The future perspective of Etesian wind patterns over Aegean Sea

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The future perspective of Etesian wind patterns over
Aegean Sea
Anagnostopoulou C., Zanis P., Katragkou E., Tolika K., Tegoulias I.
Etesian winds blow over the Aegean Sea and eastern Mediterranean during the warm period
(June to September). Etesians are widely known as the most stable localized wind systems in
the world. This research will focus on the study of the Etesian wind persistence during the
21st century using regional climate model simulations (RegCM3). An anticyclonic action
centre over central Europe or over Balkan Peninsula and the south Asian thermal Low proved
to be the synoptic patterns resulting in northerly air flow over Aegean Sea. Using an objective
statistical classification method on wind components, three distinct Etesian patterns are
defined for present and future period. Sea level pressure (SLP), geopotential height at 500hPa,
vertical velocity and wind speed at different vertical levels were used for the synoptic and
dynamic scale analysis for each Etesian pattern. The first results for the future period show a
slight increase of the Etesian wind speed in the Aegean Sea that is in consistent with a slightly
increase of the pressure gradient resulting by a deepening of the low pressure centre and slight
strengthening of the high pressure centre.
Anagnostopoulou C.1*, Zanis P.1, Katragkou E.1, Tolika K. 1, Tegoulias I.1
1 Department of Meteorology and Climatology, School of Geology, Faculty of Sciences, Aristotle University of
Thessaloniki, Thessaloniki, Greece.
*corresponding author e-mail: [email protected]
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1 Introduction
One of the most persistent localized wind systems in the world are Etesian winds over Aegean
Sea. The main flow of Etesians is northeasterly in the northern Aegean, northerly in the
central and southern Aegean, and north-westerly near the south-western Turkish coasts
(Karapiperis 1951, Kotroni et al. 2001). Anagnostopoulou et al. (2014) established three
Etesian Patterns with distinct synoptic and dynamic characteristics. The Middle East low
pressure and the Balkan Peninsula high pressure are the two action centers that control the
wind Etesian system. Also, Anagnostopoulou et al. (2014) showed that the anticyclonic action
center over central Europe or Balkan Peninsula is detached from the Azores high pressure
system (Metaxas 1977, Maheras 1980, Lionello and Sanna 2005). There are also studies
focusing on the role of the Asian Low in the Etesian winds. The linkage between Asian
monsoon and eastern Mediterranean during summer has been investigated by examining the
variations of the Etesians and the subsidence in Eastern Mediterranean (Ziv et al. 2004) and
the vertical distribution of the etesian summer circulation patterns (Tyrlis et al. 2012).
2 Data and Methodology
Different daily datasets from June to September are used in the present study:
a) daily u and v components of ECHAM simulation for the control (1961-1990) and future
(2071-2100) periods and,
b) daily u and v components of ECHAM/RegCM simulation for the control (1961-1990)
and future (2071-2100) periods.
The spatial resolution of the datasets is 25 km x 25 km for RegCM and roughly about 200
km for ECHAM5. Additionally, sea level pressure (SLP), wind speed (WS) and vertical
velocity (Omega) fields at different vertical levels were used for the synoptic and dynamic
scale analysis of the Etesian patterns.
The dominant wind patterns of Etesians were derived using the Two Step Cluster Analysis
(TSCA) statistical method. It was applied on near surface wind data (u and v components) at
three characteristic sub-regions of Aegean Sea (1st sub-region: Northeastern Aegean Sea; 2nd
sub-region: Central Aegean Sea and 3rd sub-region: southeastern Aegean Sea) for the
ECHAM/RegCM datasets for the future time period.
The procedure of TSCA follows two steps (pre-clustering and clustering). In “preclustering” step, a sequential clustering approach is applied to pre-cluster cases into many
sub-clusters. A variable in a certain cluster should be as similar as possible to other variables
in the same cluster. One by one all variables are checked if they should be merged within the
previously formed clusters or a new cluster should be started based on the distance criterion.
In the present study, the Euclidean distance is used. The results of the pre-clustering (1st step)
are used as new cases in the next step, the clustering. The clusters are determined
automatically and Schwarz’z Bayesian criterion (BIC) is used as clustering criterion, by
taking into account the lowest information criterion measure and the highest ratio of distance
measures.
Based on the derived wind clusters from TSCA, the mean and the anomalies of the SLP
and the omega vertical motion were calculated for each cluster individually.
3 Results
Following the methodology of Anagnostopoulou et al (2014), three distinct Etesian Patterns
(EPs) of the wind system blowing over Aegean Sea are detected for the future period 19712100. A detailed analysis based on the mean synoptic and dynamic fields of the future EPs is
presented here.
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The respective days from each pattern were selected and the mean SLP fields were
constructed. Two main action centers control the intensity and the duration of Etesian winds
over Aegean Sea, the Eastern Mediterranean/Middle East (EMME) low pressure and a high
pressure, the exact location of which depends on the Etesian Pattern.
a
b
c
d
e
f
Fig. 1. Mean changes in sea level pressure (hPa) for future period 2071-2100 relative to control period 1961-1990 for
each Etesian pattern based on the regional simulation ECHAM/RegCM (a, c, e) and ECHAM (b,d,f). The contours
indicate the mean sea level pressure for each Etesian Pattern (EP).
The high pressure in EP1 is located mainly over western Europe (Fig. 1a). The EP2
anticyclone shifts over central Europe and central Mediterranean (Fig. 1c), while the EP3 high
pressure is shifted further eastwards over Balkan Peninsula and eastern Europe (Fig. 1e).
Regarding the future changes in the Etesian Patterns, there is consistency between the RCM
and GCM results. The mean sea level pressure is projected to decrease over eastern
Mediterranean until the end of the 21st century for both ECHAM and ECHAM/RegCM
datasets (Fig. 1). The main differences between the three patterns are the positive anomaly
centers. The first EP is characterized by a positive anomaly center over central-eastern Europe
extending also over Balkan in the RCM while this positive center is extends further over
northern Europe in the GCM (Fig.1a and Fig. 1b). The EP2 positive anomaly center is
weakening over the study region in 2071-2100. The corresponding anomaly pattern of EP3
shows a general decrease of SLP all over eastern Europe and eastern Mediterranean. The
deepening of the EMME low presumably has a teleconnection with the weakening of Islandic
Low in the north Atlantic (Fig.1e and Fig.1f).
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20
EP1
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omega(Pa/s)
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omega(Pa/s)
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Fig. 2. (a) Longitudinal distribution of vertical velocity (Pa/s) at 500 hPa averaged over the 38oN-42oN latitudinal zone
for the future period 2071-2100 based on the regional simulation ECHAM/RegCM and (b) the mean changes in vertical
velocity (Pa/s) at 500hPa in future periods 2071-2100 relative to control period 1961-1990 for each Etesian pattern based
on the regional simulation ECHAM.
The variability of omega vertical motion was analysed for the three future EPs compared to
the control period. In Fig.2 the longitudinal distribution of vertical velocity (Pa/s) at 500 hPa
averaged over the 38oN-42oN latitudinal zone is presented for the future period 2071-2100.
Generally, omega velocity variability (Fig. 2a) presents two maxima, the primary maximum
over eastern Europe and Balkan and the second one over Atlantic. The negative vertical
motions between the two maxima indicate that the anticyclonic action center over central
Europe or Balkan peninsula is detached from the Azores high pressure system. It is apparent
from Fig. 2a that future vertical velocity over Balkan decreases for EP1, increases for EP2
and remains stable or slightly decreases for EP3. That is partially in correspondence with
ECHAM results where a tension for weakening of the subsidence over EMME is apparent
(Fig. 2b).
The weakening of the subsidence over eastern Mediterranean in combination with a
deepening of the Asian thermal low pressure centre and a slight strengthening of the Balkan
high pressure system, results in strengthening of the Etesian winds with increasing of the
northerly wind (blue color) or increasing of the southerly wind (red color) for the future
period 2071-2100 (Fig. 3). Stippling denotes areas where the changes are not statistical
significant at 95%.
4 Conclusions
In the present study, an attempt was made to study the future perspectives of Etesian winds
over Aegean. In accordance to the reference period, it is found that a seasonal low-pressure
system over East Mediterranean (the Asia thermal low) and a high-pressure system over the
Balkan Peninsula controls the direction and strength of northerly winds into Aegean Sea.
Furthermore, the detachment of upward motions between the two descending centres proves
that the anticyclonic action center over central Europe or Balkan Peninsula is detached from
the Azores high pressure system (Prezerakos 1984).
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b
c
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Fig. 3. Mean changes in near surface (at 10 m) meridional v-wind (m/s) in future periods 2071-2100: ECHAM/RegCM
(a, c, e) and ECHAM/GCM (b,d,f) relative to control period 1961-1990 for each Etesian pattern.
The relative strengths and positions of the aforementioned systems vary from year to year and
this variation results into three distinct Etesian Patterns over Aegean. It is evident that for the
future period 2071-2100 the strengthening of the Etesian winds in EP1 and EP2 is mainly
associated with a deepening of the low pressure centre and a slight strengthening of the high
pressure centre over eastern Europe and Balkan. Furthermore both RCM and GCM indicate a
weakening of the subsidence over eastern Mediterranean (especially in EP1 and EP2) which
is rather controlled from the deepening of the EMME low pressure action center in line with
the projected in future weakening of South Asian monsoon (Ueda et al. 2006).
Acknowledgments This research has been co-financed by the European Union (European Social fund - ESF) and
Greek national funds through the Operational Program “Education and Lifelong Learning” of the National
Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society
through the European Social Fund. (www.thalis-ccseawavs.web.auth.gr)
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