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Environment International
journal homepage: www.elsevier.com/locate/envint
Changing nature of Saharan dust deposition in the Carpathian Basin (Central
Europe): 40 years of identified North African dust events (1979–2018)
György Varga
Research Centre for Astronomy and Earth Sciences, Budapest H-1112, Hungary
ARTICLE INFO
Keywords:
Saharan dust events
Carpathian Basin
Dust deposition
Grain size
ABSTRACT
Several billion tonnes of mineral dust is emitted, and transported through winds every year from arid-semiarid
areas. North African dust hot spots located in the Sahara are responsible for 50–70% of the global mineral dust
budget. Dust-loaded air-masses originated from these sources can be transported over long distances and can also
affect remote areas, such as North and South Americas, Europe, and the Middle East.
In this study, we analysed 218 identified Saharan dust events (SDEs) in the Carpathian Basin (Central Europe)
during 1979 to 2018. Systematic identification of SDEs and analyses of dust emission, dust source area activity,
dust transporting wind systems, and transport routes revealed that different synoptic meteorological patterns are
responsible for SDEs, and these are occurring mostly in spring and summer. The characteristic synoptic me-
teorological background of episodes was also identified, and three major types of atmospheric pressure-system
patterns were distinguished.
In recent years, several intense wintertime dust deposition events have been recorded in Central Europe. All
of the identified unusual episodes were characterised by severe washout of mineral dust material and were
related to very similar synoptic meteorological situations. Enhanced southward propagation of a high-latitude
upper-level atmospheric trough to north-western Africa and orographic blocking of Atlas Mountains played an
essential role in the formation of severe dust storms, whereas the long-range transport was associated with the
northward branch of the meandering jet. The occurrence and southerly penetration of high-latitude upper-level
atmospheric trough to low-latitudes and the increased meridionality of the dominant flow patterns may be
associated with enhanced warming of the Arctic, leading to more meandering jet streams.
Particles size of sampled dust material of some intense deposition episodes were very coarse with a con-
siderable volumetric proportion of > 20 µm particles.
1. Introduction
Atmospheric mineral dust is a vital component of the Earth’s cli-
matic system. Similar to other aerosol constituents, dust particles are
regarded as ones of the less known drivers of recent climatic changes.
According to the reports of the Intergovernmental Panel on Climate
Change (IPCC), because of their observational uncertainties, aerosols
hamper attribution of changes of the climatic system (Boucher et al.,
2013; Regayre et al., 2018; Penner, 2019). Dust particles with diverse
sizes, shapes, mineralogies, degrees of aggregation, and optical prop-
erties play an intricate role in the energy balance of the Earth. Dust
particles can absorb, scatter, and reflect the incoming shortwave and
outgoing longwave radiation, perturbing the radiation budget of the
Earth-atmosphere system (Sokolik and Toon, 1996; Pérez et al., 2006a;
Huang et al., 2014; Nabat et al., 2015; Ginoux, 2017; Gkikas et al.,
2018, 2019). Dust particles serve as cloud condensation nuclei and ice
nuclei, affecting the formation, structure, and properties of clouds and
precipitation, indirectly altering the net radiation (Twomey, 1974;
Albrecht, 1989; Yu et al., 2006). Dust particles deposited on snow and
ice-covered surfaces can darken the surface, affect the surface energy
balance, and accelerate snow/ice melting (Drake, 1981; Reynolds et al.,
2014). Dust deposition also brings essential nutrients, which affect the
terrestrial and aquatic biogeochemical cycles and hence the climate
(Meskhidze et al., 2005; Fan et al., 2006; Schulz et al., 2012).
Several billion tonnes of mineral dust is emitted every year from
arid-semiarid areas and are transported up to several thousands of
kilometres by winds. North African dust hot spots located in the Sahara
and Sahel contribute to 50–70% of the global mineral dust budget
(Ginoux et al., 2001). Dust-loaded air-masses originated from these
sources also affect remote areas; vast amounts of mineral particles are
transported to North and South Americas across the Atlantic Ocean,
toward the Middle East, and in the direction of Europe. The PM2.5 and
https://doi.org/10.1016/j.envint.2020.105712
Received 31 December 2019; Received in revised form 19 March 2020; Accepted 31 March 2020
E-mail address: varga.gyorgy@csfk.mta.hu.
Environment International 139 (2020) 105712
0160-4120/ © 2020 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T
PM10 levels of Southern European countries (e.g., Spain, Italy, and
Greece) often exceed the European Union standards during intense
Saharan dust events (SDEs) (Rodríguez et al., 2001; Gerasopoulos et al.,
2006; Matassoni et al., 2011; Pey et al., 2013; Pandolfi et al., 2014;
Querol et al., 2009). According to Querol et al. (2019), increases in
particulate matter concentration were also associated with mixing layer
height reductions during SDEs. In addition to other health effects (e.g.,
respiratory problems and skin and eye irritation), even mortality and
morbidity increases were reported during SDEs by several authors (e.g.,
Perez et al., 2008;Tobías et al., 2011). Other environmental effects of
atmospheric and deposited Saharan dust are also widely investigated in
the Mediterranean area; cloud formation (Di Biagio et al., 2009; Klein
et al., 2010; Bangert et al., 2012; Smoydzin et al., 2012), acid rain
(Rodá et al., 1993;Rogora et al., 2004), lake salinity (Psenner, 1999),
and even soil-forming processes are considerably modified by the mi-
neral particles transported from North African deserts (Yaalon and
Ganor, 1973;MacLeod, 1980;Atalay, 1997;Yaalon, 1997;Durn et al.,
1999;Muhs et al., 2010).
Measurement campaigns of Saharan dust in the Mediterranean have
been provided both by passive satellite-borne sensors (Moulin et al.,
1998;Husar et al., 1997; Herman et al., 1997; Dulac et al., 1992), by
active spaceborne retrievals (Amiridis et al., 2013; Gkikas et al., 2016),
and by lidar measurements (Mona et al., 2006; Papayannis et al., 2008).
Daily satellite aerosol measurements have shown that Saharan dust is
an important constituent of the Mediterranean atmosphere, where
North African dust loads were identified in 23.5% of long-term daily
observations in the Western basin (Alboran and Balearic Seas), in
29.5% in the Central Mediterranean (Tyrrhenian and Ionian Seas and
Sea of Sicily), and 33.75% in the Eastern Mediterranean by Varga et al.
(2014a). Moreover, British Isles (Burt, 1991; Vieno et al., 2016), Ger-
many (Klein et al., 2010), and Scandinavian areas (Franzén et al., 1994,
Barkan and Alpert, 2010) are also affected by the mineral dust intru-
sions from North Africa. Furthermore, European countries in Central
Europe are also affected by Saharan dust (Borbély-Kiss et al., 2004,
Koltay et al., 2006, Szoboszlai et al., 2009;Varga et al., 2013, 2014b,
2016). Mineral dust deposition in the high mountainous areas of the
Alps and Carpathians could lead to accelerated snow melting as a result
of snow albedo modification of dust material and as a consequence of
snow algae formation (Di Mauro et al., 2015, 2019; Greilinger et al.,
2018). The role of past Saharan dust deposition in soil formation has
already been studied in the Central European loess-paleosol sequences
by Varga et al. (2016). Because of the missing long-term dust deposition
measurements in the region, the pedogenic importance of the Saharan
dust flux cannot be quantified. In general, Central Europe lies in the
D1b zone (meaning dust ‘admixture zone’) of the ‘Saharan dust-fall
map’ proposed by Stuut et al. (2009), implying that recent Saharan dust
material can be incorporated into the soil system and may increase its
fine silt content.
Previous long-term investigations revealed a clear seasonal pattern
of SDEs affecting Central Europe based on 130 identified episodes
(Varga et al., 2013). This analysis of daily aerosol index data from 1979
to 2011 indicated the spring- and summertime maxima of mineral dust
episodes in the broader area of the Carpathian Basin (Varga et al.,
2013). Systematic analyses of dust emission, dust source area activity,
dust transporting wind systems, and transport routes revealed that
different synoptic meteorological patterns are responsible for SDEs. The
characteristic synoptic meteorological background of SDEs was also
identified, and three major types of atmospheric pressure-system pat-
terns were distinguished.
In recent years, several intense wintertime dust deposition events
have been recorded in the investigation area (Varga et al., 2014b). In
this study, our objectives are to (1) expand and complete the previously
established simple SDE time-series (by Varga et al., 2013) for the
40 years from 1979 to 2018; (2) explain the major causes and drivers of
the recent extreme and unusual dust deposition episodes; (3) discuss the
possible role of recent climatic changes in the changing intensity of
wintertime dust deposition in Central Europe; (4) provide information
regarding dust deposition; and (5) discuss unresolved problems and
possible connections of grain size and dust deposition.
2. Methods
2.1. Study area
The Carpathian Basin (CB: 45°–48.5° N, 16°–23° E) is located in
Central Europe and its subsiding depression is framed by the Alps, the
Fig. 1. Location map of the study area (CB: Carpathian Basin).
G. Varga Environment International 139 (2020) 105712
2
Carpathians, and the Dinaric mountain ranges (Fig. 1). The geomor-
phological character of the basin is dominated by plains and gently hilly
regions with mountain ranges generally below 1000 m a.s.l.; low relief
and poor vertical dissection describe the topography. The 1,500-km
long ranges of Carpathians (adjoining to the Alps in the west and to the
Dinaric in the south) represent the high mountainous frame for the
basin. The general climatology of the study area is described by
Atlantic, Mediterranean, and continental effects. According to en-
sembles of regional climate models (considering the intermediate A1B
emission scenario), hot and dry summers, moist winters, and increased
water transport from lower latitudes are expected in the Carpathian
Basin; the dominance of the Mediterranean climate regime influence
will increase (Kis et al., 2017).
2.2. Satellite measurements
For appropriate monitoring of recent Saharan dust events, aerosol
products of several satellite campaigns were used. The long-term daily
aerosol measurements of NASA’s Total Ozone Mapping Spectrometer
(TOMS Nimbus-7 TOMSN7L3 v008; TOMS Earth-Probe TOMSEPL3)
and Ozone Monitoring Instrument (OMI – Daily Level 3 Gridded
Products; OMTO3d – source: NASA Goddard Earth Sciences Data
Information Services Center (GES DISC) via Giovanni online (Web)
environment for the display and analysis of geophysical parameters
https://giovanni.gsfc.nasa.gov/) were applied. The TOMS Aerosol
Index (AI) and OMI’s TOMS-like AI measure the relative amount of
aerosols based on the differences in the measured backscattered ultra-
violet radiation of the atmosphere (containing aerosols) and a calcu-
lated pure molecular atmosphere. Its positive values indicate absorbing
aerosols (dust, smoke from biomass burning, and volcanic aerosols). As
a result of the geographical location of the investigation area
(Carpathian Basin is situated far from active volcanoes, and in common
agricultural practices in the European countries, biomass burning is not
applied), positive values mostly indicate mineral dust. TOMS and OMI
TOMS-like AI measurement series have been used by several previous
studies to identify dust events and source areas (e.g. Prospero et al.,
2002;Washington et al., 2003; Gao and Washington, 2009; Gkikas
et al., 2009, 2013, 2016; Varga, 2012; Varga et al., 2013, 2014a).
The daily AI values of the investigation area (45°–48.5°N, 16°–23°E)
were standardised following the work of Barkan (2005); AI
st
=(AI-
AI
mean
)/σ
AI
, where AI
st
is the daily standardised AI value, AI
mean
is the
yearly regional mean AI, and σ
AI
is the standard deviation. A negative
value of AI
st
indicates below-average values, whereas positive values
represent possible dusty episodes. The periods of 2001–2004 and
2010–2011 could not be analysed in the same way because of cali-
bration problems; for these intervals, the initial selection of possible
SDEs was performed directly from daily graphical AI maps. The frac-
tional data of AI of 1993 and 1996 (caused by satellite failure) were
used in our work, but the years with missing data (1994 and 1995) had
to be excluded from the long-term analyses.
As the satellite-based dust detection over land surfaces, especially in
mid- and high-latitude regions, is rather complicated due to local
aerosol emissions and cloud cover, additional confirmation is needed to
identify SDEs. Possible SDE-days were initially selected based on the
AI
st
values, but SDE-days were only accepted after being confirmed by
backward-trajectory calculations, where the Saharan surface origin had
to be established both from the path of the trajectories and from the
vertical profiles of air-mass transport. Multiple endpoints from different
heights (1500, 3000, and 4500 m a.s.l.) were used during the 72–120 h
backward-trajectory analyses, performed by NOAA HYSPLIT (HYbrid
Single-Particle Lagrangian Integrated Trajectory) model to determine
the main dust transportation pathways (Stein et al., 2015; Rolph et al.,
2017). The meteorological input for the trajectory model was also ob-
tained from the NCEP/NCAR (National Centers for Environmental
Protection/National Center for Atmospheric Research) Reanalysis Pro-
ject dataset (Kalnay et al., 1996).
Atmospheric presence of mineral dust was also verified by true colour
satellite images of NOAA Advanced Very High Resolution Radiometer
(AVHRR), ESA Meteosat Spinning Enhanced Visible and Infrared Imager
(SEVIRI), or Terra/Aqua Moderate Resolution Imaging
Spectroradiometer and Aerosol Optical Depth data (NASA MODIS Aqua/
Terra), from 2000 dust models (BSC-DREAM8, NMMB/BSC-Dust-model,
and SKIRON; results of numerical simulations were not applied by Varga
et al., 2013), and surface observations at the proposed source areas
(visibility-reducing surface weather reports of Naval Research Labora-
tory: https://www.nrlmry.navy.mil/aerosol/#aerosolobservations) were
also applied in the verification procedure. Verification of episodes from
the ’80 s and ’90 s was based on much fewer available data sources, but
AI
st
and back-trajectories were used as the basis of identification.
Direct values of AI and AI
st
are also on aerosol optical thickness,
single scattering albedo, aerosol layer height, and underlying surface
albedo. Thus, these indices can be used only as qualitative descriptors
of mineral dust and not as direct, quantitative ones. AI
st
was used solely
for identification of dust and not for assessment of concentration or
amount. In this sense, difficult compliance of numerical values of the
different satellites and sensors (discussed in detail by Gassó and Torres,
2019) did not cause problems. Consecutive SDE-days (with the verified
atmospheric presence of mineral dust) were registered as individual
episodes.
2.3. Numerical simulations
In contrast to the dust load, there is much less information regarding
dust deposition. Although dust has long been observed in the
Mediterranean and Europe, it remains uncertain about how much dust
is transported to the region and how much and where the dust is de-
posited. There are very few direct measurements of Saharan dust de-
position in Europe. Therefore, estimates of dust flux were performed via
model calculations using the data of BSC-DREAM8b (Barcelona
Supercomputing Center’s Dust REgional Atmospheric Model) v1.0 and
v2.0 and Non-hydrostatic Multiscale Model NMMB/BSC-dust models
and a mineral dust model database. Simulation results of the BSC-
DREAM8b v1.0 are available from 1 January 2000 to 31 December
2012, whereas the results of the updated v2.0 calculations are ready for
the period between 1 January 2006 and 31 December 2014. The BSC-
DREAM8b models predict the atmospheric residence of the eroded fine-
grained aeolian material by solving Euler-type partial differential non-
linear equations. The meteorological fields are initialised every 24 h,
while the boundary conditions are updated every 6 h (Pérez et al.,
2006a, 2006b; Basart et al., 2012). The available numerical time series
is short; however, it has to be considered because modelled daily values
of BSC-dust models have already been proven to represent the atmo-
spheric dust load well (Haustein et al., 2009, 2012; Pérez et al., 2011;
Di Tomaso et al., 2017) and the simulated values are the only useable
quantitative data sources of daily dust deposition in the area (Varga
et al., 2016).
NASA’s Modern-Era Retrospective analysis for Research and
Applications, Version 2 (MERRA-2) provides modelled monthly data
from 1980 (Gelaro et al., 2017). Summarised dry and wet deposition
data of the five available dust size-bins of the model were obtained from
NASA Goddard Earth Sciences Data Information Services Center (GES
DISC) via Giovanni application for visualisation and access Earth sci-
ence remote sensing data (https://giovanni.gsfc.nasa.gov/giovanni/).
2.4. Synoptic background
To define the synoptic meteorological patterns associated with dust
intrusion episodes in the investigation area, mean geopotential height
(700 hPa), wind vector, meridional, and zonal flow maps as well as a
250-hPa jet stream wind flow map were compiled for the dusty days
using the Daily Mean Composite application of NOAA Earth System
Research Laboratory (http://www.esrl.noaa.gov/psd/). According to
G. Varga Environment International 139 (2020) 105712
3
previous studies, the 700 hPa level represents the typical dust transport
altitude (Alpert et al., 2004; Barkan, 2005; Varga et al., 2013, 2014a).
Days with meridional wind components exceeding 10 and 15 m/s at
700 hPa were investigated for every decade and season to uncover
possible strengthening of the atmospheric flow meridionality in Central
Europe and in the Central Mediterranean. Thresholds were defined
based on the compiled meridional flow maps of individual SDEs, where
10 and 15 m/s isotachs indicated the strongest flow and main pathways
of dust transport. Surface air temperature anomalies (based on
1981–2010 climatology) were also calculated using the gridded NCEP/
NCAR (National Centers for Environmental Protection/National Center
for Atmospheric Research) Reanalysis Project dataset (Kalnay et al.,
1996) for Central Europe.
2.5. Grain size of deposited dust material
Grain sizes of the collected mineral dust samples of six intense dust
episodes were measured by applying a Malvern Morphologi G3-ID au-
tomated static image analyser and a laser diffraction method of Malvern
Mastersizer 3000 with a Hydro-Lv unit. For appropriate characterisa-
tion, ~0.5 g of dust material was required for laser diffraction. At the
same time, the image analyser directly measured the size and shape
parameters of individual particles. It also provided robust statistical
data based on a few hundred thousand mineral grains (less than 0.1 g);
a detailed description of the granulometric procedure can be found in
Varga et al. (2018). In most cases, it was not possible to collect 0.5 g of
Saharan dust material, even after very intense washout episodes.
3. Results
3.1. General patterns of SDEs in the Carpathian Basin
As a result of the systematic analysis of satellite measurements
completed with the above-detailed verification process, 218 SDEs were
identified between 1979 and 2018. The previous study of Varga et al.
(2013) was completed with 88 new dust episodes. SDEs of these
40 years (years 1994 and 1995 missing due to satellite failures, while
1993 and 1996 are only represented as fractional years) showed diverse
interannual distribution with a substantial increase in the last decade.
The seasonality of the episodes is dominated by spring and summer dust
events coinciding with the dust activity of Saharan source areas. Ac-
cording to Israelevich et al. (2002), there is an almost permanent re-
servoir of atmospheric dust during summer and spring over the main
Saharan source regions. Dust transport from North Africa is primarily
governed by the synoptic meteorological situations of the broader re-
gion determining the upper-level wind flow patterns. Similar seasonal
distribution of the dust storms was also reported for the Western and
Central Mediterranean by Gkikas et al. (2009, 2013, 2016), Querol
et al. (2009), Pey et al. (2013), and Varga et al. (2014a).
SDEs were classified into three main synoptic meteorological types
based on the daily 700 hPa geopotential height, wind maps, and dust
transport pathways of SDEs. The different types were defined by spe-
cific deterministic atmospheric patterns: Type-1 SDEs were connected
to deep atmospheric troughs over Western Europe and north-western
Africa; dust transport of Type-2 episodes was caused by Central
Mediterranean cyclones; Type-3 events were defined based on the rare
dust transport, when dust-loaded air-masses approached the Carpathian
Basin from the north-western directions (Figs. 2-3.).
Type-1 episodes have been determined by the south-western flow
created by a deep trough (emanated from south-western Europe to the
Atlantic coast of N-Africa) and the subdivided subtropical high-pressure
belt over the Saharan territories.
Zonal and meridional wind components at 700 hPa define the main
dust transport patterns with a relatively long west-east directional first
phase dominated by zonal winds and northward transport over the
Western Mediterranean basins toward Central Europe by a more
enhanced meridional flow contribution (Fig. 3).
More than two-thirds (67.4%, n = 147; see Fig. 4. and Table 1.) of
the identified SDEs were classified as Type-1 episodes. In the previous
study by Varga et al. (2013) 66.2% of the total SDEs belonged to this
synoptic group. Because of the large number of Type-1 events, the
annual number of these episodes varied similarly to the total number of
SDEs. The seasonal distribution of events of this synoptic group also
showed the general spring-summer main phase. Dust material was
transported during Type-1 events over the Western Mediterranean ba-
sins from north-western African source areas.
Well-developed Mediterranean cyclones have been the main drivers
of Type-2 events. Almost a quarter (24.8%, n = 54) of the SDEs were
grouped into this type (nearly identical to the reported seasonal dis-
tributions of 25.4% by Varga et al. (2013)). The southerly flow asso-
ciated with the warm winds on the foreside of the eastward-moving
low-pressure systems is responsible for dust transport. Wind flow maps
(Fig. 3) also show the dominance of meridional flows over zonal ones.
The main transport routes of Type-2 episodes were concentrated on the
Central Mediterranean Basins. Occasionally, dust material could also be
transported from the Levant by south-easterly flows at 700 hPa. The
main period of Type-2 events was in spring with a secondary maximum
in winter (particularly in February).
Seventeen type-3 episodes (7.8% of total) were registered during the
40 year period. Varga et al. (2013) reported an 8.5% proportion of
Type-3 of SDEs between 1979 and 2011. Anticyclonic flows of a high-
pressure centre over north-western Africa and south-western Europe
carry dust material from western parts of the Sahara northward along
the coastline or over the eastern Atlantic to the higher latitudes, where
the dominant westerlies define the eastward movement of the dust-
laden air-masses toward Central Europe.
Similar general types of synoptic situations of the Mediterranean
were reported by previous studies using objective classification
methods (s‐mode factor analysis, k‐means cluster analysis, and em-
pirical orthogonal function analysis). Western Mediterranean analyses
of Salvador et al. (2014) and Schepanski et al. (2016) presented at-
mospheric patterns favouring dust transport to south-western Europe;
among them, several clusters were similar to Type-1 SDEs. Gkikas et al.
(2015) investigated the broader Mediterranean Basin, among the in-
troduced six atmospheric circulation patterns and their seasonal dis-
tributions, which can be matched with Type-1 and Type-2 SDEs of this
study. Type-3 SDEs were not discussed by these studies; however, these
rare SDEs have not been affected by the Mediterranean Basin.
In our previous study (Varga et al., 2016), data of BSC-DREAM8b
(Barcelona Supercomputing Center’s Dust REgional Atmospheric
Model) v1.0 and v2.0 dust models and the mineral dust model database
was used to assess the deposited Saharan dust. These modelled values
corrected by a few direct surface observations of published European
measurement campaigns indicated an average annual dust deposition of
3.2–5.4 g m
−2
in the Carpathian Basin (Varga et al., 2016). MERRA-2
modelled data of the present study showed a slightly lower annual
mineral dust deposition in Central Europe (1.2–3.55 g m
−2
), dominated
by wet deposition (77–93% mass of the total). The temporal distribu-
tion of Saharan dust deposition coincides with the seasonal pattern of
Type-2 events (Mediterranean cyclones bring precipitation to Central
Europe) by a dominant spring maximum with relatively high values in
autumn and in February (especially from 2003, see Fig. 4).
Interannual changes of the number of SDEs and deposited dust
material in the Carpathian Basin are obvious; however, no direct re-
lationship among these frequencies and other climatic parameters
(annual mean temperature, precipitation, and North Atlantic
Oscillation phases) could be revealed. There is no general trend in the
number of identified events; however, an increase in the annual oc-
currences in the last decade is rather apparent. In addition, the seasonal
distribution of deposition changed in the last quarter of the studied
period, and almost 25% of the Saharan dust deposition occurred during
winter (Table 1).
G. Varga Environment International 139 (2020) 105712
4
3.2. Intense dust deposition events
Several considerably intense Saharan dust depositional events
(SDDEs) were recorded after 2014. These were identified based on re-
ported surface observations of mineral dust washout situations from the
study area. An exceptionally large amount of mineral dust material was
washed out during these episodes, which also attracted the attention of
local society and (social) media. All of these SDDEs occurred between
the end of October and February; before 2009, these episodes could be
regarded as unseasonal episodes (Fig. 5).
3.2.1. SDDE #1: 19–20 February 2014
An intense unseasonal washout of Saharan dust occurred on 19–20
February 2014. A cut-off low (a small, closed depression) was left by an
upper-level atmospheric trough, associated with a remarkable southerly
meander of the jet stream leading to rain, snow, and dust storms in
western Africa and Morocco on 16 February. Simultaneously with the
north-eastward movement of the low-pressure system, its frontal winds
injected a large amount of dust from the Algerian and Tunisian source
areas into the atmosphere. An extensive high-pressure centre over Libya
and Egypt blocked the eastward spread of the depression, and conse-
quently, the dust‐laden air mass was forced to follow a southwest-
northeast flow and reached Central Europe on 19 February.
3.2.2. SDDE #2: 30 November − 1 December 2014
Synoptic backgrounds of the episode of November 2014 were also
dominated by a cut-off low stemming from an atmospheric trough
centred over south-western Europe and north-western Africa. A large
Fig. 2. Synoptic meteorological background (mean geopotential height map and wind vectors at 700 hPa) of different types of Saharan dust events.
Fig. 3. Wind flow patterns (mean meridional (a) and zonal (b) wind components at 700 hPa) and specific dust transport routes at 3000 m a.s.l. by different Saharan
dust event types.
G. Varga Environment International 139 (2020) 105712
5
amount of Saharan mineral dust material was transported north-east-
ward by the prevailing south-western flow on the warm sector of the
cyclone. Warm advection at 1000–2500 m a.s.l. associated with this
dust event (and its convergence with the Siberian air-masses) caused
freezing rain and sleet at high-altitude areas of Hungarian central re-
gions and led to severe forest damage and destruction of power trans-
mission lines.
3.2.3. SDDE #3: 22 February 2016
An intense dust storm caused severely reduced visibility conditions
and a remarkable dust washout episode in the Iberian Peninsula on 21
February 2016 (Gama et al., 2019). The dust event was generated again
by an atmospheric cut-off low separated from a deepened upper-level
trough generated by the dust event. A large amount of the mineral dust
was transported by the low-pressure system northward from salt lakes
of the high plateau region situated between the Tell Atlas and Saharan
Atlas Range, leading to an exceptionally intense wet deposition event
on 23 February in Budapest, Hungary, where the deposited reddish-
yellow dust material had blanketed parked cars and other exposed
obstacles.
3.2.4. SDDE #4: 29 February 2016
Just a week later, on 29 February 2016, another unusually intense
dust deposition event was observed in Budapest and was widespread in
Hungary. A deep cyclone centred above the western basin of the
Mediterranean Sea transported the yellowish dust material from
Algerian, Tunisian, and Libyan source areas. The main dust transport
route was over the Central Mediterranean.
3.2.5. SDDE #5: January 7–9, 2018
In the first days of 2018, an extensive cyclone developed from an
upper-level trough over the Iberian Peninsula. The penetration of cold
northerly air-mass caused intense snow in Algeria and resulted in a
deep snow-cover in the region of Ain Sefra. The 700-hPa wind speed
exceeded 25–30 m/s, and its dynamical forcing led to strong surface
winds at the lee side of the Atlas Mountains, causing intense injection of
mineral dust into the atmosphere. Warm sector winds of the cyclone
transported the dust-laden air-masses towards Central Europe, where
intense wet deposition was observed on January 9.
3.2.6. SDDE #6: 7–8 February 2018
A month later, on 5 February another cyclone was developed over
the Iberian Peninsula from a trough emanating from north-western
Europe. It was the second snow event within a month in Ain Sefra. Such
an event has never been observed before. The strong cyclonal flow of
the low-pressure system caused widespread dust storms at the foreside
of the Atlas Mountains and around the Chott regions of Algeria and
Tunisia. Dust material was transported northward over the Ionian and
Fig. 4. Annual and monthly frequencies and deposition rates of Saharan dust events.
Table 1
Decadal changes of Saharan dust episodes and dust deposition.
Decade SDEs Deposition [g/m
2
]
Events Type-1 Type-2 Type-3 DJF MAM JJA SON Annual DJF MAM JJA SON
1979–1988 48 32(66.7%) 14(29.2%) 2(4.2%) 6(12.5%) 21(43.8%) 15(31.3%) 15(12.5%) 2.1 0.3(15.8%) 1.0(47.1%) 0.3(16.6%) 0.4(20.6%)
1989–1998 27 16(59.3%) 7(25.9%) 4(14.8%) 3(11.1%) 17(63.0%) 4(14.8%) 3(11.1%) 1.9 0.2(8.3%) 0.8(43.8%) 0.4(18.5%) 0.6(29.4%)
1999–2008 53 35(66.0%) 14(26.4%) 4(7.6%) 4(8.2%) 20(40.8%) 17(34.7%) 8(16.3%) 2.6 0.4(15.9%) 1.2(44.9%) 0.5(18.7%) 0.5(20.5%)
2009–2018 90 64(71.1%) 19(21.1%) 7(7.8%) 16(18.8%) 26(30.6%) 30(35.3%) 13(15.3%) 2.1 0.5(24.9%) 0.8(40.0%) 0.3(15.0%) 0.4(20.0%)
Full period 218 147(67.4%) 54(24.8%) 17(7.8%) 29(13.9%) 84(40.2%) 66(31.6%) 30(14.4%) 2.2 0.3(16.2%) 1.0(44.1%) 0.4(17.3%) 0.5(22.4%)
G. Varga Environment International 139 (2020) 105712
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Fig. 5. General properties of the discussed intense Saharan dust depositional events (geopotential height and wind vectors at 700 hPa; modelled dust loading
(NNMB/BSC); modelled wet deposition (NNMB/BSC); jet patterns (wind vectors at 200 hPa)).
G. Varga Environment International 139 (2020) 105712
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Adriatic Seas to Central Europa and the Balkan Peninsula. The frontal
system of the Mediterranean cyclone led to washout out of mineral
particles on 7–8 February in Hungary.
3.2.7. SDDE #7: 28–29 October 2018
In the last days of October 2018, an extensive deep trough emanated
from north-north-western Europe, resulting a very intense dust emis-
sion from the dust source areas located at the lee side of the Atlas
Mountains. On October 28, the centre of the developed cyclone was
located over the Gulf of Lion and the steep pressure gradient between
the low-pressure system and the high-pressure area of Eastern
Mediterranean led to a spectacular dust event over the central region of
the Mediterranean, intensively affecting also Central Europe.
3.3. Meridional wind patterns
Seasonal intensity changes of meridional wind components were
studied for the four investigated decades. The number and relative
proportion of days when the speed of meridional wind components
exceeded 10 and 15 m/s at 700 hPa showed dominance of fall in both
cases, but more clearly for higher wind speeds (Fig. 6). Summers are
represented by a relatively low number of such days, but springs and
winters provide suitable conditions for gusty southerly atmospheric
flow patterns. Decadal variability of days with enhanced (> 15 m/s)
meridionality was rather significant, partly because of the low number
of these days; however, the increased relative proportion in the winters
of 2009–2018 (48.5% of the total decadal episodes) is remarkable.
3.4. Grain size of deposited dust material
The deposited mineral dust samples of the above presented SDDEs
were characterised by particle size and shape analysis techniques using
automated static image analysis and proved the presence of a larger
volumetric proportion of medium (6.25–20.00 μm) and coarse silt-sized
(20.00–62.50 μm) particles.
Occasionally, fine particles stacked on each other to form larger
aggregates. This phenomenon was observed during the measurements
of the samples of SDDE #1 (19–20 February 2014), when the laser
diffraction measurements resulted a modal grain size of particle size
distribution at 6.3 µm; however, images analysis results showed the
clear presence of a larger quantity of coarse silt-sized aggregates, in-
dicating that these Saharan minerals were not transported as individual
fine-medium silt-sized particles. Large aggregates were accidentally
dispersed during laser diffraction measurement, leading to the under-
estimated grain sizes. The dispersed grain sizes cannot be re-
presentative of the strength of the transport agent (e.g., wind speed);
therefore, appropriate, non-destructive sizing technique should be
chosen.
In other cases, a higher individual grain per aggregate ratio could be
identified with a large proportion of single mineral grains (quartz,
feldspar, calcium-carbonate, and dolomite) with a particle diameter
over 30 µm (e.g., during SDDEs #2 and #3; 22 and 26 February 2016),
and even larger ratios were observed during SDDE #6 on 7–8 February
2018. At this moment, because of the scarce homogeneous grain size
data and general lack of grain shape data of Saharan dust source areas,
source appointment cannot be performed solely based on granulometric
data.
The published characteristic particle sizes of Saharan dust materials
deposited in Europe are in the range of 2 to 30 µm. Particle size data
(various single statistical descriptors were used) measured by different
analytical techniques were taken from the compilation of Goudie and
Middleton (2006): Crete: 8–30 µm (modal; Mattson and Nihlén, 1996),
4–16 µm (median); Spain: 4–30 µm (mean; Sala et al., 1996); Germany:
2.2–16 µm (median); Italy: 16.8 µm (modal), 14.6 µm (median; Ozer
et al., 1998); South France: 4–12.7 µm (median; Bücher and Lucas,
1984), 8–11 µm (median; Coudé-Gaussen, 1991); France (Paris Basin):
8 µm (Coudé-Gaussen et al., 1988); Swiss Alps: 4.5 ± 1.5 µm (median;
Wagenbach and Geis, 1989); and Central Mediterranean: 2–8 µm
(modal; Tomadin et al., 1984). The identified Saharan quartz particles
from the geological samples collected from Fuerteventura, Canary Is-
lands (located significantly close to the African continent in the Atlantic
Ocean, 100 km west of Morocco), showed ~ 70 µm modal and 68.5 µm
median values (Varga and Roettig, 2018; Roettig et al., 2018).
3.5. Surface air temperature anomalies during the identified dust events
Enhanced southerly airflow patterns of SDEs also cause warm air
advection in the mid-latitudes. Intrusions of dust-loaded North African
air-masses into the Carpathian Basin caused an average warming of
3.5 °C compared to long-term (1981–2010) means. The mean decadal
warm advection values of SDE-days showed an increasing trend from
2.3 °C in 1979–1988 to 4.0 °C in 2009–2018 (Table 2).
Zonal mean surface air temperature anomalies of peak SDE-days
were calculated to show the seasonal and decadal meridional transect
anomalies from 0° to 90°N latitudes (Fig. 7). The decadal temperature
anomaly values clearly indicate the enhanced warming of high latitudes
of the Northern hemisphere, whereas seasonal values show increased
warming during winter and fall. Surface air temperature anomalies of
2009–2018 winter SDEs can be characterised by extremely high values
of high latitudes of the Northern hemisphere. The enhanced warming of
Fig. 6. Seasonal distribution of days with strong (> 10 m/s and > 15 m/s) meridional winds by decades.
Table 2
Seasonal surface air temperature anomalies of the SDE-days by decades.
Decades Winter Spring Summer Fall Mean
1979–1988 3.7 °C 1.7 °C 2.6 °C 2.6 °C 2.3 °C
1989–1998 4.4 °C 2.8 °C 3.7 °C 4.4 °C 3.3 °C
1999–2008 3.7 °C 2.9 °C 4.6 °C 5.6 °C 3.9 °C
2009–2018 4.3 °C 3.0 °C 4.3 °C 4.9 °C 4.0 °C
Full period 4.4 °C 2.6 °C 4.0 °C 4.4 °C 3.5 °C
G. Varga Environment International 139 (2020) 105712
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high-latitude regions and the reduced temperature differences of the
Arctic and lower latitude areas have an impact on other atmospheric
processes. The meridional temperature gradient determines the
strength of the jet stream partly and thus high-altitude flow patterns
(e.g., the dominance of zonal or meridional winds).
4. Discussion
4.1. Wintertime dust events
All of the identified unusual dust events characterised by severe
washout of mineral dust material in the Carpathian Basin were related
to very similar synoptic meteorological situations. The first phase of the
dust storm development was an enhanced southward propagation of a
high-latitude upper-level atmospheric trough. The orographic blocking
of Atlas Mountains played a vital role in the formation of sever surface
wind storms and dust entrainment. According to Bou Karam et al.
(2009), the low-level winds and dynamics are associated with the pe-
netration of the atmospheric trough. The northward dust transport
across the Mediterranean towards Central and south-eastern Europe
was also related to the main flow patterns and the eastward-moving
low-pressure system.
The occurrence and southerly penetration of high-latitude high-
level atmospheric trough to low-latitudes and the increased meridion-
ality of dominant flow patterns have been associated with the re-
markable southerly meander of the jet stream, often associated with
Arctic amplification (AA). AA is the enhanced warming of high-latitude
regions compared to mid- and low-latitudes. AA along with its alter-
native metric, the difference in the 1000–500 hPa thickness change in
the Arctic relative to that in mid-latitudes, is leading to more (less)
meridional (zonal) flow at high altitudes and increasing planetary wave
amplitudes (Francis and Vavrus, 2015).
Francis et al. (2018) identified the importance of more meandering
polar jets in the Saharan cyclone formation and poleward mineral dust
transport from North Africa. According to their findings, in winter, the
dust storm formation at the lee side of Atlas Mountains is generated by
high-latitude upper-level troughs and associated low-level dynamics, as
well as meridional temperature gradient-driven development of Sa-
haran cyclones (Francis et al., 2018). Therefore, the Saharan cyclone
formation and poleward dust transport from Africa are affected by the
increasing amplitude of planetary waves.
General airflow patterns of severe Saharan dust depositional events
of the last decade in the Carpathian Basin, as well as increased fre-
quency of gusty meridional flows and enhanced warming of high lati-
tudes of Northern hemisphere, coincide. Thus, here, we confirm the
findings of Francis et al. (2018), that the increasing occurrence of
extreme Saharan dust events in Central Europe has been associated with
enhanced warming of the Arctic, thereby leading to more meandering
jet streams.
4.2. Grain size uncertainties-driven deposition data issues
The general increasing trend of numbers of SDEs and the increasing
wintertime amount of deposited dust material should not necessarily
correspond to the increasing intensity of dust events, and even the
opposite can be concluded from the raw data. Intensification, however,
was indicated by direct surface observations of intense dust washout
episodes when the deposited reddish-yellow dust material had blan-
keted parking cars, roof-windows, and other exposed obstacles.
Numerical simulations of dust deposition also revealed a wintertime
increase of the number and magnitude of dust episodes. Nevertheless,
the extent of it was not equivalent to the growth of the SDE numbers. A
few previous studies confirmed the significant underestimation of de-
posited mineral dust material by numerical simulations. Quantitative
values of valuable but scarce surface measurements and satellite-based
assessments have been several orders of magnitudes larger than the
modelling results, but the spatial and temporal (e.g., interannual, sea-
sonal) patterns of dust deposition have been properly simulated by the
dust models (Gallisai et al., 2014; Varga et al., 2016; Yu et al., 2019).
According to our suggestions, grain size should be the key to resolve
this contradiction. Several recent papers reported the measurements of
giant mineral particles found in far-travelled mineral dust (Betzer et al.,
1998; Maring et al., 2003; Renard et al., 2018; van der Does et al.,
2018). Our automated static image analyses of Saharan dust material in
the Carpathian Basin also indicated that the grain size of transported
Saharan dust material could be significantly larger than the grain sizes
predicted by the model (Varga et al., 2016). Mineral grains > 20 µm are
not parametrised in the numerical simulations. Mineral grains of >
20 µm are usually not accounted for in the numerical simulations (in
the majority of global and regional dust models, only some size-bins of
a relatively narrow size range are applied; Benedetti et al., 2014)
(Fig. 8.). Because of the cubic relationship between particle diameter
and volume (mass), even a small change in applied size-bins can lead to
a significant increase of modelled dust fluxes. A deeper understanding
of grain size-related uncertainties would also be useful to explore the
relationship between uncertain granulometric parametrisation in re-
trieval algorithms of satellite observations and mineral dust flux esti-
mations.
In addition, the physical background of long-range transport of such
a giant mineral material is a matter of debate in the scientific literature
(Betzer et al., 1988; Ryder et al., 2013; Weinzierl et al., 2017; Marenco
et al., 2018; van der Does et al., 2018). This lack of understanding of
Fig. 7. Meridional transect (zonal mean) air temperature anomalies during the identified dust events by decade and by season.
G. Varga Environment International 139 (2020) 105712
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driving mechanisms makes the evaluation of radiative forcing of at-
mospheric mineral dust difficult because the net climate effect of dust
(scattering and absorption) depends on grain size distribution; larger
particles act like greenhouse gases by absorbing and emitting longwave
radiation and have a heating effect, whereas fine dust cools the atmo-
sphere (Otto et al., 2007, 2009).
5. Conclusions
The purpose of the current study was to present a long-term time
series and systematic analysis of Saharan dust episodes identified in the
Carpathian Basin and provide new information and an explanation on
changing temporal patterns of dust events. SDEs of the last 40-years in-
dicated the definite role of specific synoptic meteorological situations in
the formation of North African dust transport toward Central Europe.
Several intense episodes were recorded after 2014 when an exceptionally
large amount of mineral dust material was washed out, and all occurred
between the end of October and February, although the main period of
dust transport to Europe is the spring and summer. Synoptic analyses
confirmed that majority of the events were associated with very similar
atmospheric patterns; generally, an upper-level atmospheric trough (the
result of a remarkable meander of the jet stream) led to the development
of a cut-off low over the north-east, which deepened the low-pressure
system which transported large amounts of the mineral dust northward.
The presented individual events revealed the clearly changing
characteristics of recent wintertime Saharan dust deposition episodes,
increased number of days with gusty meridional flows at 700 hPa in
winter, and enhanced warming of high latitudes of the Northern
hemisphere during severe winter dust events of the last decade. Saharan
cyclone formation and poleward dust transport from Africa are affected
by the increasing amplitude of planetary waves. These extreme events
can be linked to AA, which is the possible cause of the development of a
more meandering jet stream (and wavy polar vortex) as a result of
decreasing temperature differences of the Arctic and lower latitudes
driven by the faster warming high latitudes.
CRediT authorship contribution statement
György Varga: Conceptualization, Data curation, Investigation,
Methodology, Validation, Visualization, Writing - original draft,
Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgement
Support by the National Research, Development and Innovation
Office (NKFIH K120620 and KH130337) is gratefully acknowledged.
The research was additionally supported by the MTA research fund
KEP-08/2018. The author would like to acknowledge the COST Action
inDust (COST Action CA16202).
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