A Quick Look at the Atmospheric Circulation Leading to Extreme Weather Phenomena on a Continental Scale

Abstract

The study delves into the primary large-scale atmospheric features contributing to extreme weather events across Europe during early September 2023. The period was examined using a dataset composed by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis and satellite imagery. In early September 2023, an omega blocking pattern led to the development of a low-pressure system over the Iberian Peninsula producing heavy precipitation and flooding over Spain and acting as a mechanism for a mineral dust outbreak. A second low-pressure system developed over Greece. Extreme precipitation was recorded across Greece, Turkey, and Bulgaria as the system gradually shifted southward over the Mediterranean. The system earned the name “Storm Daniel” as it acquired subtropical characteristics. It caused floods over Libya and its associated circulation favoured the transport of mineral dust over Northern Egypt as it moved eastward. Meanwhile, the high-pressure blocking system associated with the omega pattern induced heatwave temperatures in countries further north. This period was compared with the large-scale circulation observed in mid-September 2020, when severe weather also affected the Mediterranean region. However, the weather systems were not directly connected by the large-scale circulation, as shown in September 2023. Although mesoscale conditions are relevant to formation and intensification of some atmospheric phenomena, the establishment of an omega blocking pattern in early September 2023 showed how large-scale atmospheric dynamics can produce abnormal weather conditions on a continental scale over several days.

1. Introduction

Our dynamic atmosphere is responsible for forming a number of distinct weather systems, which differ in terms of spatial and temporal scales and may be associated with diverse physical phenomena. On a continental scale, many extreme weather events can occur simultaneously without a direct connection, even when they affect large regions. For instance, while a dust outbreak was influencing the weather in the Iberian Peninsula on 23 February 2017 [1], the United Kingdom (UK) and Ireland were hit by “Storm Doris” [2]. This storm brought strong wind gusts across the UK, but was not associated with the cut-off low (COL) that was transporting high amounts of mineral dust toward Portugal.

For some weather systems, stratosphere-troposphere coupling is the main mechanism for their development. In the case of cut-off low-pressure systems, undulations of the westerlies may increase in amplitude so that air masses can be separated from the main westerly airflow. Rossby wave breaking events favour the equatorward transport of cold and dry air, high potential vorticity and ozone from the stratosphere into the troposphere [3,4,5], producing the development of a slow-moving cyclonic PV anomaly [6], also known as mid-tropospheric low-pressure systems (e.g., [7]). Concerning the evolution of COLs and using the eddy kinetic energy budget, Pinheiro [8] demonstrated that, in the downstream baroclinic development, the ageostrophic flux convergence is an important factor in the genesis and intensification phases of these systems, while baroclinic conversion acts for their maintenance. The downstream development of a baroclinic wave was also found prior to the development of South African COLs [4,7]. The South African COLs have been extensively studied due to their impact at the surface and their important role in the annual precipitation regime (e.g., [4,7,9,10,11]).

Cut-off lows are well-recognised for producing episodes of significant precipitation. For example, the dynamics behind a slow-moving COL, i.e., cold upper-level air and moisture supply, favoured moist convection over the Middle East and caused a flash flood in Israel in April 2018 [12]. Cut-off systems play a significant role in seasonal and interannual precipitation over Iraq, especially in extreme events, such as that observed in 2013 during the largest flood in a 12-year period in Baghdad [13]. Furthermore, a COL centred over Iraq was the main factor causing heavy precipitation in southwestern Iran in March 2019 [14]. In Madeira Island, besides the orographic effects producing heavy precipitation [15,16], cut-off lows can also produce significant precipitation events on the island, as observed in November 2012 [17].

Nevertheless, the development of COLs can lead to other extreme weather events. Francis et al. [18] analysed an intense period of dust activity over the Middle East and the Arabian Peninsula in September 2015. The authors found that the dust storm was associated with a large dry cyclone centred over Iraq, which had developed from a COL located over Turkey several days earlier. Cut-off lows also play an important role in transporting mineral dust outward of the Sahara Desert [1,19].

Studies also show that COLs can cause severe weather conditions, namely hailstorms, as identified in July 2017 in Istanbul, Turkey [20], or as a synoptic condition propitious for the occurrence of hailstorms in north-eastern Romania [21].

In contrast, the persistence of a high-pressure system can cause drought and heatwave conditions that enhance the wildfire risk depending on the region (e.g., [22,23]). In January 2022, a high-pressure system centred on the Biscay Gulf for several days favoured warmer winter conditions over the Iberian Peninsula and the development of a large wintertime fire in the northwestern region [24]. On the other hand, spring frosts in Iran are caused by blocking patterns over central Europe and the Black Sea basin, resulting in cold air advection from the north towards Iran [25]. Miri [14], in turn, identified a deep trough linked to an omega-shaped blocking system that favoured intense precipitation in the south and southeast of Iran in January 2020.

Sometimes, the large-scale circulation may be organised in a way that allows the synoptic weather systems to remain semi-stationary in a blocking configuration. In such cases, if favourable conditions for extreme weather phenomena exist, they tend to persist for a prolonged period as the system stays over the same geographic location, typically 4 or 5 days [26]. However, climate change brings uncertainty regarding these blocking systems’ location, intensity, and frequency [26,27].

This can lead to severe droughts, changes in the wildfire seasons, extreme precipitation followed by flash floods or inundations in large areas. In this sense, there are many efforts to determine blocking events (e.g., [28,29]). Two main types of blocking systems can be identified: (a) high-over-low and (b) omega. The latter can be characterised by two lows that lie southeast and southwest of the blocking high in the northern hemisphere [29]. However, the dynamic processes involved in blocking and their association with extreme phenomena are not yet fully understood due to several factors leading to them in different regions of the globe. For instance, the omega blocking system can occur over Eurasia, with extreme phenomena occurring simultaneously only in some sporadic situations [27].

Documenting and increasing knowledge of extreme weather phenomena can be useful for meteorologists and the scientific community worldwide. This paper aims to investigate the main large-scale atmospheric features behind the occurrence of extreme weather phenomena surrounding the Mediterranean Sea and across Europe in early September 2023. This period was compared with the large-scale circulation observed in mid-September 2020, when severe weather also affected the Mediterranean region. Section 2 outlines the dataset and methodology used, whereas the results of the large-scale circulation are examined in Section 3, followed by discussion in Section 4. The concluding remarks are provided in Section 5.

2. Period Studies and Dataset

2.1. Period Studies and Large-Scale Data

The study considers two periods marked by several extreme meteorological phenomena: (1) 1 to 12 September 2023 and (2) 14 to 21 September 2020. The following section presents an analysis of the periods. In the first period (September 2023), we consider the extreme weather phenomena recorded surrounding the Mediterranean Sea and across Europe, as displayed in Figure 1. In the second period (September 2020), landfall and tornadoes were observed in Portugal, and extreme precipitation produced floods in Greece. The second period is chosen to verify if there was some similarity between the periods in terms of large-scale circulation patterns.

The large-scale analysis is based on the dataset located in ECMWF’s Meteorological Archival and Retrieval System—MARS archive [30], accessed through the interactive website and available in NetCDF format. The data were obtained with a horizontal resolution of 0.125 × 0.125 degrees and downloaded for the four main synoptic hours 0000 UTC, 0600 UTC, 1200 UTC, and 1800 UTC. Data were produced at the surface and pressure levels (850 hPa, 500 hPa, 250 hPa) and for the following parameters: geopotential height, potential vorticity (PV), ozone mass mixing ratio, U and V component of wind, air temperature, mean sea level pressure (MSLP), sea surface temperature (SST), and total column water vapour (TCWV).

2.2. Satellite Data

The satellite images were obtained from the Eumetview platform [31], which allows access to various meteorological products derived from satellite data, updated every 15 min.

The cloud top height (km) product from the Meteosat Second Generation (MSG)—0 degree has been used and indicates the height of the highest cloud. This product is based on a subset of the information derived during Scenes and Cloud Analysis, but also makes use of other external meteorological data [32].

In addition, the Airmass RGB imagery is used to diagnose the environment surrounding the large-scale systems. The Airmass product is an RGB (Red, Green, Blue) composite based upon data from infrared and water vapour channels from the Spinning Enhanced Visible Infra-Red Imager (SEVIRI) instrument [33]. These data are indicated for distinguishing air masses, detection of ongoing cyclogenesis, as well as the identification of areas with descending dry stratospheric air, for example, behind cold fronts and in the centre of cyclones/upper-level lows. The Airmass RGB is composed from data from a combination of the SEVIRI WV6.2, WV7.3, IR9.7 and IR10.8 channels [33].

The satellite images dataset is also composed of Moderate Resolution Imaging Spectroradiometer (MODIS) Corrected Reflectance images obtained from the Worldview platform [34]. The MODIS Corrected Reflectance (True Color: Red = Band 1; Green = Band 4; Blue = Band 3) imagery is called true-colour because the combination of wavelengths is similar to what the human eye would see. The sensor resolution is 500 m and 250 m (Bands 1 and 2 have a sensor resolution of 250 m, Bands 3–7 have a sensor resolution of 500 m, and Bands 8–36 are 1 km. Band 1 is used to sharpen Band 3, 4, 6, and 7), imagery resolution is 250 m, and the temporal resolution is daily [34].

3. Results

3.1. September 2023: Omega Blocking Pattern

3.1.1. Heavy Precipitation, Flooding, and Dust Outbreak

The first extreme weather phenomenon considered is Spain’s heavy precipitation and flooding event which occurred on 3 September 2023. Figure 2 shows the development of a cut-off low over the Iberian Peninsula between 1 and 3 September 2023. The geopotential height field (contour) at 1800 UTC on 1 September shows the establishment of a positively tilted upper-air trough (Figure 2a), which was moving southward. It is worth noting that the jet stream at 250 hPa on 2 September (0000 UTC), identified from the coloured arrows, presents a meridional evolution and configures a trough extending its axis over the Western Iberian Peninsula (Figure 2b). Stronger winds are located west of the trough (Figure 2b), typical of a diffluent trough. This configuration generally indicates stratospheric air intrusion as the trough deepens. This situation is confirmed in Figure 2a,b, since the left side of the trough is characterised by higher ozone values and positive PV, representing a region where there is exchange between the troposphere and stratosphere.

The trough deepens further the following day, giving rise to a low-pressure system (Figure 2c,d). This system corresponds to the lower geopotential height values, higher ozone concentration (Figure 2c), and higher positive PV (Figure 2d). Figure 2c,d show the development of a cut-off low at upper levels, which is centred south-westward of the Iberian Peninsula (36° N, 10° W) and appears separated from the westerly airflow (jet stream) at around 60° N and with a closed counterclockwise circulation (Figure 2d). In general, Figure 2 shows how the undulations of the westerlies can become large enough to lead air masses to separate from the main westerly airflow.

As the upper trough deepens, it locally lowers the tropopause height, resulting in a local PV maximum and indicating the intrusion of stratospheric air rich in ozone concentrations into the upper troposphere. Figure 3a shows that the upper-air wind (coloured arrows) from higher latitudes intensified the cold air advection southward, leading to a COL of cold core, as seen at 500 hPa on 3 September at 1800 UTC. The low-pressure system intensified on 3 September, with an increase in wind speed on its eastern side.

This upper air dynamic provides the necessary conditions for developing the low-pressure system in the lower troposphere. Figure 3b shows a cyclonic circulation (coloured arrows) at 850 hPa centred southwest of the Iberian Peninsula at 1800 UTC (lower geopotential height contour at 850 hPa, around 1500 m) on 3 September and accompanying the low-pressure system in upper levels (Figure 2d).

Regarding the conditions near the surface, Figure 4a shows the precipitable water content represented by the TCWV variable. In the early morning, a significant amount of precipitable water, around 50 mm, is found in the Western Mediterranean surrounding the Balearic Islands. The MSLP (Figure 4b) displays a high-pressure region covering North Europe, while a low-pressure region is seen southwestward from the Iberian Peninsula, as well as in North Africa. In the late afternoon, the cyclonic circulation associated with the cut-off low is evident at the surface (Figure 4c). The cut-off position and its counterclockwise circulation encompass the Iberian Peninsula and North Africa at the lowest tropospheric levels in the late afternoon (Figure 3b and Figure 4c). At this moment, however, the COL is verified extending throughout the troposphere (Figure 2d and Figure 3a; upper-level winds).

It is noteworthy that strong easterly winds at the surface with a speed of around 14 m·s⁻¹ (Figure 4c) and confluent flow at 850 hPa (Figure 3b) keep the moist air advection from the western Mediterranean Sea toward Spain and, consequently, into the system. In addition, Figure S1a displays the sea surface temperature at 1200 UTC, with values above 26 °C in the western Mediterranean Sea.

In Figure 5a, the satellite image displays an agglomerate of convective clouds over the Iberian Peninsula, especially over Spain. The cloud system takes on a comma-shaped cloud, with convective cores indicating deep convection within the system and cloud tops reaching around 12 km altitude. The thunderstorms caused heavy precipitation and flash floods in Toledo (Central Spain) in the late afternoon and early evening of 3 September. The cloud band lies in the region of diffluent flow in the upper levels on the northeastern side of the upper air low, which is centred over the southern coast of Portugal and swirls counterclockwise (Figure 2d and Figure 3a).

Due to its position, the counterclockwise circulation associated with the COL induces the transport of mineral dust from the Sahara Desert toward the north and out of North Africa. The mineral dust is carried along with the main system circulation, spiralling around the COL, and can be observed over Eastern Spain and Southwestern France (Figure 5b). This dust outbreak represents the second extreme weather phenomenon displayed in Figure 1. On 4 September, the cut-off low moved slightly north-westward. It still had a cold core and strong southerly winds over the Iberian Peninsula (Figure 6a). The low-pressure system remained in the northwestern Iberian Peninsula during the following day.

Thus, the large-scale mechanism leading to heavy precipitation and flooding in Spain on 3 September was the development of a cut-off low. This system also facilitated the outward transport of mineral dust from the Sahara Desert on 4 September. While the cut-off low continued to affect the weather over the Iberian Peninsula, two additional systems developed in the following days.

3.1.2. Extreme Precipitation over Eastern Mediterranean and Dust Outbreak

Between 4 and 5 September, the jet stream that originated the COL southward across Spain remained northward, curving across the top end of the UK and Scandinavia, back down south towards Greece, in a shape that looks like the Greek letter omega (e.g., Figure 2d and Figure 6a,b).

A cold upper-air trough is configured over Eastern Europe, and another low-pressure system starts to develop (Figure 6a). This situation is similar to the one presented in the previous subsection. The upper-level low-pressure system, i.e., the COL, is embedded in the main flow of the upper tropospheric jet stream as it becomes distorted as an upper-air trough elongated meridionally. The upper-level low-pressure system develops over southwestern Greece and, as it deepens (lower geopotential height contour), it isolates cold air from the higher latitudes (Figure 6b,c).

Besides the trough presenting cold stratospheric air, it is also characterised by higher values of PV and ozone (Figure 7). Figure 7a,b confirm the advection of positive PV and ozone at 250 hPa, respectively. The COL maintained the high positive PV and ozone concentration during 6 September (Figure 7c,d). Although the low-pressure systems appear to be separated from the westerly flow, the upper-air jet stream (coloured arrows) keeps the two low-pressure systems connected. Figure 6a,b display at 500 hPa an omega blocking pattern over Europe, associated with two low-pressure systems. One is located westward of the Iberian Peninsula, and the other is over the Mediterranean with a centre around 35° N and 20° E.

On 5 September, the omega pattern remained quasi-stationary, and the low-pressure system centred in southwestern Greece (Figure 8a) produced predominantly easterly-northeasterly flow over mainland Greece due to the associated counterclockwise circulation. Due to the precipitable water values above 40 mm (Figure 8b) and wind speed of 12 m·s⁻¹ in the Western Black Sea and above 14 m·s⁻¹ over the Aegean Sea (Figure 8c), the dynamic behind the intensification of this cyclone indicates an advection of moist air from the Aegean Sea and Black Sea at the lowest levels (Figure 8b,c).

As Figure 6, Figure 7 and Figure 8 show, the cyclonic circulation extends throughout the troposphere. This circulation produced a convective cloud system with cloud tops above 10 km in the afternoon of 5 September (Figure 9a).

Figure 10 depicts some key moments during 6 September that aid in understanding the maintenance of the cyclone on that day. For instance, a significant amount of precipitable water (>40 mm) is present in the Aegean Sea and Central Mediterranean (Figure 10a), where the low-pressure system remains centred and visible at middle levels (Figure 10b). In the late afternoon (Figure 10c), the cyclone still exhibits a well-defined low-level jet with mean wind speeds of approximately 10 m·s⁻¹. This jet extends from the Western Black Sea and crosses the Aegean Sea towards the cyclone’s centre in southwestern Greece. Figure 10d shows that this low-level jet is influenced by the clockwise circulation of the high-pressure system centred northward, which contributes to feeding the low-pressure system with moist air. The convective activity was favoured by such an environment from the afternoon. Moist convection produced cloud systems with tops above 13 km (Figure 9b). It is important to note that Greece was located beneath a region of weak and diffluent winds at upper levels (e.g., Figure 10b), as well as in a region marked by the confluence of the northeast low-level jet and a southeasterly wind associated with the cyclonic circulation in the lower troposphere (Figure 10d).

The thunderstorms were accompanied by extreme rainfall that impacted the eastern and central parts of Greece, as well as neighbouring regions of Bulgaria and Turkey. This event caused extensive flooding in Greece and is the third extreme weather event highlighted in Figure 1. However, it is noteworthy that the low-pressure system developed as part of an omega blocking pattern established over Central Europe.

On the following day, the low-pressure system remained almost stationary over the Ionian Sea, with the cyclonic circulation affecting the southern parts of Italy and Greece (Figure 11a). Figure 11b shows that the precipitable water content was above 40 mm in a large area of the central Mediterranean Sea and around 50 mm on 8 September at 1200 UTC (Figure 11c). The figures indicate that the system probably starts intensifying due to strong diabatic forcing over the warmer waters of the Southern Mediterranean. The low-pressure system, named Daniel, gradually shifts south towards Libya, and undergoes a kind of sub-tropical transition. Figure S1b illustrates that the storm system encountered warmer waters off the coast of Libya with SST values of almost 28 °C (8 September).

The storm system’s pressure continues to drop the following day (Figure 12a). At this moment, the system contains a precipitable water amount of above 55 mm (Figure 12b). As the system approached the Libyan coast in the late afternoon of 9 September (Figure 12c), the storm intensified and the high amount of precipitable water (>55 mm) favoured the development of an agglomeration of thunderstorms. Additionally, the storm system also presents intense rotating winds of magnitude above 15 m·s⁻¹ close to the storm’s centre at the surface (Figure 12d).

The storm temporarily stalled in its east-southeastward progression and remained above the coastal regions for almost 24 h. Figure 13a,b depict the low-pressure system over Libya on 10 September at 1800 UTC, with precipitable water of around 55 mm. In the middle and upper tropospheric levels, the steering of the large trough over Eastern Europe is still visible in Figure 13c. The influences of the subtropical jet stream can be seen in the right part of the trough (coloured arrows, Figure 13c). The slowdown of the system’s movement caused quasi-stationary thunderstorms over the region, which was responsible for extreme precipitation in a short period and catastrophic flooding over Libya. This is the fourth extreme weather event shown in Figure 1.

However, late on 10 September, the cyclone finally started dissipating and moving further east. Figure 14a shows the cyclonic circulation at lower levels on 11 September at 0000 UTC. At this moment, another phenomenon is identified in Northern Egypt. The counterclockwise air circulation around the storm system facilitates the transportation of mineral dust from the western Egyptian desert, as shown in the satellite image (Figure 14b). This dust outbreak is the fifth weather phenomenon displayed in Figure 1. The mobilization and transportation of dust persists in the following day as the system moves eastward during its dissipating phase.

3.1.3. Heatwave Conditions in the UK and France

While the regions surrounding the Mediterranean Sea experienced torrential rains due to the development of two low-pressure systems, the regions to the north were affected by an abnormal period of high air temperatures.

The previous subsections showed the presence of the jet stream well to the north of the UK, with an upper high-pressure system located over northern Europe. The omega blocking pattern was configured by two low-pressure systems located southeast and southwest of the high-pressure system (e.g., Figure 6). The high-pressure system at lower levels, with clockwise circulation (coloured arrows), also known as a blocking high, remained in the same region for several days, as shown in Figure 3b (3 September at 1800 UTC), Figure 10d and Figure 11a (7 September at 0000 UTC and 1200 UTC, respectively).

The circulation pattern at a large scale favoured warm air advection from the south between the two low-pressure systems (e.g., Figure 3b). The blocking high centred over Eastern Europe also impacted Northern France and Britain, allowing some hot days, as displayed by the warm air mass observed at 850 hPa on the western side of the clockwise circulation (Figure 15 and Figure 10d). Moreover, this persistent weather pattern tends to induce stable subsiding air and suppress convective activity and associated precipitation. This blocking pattern also kept the lows in their respective positions. The sixth extreme weather phenomenon considered here (Figure 1) is represented by the heatwave conditions that were favoured by the large-scale blocking high pattern.

3.2. September 2020: Landfall and Tornadoes in Portugal, Floods in Greece

This section aims to understand the large-scale circulation patterns that contributed to the extreme weather events in Portugal on 18 September and Greece between 17 and 20 September 2020.

The upper air circulation is displayed in Figure S2a–f with an upper-level trough identified at 250 hPa on 14 September, north of the Azores Archipelago and with high ozone concentration (Figure S2a). The trough presents strong winds on the western side of its axis and has high positive PV values (Figure S2b). In the following days, as the trough deepens southeastward, it continues to transport stratospheric air to lower latitudes, as evidenced by the presence of ozone and positive PV. This process helps to form and maintain a cut-off low in the east of the Azores archipelago (Figure S2c,d). The upper-level cut-off low reaches Portugal in the afternoon on 18 September (Figure S2e) and weakens as it moves towards the Biscay Gulf (Figure S2f).

At 500 hPa, the low-pressure system over the Atlantic Ocean is visible due to the cold air mass core being separated from the jet stream on 16 September (Figure S3a). At this moment, a relatively small low-pressure core can also be identified in the central Mediterranean Sea. The weaker geopotential gradient associated with the low-pressure system over the Atlantic on 18 September at 0000 UTC (Figure S3b) indicates that the system weakened during the previous day. As the system approaches Portugal, the low-pressure core over the Mediterranean reaches the Western coast of Greece (Figure S3b). In the lower troposphere (Figure S3c,d), the cyclonic circulation over the Atlantic Ocean confirms that the low-pressure system extends throughout the troposphere with a low-pressure core located westward of Southern Portugal. In the Mediterranean Sea, strong rotating winds are identified on the Western coast of Greece (Figure S3c,d). This storm system, named Medicane Ianos, continued to affect the weather over southern Greece in the following days.

At the surface, the presence of both systems is easily identified from the MSLP field on 17 September at 1800 UTC (Figure S4a). Over the Atlantic Ocean, the slow-moving system from the higher latitudes is displaced over unusually warm sea surface temperatures to the west of mainland Portugal, as seen on 17 September from SST of around 22 °C (Figure S4b).

Even with weaker conditions, the low’s core still contained moist air and convective activity, as shown in Figure S4c and S4e, respectively. The convective clouds became a well-organised system on 17 September and exhibited sufficient organization to be classified as a subtropical cyclone. From this moment on, the storm, also known as Storm Alpha, began its subtropical phase and intensified as it moved to the northeast. This situation is similar to the development of Storm Daniel described in Section 3.1.2. In this case, convection in the low’s core presents a kind of subtropical transition, with a subsequent intensification of convective activity.

Figure S4c also indicates that the low-pressure core identified over the Mediterranean (Figure S4a) has a high amount of precipitable water, with values above 55 mm, as well as a cyclonic circulation with wind speeds of around 20 m·s⁻¹ (Figure S4d). The cloud system associated with Medicane Ianos can be seen in Figure S4e near the western coast of Greece. It is important to note that Ianos developed over warm waters in the Mediterranean Sea with SST around 27 °C (Figure S4b).

In the late afternoon of 18 September (Figure S5), Storm Alpha reached the western Iberian Peninsula. The low-pressure core (Figure S5a) and the small spiral cloud system (Figure S5c) were located in the Portuguese coastal zone at 1800 UTC, also associated with precipitable water of above 30 mm (Figure S5b). The storm system produced extreme local phenomena in Portugal, including landfall in Figueira da Foz. The development of tornadic supercells was also verified on that day. The small low cyclonic circulation associated with Alpha started to decay as it moved inland, becoming a subtropical depression at 0000 UTC on 19 September.

Figure S5a also shows Medicane Ianos over the western coast of Greece, still with a significant amount of precipitable water above 45 mm (Figure S5b). Figure S5c displays the organization of the cloud system. The system remains semi-stationary over the Mediterranean for days, producing intense precipitation over Greece.

It is noteworthy that both events, Storm Alpha and Medicane Ianos, developed simultaneously, but without a significant large-scale connection like the period presented in Section 3.1, where the large-scale circulation was able to sustain the weather systems for several days on a continental scale.

4. Discussion

The study shows how extreme weather phenomena occurring simultaneously on a continental scale can be connected by large-scale circulation. The weather pattern in Europe and surrounding regions of the Mediterranean Sea was determined by a large-scale circulation pattern known as an omega blocking pattern. This system is recognized by a high-pressure system remaining stuck between two low-pressure systems to the southwest and southeast, resembling the Greek letter omega due to its shape.

The development of an omega blocking pattern in September 2023 revealed key aspects of stratosphere-troposphere interaction during trough deepening and cut-off low development in the mid-latitudes. However, the position of the low-pressure systems was a significant aspect of the omega blocking configuration. The cyclonic circulation centred near the Mediterranean Sea favoured ascending air and cloud development, which was fed by moisture from the warm waters of the Mediterranean. This contributed to intensifying the low-pressure systems, both in Spain and Greece. In Greece’s case, the study also highlighted the significance of the position and clockwise circulation of the high-pressure blocking system in relation to the low-pressure system. This configuration facilitated moist advection from the Black Sea towards the cyclone.

As this large-scale circulation pattern persisted over several days, the low-pressure systems produced thunderstorms and precipitation records. Heavy precipitation was observed across Spain, Greece, Turkey, Bulgaria, and Libya. The position and movement of the low-pressure systems also caused dust outbreaks in Spain, southwestern France, and northern Egypt. In the north, a high-pressure system associated with subsiding air inhibited thunderstorm development. Warmer and drier conditions prevailed over a large area of Central Europe, characterised by heatwave conditions, particularly in the UK and France. The configuration presented in this study is slightly different from the schematic illustration presented by [27] for the warm season, where thunderstorms were indicated occurring northward of Greece and temperatures extremes found to the north, namely over Scandinavia.

In the Mediterranean region, extreme events are expected to increase coastal risks, resulting in damage and significant socio-economic impacts [35]. Storm development mechanisms in this region are complex and strongly dependent on the SST, which can alter system intensity (e.g., [36]). For instance, explosive cyclones and medicanes can develop over the region throughout the year. There are also instances where explosive cyclones are observed transitioning to medicanes (e.g., [37]). In mid-September 2020, the weather over the western Iberia Peninsula was marked by Storm Alpha that was associated with intense lightning activity [38], while the eastern Mediterranean was affected by the remarkable Medicane Ianos, which caused floods and numerous landslides in Greece [39,40].

On the other hand, atmospheric blocking is a worldwide issue [6,27,41,42,43,44]. In Scandinavia, for instance, hydrological extremes over the last two millennia have been connected to the presence of atmospheric blocking regimes [45].

Blocking situations associated with high-pressure systems can lead to heatwaves [46,47], influencing air quality [48,49] or exacerbating extreme events that in a global climate change context can produce unprecedented hot periods [50]. In the higher latitudes, Wang [51] showed three melting types caused by Greenland blocking. In general, this high-pressure blocking can cause extreme temperature events and intense melt rates [52].

In the southern hemisphere, O’Kane [53] highlights changes in the South Annular Mode leading to major blocking regions. For a 37-year period, Mendes [54] observed that blocking events in the south Atlantic and southeastern Pacific were characterised respectively by an omega and a dipole-type pattern. However, modelling blocking events remains challenging [55,56], in particular representing blocking events in the southeastern Pacific and southwestern Atlantic by global climate models [57].

In the present study, we present an analysis of a period during which a specific atmospheric blocking regime connected several weather systems that produced extreme weather phenomena on a continental scale for several days. Generally, the weather patterns described in this study can usually be observed developing simultaneously and in isolation, as was shown during mid-September 2020. During this time, a cut-off low developed over the Atlantic Ocean and as it moved slowly south-eastward over relatively warmer waters, the low-pressure system presented a subtropical transition and the storm system in its core intensified as it approached mainland Portugal. During the same period, Medicane Ianos was developing over the warm waters of the Mediterranean Sea and was responsible for producing extreme precipitation over Greece.

Figure 16 summarises the main features of the period that led to the extreme weather phenomena highlighted in Figure 1. The period began with the development of a cut-off low over the Iberian Peninsula between 1 and 3 September, caused by an upper-air trough that deepened over the western Iberian Peninsula (Figure 16a). The low-pressure system was associated with thunderstorms that produced heavy precipitation and flooding in Toledo, Spain, in the late afternoon of 3 September (Figure 16b). On 4 September, the same weather system favoured the transport of mineral dust from the Sahara Desert to Spain and Southwestern France (Figure 16b). On the other hand, as the jet stream turned northward, curving across the top end of the UK and Scandinavia, and back down south toward Greece, the omega blocking pattern began its establishment on 5 September with the development of a low-pressure system over Greece (Figure 16c). The surface wind favoured moist air advection into the system and intensified cloud development processes. The thunderstorms produced extreme precipitation and flooding in Greece, Bulgaria, and Turkey. Simultaneously, the high-pressure system produced heatwave conditions over Northern France and the UK (Figure 16c).

On 8 September, the low-pressure system over Greece moved towards Libya and underwent a kind of subtropical transition. The intensification of the storm led to extreme precipitation in Libya on 9 September (Figure 16d). As the storm was displaced eastward, its circulation favoured dust outbreaks in Northern Egypt on 11 September (Figure 16e). This event marks the final stage of the omega blocking pattern.

5. Conclusions

This paper demonstrates the significant impact of strongly meridional westerly waves on weather conditions, resulting in an omega blocking circulation pattern at a continental scale. The extreme precipitation events, floods, heatwave conditions, and mineral dust outbreaks experienced in early September 2023 were attributed to this blocking circulation pattern.

Regarding the low-pressure systems associated with the omega blocking pattern, the study highlights the importance of their positioning in their intensification, due to the advection of moist air from the Mediterranean Sea. In the case of the low-pressure system over Greece, there was also an advection of moist air from the Black Sea, which was influenced by the position and clockwise circulation associated with the high-pressure system centred to the north. The positioning of the lows was also relevant to the outward transport of mineral dust from North Africa.

The study also analysed the extreme weather phenomena occurring simultaneously over the Iberian Peninsula and Greece in mid-September 2020, showing that their development was not directly connected on a large-scale. While Greece was affected by Medicane Ianos with intense precipitation that resulted in floods and landslides, Portugal was under a conditionally unstable air mass that favoured the development of subtropical Storm Alpha, leading to landfall, and tornadic supercells.

The study shows that omega blocking is a complex phenomenon involving a range of different processes that need to work simultaneously to cause extreme weather conditions on a continental scale. The position of both low-pressure systems was essential to the extreme precipitation events, with the Mediterranean Sea acting as a moisture source for the convective clouds. Furthermore, this case study highlights the importance of the omega blocking pattern location and mechanisms for the occurrence of extreme precipitation events, dust outbreaks, and heatwave conditions, which can be influenced by several processes on a continental scale.

Although the present study identifies the main large-scale environment that induces extreme weather events, it is important also to consider the role played by smaller-scale phenomena in intensifying the conditions associated with these systems. For instance, warmer waters may be contributing in conjunction with local effects. This is an initial step to understanding more about the coupling of dynamic features and surface interactions. A current study is under way to assess the different atmospheric phenomena identified during early September 2023 from a set of convection-permitting simulations. The high-resolution numerical simulations will allow us to explore the low-pressure systems and their interaction with the surface. Finally, future studies should investigate how climate change may affect the frequency, duration, and geographic distribution of large-scale omega blocking patterns, and consequently, the occurrence of extreme weather events.