Local and non-local land surface in uence in …...Con dential manuscript submitted to Geophysical...

Confidential manuscript submitted to Geophysical Research Letters

Local and non-local land surface influence in European1

heatwave initial condition ensembles2

A. L. Merrifield1,4, I. R. Simpson2, K. A. McKinnon3, S. Sippel1, S.-P. Xie4,3

C. Deser24

Key Points:5

• By constraining atmospheric circulation, a seasonally persistent European heat-6

wave is imposed over different initial land surface states.7

• The prescribed heatwave is amplified by local dryness directly and indirectly through8

the modification of the near-surface atmosphere.9

• The atmospheric response to surface dryness allows for the propagation of land-10

atmosphere interactions and non-local heatwave amplification.11

Plain Language Summary12

Seasonally persistent European heatwaves are socioeconomically costly. Though13

established by atmospheric circulation patterns, heatwaves can be intensified by dry land14

surface conditions, increasing societal risks. Here, we show that the manner in which dry15

land surface conditions affect European heatwave intensity is twofold: through local heat-16

ing and through changes to the structure of the atmosphere. The additional influence17

of the land surface on the atmosphere can make the same European heatwave up to 0.4◦C18

hotter overall and can cause surface drying in other regions. This non-local mechanism19

leads to hotter conditions in those regions in subsequent months.20

Corresponding author: Anna L. Merrifield, [email protected]

–1–


Abstract21

Dry land surface conditions have been shown to amplify extreme heat events in Europe22

but the extent to which this influence involves modification of the overlying atmospheric23

circulation has yet to be fully established. Here, this issue is addressed using two Com-24

munity Earth System Model ensembles, with the same heatwave-inducing atmospheric25

circulation pattern imposed over different land surface states. These two ensembles dif-26

fer in the vertical level above which the circulation is constrained (surface vs. upper tro-27

posphere). Soil moisture anomalies are found to play an important role in dictating heat-28

wave intensity among ensemble members. The heatwave is approximately 0.1◦C hotter29

per standard deviation soil moisture reduction when the troposphere is free to respond30

to surface conditions than when it is constrained, implying that a portion of the land31

surface influence involves feedbacks through the atmospheric circulation. The additional32

atmospheric response also allows for non-local heatwave amplification in subsequent months.33

1 Introduction34

In recent decades, European summers have been punctuated by record-breaking35

extreme heat events [e.g. Barriopedro et al., 2011; Fischer , 2014]. Such events are eco-36

nomically costly; they adversely affect carbon uptake by vegetation, strain water resources,37

tax electrical grids, and set conditions that are favorable to wildfires [e.g. Ciais et al.,38

2005; von Buttlar et al., 2018; Perkins, 2015; Miralles et al., 2018]. They can also be deadly39

with tens of thousands of heat-related deaths reported during the European heatwaves40

of 2003 and 2015 and the Russian heatwave of 2010 [Robine et al., 2008; Muthers et al.,41

2017].42

Heatwaves are often caused by persistent, stationary high pressure systems, or block-43

ing highs [e.g. Charney and DeVore, 1979; Pfahl and Wernli , 2012; Hurrell , 2015] which44

are accompanied by clear skies, light winds, subsidence, and warm air advection i.e., pro-45

cesses that contribute to heatwave conditions [Meehl and Tebaldi , 2004; Fischer et al.,46

2007a]. Europe resides within a geographically-preferred blocking region [Barriopedro47

et al., 2006], where blocking highs tend to form in the quiescent region between splits48

in the polar jet [Egger , 1978; Barnes and Screen, 2015], can persist for several weeks [Rex ,49

1950; Brunner et al., 2018], and can be amplified by the quasi-stationary component of50

both free and thermally/orographically forced Rossby waves [Schubert et al., 2011; Petoukhov51

et al., 2013; Coumou et al., 2015]. However, the considerable unpredictable, internal at-52

mospheric variability in the Northern midlatitudes hinders the one-to-three month pre-53

dictability of blocking events and accompanying heatwaves [Wallace et al., 1995, 2015;54

Xie et al., 2015].55

Warning of heatwave risk a month in advance must thus come from fields that evolve56

on longer-than-synoptic timescales, such as sea surface temperature and soil moisture57

[e.g. McKinnon et al., 2016; Schlosser and Milly , 2002; Dirmeyer , 2003]. Soil moisture58

(SM) has been identified as a key contributor to the intensity and duration of European59

heatwaves in both observational [e.g. Hirschi et al., 2011; Quesada et al., 2012; Miralles60

et al., 2014] and model frameworks [e.g Vautard et al., 2007; Lorenz et al., 2010; Jaeger61

and Seneviratne, 2011]. Using a regional climate model, Fischer, Seneviratne, Vidale,62

Luthi, and Schar [2007a] concluded that wetter soils could have mitigated the exceptional63

summer temperatures during the 2003 heatwave by up to 40%; climatological SM would64

have offered up to 2◦C of relief. Primarily, SM influences surface climate through the par-65

titioning of outgoing surface energy between the latent and sensible heat fluxes [Senevi-66

ratne et al., 2010; Berg et al., 2014]. Under typical circumstances, this influence affects67

surface air temperature (SAT) only in not too wet, not too dry “hot spots” of land-atmosphere68

interaction [Koster et al., 2004; Schwingshackl et al., 2017], such as southern central Eu-69

rope [Seneviratne et al., 2006; Diffenbaugh et al., 2007]. During heatwaves, however, as70

–2–


soils desiccate under above average SAT, the land surface feedback can occur on a con-71

tinental scale [Fischer et al., 2007b].72

Though numerous studies have assessed the initiation, evolution and impacts of Eu-73

ropean heatwaves, open questions remain, particularly with regards to how a dry land74

surface, exacerbating a heatwave locally, could modify the overlying atmospheric circu-75

lation pattern and induce a remote temperature response. Analogous to ocean-atmosphere76

teleconnectivity [Gill , 1980; Wallace and Gutzler , 1981], the land surface feedback in one77

region may modify the distribution of heat and moisture on a continental-scale, expand-78

ing a heatwave or drought event [Koster et al., 2016; Miralles et al., 2018]. Fischer, Senevi-79

ratne, Vidale, Luthi, and Schar [2007a] found that dry spring soil conditions ahead of80

the 2003 European heatwave were associated with a thermal low signature at the sur-81

face and enhanced anticyclonic ridging aloft, suggesting that dry soils served to reinforce82

the attendant blocking high pattern.83

Here, we investigate local and non-local land-atmosphere relationships during a Eu-84

ropean heatwave using two initial condition ensembles with imposed atmospheric circu-85

lation. Using these constrained circulation ensembles or CCEs, we assess to what extent86

SM, subsidence, and cloud cover independently affect heatwave intensity in central Eu-87

rope. We also demonstrate that the ability of the land surface to amplify or damp a heat-88

wave goes beyond the direct local influence of surface heat flux partitioning on SAT, and89

depends on the modification of continental-scale atmospheric circulation patterns as ex-90

emplified by the relationship between southeastern central European SM conditions and91

the strength of blocking highs to the northwest.92

2 Experimental Design93

The CCEs are constructed from a Community Earth System Model version 1 (CESM1)94

free running pre-industrial control simulation (CTL) [Hurrell et al., 2013]. Year 200 of95

this simulation was selected as our surrogate European heat wave. This featured a seasonally-96

persistent blocking event that resembles the observed 2003 event [Black et al., 2004; Fer-97

ranti and Viterbo, 2006; Della-Marta et al., 2007]. The CCEs consist of 44 members, ini-98

tialized from random years of CTL, with the atmospheric circulation constrained via a99

nudging technique to follow the seasonal evolution of Year 200. Each member differs in100

the land-surface initial conditions, but experiences near-identical heatwave-inducing at-101

mospheric circulation, which allows us to investigate the physical mechanisms underly-102

ing the land-atmosphere coupling during the event and to assess the role of the tropo-103

spheric circulation in that coupling.104

The timeseries of CTL June-July-August (JJA) averaged central European SAT105

anomalies (◦C) is shown in Figure 1a. Central European or CEU anomalies are area-averaged106

over 41.95◦-53.25◦N, 1.25◦W-23.75◦E. JJA SAT in Year 200 is 2.5◦C above the long-term107

average (Fig.1a,b, red). The 44 years from which the CCEs are initialized (Fig.1a, black108

dots) provide a representative distribution of initial land surface conditions and are also109

used as an accompanying 44-member CTL ensemble (Fig.1b, black). Each CCE mem-110

bers is branched from June 1 of these 44 years and the zonal (u) and meridional (v) winds111

are linearly relaxed or “nudged” towards Year 200 values (the linear interpolation be-112

tween 6 hourly instantaneous values at 00h, 06h, 12h, and 18h).113

In the first CCE, CCEtop (Fig.1b, blue), nudging is applied solely in the upper at-114

mosphere (above ∼322 hPa). Constraining only the upper atmosphere allows us to cre-115

ate an ensemble of spatially near-identical heatwaves, while still retaining the land sur-116

face’s potential ability to modify the atmospheric circulation pattern beneath the con-117

straint. In the second CCE, CCEfull (Fig.1b, orange), nudging is applied throughout the118

atmospheric column (above ∼993 hPa). This allows us to quantify the direct local in-119

fluence of the land surface on SAT, but prevents the subsequent influence of the land sur-120

–3–


face on atmospheric circulation. The distributions of JJA CEU SAT in Fig.1b indicate121

that the SAT range in CCEtop is roughly double that in CCEfull with CEU anomalies122

ranging from 1.3 to 2.9 ◦C in the CCEtop and from 1.7 to 2.5◦C in the CCEfull. The dif-123

ference in SAT spread reflects the differing tropospheric circulation constraints.124

3 Subseasonal Evolution125

Our CCE ensembles provide a range of heatwave intensities for near-identical at-126

mospheric circulation conditions. Figure1(c, d) shows the maximum and minimum heat-127

wave cases, measured by the CEU JJA SAT, in the CCEtop ensemble. In both cases, a128

persistent ridge of high pressure (shown in contours of geopotential height at 500hPa (Z500)129

anomaly in Fig.1c,d ii) is established over central Europe in June, elongates to bring above130

average SAT to the whole of continental Europe in July (Fig.1c,d iii) and culminates cen-131

tered over western Europe in August (Fig.1c,d iv). The maximum and minimum (and132

intermediate; not shown) heatwaves that result from the persistent blocking conditions133

are similar in spatial extent, but differ in intensity. During the maximum CCEtop heat-134

wave (Fig.1c ii-iv), SAT anomalies exceed 5◦C over the majority of central Europe in June135

and over east central Europe in July. In August, SAT anomalies reach a maximum over136

France, exceeding 4.5◦C. In contrast, during the minimum CCEtop heatwave (Fig.1d ii-137

iv), SAT anomalies only reach 2.5-3◦C in the same regions. Consistent with the differ-138

ence in SAT magnitude, there is a larger (smaller) amplitude local Z500 anomaly over139

the maximum (minimum) CCEtop heatwave.140

The differences between the CCEtop “end members” reflect differences in soil mois-141

ture initial conditions. In this study, we represent the state of the land surface with SM142

or soil liquid water (SOILLIQ) summed over the top 8 levels (∼ 1 m) of CESM1’s land143

model, CLM4.5, which approximately corresponds to the root zone [Shukla and Mintz ,144

1982; Hirschi et al., 2014]. The maximum CCEtop heatwave follows a May with nega-145

tive precipitation anomalies, while the minimum CCEtop heatwave follows a May that146

is wetter-than-average east of the CEU (Fig.1c,di). Both members feature maximum May147

SM anomalies in southern central Europe (Fig.1c,div), a region identified as a “hotspot”148

of land-atmosphere interaction, where the absence of SM can shift the partitioning of149

surface heat flux towards sensible heat flux (QH) at the expense of the latent heat flux150

(QE) [Seneviratne et al., 2006; Vidale et al., 2007; Fischer and Schar , 2009]. We rep-151

resent this partitioning by the QH fraction, defined as QH / (QH + QE). Positive (neg-152

ative) anomalies in the QH fraction within the CEU are shown in red (blue) contours153

in Fig.1c,d, panels v-viii.154

Soils dry through the summer beneath the maximum SAT anomalies in both the155

maximum and minimum CCEtop heatwave cases (Fig.1c,dv-viii) with CEU SM anoma-156

lies decreasing from -20.8(-13.6)kg/m2 in June to -44.6(-20.9)kg/m2 in August in the the157

maximum (minimum) CCEtop heatwave. The seasonal desiccation of SM is accompa-158

nied by increases in the QH fraction in non-alpine regions of central Europe, with pos-159

itive anomalies in the east of the domain expanding to include the northwest by August.160

Starting from wetter initial conditions, the magnitude and spatial extent of QH fraction161

anomalies during the minimum CCEtop heatwave lag those in the maximum CCEtop heat-162

wave case by one month. In terms of CEU averages, there are strong (r2 > 0.8) linear163

relationships between SM and QH fraction throughout the summer in both the CCEtop164

and CCEfull (Figure S1), indicating that SM conditions are representative of the influ-165

ence of the land surface, via the surface energy budget, on near surface climate.166

4 Controls on Heatwave Intensity167

While the signal of interest in this study is the land surface influence on a seasonally-168

persistent European heatwave, other factors, such as cloud cover and subsidence, also169

affect heatwave intensity [Fischer et al., 2007a]. To quantify the relative contributions170

–4–


of cloud cover, subsidence, and soil moisture to CCE heatwave intensity, we employ the171

statistical technique of hierarchical partitioning (further description in the supplement)172

[Chevan and Sutherland , 1991]. Cloud cover is represented by the cloud radiative effect173

(CRE; W/m2): the difference between all-sky and clear-sky downward shortwave and174

longwave radiation at the surface [Cheruy et al., 2014]. Subsidence is represented by 700hPa175

vertical (pressure) velocity (ω700; Pa/s) [Trenberth, 1978]. We apply the method to CCE176

SAT using CRE, ω700, and SM as predictors in a hierarchy of multivariate regression mod-177

els. The independent contribution of each predictor to SAT spread is determined at each178

grid point in the CEU and presented in terms of percent variance explained in Figure179

2.180

Along with the percent variance explained by each predictor, the overall magni-181

tude of SAT spread must be taken into account when assessing strength of influence. There-182

fore, the standard deviation of June through August (i-iii) CCEtop and CCEfull SAT are183

shown in Fig.2 a and b respectively. In the CEU, CCEtop SAT spread is ∼1.5-2.5 times184

larger than CCEfull SAT spread in non-alpine regions and ∼2-5 times larger in the vicin-185

ity of CESM1’s representation of the Alps. The two ensembles have strikingly similar186

spatial patterns of SAT spread: ensemble members differ from one another most in the187

eastern portion of the CEU domain in June (Fig.2a,bi) and the western portion of the188

domain in August (Fig.2a,biii). In July (Fig.2a,bii), SAT spread is elevated in both the189

eastern and western portions of the CEU domain in both ensembles. The spatial sim-190

ilarity of SAT spread suggests that the atmospheric response (or lack thereof) to pro-191

cesses that modulate SAT does not alter the locations of influence. Regions of elevated192

SAT spread in the CCEs also correspond to the regions of maximum SAT anomaly the193

following month, suggesting a relationship between modulating processes and heatwave194

intensity.195

The candidate controls exert varying influence on SAT spread, depending on en-196

semble, month, and region. As with ensemble spreads, the CCEtop and CCEfull have sim-197

ilar patterns of predictor influence through the summer. In June (Fig.2c,di,vii), heat-198

wave intensity is related most closely to soil moisture (cloud cover) in the south and east-199

ern (northwestern) CEU, where overall SAT spread is larger (smaller). In July, soil mois-200

ture explains the majority of CEU SAT variance, except over the Alps where ensemble201

spread is limited (Fig.2a,b ii). By August, a soil moisture-only regression model explains202

nearly 80% of the CCEfull SAT variance in CEU (Fig.2dix). Because regions of elevated203

SAT spread in the CCEs coincide with regions where soil moisture makes the largest in-204

dependent contribution to heatwave intensity, we conclude the land surface plays the pri-205

mary role in the amplification or damping of CCE heatwaves. In contrast, for summers206

in the CTL simulation that are not characterized by persistent blocking, cloud cover ex-207

plains the majority of summer SAT variability in the CEU (Figure S2b).208

5 Atmospheric Response to the Land Surface Feedback209

Because soils modify SAT during a heatwave, they may also modify large-scale at-210

mospheric circulation patterns in the free troposphere. A potential local mechanism that211

links dry soil conditions to an enhancement of the attendant blocking ridge is through212

atmospheric column expansion resulting from enhanced diabatic heating in the presence213

of enhanced sensible heat fluxes [Fischer et al., 2007a]. It is also possible, however, for214

soil moisture anomalies in remote regions to influence a heatwave elsewhere by perturb-215

ing the large-scale atmospheric circulation. For example, non-local soil conditions may216

induce thermal low pressure features that advect warm, dry air into regions prone to at-217

mospheric blocking. Drier Mediterranean soil conditions have been shown to steer con-218

tinental weather into central Europe by this mechanism, as the thermal low induced over219

the region enhances easterly atmospheric flow on its northern flank [Vautard et al., 2007;220

Haarsma et al., 2009]. Additionally, a large-scale atmospheric response to regional land221

–5–


forcing has also been shown in CESM1 through the prescription of North American soil222

moisture anomalies [Teng et al., 2019] and afforestation [Lague and Swann, 2016].223

Relationships between soil conditions and atmospheric circulation can be quanti-224

fied in a variety of ways, most simply by quantifying to what extent preseason soil mois-225

ture conditions dictate overall heatwave intensity in the CCEfull where the atmosphere226

is constrained throughout its entire depth and the CCEtop where it is free to respond,227

to the extent that it can, in the presence of nudging above 322 hPa. The relationship228

between initial soil moisture (represented by a normalized May CEU average) and heat-229

wave magnitude (represented by the deviation of JJA CEU SAT from its ensemble mean230

value) indicates that the June through August heatwave is 0.24±0.06 (0.15±0.03) ◦C hot-231

ter per σSM reduction in CCEtop (CCEfull) (Figure 3a). This difference of ∼ 0.1◦C per232

σSM (0.4◦C for the driest initial reflects theconditions) magnitude of the influence of soil233

moisture that depends on the additional atmospheric response that is allowed to occur234

in the CCEtop compared to the CCEfull. In both cases, the linear relationships are sig-235

nificant at 95% by two-tailed student T-test.236

To investigate local and non-local relationships between the land surface and the237

atmosphere, we consider the differences between the 11 ensemble members with the dri-238

est antecedent soil moisture conditions (dry25th) and the 11 ensemble members with the239

wettest antecedent soil moisture conditions (wet75th). With 44 members, the driest and240

wettest 11 members comprise the 25th and 75th quartiles of the ensemble; driest and wettest241

are determined in terms of CEU average values. Quartile differences are shown in Fig.3b-242

d for antecedent soil moisture (i,ii), subsequent Z500 (iii,iv), and subsequent atmospheric243

temperature across a vertical longitude section (v,vi) for the CCEtop and CCEfull respec-244

tively.245

The CCEtop and CCEfull share initial soil moisture conditions, with the dry25th mem-246

ber average ranging from ∼20 kg/m2 drier than the wet75th member average in the north-247

west CEU to ∼40 kg/m2 drier than the wet75th member average in the south and east-248

ern CEU. Due to the atmospheric nudging protocol, which is initiated from June 1, June249

Z500 in the CCEfull differs slightly in concert with the dry25th - wet75th May soil mois-250

ture pattern (Fig.3biv). This difference of 2 m at 500 hPa qualitatively reflects the un-251

certainty associated with prescribed circulation technique. Dry members of the CCEtop,252

however, have June Z500 values that exceed wet member values by, on average, 5 to 9253

m in the CEU (Fig.3biii). This difference in dry25th - wet75th June Z500 cannot be solely254

attributed to surface forcing though, as Z500 differences of up to 6 m occur elsewhere255

in the domain, indicating that internal variability in the atmosphere below the upper tro-256

pospheric constraint in the CCEtop is not negligible.257

The largest difference in dry25th - wet75th June Z500 is not colocated with the SM258

maximum. Instead, it is centered over the northwestern CEU, a region where May soil259

moisture differs the least. The shift suggests the surface influence on the free troposphere260

has a non-local component beneath the CCEtop upper atmospheric circulation constraint.261

The non-local SM-atmosphere relationship in June can be explored further by consid-262

ering the vertical structure of temperature in dry vs. wet members of the CCEtop and263

CCEfull (Fig.3bv-vi). On average, dry25th CCEtop members are between approximately264

0.6 to 1.2◦C warmer than wet75th members from the surface to 700 hPa. Dry25th CCEfull265

members are more similar to wet75th members, with differences ranging from 0.2◦C warmer266

in the western CEU to 0.8◦C warmer in the eastern CEU below 700 hPa. The CCEtop267

and CCEfull share a region of maximum temperature difference: the non-alpine region268

in the eastern CEU above the region of maximum May dry25th - wet75th soil moisture269

difference. This implies there is soil moisture influence that is stronger when the atmo-270

sphere is free to respond, but is reflected in local temperature in either case.271

Additionally, the CCEtop has a second temperature difference maxima, originat-272

ing over the Alps and extending westward with height (Fig.3bv). The CCEfull also fea-273

–6–


tures a westward extension in temperature difference at 800 hPa, where dry25th mem-274

bers are 0.4◦C warmer than wet75th members. Several factors suggest that this westward275

extension represents a non-local pathway through which SM anomalies in the east of the276

CEU propagate their influence westward, as opposed to a secondary local pathway where277

SM anomalies in the west influence the local atmosphere. First, the local SM difference278

in the CCEs is at a CEU minimum over the Alps. Secondly, the dry25th - wet75th tem-279

perature difference extends both higher into the atmosphere (with a difference of 0.4◦C280

at 600 hPa in the CCEtop) and is larger at approximately 800 hPa than at the surface281

in the western CEU. Thirdly, the temperature difference maxima is much less prominent282

in the CCEfull. In combination, these three lines of evidence suggest that dry soil con-283

ditions in southeastern central Europe affect heatwave intensity by modifying the struc-284

ture of the lower atmosphere so as to enhance high pressure centers situated to the north-285

west.286

In July, dry25th - wet75th Z500 and temperature vertical profile differences track287

locally with June soil moisture anomaly differences (Fig.3c,d). June dry25th - wet75th SM288

differences feature similar patterns in the CCEtop and CCEfull, with a magnitude dif-289

ference of 10 kg/m2 in the north and western CEU. In the CCEtop, the maximum dry25th290

- wet75th Z500 differences of 4 m occurs in the eastern non-alpine region of the CEU. In291

the CCEfull, Z500 differences only reach 1 m or less over the whole domain. Maximum292

temperature differences also occur in the eastern non-alpine region of the CEU for both293

the CCEs, though differences are about 0.2◦C larger in the CCEtop than in the CCEfull.294

As in June, temperature differences are larger at height over the longitude domain in the295

CCEtop, both east from the eastern CEU and west from the western CEU. The west-296

ern CEU temperature difference maxima is larger in the CCEtop, consistent with the larger297

soil moisture difference in the region.298

By August, there is little difference between dry25th - wet75th Z500 and temper-299

ature in the CCEfull, while differences in the CCEtop remain similar in magnitude to July300

values (Fig.3d iii,iv). Western CEU soil moisture has continued to dry more in the CCEtop301

than the CCEfull, and this desiccation reinforces the enhancement of August Z500 in drier302

CCEtop members. The temperature difference maximum also occurs, again tilting west-303

ward with height, in the western CEU. Overall, the atmospheric response to land sur-304

face conditions in one region allowed for a heatwave to be amplified and soils to dry in305

another region, creating a local land surface feedback there two months later. Without306

the response of the atmosphere, local, coincident heatwave amplification can still occur,307

but risk of remote, temporally lagged amplification is eliminated.308

6 Conclusion309

Using initial land surface condition ensembles with constrained atmospheric cir-310

culation, we assess the direct local influence of and non-local effects induced by soil mois-311

ture anomalies that precede a seasonally-persistent European heatwave in CESM1. Mem-312

bers of the CCEs considered experience the same heatwave-inducing atmospheric circu-313

lation pattern from June through August, prescribed either through the full atmosphere314

(to isolate direct local influence of the land surface on heatwave intensity) or solely in315

the upper atmosphere (to elucidate the additional influence that arises). It is shown that316

CCE heatwaves following Mays with drier-than-average SM anomalies evolve to be hot-317

ter than CCE heatwaves following wetter years. Linear relationships between SM and318

QH fraction further demonstrate that the canonical land surface feedback operates in cen-319

tral Europe beneath the high pressure centers prescribed in the atmosphere. Constrain-320

ing the full atmosphere reduces the range of possible heatwave intensity (in terms of JJA321

SAT anomalies, averaged over central Europe) by approximately half, with the maximum322

(minimum) heatwaves in the CCEtop being 0.4◦C hotter (cooler) their CCEfull counter-323

parts. The two CCEs have spatially similar patterns of SAT spread, however, suggest-324

ing atmospheric response does not substantially alter regions of influence on SAT.325

–7–


While the CCEs are set up to interrogate the influence of the land surface, other326

factors also affect heatwave intensity. Using a set of hierarchical regression models, we327

establish that differences in soil moisture and cloud cover both contribute to SAT spread328

in the CCEs, with the former (latter) being the primary control in regions where SAT329

spread is largest (smallest). This confirmation that the land surface plays a primary role330

in the amplification or damping of the prescribed heatwave allows us to further inves-331

tigate direct local vs. non local land surface influences. In an aggregate sense, prescribed332

central European heatwaves in CESM1 become 0.15±0.03◦C hotter per standard devi-333

ation of SM reduction as a result of direct land surface influence and 0.24±0.06◦C hot-334

ter as a result of direct land surface influence and subsequent atmospheric response. The335

0.1◦C per σSM difference appears, in part, to stem from a modification of the atmosphere336

to the northwest of the region of maximum soil moisture anomaly difference between wet337

and dry members. In following months, this amplification pathway results in a larger west-338

ern CEU soil moisture difference between wet and dry members in the CCEtop than in339

the CCEfull. Along with this soil moisture difference is an amplification of western CEU340

temperature below 700 hPa in July and enhanced ridging among drier members in the341

prescribed western European high pressure center in August.342

While the CCEs allow us to establish the influence of the land surface on SAT, with343

or without an accompanying atmospheric response, it is important to emphasize that re-344

lationships depend on 1) the CESM1 framework used and 2) the presence of the partic-345

ular high pressure system that set heatwave conditions there in the first place. To be-346

gin to address the question of whether or not the model framework is realistic, we con-347

firm that May CEU soil moisture anomalies used as CCE initial conditions are distributed348

similarly to observational estimates of May CEU soil moisture (Figure S3). Consistent349

with the dry preseason soil moisture-amplified heatwave relationship found in the CCE,350

observed May CEU SM prior to the 2003 European heatwave was 1.2 σ below the av-351

erage. In regards to the dependence on particular heatwave-inducing circulation patterns,352

climate models have been shown to underestimate both the occurrence and persistence353

of European blocking events [Woollings et al., 2018]; events similar to our surrogate heat-354

wave may occur more often in observed climate. Extreme European heat events reported355

in 2006, 2007, 2010, 2015, and 2018 point to a need to understand non-local land-atmosphere356

interactions, as mean warming may allow for these non-local pathways to be triggered,357

regardless of the overlying atmospheric circulation pattern.358

Though our results cannot fully elucidate the nature of the pathway for southeast-359

ern central European soil moisture’s influence to be felt remotely, “phase-locking” re-360

lationships between dry soils, orography, and large-scale atmospheric circulation patterns361

have been shown to occur over North America and the Middle East with stationary wave362

model experiments [Wang et al., 2019; Teng et al., 2019]. We feel it is reasonable to in-363

fer similar relationships may exist over continental Europe under certain circumstances,364

such as during persistent blocking events. Further assessment of the ability of the land365

surface to modify atmospheric circulation over Europe is warranted, particularly because366

“land-atmosphere teleconnectivity” may have implications for seasonal forecasting.367

Acknowledgments394

We would like to thank Ruth Lorenz, Iselin Medhaug, Lukas Brunner, Erich Fischer, and395

Reto Knutti for their helpful comments on this manuscript. A.L.M. was supported by396

National Science Foundation (NSF) Graduate Research Fellowship Program under grant397

DGE-1144086. The National Center for Atmospheric Research is sponsored by NSF.398

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AugustMay June July

d) Minimum CCEtop Heatwave

c) Maximum CCEtop HeatwaveControl Run Year

Heatwave (Year 200)

44 years used for the CTL, CCEtop, and CCEfull Ensembles

a) Box-Averaged JJA SAT, Central Europe (CEU)

SAT

Anom

aly

(˚C

)

b) Ensembles, JJA CEU SAT

CTLCCEtop

CCEfull

v. vi. vii. viii.

i. ii. iii. iv.

i. ii. iii. iv.

v. vi. vii. viii.

AugustMay June July

0-5 5 0-4 4 0-80 80Precipitation Anomaly (mm/day) SAT Anomaly (˚C) SM Anomaly (kg/m2)

[10 : 30 : 100]

Z500 Anomaly (m)

[-100 : 30 : -10] [10%, 30%]

QH Fraction Anomaly

[-30%, -10%]

Figure 1. a) JJA SAT anomalies (◦C), averaged over central Europe, in the CTL (black)

and the Year 200 heatwave (red). The 44 years used to construct the CCEtop and CCEfull are

indicated with black dots. b) Box-and-whisker plot, showing the interquartile range (between the

25th and 75th percentiles; box) and the maximum and minimum values (whisker) for the CTL

(black), CCEtop (blue) and CCEfull (orange) distributions of JJA CEU SAT anomalies. Year 200

is shown in red. c) The maximum CCEtop heatwave. Panel i shows May precipitation anomalies

and panels ii-iv show June-August SAT (color) and Z500 (black contours) anomalies. Panels

v-viii show May-August SM (color), QH fraction (blue and red contours) and the CEU averaging

region (boxes). d) As in c), but for the minimum CCEtop heatwave.

368

369

370

371

372

373

374

375

376

–13–


Percent Variance Explained100%80%60%40%20%0%

June July August

CR

Eω

700

SM

c) Independent Contribution to SAT spread, CCEtop

i. ii. iii.

iv. v. vi.

vii. viii. ix.

a) SAT Ensemble spread, CCEtop

Std.

Dev

. (˚C

) June July August

0

1

0.5

Percent Variance Explained100%80%60%40%20%0%

June July AugustC

RE

ω70

0SM

d) Independent Contribution to SAT spread, CCEfull

i. ii. iii.

iv. v. vi.

vii. viii. ix.

b) SAT Ensemble spread, CCEfull

Std.

Dev

. (˚C

) June July August

0

1

0.5

i. ii. iii. i. ii. iii.

Figure 2. a) Ensemble standard deviation (i-iii) of CCEtop SAT (◦C) in the CEU (box). Dot-

ted contours show orographic features above 600 m at 200 m intervals. b) As in a), but for the

CCEfull c) The independent contribution to CCEtop SAT spread of processes related to heatwave

intensity: the cloud radiative effect (CRE; i-iii), vertical velocity at 700 hPa (ω700; iv-vi) and SM

(vii-ix). d) As in c), but for the CCEfull.

377

378

379

380

381

–14–


July T Profile CCEfullCCEtop

v. vi.

July Z500 CCEfullCCEtop

iii. iv.

June SM

i. ii.

CCEfullCCEtop

Latit

ude

Latit

ude

Pres

sure

(hPa

)

65˚N

55˚N

45˚N

35˚N

65˚N

55˚N

45˚N

35˚N

900800700600500

400

300

Longitude10˚W 20˚E0˚ 10˚E 30˚E


a) May SM, JJA SAT J

JA C

EU S

AT d

evia

tion

from

m

ean

anom

aly

(˚C)

Normalized May CEU Soil Moisture (σSM)La

titud

eLa

titud

ePr

essu

re (h

Pa)

65˚N

55˚N

45˚N

35˚N

65˚N

55˚N

45˚N

35˚N

900800700600500

400

300

i. ii.

iii. iv.

v. vi.

b) Differences, May SM & June Atmosphere



c) Differences, June SM & July Atmosphere

CTL CTLMay SM

CCEfullJune Z500 CCEtop

CCEfullJune T Profile CCEtop

r2 = 0.73

CCEtop-0.24±0.06˚C / σSM

r2 = 0.59

CCEfull-0.15±0.03˚C / σSM

0

40

-40

SM (kg/m

2)

0

8

-8

Z500 (m)

T Profile (˚C)

0

0.8

-0.8

d) Differences, July SM & August Atmosphere

0

40

-40

SM (kg/m

2)

0

8

-8

Z500 (m)

T Profile (˚C)

0

0.8

-0.8v. vi.

August T Profile CCEfullCCEtop

iii. iv.

August Z500 CCEfullCCEtop

i. ii.

July SM CCEfullCCEtop

Latit

ude

Latit

ude

Pres

sure

(hPa

)

65˚N

55˚N

45˚N

35˚N

65˚N

55˚N

45˚N

35˚N

900800700600500

400

300



Figure 3. a) Relationship between normalized May CEU Soil Moisture and JJA CEU SAT,

shown in terms of deviation from mean anomaly (◦C) in the CCEtop (blue) and CCEfull (orange).

R2 values and least-squares regression coefficients are given in the legend in the upper right. b)

Field differences between the average of the ensemble members with the driest (dry25th) and

wettest (wet75th) antecedent soil conditions. (i,ii) dry25th - wet75th May soil moisture from the

CTL ensemble, with the CEU region (box) and latitude chosen for the vertical temperature sec-

tion (47.6◦N; dotted) indicated. (iii,iv) dry25th - wet75th June Z500 in the CCEtop and CCEfull,

respectively. (v,vi) A vertical section of dry25th - wet75th June temperature in the CCEtop and

CCEfull, respectively. A cross section of topography (black) and the longitudinal boundaries of

the CEU (1.25◦W and 23.75◦E; dotted lines) are indicated. c) as in b), but for June CCEtop and

CCEfull soil moisture and July atmospheric fields. d) as in b) and c), but for July CCEtop and

CCEfull soil moisture and August atmospheric fields.

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387

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391

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393

–15–

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