RESEARCH ARTICLE Twenty‐First‐Century Changes in the Eastern
10.1029/2019JD031203 Mediterranean Etesians and Associated
Special Section: Midlatitude Atmospheric Circulation
Long‐term Changes and
Trends in the Middle and Stella Dafka1 , Andrea Toreti2 , Prodromos Zanis3, Elena Xoplaki1,4 ,
Upper Atmosphere and Juerg Luterbacher1,4
1
Key Points: Climatology, Climate Dynamics and Climate Change, Department of Geography, Justus‐Liebig‐University of Giessen,
Germany, 2• The impacts of large‐scale Joint Research Centre, European Commission, Ispra, Italy, 3Department of Meteorology and Climatology,
atmospheric circulation changes in School of Geology, Aristotle University of Thessaloniki, Thessaloniki, Greece, 4Centre of International Development and
the Eastern Mediterranean winds Environmental Research, Justus Liebig University of Giessen, Germany
over the twenty‐first century is here
assessed
• Projections show a reinforced
westerly flow, a decreased Abstract The Etesians are the dominant synoptically driven winds observed in the Eastern
meridional wave amplitude, and an Mediterranean, usually from late spring to late summer. Due to the complex topography, the Etesians can
intensified‐poleward shifted be very strong and pose significant environmental hazards, especially over wildfire incidents. This study
Subtropical Jetstream
A progressively increase of Etesians assesses the impacts of climate change on future Etesians by analyzing the response of the most recent•
frequency and intensity induced by EURO‐CORDEX regional climate simulations at the 12‐km grid resolution over the twenty‐first century.
large‐scale circulation processes is The mean model ensemble projects a significant increase of the Etesians' frequency and intensity under the
projected for the twenty‐first century
two emission scenarios RCP4.5 and RCP8.5. This response is connected to an increase in the zonal wind at
Supporting Information: 200 hPa, a reinforcement of the midlatitude westerly flow, and a decrease in the wave amplitude. These
• Supporting Information S1 circulation changes accelerate the mid‐to‐high latitude eastward propagation of the large‐scale circulation
systems which can favor enhanced ridges over the Balkans. A strengthening and poleward shift of the
subtropical jet stream is also projected, connected with stronger subsidence over the Eastern Mediterranean.
Correspondence to: The projected changes will have profound environmental and societal implications, including the
S. Dafka,
styliani.dafka@geogr.uni giessen.de lengthening of the wildfire season and increasing air pollution risk in the region. On the other hand, the‐
current estimate of future wind power potential in the Aegean Sea will be significantly increased by the end
of the century, which might have positive impact in the regional economy.
Citation:
Dafka, S., Toreti, A., Zanis, P., Xoplaki, Plain Language Summary The Etesians are one of the most prominent wind systems in the
E., & Luterbacher, J. (2019). Twenty‐
first‐century changes in the Eastern world. They are persistent, northerly, regional‐scale winds that blow every summer over the Aegean Sea
Mediterranean Etesians and associated and the Eastern Mediterranean. They are known since antiquity, with the first reference dating back to the
midlatitude atmospheric circulation. eighth century B.C. Recent studies based on climate model projections have pointed to a decrease in the
Journal of Geophysical Research:
Atmospheres, 124, 12,741 12,754. wind speed in most of Europe for the coming decades, except for the Aegean Sea. Thus, this study assesses–
https://doi.org/10.1029/2019JD031203 the impacts of climate change on Etesians and identifies the associated dynamical mechanisms. The main
findings show an increase in the Etesians' frequency and intensity over the twenty‐first century mainly
Received 19 JUN 2019 attributable to changes in the large‐scale atmospheric circulation. The projected changes will have profound
Accepted 1 OCT 2019
Accepted article online 22 OCT 2019 societal and environmental implications in response to the associated lengthening of the wildfire season
Published online 9 DEC 2019 and the increased risk of air pollution in the region. At the same time, a positive impact on the regional
economy due to the increase in wind power potential is also expected.
1. Introduction
The Etesians are an integral summer feature of the regional climate of the Eastern Mediterranean (EMED)
that is influenced by interactions between midlatitude and tropical processes (Cherchi et al., 2014;
Logothetis et al., 2019; Tyrlis et al., 2013). Tyrlis and Lelieveld (2013) suggested that both the Etesians and
the upper level subsidence over the EMED are manifestations of the Rossby wave structure induced by
©2019. The Authors. the monsoon convection. The recurrent EMED summer circulation and, thus, the Etesians are associated
This is an open access article under the to the stability of the monsoon signal.
terms of the Creative Commons
Attribution License, which permits use, The conditions necessary for the establishment of the Etesians are a stationary high‐pressure system cen-
distribution and reproduction in any
medium, provided the original work is tered over central Europe and the Balkans and a thermal low (Anatolian low) that extends westward from
properly cited. the Arabian Peninsula over the EMED. This dipole induces a synoptic pressure gradient over the Aegean
DAFKA ET AL. 12,741
Journal of Geophysical Research: Atmospheres 10.1029/2019JD031203
Sea that drives surface winds into the Archipelago from the northeast quadrant. Air flowing over the
complex topography is channeled and accelerated through passes and rocky mountains, notably in the
Aegean islands (Kotroni et al., 2001). The Etesians regime displays distinct seasonal variation, blowing
persistently from May through September, and usually peaks during July and August (Dafka et al.,
2016). Episodes are typically associated with strong, gusty winds and very low relative humidity (Tyrlis
& Lelieveld, 2013).
Among the most notorious environmental hazards associated with Etesians are wildfires. The Etesians
induce the greatest fire risk when they coincide with low humidity during the peak period (Amraoui
et al., 2013). The intense winds can rapidly spread the fire rendering the wildfire predictions and firefighting
extremely difficult, which could result in significant loss of life and property (Koletsis et al., 2009; Kotroni
et al., 2001). Another environmental impact concerns the contribution to the transport of pollutants such
as ozone and other anthropogenic aerosols, from central Europe and the Balkans over the EMED
(Georgoulias et al., 2016; Zanis et al., 2014).
Due to the complex topography and the scarcity of the data, only few studies have investigated the projected
changes in regional wind systems over the Mediterranean basin. For instance, Regional Climate Model
(RCM) simulations project a significant decrease in Tramontane frequency over the gulf of Lion
(Obermann‐Hellhund et al., 2017) and in the number of Bora events over the Adriatic region (Belušić
Vozila et al., 2019) during the twenty‐first century. As for the eastern Mediterranean, to date as far as we
know, only Anagnostopoulou et al. (2014) and Tolika et al. (2015) based on the RCM RegCM3 and Ezber
(2018) using a set of EURO‐CORDEX and CMIP5 models assessed future changes of Etesians. They have
found an intensification of the Etesians, which was mainly attributed to the strengthening of the high pres-
sure system over the Balkans and the deepening of the Anatolian low. Nevertheless, both studies did not
investigate the forcing mechanisms underlying the changes. Our approach goes beyond these findings and
provides new evidence of possible changes in Etesians at monthly scale, considering an ensemble of
EURO‐CORDEX twenty‐first century simulations under the two Representative Concentration Pathways
(RCP) 4.5 and 8.5.
Changes in the Etesians depend on several competing factors, including changes in the midlatitude circula-
tion (atmospheric blocking activity, midlatitude westerlies), in the jet streams patterns as well as in the
Indian summer monsoon variability. Therefore, the major challenge in investigating the Etesians under a
changing climate stems from the need: to improve our understanding of the dynamical processes behind
the phenomenon and their representation in the RCMs. Indeed, the RCMs should be able to resolve not only
the large‐scale atmospheric features, such as the development and movement of the large‐scale dipole that
controls the Etesians, but also the local circulation features that are affected strongly by the complex topo-
graphy of the Aegean Sea (Dafka et al., 2017). This contribution goes beyond the state of the art through
studying changes in the large‐scale processes associated with Etesian events and identifying the mechanism
behind these changes.
The paper is organized as follows: sections 2 and 3 deal with the description of the data and the analysis
performed. Section 4 provides the main results in terms of (1) differences between twenty‐first and
twentieth‐century climatologies, (2) analysis of meridional circulation changes in the area, and (3) changes
in the locations and strength of the jet stream patterns. Finally, sections 5 and 6 present the discussions and
the conclusions.
2. Data
Our analysis uses historical simulations and future projections driven by nine General Circulation Models
(GCMs) available from the fifth phase of the Climate Model Intercomparison Project (CMIP5, Taylor
et al., 2012). The selected GCMs provide boundary conditions to four RCMs from the EURO‐CORDEX initia-
tive (Jacob et al., 2014) with a spatial resolution of 0.11° (around 12 km). Table 1 provides an overview of all
GCM‐RCM combinations used in this study. A subset of six models (gray box; Table 1), driven by those
EURO‐CORDEX RCMs (i.e., the RCA4 and HIRHAM5) that have been already proven to capture the
Etesian wind pattern and the associated large‐scale circulation within the observational period 1989 to
2004 (Dafka et al., 2017), is selected and used for the analysis.
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Table 1
List of GCM‐RCM Combinations, Along With the Institution Name and the Corresponding References
Driving GCM RCM Institution Reference
CNRM‐CM5 RCA4 Swedish Meteorological and Samuelsson et al. (2011)
EC‐Earth RCA4 Hydrological Institute (SMHI)
HadGEM2‐ES RCA4
IPSL‐CM5A‐MR RCA4
MPI‐ESM‐LR RCA4
EC‐Earth HIRHAM5 Danish Meteorological Institute (DMI) Christensen et al. (2006)
IPSL‐CM5A‐MR WRF3.3.1 Institut Pierre Simon Laplace/Institut National de l'Environnement Industrie Skamarock et al. (2008) and
CNRM‐CM5 ARPEGE5.2 et des Risques (IPSL/INERIS) Menut et al. (2013)
CNRM‐CM5 ALADIN5.3
Note. The subset of six models that has been used for the mean ensemble analysis is identified by the gray box.
We study future conditions of mean Sea Level Pressure (SLP), surface zonal (u) and meridional (v) wind
components (10 m above surface), geopotential height at 500 hPa (Z500), and zonal and meridional wind
components at 200 hPa (U200 and V200, respectively). Changes are calculated for 2021–2050 (mid‐21c)
and 2071–2100 (late‐21c) relative to 1971–2000 (late‐20c), under the midrange mitigation (RCP4.5) and
high‐end (RCP8.5) emission scenarios (Moss et al., 2010). All variables are taken from May to September,
at 12 UTC (15:00 local time) when the wind is supposed to reach the highest intensity, due to the daytime
deepening of the Anatolian thermal low (Tyrlis & Lelieveld, 2013). In addition, 3‐hourly SLP data for the sta-
tions of Elliniko and Rhodes are taken from the National Hellenic Meteorological Service.
3. Methods
3.1. Classification of Etesians
The literature describes a wide range of approaches to objectively classify the Etesians (Tyrlis & Lelieveld,
2013). Here in order to be consistent with our previous studies (Dafka et al., 2016, 2017, 2018), we define
intense Etesians as days exceeding the third‐quartile (Q3) threshold of 12 UTC observed/simulated SLP dif-
ferences (ΔP) between Elliniko (PE) and Rhodes (PR; ΔP = PE− PR ≥Q3). Likewise, days such thatmedian
≤ ΔP < Q3 are defined as moderate Etesians. Days with negative ΔP indicate southerlies and are excluded
from the analysis. Finally, following Tyrlis and Lelieveld (2013), an Etesian episode is defined as a sequence
of intense Etesian days ≥2. Table 2 presents Etesians' classification as derived from the observed Etesian
days. Note that the selection of the Etesians was done using the corresponding value of Q3 and median of
each period (late‐20c, mid‐21c, and late‐21c). A detailed description of the classification method can be
found in Dafka et al. (2016, 2017, 2018).
3.2. Mean Model Ensemble
TheMean‐Model Ensemble (MME) of six GCM‐RCM combinations (gray box; Table 1) is analyzed regarding
changes of Etesians frequency, wind speed, SLP, Z500 anomalies, and U200 under the two RCPs 4.5 and 8.5
for the mid‐ and late‐21c. The magnitude of the change is calculated as the difference between the reference
30‐year period (1971–2000) and the two future 30‐year periods (2021–2050 and 2071–2100). Model ensemble
allows to sample part of models' structural uncertainty (Knutti et al., 2010, 2017), to evaluate the model
spread and investigate the robustness of signals. In this study, the projected changes are robust and consis-
tent if at least 66% of all simulations agree in the direction of change and at least 66% of the simulations show
significant changes at the 90% level using theWilcoxon–Mann–Whitney test (Pfeifer et al., 2015). Our analy-
sis considers also the results of individual models (apart from the MME). We extend prior findings
(Anagnostopoulou et al., 2014; Ezber, 2018) by considering changes in the waviness of the atmospheric cir-
culation as well as in the strength and locations of the jet streams and their implications on Etesians, as
described in the following sections.
3.3. Z500 Wave Extent
In order to assess whether the wave amplitude (i.e., the meridional extent) of the 500 hPa field of the Etesian
days is projected to change, we use the metric proposed by Barnes (2013). More specifically, a single isopleth
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Table 2
Classification of Observed Etesian Days for the Extended Summer Season May to September in the Period 1971–2000
Percentage of observed Etesian days and number
Condition Name of class of episodes in late‐20c
ΔP < 0 Southerly flow 10.3%
Median ≤ ΔP < Q3 Moderate Etesians 22.0%
ΔP ≥ Q3 Intense Etesians 22.5%
Intense Etesian days ≥ 2 Etesian episodes 253
for each summer month is used and its maximum and minimum latitudes (LatðmaxÞ and LatðminÞ;
respectively) are calculated over the EURO‐CORDEX domain. The meridional wave extent is then defined
as the difference (ΔLat) of the averaged maximum and minimum latitudes, LatðmaxÞ and LatðminÞ;
during the intense Etesian days:
ΔLat ¼ LatðmaxÞ−LatðminÞ
Following the methods of Francis and Vavrus (2012, 2015) and Barnes (2013) single isopleths at 500 hPa
were selected. The MME climatology of the geopotential height at 500 hPa and zonal wind at 200 hPa from
the model simulations are calculated. Then, those Z500 isopleths that exist in the strongest mean U200 sum-
mer climatology and, thus, could be used as a proxy for the trajectory of the upper level jet stream are
selected. Results are presented as the average meridional extent of these isopleths (in our case are the
5650, 5700, 5750 m for each month). This analysis is performed from July to September.
3.4. Meridional Circulation Index
A key question that remains unexplored in the linkage between Etesians and the midlatitude atmospheric
circulation is whether a stronger midlatitude westerly flow can both accelerate the eastward propagation
of synoptic systems and amplify the high‐pressure system over the Balkans. In addition, a stronger subtro-
pical jet stream could be associated with an amplified trough over the EMED that enhances the Etesians.
To address these questions, we calculated the Meridional Circulation Index (MCI) following Francis and
Vavrus (2015). The MCI is a simple metric that estimates departures from the zonal flow of the upper level
winds and it is defined as
¼ V jV jMCI
U2 þ V2
where U and V are the zonal and meridional components of the wind at 200 hPa, respectively. When MCI =
0, the wind is zonal, and when MCI = 1 (−1), the wind is blowing from the south (north). Positive (negative)
differences of |MCI| in the projected future time periods relative to the historical simulations indicate more
(less) undulations, suggesting an increased meridional (zonal) circulation.
3.5. Jet Strength and Position
Our analysis derives daily time series of the strength and the latitudinal/longitudinal position of the Polar
and Subtropical Jet streams (PJ and STJ, respectively). We examine the projected changes under both
RCPs and future periods and we quantify the trends in their position (meridional and zonal location)
and strength.
Both jets are best diagnosed from the wind at higher levels. Therefore, the latitude/longitude and wind speed
of the jet streams are identified in daily U200 over the extended summer period (May to September). In order
to select the preferred position of the jet streams during the Etesians, theMME climatology of the zonal wind
at 200 hPa from the historical simulations is calculated. Results have shown that, during the Etesians and in
all months, the PJ is confined to the area 0°–20°E and latitude from 50°–60°N. The STJ has a preferred posi-
tion, ranging 18°–38°E and 32°–42°N (in agreement with Dafka et al., 2016). Generally, both jets tend to be
weaker in summer as compared to winter. Using the method of Woollings et al. (2010), our analysis consists
of the following steps:
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Figure 1. The 15‐year running mean of intense Etesian days per year in historical runs and (a) RCP4.5 and (b) RCP8.5 runs. Horizontal dotted line shows the mean
of observed Etesian days during the period 1971–2000. The vertical dotted line marks the starting point of climate projections. Gray lines show the MME linear
trend.
1. The daily mean zonal wind at 200 hPa is zonally and meridionally averaged for the identification of the
latitude and longitude, respectively, over the longitudinal sector 0°–20°E (18°–38°E) as a proxy for the
location of the PJ (STJ), neglecting winds poleward of 60° (42°) and equatorward of 50° (32°N);
2. The intense Etesian days are selected;
3. The jet speed is then identified as the maximumwesterly wind speed of the selected days. The jet latitude
and longitude are defined as the latitude/longitude at which the maximum jet speed is found.
4. Results
4.1. Intense Etesian Days
Figure 1 shows the 15‐year runningmean of intense Etesian days per year for each GCM‐RCM and theMME
in historical runs, RCP4.5 (Figure 1a) and RCP8.5 (Figure 1b).
The MME projects a statistically significant (p value ≤0.05) annual increase of about 0.3% (0.15%) in the
number of Etesian days for the twenty‐first century compared to late‐20c under RCP8.5 (RCP4.5). The
increase under RCP4.5 can be seen only until mid‐21c, while the Etesian days remain constant thereafter
(Figure 1a). It is worth to note that RCP4.5 emissions peak around 2040 and diminish afterward. At monthly
scale (where the full period is May–September), the MME shows a robust and statistically significant
increase in the Etesian days in September of about 2 and 4% for mid‐ and late‐21c, in both RCPs. A robust
and statistically significant increase (of about 3%) in the Etesian days is found also for May and June, but
only in late‐21c under RCP8.5. In contrast, a robust and statistically significant reduction of about 5% is iden-
tified in August in late‐21c under RCP8.5. While the frequency of the Etesian episodes is increased (except
for August), there is no consistent evidence for changes in the episodes' duration (i.e., the number of conse-
cutive days) in any of the RCPs or future periods.
Generally, these results show that the contribution of intense events to the summer seasonal amounts will
significantly increase over the century. This phenomenon is more pronounced in May, June, and
September in late‐21c under RCP8.5, suggesting a strengthening of Etesians activity outside of the traditional
peak period (July–August). The projected changes under the RCP8.5 are approximately double than the ones
from the midrange mitigation emission scenario RCP4.5 (Figure 1).
4.2. Mean Model Ensemble
We investigate projected changes of the surface wind and the large‐scale circulation patterns associated with
Etesians. Figure 2 shows the MME future changes in wind speed (Figures 2a–2d), SLP (Figures 2e–2h), Z500
anomalies (Figures 2i–2l), and U200 (Figures 2m–2p) for the intense Etesian days for each month (the peak
months, July and August, present similar characteristics and, thus, are considered together), for late‐21c
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Figure 2. MME future changes in (a–d) wind speed (m/s), (e–h) SLP (hPa), (i–l) Z500 anomalies (gpm), and (m–p) U200 (m/s), for the late‐21c (2071–2100) relative
to the late‐20c (1971–2000) under RCP8.5 for the intense Etesian days per month. Contours indicate the MME climatological values in RCP8.5 for (i–l) Z500 and
(m–p) U200. Stippling indicates robust (66% of the models) and statistically significant changes at 90%.
relative to late‐20c under RCP8.5. Projections indicate a strengthening of the Etesian wind speed over the
Aegean Sea of about 0.5–1 m/s, which is robust and statistically significant in the period June to
September (Figures 2b–2d; see also Figure S1 for a spatial focus over the Aegean Sea). On the contrary, in
July to September (Figures 2c and 2d) near‐surface wind speeds over most of the European areas are
projected to decrease.
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Sea level pressure patterns associated with Etesians for late‐21c compared to late‐20c under RCP8.5 indicate
robust and statistically significant increase of SLP over the British isles, the North Sea, and central Europe in
July–August (Figure 2g) and over western and northern Europe in September (Figure 2h). The SLP is pro-
jected to decrease in the EMED in the period June to September (robust and statistically significant results;
Figures 2f–2h). Indeed, over most of central and western Europe, the subtropical high pressure system is pro-
jected to strengthen by the end of the century by ~1.5–2 hPa, while the MME projects a deepening of the
EMED low‐pressure system by ~1–1.5 hPa, indicating a strengthening of the westward extension of the
Anatolian thermal low. Overall, from July to September the dipole that sustains the Etesians is enhanced,
leading to an intensification of the pressure gradient over the Aegean, and therefore to stronger
surface northerlies.
All GCM‐RCM combinations represent the typical 500‐hPa atmospheric dipole that is associated with
Etesians very well in the late‐twentieth century (not shown). The configuration is characterized by strong
and broad positive 500‐hPa geopotential height anomalies over central Europe and a deepening of the
500‐hPa trough over the EMED (not shown). Late‐21c projections relative to late‐20c show that the anom-
alous ridge weakens and extends southwestward and northeastward, connected to stronger midlatitudes
westerlies at 500‐hPa level (Figures 2j–2l). The central European ridge is shifted northward, associated with
negative anomalies over central and southern Europe. Z500 patterns are consistent with the regional SLP
signal, which shows an intensification over central and western Europe in July–August and September
(Figures 2g, 2h, 2k, and 2l). Except for May (Figure 2i), no statistically significant changes are found in
the 500‐hPa anomalous EMED trough.
In agreement with the lower and midtropospheric changes, large‐scale circulation in the upper troposphere
will also experience significant changes in the future. The MME shows a notable increase in U200 (larger
than 3 m/s) around 60°N and along 35°N, which is robust and statistically significant for the period July
to September (Figures 2o and 2p). In May and June, results show a strengthening of U200 in late‐21c com-
pared to late‐20c especially over the central Europe and the Balkans (Figures 2m and 2n). All variables exhi-
bit the same direction of change, of weakenedmagnitude under the moderate scenario (RCP4.5; not shown).
Six out of nine GCM‐RCM combinations (except for IPSL‐CM5A‐MR‐WRF331, IPSL‐CM5A‐MR‐RCA4, and
CNRM‐CM5‐ARPEGE5.2 from Table 1) agree in the direction and magnitude of change regarding the
intense Etesians wind speed in late‐21c under RCP8.5 (not shown). The projected changes regarding the
wind speed and the SLP (U200) are already apparent in the mid‐21c for both RCPs, while the signal is robust
and statistically significant from June to September (July to September).
To assess theMME projected changes with respect to the pressure gradient over the Aegean Sea, we consider
also the moderate Etesian days. The MME projected changes have the same sign in both future periods and
RCPs (for RCP8.5; see Figure S2) and demonstrate enhanced magnitude for each variable under moderate
Etesians compared to the intense Etesians. The latter strengthens confidence that the projected changes
are robust and independent of the pressure gradient.
4.3. Z500 Wave Extent
We extend our analysis by presenting atmospheric waviness as a function of 500‐hPa wave extent to identify
potential changes in the midlatitude meridional circulation across the EURO‐CORDEX domain. Table 3
shows the projected changes in ΔLat in late‐21c relative to late‐20c for each ensemble member and the
MME in July, August, and September.
All GCM‐RCM combinations (except for the EC‐Earth‐HIRHAM5) show a decrease in the meridional wave
amplitude in both RCPs. The MME suggests a decrease in late‐21c (mid‐21c; not shown) of about 1.3–1.7
under the RCP8.5 and 0.7–1.1 under the RCP4.5, compared to late‐20c, which potentially indicates a ten-
dency of less undulations (waviness) in the future. This phenomenon can promote the eastward progression
of the disturbances reaching central Europe and the Balkans, which is also coherent with the intensification
of the SLP dipole seen at the surface (Figures 2g and 2h).
Figure 3 shows the time series of the meridional wave extent (ΔLat) of Z500. The MME shows a downward
trend of ΔLat in both future periods and RCPs; however, statistically significant results are obtained only in
mid‐21c under RCP4.5 (0.6° decade−1; Figure 3a) and in late‐21c under RCP8.5 (0.5° decade−1; Figure 3b).
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Table 3
Future Changes in ΔLat (°) in Late‐21c Relative to Late‐20c for Each Ensemble Member and MME Under RCP4.5
and RCP8.5 for July, August, and September
RCP4.5 RCP8.5
ΔLat (°) July August September July August September
CNRM‐CM5‐RCA4 −4.5 −1.2 −3.1 −1.3 −1.4 −1.7
EC‐Earth‐RCA4 −0.5 −0.6 1.4 −2.7 −1.7 −2.5
HadGEM2‐ES‐RCA4 −0.6 −1.7 −1.7 −2.2 −2.5 −5.1
IPSL‐CM5A‐MR‐RCA4 −0.1 −0.1 −1.5 −1.0 −0.3 −3.1
MPI‐ESM‐LR‐RCA4 −1.3 −2.1 −1.6 −1.6 −3.3 −2.9
EC‐Earth‐HIRHAM5 0.6 1.1 2.5 −0.3 1.5 5.5
MME −1.1 −0.8 −0.7 −1.5 −1.3 −1.7
4.4. Meridional Circulation Index
Here we analyze theMCI index in order to obtain more information on the spatial variation of waviness over
the European sector during the intense Etesian days (Figure 4).
The MME projected changes of |MCI| at 200 hPa for the late‐21c in both RCPs show predominantly decrease
over the EMED (robust and statistically significant in the period June to September; Figures 4b–4d and 4f–
4h). These areas are closely matched by the spatial pattern of U200, as seen in Figures 2n–2p, indicating
increased zonal circulation and suggesting, thus, an intensified STJ which might be one factor driving more
subsidence and northerly flow over the Aegean Sea. The identified areas are also broadly consistent with the
surface pressure signals presented in Figures 2f–2h. The EMED sector exhibits a reinforced low‐pressure sys-
tem. In addition, in July–August the Euro‐Atlantic sector exhibits negative |MCI| values which correspond
to a reinforced westerly flow in late‐21c under RCP8.5 (Figure 4g).
Overall, the zonal flow at 200 hPa shows enhanced westerly jet over the EMED near 35°–40°N in June to
September, stretching across most of the Mediterranean Sea in July and August. The response at approxi-
mately 50°N shows similarities in the peak period with either a slightly reinforced westerly flow, potentially
associated with decreased waviness. Finally, similar |MCI| changes are projected during the mid‐21c,
although the signal is not statistically significant.
4.5. Jet Strength and Position
In order to better understand the changes identified in the large‐scale circulation, the Z500 wave amplitude,
and the MCI, the link between the intense Etesians and the jet streams' strength and position is analyzed.
Figure 3. Interannual variability of ΔLat (°) in (a) mid‐21c under RCP4.5 and in (b) late‐21c under RCP8.5 for each ensemble member. Gray lines show the MME
linear trend.
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Figure 4. MME projected changes of |MCI| at 200 hPa for late‐21c relative to late‐20c under (a–d) RCP4.5 and (e–h) RCP8.5 for the intense Etesian days per month.
Stippling indicates robust and significant changes at 90%.
The jet stream response under RCP4.5 and RCP8.5 is examined in each GCM‐RCM combination by looking
at speed, latitude, and longitude and the MME annual trends. In the RCP8.5, all models project an average
increase in the STJ strength in late‐21c (mid‐21c) of about 1.4 m/s (0.3 m/s) compared to late‐20c in July to
September. The MME shows a positive trend in the STJ strength under RCP8.5, which is found robust and
statistically significant only in the mid‐21c (0.6 m/s per decade; Figure 5a). Results under RCP4.5 suggest a
strengthening in the STJ wind speed of up to 0.9 m/s in late‐21c and in July to September (not shown).
August and September are the months with the highest strengthening under both RCPs and future periods.
As for the PJ strength, the MME projects an average increase in the jet wind speed by the end of the century
(mid‐21c) of 3.5 m/s (1.4 m/s) compared to late‐20c. The IPSL‐CM5A‐MR‐RCA4 shows the highest increase
in the PJ wind speed. In addition, theMME suggests a robust and statistically significant positive trend in the
PJ strength of 0.8 m/s per decade for late‐21c under RCP8.5 (Figure 5b). The MME under RCP4.5 shows also
an increase in the PJ wind speed of approximately 1.5 m/s in both future periods; however, no significant
trends are found under this scenario (not shown). The greatest increase in the PJ wind speed is found in
June (June and September) under RCP8.5 (RCP4.5) for both future periods.
The STJ during the extended summer period of late‐20c displays an average meridional (zonal) location at
37°N (31°E) and shifts northward (eastward) during mid and late‐21c at 38°N (32.5°E). The mean position
of the PJ (54.5°N and 9.3°E) shows no or little variation in the future compared to the recent past.
Figure 6 shows the projected changes in meridional location of STJ in mid‐21c and late‐21c relative to
late‐20c under RCP4.5 and RCP8.5 for the MME and each ensemble member per month.
Most of the models suggest a poleward shift (~0.3°) of the STJ under both RCPs and future periods compared
to late‐20c. This can be seen, with some exceptions (especially in EC‐Earth‐RCA4 and IPSL‐CM5A‐MR
RCA4) in all months (Figures 6a–6d); however, it is more pronounced in May and September. Regarding
the longitude, the MME shows an eastward shift of the STJ at a rate of ~0.8° (more pronounced in
September; not shown). The MME shows a statistically significant negative trend of of about 0.4° STJ‐Lon
in late‐21c under RCP8.5.
The projected changes in the PJ latitude and longitude show large variation among the models and months
(not shown). Most models show an eastward (westward) displacement in August (May and September)
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Figure 5. Interannual variability of mean wind speed (m/s) of (a) STJ in mid‐21c and (b) PJ in late‐21c under RCP8.5 for each ensemble member. Gray lines show
the linear trend.
Figure 6. Projected changes in meridional location of STJ Lat (°) for theMME and each ensemblemember in (a and c) mid‐21c and (b and d) late‐21c relative to the
late‐20c under (top panel) RCP4.5 and (bottom panel) RCP8.5 for each ensemble member and month.
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under RCP8.5 (both RCPs) and both future periods. There is an agreement on the southward (poleward) PJ
shift in May (June–September) in late‐21c under RCP8.5. In addition, the MME shows a statistically signifi-
cant upward trend of PJ‐Lon in late‐21c under both RCPs. A negative trend in the PJ‐Lat in late‐21c under
both RCPs is also indicative, though not significant.
Overall, the MME shows statistically significant intensification and an eastward shift of the PJ under both
RCPs and future periods. In addition, the STJ is projected to increase especially in July to September and
moves northward in both RCPs and future periods. The dominant signal implies acceleration in eastward
wave propagation and increased subsidence over the EMED.
5. Discussion
Driven by strong emission‐related forcing, the simulated surface wind and the large‐scale atmospheric cir-
culation associated with Etesians features major changes by the end of the century.
Our analysis shows that most GCMs‐RCMs agree on the progressive increase of Etesians days. This increase,
which is clearly evident in the twenty‐first century under RCP8.5, is particularly noticeable in September
and less pronounced in May–June. In contrast, a considerable decrease in Etesians frequency is found in
August. The MME projects an increase in the Etesians wind speed in the period June to September in both
future periods under RCP8.5. Extreme wind events are expected to become more (less) frequent and more
intense in May, June, and September (July–August) by the end of the century. Changes are associated with
an amplification of the large‐scale SLP dipole that controls the pressure gradient over the Aegean Sea and,
thus, the Etesians. We expect a large‐scale decrease of SLP over the EMED, associated with a deepening
of the Anatolian thermal low (in agreement with Anagnostopoulou et al., 2014 and Ezber, 2018). Sandeep
and Ajayamohan (2015) investigated the SLP linear trend during the period 2006–2099 under RCP8.5 using
a MME of 23 CMIP5 simulations and identified also a significant decrease over the EMED. In addition,
Hochman et al. (2017) analyzed an ensemble of eight CMIP5 models and found that in summer the occur-
rence of the Persian Trough (Anatolian low) over the EMED is expected to be lengthened by 49% at the end
of the century under RCP8.5.
Changes seen at the surface wind field are inherited from the large‐scale circulation processes. We find that
the midlatitude sinuosity (seen as reduced north‐south deviations of the geopotential height isopleths at 500
hPa) associated with Etesians is projected to decrease in response to climate change according to the EURO‐
CORDEX simulations. The EC‐Earth‐HIRHAM deviates from the MME; however, its driving GCM exhibits
biases in the representation of the mean state of the geopotential height at 500 hPa over the Euro‐Atlantic
region (Hartung et al., 2017), and thus, this model should be treated with caution. Di Capua and Coumou
(2016) found significant downward trends in meandering over the Eurasian sector during summer, which
is consistent with the recently reported decrease in summer synoptic activity (Coumou et al., 2015). In addi-
tion, many previous studies based on CMIP5 model runs have found a robust decrease in wave extent
(Barnes & Polvani, 2015; Cattiaux et al., 2016) and a decrease in European blocking frequency by the end
of the twenty‐first century (Barnes & Polvani, 2015; Masato et al., 2013). We extend previous analyses, by
assessing the implications of changes of the midlatitude circulation to a specific regional wind system, that
is, the Etesians.
Atmospheric circulation changes in the mid‐troposphere are already noticeable in late‐20c and were mainly
attributed to anthropogenic warming (Christidis & Stott, 2015). As for the future, climate models predict a
robust poleward shift of the jet streams during the warm season in response to anthropogenic forcing
(Barnes & Polvani, 2015; Peings et al., 2017; Woollings & Blackburn, 2012). Nevertheless, there is a lack of
consistency among the CMIP5 models regarding the zonal wind responses at 200 hPa (Barnes & Polvani,
2015; Yim et al., 2016). We find the strengthening of both jet streams and the northward displacement of
the STJ in both RCPs and future periods, when the Etesians are blowing. It is noteworthy to mention that
both jet streams are sufficiently captured in the EURO‐CORDEX simulations within the historical period
1989–2004 (Dafka et al., 2017). This increases the confidence in the projected signal. Time series of jet loca-
tions from individual models (not shown here) show that at least 66% of models used for the study reproduce
STJ's poleward shift in the period July to September. The projected zonal mean changes seem favorable for
an intensification of the Etesians. Our findings are consistent with Coumou et al. (2014) who showed that
the July–August thermal gradients are generally projected to increase northward of 50°N leading to
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strengthening of the polar jet. In addition, Coumou et al. (2018) reported that during summer the subpolar
westerlies around 70°N along with the distinct subtropical jet stream form a double‐jet regime, which pro-
motes the development of resonant flow regimes. While recently, Mann et al. (2017), using CMIP5 simula-
tions have shown that the double‐jet patterns have been turning upmore often in the summermonths due to
high‐latitude land warming. Barnes and Polvani (2015) have found a northward displacement of the PJ over
the North America/North Atlantic sector in summer. In contrast, our results regarding the PJ shift in lati-
tude and longitude vary across models and months, and the signal is not clear, at least during the
Etesians. We cannot confirm that the jets will form a double‐jet regimemore often in the upcoming decades.
Nevertheless, the concurrent variation between the STJ and PJ is proven to act as an important signal asso-
ciated with increased subsidence and extreme wind events over the Aegean Sea during the boreal summer.
Overall, the Etesian response to climate change is associated to stronger zonal flow, reduced waviness, and
northward displacement of the STJ. This mechanism is consistent with the upper tropospheric warming in
the tropics that induces both a poleward shift and a strengthening of the westerly flow (Cattiaux et al., 2016;
Peings et al., 2017), and with the projected decrease in European blocking frequency seen in the twenty‐first
century based on CMIP5 model runs (Barnes & Polvani, 2015; Masato et al., 2013; Matsueda & Endo, 2017;
Woollings et al., 2018). The reduced blocking activity in summer facilitates the easterly propagation of the
synoptic activity and, therefore, favors the formation of the Etesians (Tyrlis et al., 2015).
Atmospheric circulation changes both in the middle to upper troposphere vary insignificantly (to some
extent) from one ensemble member to the other, highlighting low uncertainties in projected changes in
the midlatitude circulation.
Large‐scale circulation changes pose also a significant challenge for wind energy production. Previous stu-
dies have shown a robust signal for increase wind power generation potential over the EMED in summer,
and in particular over the Aegean Sea associated with the Etesian winds (Bloom et al., 2008; Hueging
et al., 2013; Koletsis et al., 2016; Moemken et al., 2018; Tobin et al., 2014, 2016). Indeed, the Aegean Sea
enjoys a remarkable wind resource, while the average wind speeds (at wind turbines hub height of 80 m)
often exceed the 11 m/s, during the intense Etesians (Dafka et al., 2018). Future anthropogenic emissions
will change the strength of near‐surface winds, at desirable wind turbine sites. It is, therefore, very important
to take into account the potential impacts of climate change in the preconstruction wind energy assessment
over the Aegean Sea.
6. Conclusions
In this study, we used an ensemble of six GCM‐RCMmodel chains from EURO‐CORDEX to estimate future
changes of Etesians for the mid‐ and late‐21c under RCP4.5 and RCP8.5. More specifically, we analyze the
ensemble mean response regarding changes in Etesians' frequency, intensity, and the associated large‐scale
atmospheric circulation. We also examine processes that take place throughout the troposphere (from the
low to middle and upper levels), providing thus, more robust results that go beyond the state of the art.
In June to September all models projected increase in Etesians' wind speed ranging from 0.5 to 1m/s over the
same coastal region in late‐21c under RCP8.5 (evident in June–August for RCP4.5). Consistent with previous
studies, we found that the SLP dipole that controls the Etesians strengthens with increasing forcing in both
RCPs (more pronounced in RCP8.5). Climate projections suggest shifts in mean conditions and also changes
in climate variability, since the Etesians are forecasting to become more intense (in June to September) and
frequent (in May, June, and September).
Under the high‐emission scenario and by the end of the twenty‐first century, the July to September zonal
wind at 200 hPa is generally projected to increase northward of 55°N and southward of 45°N, leading poten-
tially to strengthening of the PJ and STJ. Most of themodels project a robust reinforcement of the jet streams,
decrease in waviness, and a poleward shift of the STJ.
Overall, our study illustrates that during the Etesians, the European sector exhibits a reinforced westerly
flow and a decreased meridional wave amplitude (midlatitude waviness), which might accelerate the east-
ward progression of the large‐scale circulation systems, favoring the establishment of enhanced ridges over
the Balkans. In addition, the EMED sector exhibits a stronger 200‐hPa zonal flow, and an intensified STJ
which shifts poleward by the end of the twenty‐first century in both RCPs. Most changes are to a lesser
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Journal of Geophysical Research: Atmospheres 10.1029/2019JD031203
extent apparent already in mid‐21c under both scenarios, with the exception of changes in the meridional
circulation index. Finally, all projected changes are more pronounced in September and in late‐21c
under RCP8.5.
Our results have important societal implications associated with prevention, risk reduction of wildfires, and
air pollution. The analysis suggests that the Etesians may significantly increase during summer fire season,
and more specifically, the episodes are likely to be more frequent but not longer‐lasting than at present. In
terms of fire weather threats in the future, this study suggests that the Etesians increases' during critical dry
periods, especially late in the extended summer season (September), that could potentially lead to more
extensive wildfires in the future. Finally, our results show that climate change will greatly impact the poten-
tial for wind energy deployment, which is projected to be significantly increased by the end of the century.
Therefore, we suggest policy makers and wind energy companies to take into consideration future climate
changes which may alter the pattern and strength of near‐surface wind at desirable locations for wind farms
over the Aegean.
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