TCDBs). Thus, future climate modelprojections of large-scale oceanic conditions have important implications for whether—andhow—droughts and landfalling TCs will intersect in the Southeast. Climate models project aweakening in the Atlantic thermohaline circulation for the next century (Cook et al., 2007),which would lead to cooling of the subtropical North Atlantic, implying a long-term increasein mean central pressure in the surface NASH. Numerical climate simulations indicate thatfuture warming would result in an increasingly intense NASH with a stronger westwardexpansion, especially southwestward (Li, L. et al., 2012; Li, W. et al., 2012). However, recentanalyses of observed trends in the location of the western f lank of the NASH have producedcontradictory results, with some evidence for westward displacement since the 1970s and someevidence for southeastward displacement during the same period (Diem, 2013a, 2013b; Li et al.,2011; Li et al., 2013). Further work is needed to fully resolve the physical connections andspatiotemporal associations between the main components of North Atlantic climate variability,particularly the relationship between the NASH, the AMO, and global temperature increases.Additionally, the evidence that the AMO and the Bermuda–New Orleans SLP gradient aresignificantly correlated requires physical description andmust be effectively reproduced in coupledclimate models if enhanced predictability is to be achieved (e.g., Cook et al., 2007). Clearly, this isan important topic for future research related to drought variability in the Southeast.




Many climate models project increases in global average temperature but projections offuture precipitation are far less certain (Wehner et al., 2011). Increases in temperature could leadto increases in evapotranspiration and thus lead to future drying, which most of the climatemodels project (Wehner et al., 2011). However, the expected moistening and enhanced globalmean precipitation of a warmer atmosphere may to some degree offset projected losses of soilmoisture to evapotranspiration. The uncertainty of precipitation projections makes it difficultto draw firm conclusions on future soil moisture conditions.Climate models have similar uncertainties in predicting future TC characteristics. The strong

connection between ocean SSTs and TC frequency and intensity in the observed record(Elsner et al. 2000b; Elsner et al. 2000a; Elsner et al., 2008; Miller et al., 2006; Mora et al.,2007) indicates that higher global temperatures would lead to more frequent TCs, althoughmost models show a slight decrease or no change in TC frequency (Chauvin et al., 2006;Emanuel et al., 2008; Knutson et al., 2008). TC intensity has a relationship to SSTs(Emanuel, 1987; Holland, 1997), which lends credibility to the climate model projections ofgreater intensity TCs with warmer global temperatures (Emanuel, 2007; Oouchi et al.,2006). However, there is high uncertainty when downscaling simulated large-scale oceanic–atmospheric conditions to the basin or regional level, (Knutson et al., 2010; Zhao et al.,2009). Even more uncertainty plagues the projections of the location of TC formation, TCtracks, and TC-generated storm surge (Knutson et al., 2010). However, climate modelsgenerally agree that future TCs will most likely produce more rainfall, but a documented trendin the observed record has not been established (Knutson et al., 2010).Uncertainties in future large-scale oceanic–atmospheric conditions and in the net effects of

feedbacks related to, for example, evaporation, clouds, atmospheric moisture, and troposphericaerosols make it challenging to forecast the relationship between droughts and TCs. Of greatimportance is the ability for Global Climate Models to be downscaled and accurately representregional conditions, signifying an area in need of improvement. Of specific importance to theNorth Atlantic Basin and Southeast is the potential coupling between the AMO, NAO, andNASH and their possible teleconnections to the tropics and other ocean basins.

Suggestions and Conclusions

From a climatological standpoint, the fact that droughts, TCs, and drought-TC co-occurrencesappear to be inf luenced by some of the same larger-scale forcing mechanisms makes drought-TC intersection research a prime focal point from which climate scientists may acquire anenhanced general knowledge of overall long-term climate variability in the Southeast. Weidentify two main areas of drought-TC associations that remain relatively understudied. First,some recent severe weather events in the eastern USA (e.g., TC Marco, 1999; TC Erin,2007; TC Gustav, 2008; TC Sandy, 2012) have involved overland interactions betweenlandfalling TCs and mid-latitude cyclonic storms, producing extreme precipitation andf looding. The climatology of these interactions between tropical and nontropical systems andtheir impacts deserves analysis. Second, some forecast models indicate a probability that futureTCs may produce more precipitation in a warmer andmore humid atmosphere, even if landfallfrequency and storm characteristics do not change. Such a trend has yet to be documented butcould carry significant implications for the hydroclimate of the Southeast.This article has summarized the growing body of research on variability in droughts, TCs, and

their co-occurrence in the Southeast. Both droughts and TCs are associated with negativeeconomic and societal impacts in the region. However, recent evidence indicates that thetwo phenomena coincide frequently enough that the long-term societal costs of drought inthe Southeast appear to be less than they would be without the counteracting inf luence of




rainfall from landfalling TCs. Similarly, while landfalling TCs can impose devastating costs inwind and f lood damage, their moisture balance benefits related to drought mitigation—althoughalmost certainly not equivalent to their costs—should be included in assessments of theiroverall impacts.

Acknowledgement

We would like to thank two anonymous reviewers and editor Dr. Scott Curtis for insightfulcomments and suggestions that helped us improve earlier versions of the manuscript. We alsothank Jeremy Mullins for valuable feedback.

Short Biographies

Jason Ortegren’s research focuses on the large-scale oceanic–atmospheric forcing on eastern USclimate. He has authored or co-authored papers in these areas for Professional Geographer, Journalof Climate, Agricultural and Forest Meteorology, and the Annals of the Association of AmericanGeographers. His current research focuses on the inf luence of the Bermuda Subtropical Highon precipitation in the eastern USA. He holds an MA and PhD in Geography from theUniversity of North Carolina at Greensboro. He taught atWesternMichiganUniversity Beforecoming to the University of West Florida, where he is currently an assistant professor inEnvironmental Studies.Justin Maxwell’s research focuses on documenting the climatology of extreme weather

events and examining how these events interact. Further, he examines the inf luence of climateon tree growth and agricultural yields. He has authored or co-authored papers in these areas forJournal of Climate, Agricultural and Forest Meteorology, Ecology and Evolution, Agriculture, Ecosystemsand Environment, Annals of the Association of American Geographers, and Climate Research. Currentresearch involves examining the interaction between tropical cyclones and mid-latitude wavecyclones and comparing the climate responses of co-occurring species in the Eastern DeciduousForest. Maxwell holds a MA in Geography from Appalachian State University and PhD inGeography from the University of North Carolina at Greensboro. He is currently an assistantprofessor of Geography at Indiana University.

Notes

* Correspondence address: Justin T. Maxwell, Department of Geography, Indiana University, Student Building 120, 701E. Kirkwood Ave. Bloomington, IN 47405, USA. E-mail: [email protected]

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