Impacts of three severe storms on Aegean Sea oceanography (2018–2020)

Abstract

This study presents a comprehensive analysis of the impacts of severe storms on the oceanography of the Aegean Sea, with specific focus on storms occurring on 15 November 2018, 14 September 2019, and 7 January 2020. Notable changes in oceanographic variables are observed on the storm day and in the subsequent days, gradually returning to pre-storm conditions. During storms, Aegean Sea surface temperatures were 2–3 °C below average, with cooling particularly evident along the west coast and northern part. Salinity increases of 0.5–1 PSU are observed during storms, with the most significant effects noted on 7 January. Storms consistently led to a decrease in sea level in our study area, with the western coast and north Aegean Sea experiencing levels 6–10 cm lower than the monthly average. Surface currents along the west coast and within latitudes 36–40° are 0.3 to 0.4 m/s stronger than average during storms. Storm-induced changes in the Thracian Sea’s anticyclonic gyre on 5 January reveal a speed increase of 0.3–0.4 m/s. Notably, both temperature and salinity fluxes exhibit significant changes during storms, characterized by a shift from east-to-west transport, contrasting with the west-to-east pattern observed before the storms. This shift is mainly influenced by Ekman transport, which predominantly directs water westward and north-westward when the wind comes from the northeast to southwest. Storms contribute to heightened salinity, increased kinetic energy, and enhanced transport, accompanied by a decrease in temperature, with the most pronounced effects near the surface along vertical layers.

1 Introduction

The Aegean Sea is located between Greece and Turkey (Fig. 1). This water basin has unique characteristics, such as its azure waters, irregular coastline, and extensive archipelago of islands (Olsen et al. 2007). The area of this basin is 1.8 × 10 m⁶, containing a volume of approximately 7.4 × 10¹¹ m³ (Hopkins 1978). Due to these characteristics, this water basin is important for trade and tourism, drawing millions of visitors annually. Not only this, but also the Aegean Sea is an interesting topic for oceanography research.

Fig. 1

Geographic area of study—the Aegean Sea and its sub-basins

Aegean Sea hydrodynamics and physical characteristics, particularly in the upper ocean, are influenced by prevailing meteorological conditions and buoyancy-driven currents. A major component of hydrodynamic circulation is wind-induced, governed by the Etesians, with a significant north wind component shifting from northeastern in the northern Aegean to northwestern in the southern region (Valioulis and Krestenitis 1994). The impacts of the Black Sea and its water exchange with the Aegean Sea is another factor which plays an important role in defining the complex dynamics of the Aegean region (Poulos et al. 1997; Androulidakis et al. 2017; Zervakis and Georgopoulos 2002; Kourafalou and Barbopoulos 2003; Androulidakis and Kourafalou 2011; Androulidakis et al. 2012).

Furthermore, an intricate relationship exists between the general circulation of the Aegean Sea and the region’s complex bathymetry (Nittis and Perivoliotis 2002). This is mainly because the bathymetry control current’s paths and forming gyres and eddies in the Aegean Sea. Various mesoscale phenomena with spatial radius between 10 and 100 km, contribute to the dynamic nature of the water basin (Della and Gaube 2019; Spondylidis et al. 2020). Herein, a key question is what happens to the circulation, eddies, and gyres in the Aegean Sea during storms. Additionally, how the temperature and salinity distribution has changed in the windstorm, as one role of eddies and gyres is to transport heat and salt flux.

Natural events such as storms make large waves in the Aegean Sea, causing catastrophic problems. During storms, these waves are formed by strong winds and pose a serious threat to coastal areas. They have the power to cause substantial damage to buildings and put people’s lives at risk. All of us are familiar with these effects but we have limited information about how storm can change the oceanography of the Aegean Sea. We try to address these scientific questions: How does the storm influence the circulation patterns, temperature, salinity distribution, and surface level of the Aegean Sea? What impact will the storm have on the vertical structure of the Aegean Sea? How do storms at different times of the year impact the Aegean Sea differently?

This paper is pivotal for comprehending the impacts of natural hazards, particularly storms, by delving into the intricate interactions between meteorological conditions and the oceanography of the Aegean Sea. Through a meticulous examination of how storms influence circulation patterns, temperature, salinity distribution, and surface levels, the study offers valuable insights into how this water basin responds to extreme weather events. The knowledge gained not only enhances our current understanding but also holds implications for the future, notably in the context of climate change, which often correlates with an increased frequency of storms. Moreover, extreme events can redistribute pollution from one area to another, and understanding their effects aids in predicting pollution paths. Extreme storms can be significant as they rapidly increase water transport, playing a pivotal role in redistributing pollution in a short amount of time. Understanding how water transport and circulation change enables us to better track the paths of pollution.

Additionally, during extreme events, water masses can transport nutrients across different parts of the Aegean Sea, crucial for phytoplankton growth, particularly given the region’s complex circulation with numerous eddies and gyres. We should consider that the temperature and salinity of the water serve as preparatory stages for the growth of phytoplankton, along with nutrient availability. Furthermore, extreme events, especially in the cold season, lead to evaporation and the sinking of water, contributing to the formation of dense water and altering the vertical structure, which affects oxygen distribution from the surface to the bottom. These compelling reasons drive our motivation to explore this topic thoroughly.

In this paper, we use an ocean model for modelling oceanography of the Aegean Sea. The rest of the paper is structured as follows. The second section explains the model configuration, the third section discusses model verification with satellite and reanalysis data, and the fourth section presents our methodology for selecting days for investigation. The fifth section presents results and discuss how storms affect oceanography in the Aegean Sea. In the conclusion, the main points are summarized, and conclusions are drawn.

2 Model set-up

The regional ocean modeling system (ROMS) is a numerical ocean model widely used for simulating and studying the dynamics of regional oceanic processes. ROMS can model a range of scales, from coastal regions to larger basins. The model employs finite-difference methods to solve the primitive equations governing fluid motion and includes modules for various physical processes, such as turbulence, heat and salinity transport, and sea-ice interactions (Shchepetkin and McWilliams 2005; Moore et al. 2011a, 2011b). ROMS has found applications in a diverse array of oceanographic studies, including coastal circulation, regional climate variability, and ecosystem dynamics (Robertson, and Hartlipp 2017; Babagolimatikolaei and Layeghi 2022; Nyamweya et al. 2016). Its versatility and capability to represent both physical and biological components make it a valuable tool for researchers and scientists working in the field of oceanography.

In this paper, the model simulation covers a six-year period, ranging from January 2015 to December 2020. The initial three years are designated for warm-up and spin-up processes, with a focus on the subsequent years (2018–2020) for the analysis in this paper. The horizontal grid configuration of the model consists of 239 grid points along latitude and 334 along longitude, resulting in a grid spacing of 2 km. Utilizing the General Bathymetric Chart of the Oceans (GEBCO) dataset with a 15 s resolution, a grid file is generated for the simulation domain (www.gebco.net; GEBCO Compilation Group 2022). The model domain is bounded by four boundaries, where the north, east, and west boundaries are closed, the southern boundary is open. Climatological data for boundary conditions are sourced from the International Comprehensive Ocean–Atmosphere Dataset (ICOADS) (www.icoads.noaa.gov). We implement the nudging technique on open boundary, a method employed to steer the model’s solution towards climatological data. This technique relies on incorporating monthly data near the boundaries of the simulation domain, effectively constraining the model’s behavior to align with observed climatic patterns. Through nudging, the model is nudged or adjusted towards the desired climatological state, enhancing its ability to accurately represent long-term climatic trends and variability. Temperature and salinity data, crucial for model initialization, are extracted from the World Ocean Atlas, utilizing the datasets provided by Locarnini et al. (2013) and Zweng et al. (2013), respectively. For the atmospheric forcing file, we employ ERA5 data with a 0.25° × 0.25° grid resolution per hour (www.ecmwf.int; Hersbach et al. 2020). Additionally, global monthly river discharge data are incorporated to account for river runoff in the model simulation. All files serve as input for running the model in the Aegean Sea, and the model outputs are saved as daily average data after running for 20 days using a 32-core supercomputer.

3 Verification

The model’s accuracy is validated through a comparison with the monthly averaged Aqua/Modis sensor data featuring a 4-km grid spacing (Fig. 2). When examining January, the model tends to overestimate water temperatures by approximately 1 to 2 °C in central Aegean Sea. The eastern coast, particularly at latitudes below 38°, showcases the model’s best performance for this month, with errors increasing at higher latitudes. In July, the model estimates surface water temperatures to be about 1 to 2 °C lower. Notably, the model aligns closely with observations in the central Aegean Sea and along the eastern coast. Moving to September, the model predicts colder water by about 1 to 1.5 °C. Nevertheless, the results from the model and satellites demonstrate strong agreement, with any differences being negligible.

Fig. 2

a Sea surface temperature of the Aegean Sea based on satellite data. b Sea surface temperature derived from the ROMS model for selected months

For the comparison of vertical model performance, Mediterranean Reanalysis data were utilized for the study area in daily average data. A total of 300 samples were analyzed. An illustrative example of this comparison is presented in Fig. 3, depicting conditions on 16 November 2018, at 39°N and 25°E. The comparison reveals that the model tends to underestimate salinity by 0.2–0.4 PSU and temperature by 0.1–2 °C. Specifically, at the surface, the model indicates a salinity of 38.1 PSU, whereas the reanalysis data show 38.28 PSU, resulting in a difference of 0.2 PSU. This difference decreases to 0.1 PSU at a depth of 400 m. Regarding temperature, at the surface, the model registers 1 °C, which rises to 2 °C at a depth of 100 m before decreasing to 0.3 °C at depths greater than 300 m. Overall, as depth increases, the accuracy of the model improves.

Fig. 3

Comparison of Salinity and Temperature Profiles between model and reanalysis data on 16 November 2018, at 39°N and 25°E

Similar to Fig. 3, all comparisons have been conducted for 300 samples using statistical analysis. Table 1 displays a summary of this comparison. In terms of salinity, the model exhibit relatively low errors, with mean absolute error (MAE) of 0.2618. This suggests that the ROMS model’s estimations closely align with the recorded salinity values. The mean squared error (MSE) and root mean squared error (RMSE) further corroborate this, with values of 0.0855 and 0.2924, respectively. These metrics collectively indicate a strong agreement between the ROMS model and the reanalysis data in terms of salinity estimation, highlighting the model’s robustness in simulating salinity distributions.

Table 1 Statistical comparison of model performance and reanalysis data

Conversely, when assessing temperature metrics, the ROMS model shows higher errors compared to salinity, with a mean absolute error (MAE) of 0.7032. This suggests a larger difference between the ROMS model’s temperature estimations and the observed values. The mean squared error (MSE) and root mean squared error (RMSE) further underscore this difference, with values of 0.7491 and 0.8655, respectively. These metrics collectively indicate a comparatively weaker performance of the ROMS model in capturing temperature dynamics, implying potential areas for improvement in its temperature simulation capabilities.

In summary, the model’s performance is deemed appropriate for our study, as evidenced by comparisons with satellite and reanalysis data. Therefore, we utilize these results to study the hydrodynamics of the Aegean Sea during extreme events.

4 Method

The main step in is selection of the appropriate days during 2018–2020. The primary goal is to choose a single severe storm from each of the years 2018, 2019, and 2020. We review some metrological archive news and analysis of ERA 5 (applied on the model) data for finding these days. Consequently, 15 November 2018, 14 September 2019, and 7 January 2020 have been selected. On these days, extreme storms occurred in the Aegean Sea. To enhance comprehension, one and three days following the main storm, as well as two days before the primary storm, are studied. This selection aims to provide insight into the state of the Aegean Sea’s oceanography before, during, and after the storm by observing these five days. To facilitate a more robust evaluation of changes, the data for the chosen days has been subtracted from the monthly average for all values. In Sect. 6, we examine the wind field as it serves as a key parameter to indicate storm conditions in the Aegean Sea. Although changes occur in various atmospheric parameters such as air pressure and flux during storms, these alterations are not explicitly presented in this paper. ROMS, however, integrates all these atmospheric variables in its computations as bulk fluxes.

5 Storm-induced wind patterns

This section examines storm characteristics for selected days. The wind field deviation from the monthly average around the center of the Aegean Sea and the Thracian Sea is minimal on 13 November 2018 (Fig. 4). On the other hand, considerable variation is observed in the southern part of the Aegean Sea and the Sea of Crete, with a speed difference of approximately 2 to 3 m/s from monthly average data. Wind in these parts blows from east to west. On 15 November, however, there is a remarkable change, as a storm with a speed deviation of 12 to 13 m/s occurred. From northeast to southwest, the storm impacted the central and northern Aegean Sea, as well as the central and southern parts of the Sea of Crete. Moreover, the western coast experiences the most substantial deviations, ranging from 13–15 m/s. The wind field strength decreases by approximately 2 to 4 m/s on 16 November. The wind field deviation decreases to less than 7 m/s on 18 November, and the prevailing direction shifted from northwest to southwest.

Fig. 4

a The wind field on selected days in the years 2018, b 2019, and c 2020. The wind values on these days are calculated by subtracting the corresponding monthly averages, providing a comparative view. For example, on 15 November, the wind values are obtained by subtracting the daily average wind from November 2018

On 12 September, the deviation of the wind field from the monthly average is unremarkable, remaining below 4 m/s. On 14 September 2019, a deviation is from northeast to southwest in the central and northern Aegean Seas and the Thracian Sea, while the southern Aegean Sea and the Sea of Crete experiences a different pattern. On 15 September, conditions are similar, but with a decrease in wind speed, especially in the northern Aegean Sea. On 17 September, the deviation from the average value has become minimal, typically less than 4 m/s.

The strongest storm occurred in January 2020. The wind field deviation from the average on 5 January is not remarkable, less than 5 m/s. On 7 January, the wind speed difference from monthly average has reached over 19 m/s, with a prevailing direction from northeast to southwest. A cyclonic wind field with a scale of more than 100 (km) is observed on the east coast which is less than 10 m/s. On 8 January, the deviation of the wind field in the north and center of the Aegean Sea decreases to less than 15 m/s. A southerly wind field, on the other hand, is in the southern regions and in the Sea of Crete. Also, a cyclonic wind field is also visible in the east coast. It is by 10 January that the wind field deviation from the monthly mean reached less than 10 m/s.

6 Results

The purpose of this section is to explore the influence of storms on oceanographic parameters, including temperature, salinity, sea level, and circulation, as well as salinity and temperature flux. This section attempts to explore by subtracting the daily average values of selected days from the corresponding monthly averages. The resulting values are then visually presented, offering a clearer insight into the anomalies linked to storm impacts on oceanographic parameters.

6.1 Temperature variation

On 13 November 2018, a detailed analysis of sea surface temperature deviations reveals a range of –2 to 3 °C from the corresponding monthly average (Fig. 5). In the western part of the central Aegean Sea and the Thracian Sea, there is a 2-degree reduction in temperature. In contrast to the monthly average, the eastern part of the central Aegean Sea exhibits a significant 2-degree increase in temperature.

Fig. 5

a Temperature anomalies on selected days compared to monthly average temperatures for the years 2018, b 2019 and c 2020. The daily average temperature is determined by subtracting it from the corresponding monthly value

A temperature decreases of 1–2 °C is observed along the west coast, the north Aegean, and the Thracian Sea on 15 November. In contrast, relatively insignificant changes are observed in the central Aegean Sea. On the west coast, specifically in the latitude range of 36–38°, temperatures reduce by 1–2 °C. Similar trends are observed in the Sea of Crete, where the temperature decreases by 2 °C compared to 13 November. As temperatures continue to decline on 16 November, the Thracian Sea, and the west coast between latitudes 38 and 40°, as well as the Sea of Crete, experiences a decrease of 0.5 to 1 °C. Surface temperatures reduce by 3.5 °C from average on 18 November. The west coast between latitudes 23 and 24°, the east coast between latitudes 40 and 42° are particularly affected.

On 12 September, temperature differences ranging from – 3 to 2 °C from the monthly average. The temperature on 12 September is 2–3 °C lower than the monthly average in the north and north-west of the domain. Further, temperatures are 2 °C higher than average in regions along the central and western coasts below latitude 38°. As of 14 September, a decrease of 1 to 2 °C is observed in the North Aegean and the Thracian Sea, particularly along the west coast between latitudes 39 and 40°. In September, the downward trend in sea surface temperatures persists throughout almost the entire domain of the sea, marking the lowest sea surface temperatures recorded in comparison to the monthly average.

The temperature fluctuates within – 2 to 2 °C of the average monthly temperature on 5 January 2020, with most areas experiencing warmer temperatures than average, except for the west coast. On 7 January, temperatures decrease by a negligible amount, while the most significant changes occur along the west coast between latitudes 36 and 38°. Between 8 and 10 January, minimal fluctuations are observed, with the water being approximately 1 °C colder on 10 January than it was on 8 January.

Consequently, storm events in the Aegean Sea leads to sea surface temperatures anomalies compared to the monthly average data. Several regions experience remarkable temperature reductions following the storms, particularly along the west coast and north part.

6.2 Salinity variability

On 13 November 2018, salinity levels exhibit deviations ranging from – 2 to 2 PSU from the monthly average. The most notable variations are observed in the north Aegean, where salinity levels reach up to 2 PSU above the average (Fig. 6). Conversely, latitudes below 39.5° shows a decrease in salinity of up to 1.5 PSU below the monthly average, while latitudes less than 36° presents negligible changes. By 15 November, salinity rises notably, especially in the middle Aegean. A salinity increase of 1 PSU is observed between latitudes 38 and 39°. In contrast, changes in salinity are negligible in the Sea of Crete and the southern Aegean. The trend of increasing salinity continues 16 November. As of 18 November, the deviations from the monthly data decreases.

Fig. 6

a Salinity anomalies on specific days relative to monthly average salinity for the years 2018, b 2019 and c 2020. The daily average salinity is calculated by subtracting it from the corresponding monthly salinity

On 12 September 2019, salinity shows fluctuations of 1 to 2 PSU from the September monthly average. Some regions, notably in the middle and north, have salinity levels higher than the monthly average, while others, especially on the west coast, exhibit levels 2–3 PSU below the average. The most significant effects are observed on 14 September in the northeast part of Saros Gulf (40°N and 27°E), where salinity increase by 1 PSU compared to 12 September. Furthermore, there is a 0.5 PSU increase in salinity in the western part of the central Aegean Sea. On 17 September, there are no noteworthy changes in salinity compared to the pre-storm conditions.

As of 5 January 2020, salinity variations from the monthly average in the Aegean Sea range from – 1 to 3 PSU. A significant salinity rise is observed on 7 January, with levels increasing by around 1–2 PSU, especially in the northern and central Aegean Sea. Salinity levels in the middle and north Aegean Sea increases by 0.1 to 0.3 PSU on 8 January. Despite this, salinity levels remain higher than normal on 10 January compared to the 5 January.

The storms observed in November 2018, September 2019, and January 2020 caused distinct changes in salinity in the Aegean Sea. Notable variations include increased salinity in the north Aegean, localized changes along the west coast, and broader impacts on the northern and central Aegean. In summary, storms tend to elevate salinity, especially during colder months.

6.3 Storm-driven sea levels

On 13 November 2018, there is a notable variation in the sea surface level, with a height 6 to 10 cm above the monthly average (Fig. 7). There is, however, a distinct pattern on the western coast between 38.5°and 39.5°, where the sea level is 6 cm below the average. With a sea level elevation 12 cm higher than the monthly average, the northwestern region of the Sea of Crete displays the largest deviation. As the storm begin on 15 November, a decline in sea level is observed, particularly in the Thracian Sea and north Aegean. There is a 5 cm decrease on the west coast between 39–40°, whereas other areas show a 3 to 5 cm reduction. As of 18 November, the surface level has risen by approximately 2 to 3 cm in comparison to 16 November.

Fig. 7

a Anomalies in Sea surface level on specific days compared to the monthly average Sea surface level for the years 2018, b 2019 and c 2020. The daily average sea surface level is derived by subtracting it from the corresponding monthly sea surface level

As of 12 September 2019, surface levels exceed the monthly average, with the most notable increase occurring on the west coast within 37°, reaching a height of 12 cm. Between 39 and 40°, there is an 8 to 10 cm rise in sea level compared to the monthly average. Other regions experience an increase of 4–5 cm compared to the monthly average. In most areas, a decrease of 4–5 cm is observed on 14 September compared to 12 September, while on 15 September, a slight decrease of 1–2 cm is noted compared to 14 September. However, on 17 September, there is an increase.

Relative to the monthly average, surface levels show an elevation of 5 to 8 cm on 5 January 2020. The Thracian Sea and the west coast exhibit the highest sea levels, surpassing the average by 10 cm. On 7 January, there is a significant decrease of 20 cm on the east and west coasts of the north Aegean Sea compared to 5 January, with other areas experiencing decreases of 5–10 cm. 8 January sees an increase of 2–3 cm in the north and middle Aegean Sea, and on 10 January, an increase of 6–10 cm is noted, though it remains 3–5 cm less than on 5 January.

Consequently, examining sea surface levels in November 2018, September 2019, and January 2020 reveals dynamic fluctuations influenced by Aegean Sea storm events. Notably, storms are observed to contribute to a reduction in sea level, with the most significant changes occurring in January 2020.

6.4 Storm-driven current

A significant contrast in surface currents is observed on 13 November 2018, particularly along the west coast and within the latitudinal ranges of 36–37° and 38–40°, in comparison to the monthly average (Fig. 8). During this period, the current speed is elevated by 0.3 to 0.4 m/s compared to the monthly average. A noticeable variation of 0.2–0.3 m/s is observed in the Thracian Sea relative to the monthly average. Additionally, the formation of eddies is observed in the northern part of the Sea of Crete. However, the differences between 13 November and the monthly average are minimal in other parts, generally remaining below 0.1 m/s.

Fig. 8

a Anomalies in sea surface circulation on specific days relative to the monthly average sea surface circulation for the years 2018, b 2019 and c 2020. The daily average sea surface circulation is calculated by subtracting it from the corresponding monthly sea surface circulation

On 15 November 2018, surface currents intensify in the north Aegean Sea and the Thracian basin, resulting in a 0.1 to 0.2 m/s increase in speed. A significant change in both speed and direction occurs in the middle Aegean region between latitudes 38 and 39°, showing a 0.3 to 0.4 m/s increase compared to 13 November. A rise in speed is also observed at 36° latitude and 24° longitude. On 16 November, there is a decrease in surface velocity by approximately 0.1 m/s in the central and northern Aegean. However, by 18 November, sea conditions have largely returned to those observed on 13 November.

Compared to 12 September 2019, both anticyclonic and cyclonic gyres exhibit increased strength in the north Aegean, reaching speeds of approximately 0.6 m/s. Similarly, the middle Aegean experiences a rise in speed, while the Sea of Crete undergoes a shift in current from east to west. Notably, the strength of the current exhibit higher speeds in the Aegean Sea on 17 September, even after the storm, compared to the pre-storm conditions.

On 5 January 2020, significant variations occur, particularly in the Thracian Sea before the onset of a storm compared to the monthly average. The current strengthens by 0.1 m/s, especially along the western coast below 38° latitude, with negligible differences in other areas. Notable changes are observed on 7 January where the anticyclonic gyre in the Thracian Sea strengthens, and the east–west flow increases by 0.3 to 0.4 m/s across the entire basin from northeast to southwest. A robust current along the western coast transports water southward at a speed of 0.7 m/s. On 8 January 2020, the general circulation shows no significant changes, but the current speed decreases by 0.2–0.3 m/s. Meanwhile, the current in the north of the Sea of Crete remains 0.1 m/s stronger than on 7 January. As of 10 January, the current speed further decreases, with the Thracian anticyclonic gyre and the west coast current in the latitude range of 36–38° exhibiting the most significant change compared to 5 January.

Overall, the most changes occur on 7 January 2020, reveals remarkable fluctuations in both speed and direction of surface currents when compared to other storm periods. Among all the study area, remarkable effects are observed along the west coast, in the Thracian Sea, and along the north Aegean, highlighting pronounced regional differences.

6.5 Variability in temperature flux patterns

This section investigates alterations in temperature flux within the study area during storm events. Temperature flux is determined by multiplying temperature data and cell volume, with components analyzed independently along longitude (Huon_temp) and latitude (Hvom_temp).

Compared to the monthly average data, notable differences in temperature flux are observed on 13 November 2018, particularly along the west coast and north of the Sea of Crete (Fig. 9). However, on 15 November, the most important changes are in the east–west movement of the temperature flux. Increase of 10,000–20000 °C m³/s are observed particularly in the north and center of the Aegean Sea below 36° latitude. In the south of the Sea of Crete, distinct variations near the open boundary result in a westward flux of 30,000–40000 °C m³/s and an increase of 5000 – 10,000 °C m³/s in the Thracian Sea. The east–west temperature flux decreases by approximately 10,000 °C m³/s on 16 November, particularly in the Thracian and middle Aegean seas. Temperature flux distribution patterns return to pre-storm on 18 November.

Fig. 9

a Anomalies in temperature flux along longitude on specific days compared to the monthly average temperature flux for the years 2018, b 2019 and c 2020. The daily average temperature flux is derived by subtracting it from the corresponding monthly temperature flux

The most marked differences occur at latitudes below 38° on 14 September 2019, when the temperature flux increases by 15,000–20,000 °C m³/s compared to the monthly average and by 10,000–20,000 °C m³/s compared to 12 September. By 15 September, the increase in east–west flux has decreased, and on 17 September 2019, conditions in the Aegean Sea are similar to those on 12 September. The greatest difference occurs on 7 January 2020, compared to 5 January. Across the entire study area, the temperature flux increased by 20,000–40,000 °C m³/s from east to west, except for minimal differences along the east coast of the Sea of Crete. The temperature flux from east to west has decreased by 8000 °C m³/s on 8 January. Despite a decrease in flux transport on 10 January, there is still an increase in transport from east to west compared to 5 January.

Temperature fluxes are generally less affected in latitude direction (Hvom_temp). On 15 November 2018, the temperature flux moves 5,000–10,000 °C m³/s more towards the north in the center and north of the Aegean Sea (Fig. 10). Furthermore, on 14 September, the temperature flux difference between 14 and 12 September is not substantial. However, there is a notable movement of temperature flux from the north to the south on 7 and 8 January, with 30,000–40,000 °C m³/s, particularly on the west coast.

Fig. 10

a Anomalies in temperature flux along latitude on specific days relative to the monthly average temperature flux for the years 2018, b 2019, and c 2020. The daily average temperature flux is calculated by subtracting it from the corresponding monthly temperature flux

In summary, the analysis indicates that variations in temperature flux are more pronounced along longitude (Huon_temp) than along latitude (Hvom_temp). Furthermore, the January storm exhibits the most significant and distinctive changes in temperature flux when compared to other storm periods.

6.6 Variability in salinity flux patterns

The calculation of salinity flux involves multiplying salinity by the volume of each cell, with components separated along both longitude (Huon_salt) and latitude (Hvom_salt). The findings indicate that Huon_salt exhibits more pronounced fluctuations in its pattern during the storm.

On 13 November 2018, the salinity flux shifts predominantly from west to east, ranging between 30,000 and 70,000 PSU m³/s compared to its average (Fig. 11). On 15 November, a marked shift in the direction of the salinity flux is observed along longitude in the entire study area. The salinity masses move from east to west at rates ranging from 30,000 to 90,000 PSU m³/s. With the greatest movement taking place at latitudes between 36 and 39°. Even though the salinity flux slows to the west on 16 November, there are still differences from the pre-storm conditions. On 18 November, the salinity flux distribution conditions are similar to those on 13 November.

Fig. 11

a Anomalies in salinity flux along longitude on specific days in comparison to the monthly average salinity flux for the years 2018. 2019 (b) and 2020 (c). The daily average salinity flux is calculated by subtracting it from the corresponding monthly salinity flux

The salinity flux typically moves along longitude from east to west at a rate of 30,000–60,000 PSU m³/s on 12 September. On 14 September, there is a noticeable change in salinity flux direction, with a predominant movement from east to west across the entire study area, ranging from 30,000–70,000 PSU m³/s. Although the salinity flux slows down on 15 September, it continues to move from east to west. On 17 September, however, sea conditions return to their pre-storm state. As of 5 January, the pre-storm salinity flux moves from west to east at a rate of 30,000 to 60,000 PSU m³/s. At latitudes above 36°, a southward movement is observed on 7 January, ranging from 30,000–90,000 PSU m³/s. The westward movement of salt mass decreases on 8 January and continues to decrease until 10 January, although it remains different from 5 January.

Salinity flux along latitude (Hvom_salt) on 13 November 2018 shows a southward movement ranging from 30,000–80,000 PSU m³/s (Fig. 12). On 15 November, the salinity mass moves primarily southward at latitudes below 37°, while it moves more rapidly towards the north at latitudes above 37°. Following the storm, sea conditions return to their pre-storm condition. The storm causes the salinity flux to move southward from the west coast in September, continuing until 17 September. The salinity flux on 7 January, as compared to 5 January, exhibits a southward movement of approximately 30,000 PSU m³/s, continuing in the days following the storm, with a more saline flux moving along the western coast from the north to the south.

Fig. 12

(a) Anomalies in longitudinal salinity flux on specific days for the years 2018, 2019 (b) and 2020 (c). The daily average salinity flux is determined by subtracting it from the corresponding monthly salinity flux

As a result, salinity flux shows remarkable changes along longitude. Comparatively to other storm events, the January storm induces remarkable changes in the direction and magnitude of salinity flux.

6.7 Vertical structure

This section aims to assess the impact of storms on the vertical structure of the Aegean Sea, focusing on temperature, salinity, water volume transport, and kinetic energy across different layers. The methodology involves subtracting each parameter from the corresponding monthly average and calculating the average within each layer. Water volume is computed by squaring the volume in each direction, summing them, and dividing the result by 2. Kinetic energy is determined similarly, with the velocity squared in each direction, summed, and the total divided by 2.

In November 2018, the profile of values shows that the storm has the greatest impact on the temperature from surface to layer 15(Fig. 13). At the surface layer, it is 1.5 °C lower than the average on 13 November, but with the beginning of the storms, the average temperature decreases 0.5 °C. On 18 November, the temperature is 1 °C colder than 5 days before. In deeper layers, the influence of storms on temperature diminishes. However, salinity exhibits two different behaviors: in the first five layers, salinity increases by 0.1 PSU on 15 November compared to 13 November. After storm, the differences are 0.15 and 0.25 PSU on 16 and 18 November respectively. However, in the layers 1 to 20, salinity decreases during storm. We observe an increase in the volume of water transferred at the surface on 15 November compared to two days before. This shows an increase of 0.2 × 10⁸ m³/s, but we can only see this behavior in the first five layers(from surface), which indicates that the storm has the greatest effect near the surface. At the bottom, 0.2 × 10⁸ m³/s increase in water can be seen on 16 November compared to 13 November. The highest kinetic energy related to the surface layer is on 15 November, 600 J/kg higher than on 13 November. This difference decreases up to 10th layer although still the kinetic energy is higher after 13 November.

Fig. 13

a Differences in temperature, salinity, volume, and energy profiles between daily data and their corresponding monthly averages for the years 2018, b 2019 and c 2020. The subtraction of daily data from monthly data is performed to illustrate these distinctions

Compared to 12 September, the temperature on 14–17 September has decreased to 1 °C on the surface, but we do not see this difference in layers below 20. In the salinity profile, we can see that the storm caused an increase in salinity on days 14 to 17 compared to 12 September, which is up to 0.1 PSU, but this difference is decreasing until layer 15. After that the trend is different and the salinity on 12 September is slightly higher. The volume of transferred water in the storm increases compared to 12 September, while in the surface layer on 14 September, it is 2 × 10⁸ m³/s more than 12 September. But in the lower layers, 12 September also has the lowest volume. The highest kinetic energy is on 14 September, which is 1,250 J/kg higher than the average and 700 J/kg higher than 12 September. But in layers less than 15, this difference decreases.

After 5 January, the temperature experiences a decrease due to the storm. The difference is 0.5 °C compared to 5 January. During the storm there is a rise in salinity. Specifically, on 7 January, salinity exhibits an increase of 0.2 PSU. Subsequently, until 10 January, the surface salinity displays a rise of 0.4 PSU, with a diminishing trend as we move towards deeper layers. The volume of water in different layers has increased by about 0.5– 0.8 × 10⁸ m³/s compared to 7 January. Notably, we can see this difference from the surface to the bottom. Moreover, 7 January has the highest kinetic energy, approximately 2000 J/kg above the monthly average, and approximately 1500 J/kg above 5 January, and this difference decreases with the decrease in layer thickness.

In summary, storm impacts on the Aegean Sea’s vertical structure are most prominent near the surface, affecting temperature, salinity, water volume transport, and kinetic energy. Temperature changes extend to layer 15, while salinity behaves differently in various layers. Water volume transport increases significantly near the surface during storms. Kinetic energy peaks near the surface, gradually diminishing with depth.

7 Summary and conclusion

To assess the impact of severe storms, the study selected three storms occurring on 15 November 2018, 14 September 2019, and 7 January 2020. Analyses are conducted one and three days after each storm, as well as two days prior. This study allows for a detailed examination of variations in physical oceanographic properties by subtracting the data for these days from the corresponding monthly average.

During storms, the sea surface temperature sees a decrease of 2–3 °C compared to before the storm. Salinity experiences a rise of 0.5–1 PSU due to storms, with the most pronounced impacts observed on 7 January. These findings can be attributed to two primary factors. Firstly, the cooling air system induces a temperature decrease, and storms enhance evaporation, contributing to the observed increase in salinity. Additionally, storms can lead to the mixing of surface water with deeper water, resulting in a decrease in temperature and an increase in salinity, as deep water tends to be colder and more saline. One of the reasons supporting this perspective is that in the vertical layers (Fig. 13) while storms increase salinity at the surface during the storm, in deeper layers, storms result in a slight decrease in salinity. This could indicate an increase in vertical mixing.

The observed pattern of increased salinity and concurrent reduction in temperature, especially noted in the month of January, holds notable significance. This unique combination of environmental conditions during severe storms creates an optimal scenario for the formation of dense water in the Aegean Sea. The increased salinity resulting from storm-induced processes, such as enhanced evaporation and mixing of surface and deeper waters, contributes to the density of the water mass. Simultaneously, the decrease in temperature, driven by factors like cooling air systems and storm-induced evaporation, further enhances the potential for water mass densification. The formation of dense water has implications for ocean circulation, nutrient distribution, and broader ecological processes in the Aegean Sea.

During a storm, the sea level experiences a reduction of 5–10 cm. The reduction in sea level during a storm in the Aegean Sea is primarily instigated by the forceful winds. These strong winds induce a phenomenon referred to as wind setdown, where water is displaced away from the storm’s center or the direction of prevailing winds. The synergistic impact of these factors results in a temporary decline in sea level in specific regions, notably along the north and middle of the Aegean Sea. Consequently, we have observed a decrease in sea level.

Surface currents displayed an increase of 0.3 to 0.4 m/s along the west coast and within latitudes 36–40° on 13 November, surpassing the average currents. On 5 January, storm-induced alterations in the Thracian Sea’s anticyclonic gyre are identified. The heightened current strength and intensified gyre activity can be attributed to the increased atmospheric energy and momentum transfer to the surface water. During storms, atmospheric forces transfer heightened energy and momentum to the ocean, inducing a more vigorous circulation of surface currents and reinforcing the gyre dynamics in the studied regions. This enhanced interaction between the atmosphere and the sea surface amplifies the water response, resulting in notable changes in current patterns and gyre strength during the specified storm events in the Aegean Sea.

During storm events, substantial changes in temperature and salinity flux are observed. The storm of January 2020 exhibited the most significant and distinctive changes, particularly along longitudinal: why? Examining the storm’s direction reveals a noteworthy phenomenon: while the storm itself moves from northeast to southwest, the transport of water occurs predominantly towards northwest and west directions. This can be attributed primarily to Ekman transport, a phenomenon arising from the interaction of wind stress with the ocean surface layer. As the wind-driven movement imparts momentum to the surface water, the Coriolis effect induces a net transport of water at an angle to the wind direction. In this specific context, the resulting Ekman transport leads to a distinct divergence from the storm’s trajectory, showcasing the complex dynamics influencing the oceanic response to meteorological forces.

The in-depth analysis of the vertical structure during storm events underscores an important finding: the most pronounced effects of the storm are concentrated near the sea surface and within the mixed layer of the ocean. This phenomenon can be attributed to the dynamic interactions occurring in the upper layers of the sea, where the storm’s energy and atmospheric disturbances have a more direct and immediate impact. As storms unfold, the mixing and turbulence induced by strong winds and other meteorological factors primarily influence the upper layers of the ocean. This heightened sensitivity of the surface and mixed layer to storm-induced changes is a result of the intricate interplay between atmospheric conditions and water responses.

This study delves into the substantial impact of storms on the oceanographic characteristics of the Aegean Sea, particularly emphasizing their importance during colder months in influencing the formation of dense flows. The alterations in currents, especially in the middle and northern Aegean, play a crucial role in the rapid transport of pollutants and nutrients within a short timeframe, typically a few days. While storms can generate both waves and currents, understanding the specific role of currents is essential, as they serve as key parameter in transporting pollutants and nutrients. This comprehension is crucial for unraveling the complex processes influenced by storms in the Aegean Sea and their implications for the marine environment.

While the study primarily focuses on analyzing specific storm events in the Aegean Sea, it also provides valuable insights that can be generalized and applied to broader oceanographic research. Despite examining only three storms, it’s essential to recognize that these results offer insights into the general behavior of the Aegean Sea. For example, the occurrence and direction of Etesian winds, a significant wind component, closely align with the main wind patterns observed during the studied storms. Given that these winds persist throughout the year, particularly in September, and share similar directional characteristics, this study can provide useful general results for understanding the hydrodynamics of water in the region. Therefore, these findings can be considered representative of the broader characteristics of the Aegean Sea.

However, it’s crucial to acknowledge the inherent limitations and uncertainties in the study. The findings are based on a specific set of storm events and may not fully capture the entire range of variability present in the Aegean Sea. Additionally, uncertainties associated with model simulations and data analysis methods may affect the generalizability of the results. Future studies could address these limitations by incorporating a wider range of storm events, utilizing ensemble modeling approaches, and conducting sensitivity analyses to assess the robustness of the findings. In conclusion, while the study offers valuable insights into the impact of severe storms on oceanographic processes, caution is warranted when extrapolating the findings to other regions or drawing broad conclusions for the scientific community.