*3.2. Sea Warming*

The sea surface temperature (SST) is a second important factor for coastal tourism. The SST in the Black Sea, like the air temperature, has significantly increased over the past 40 years. Since the end of the 1990s, the maximum monthly average summer SST values have increased. Summer highs in most cases exceeded 25 ◦C with an extreme of 26.94 ◦C in August 2010. The maximum/minimum summer SST values correspond to approximately the same years as for the air temperature. The SST linear trend for the whole Black Sea for 1982–2020 is equal to + 0.052 ◦C/year, which is identical to the air temperature rate of change. The SST trends are unevenly distributed over the Black Sea area: the highest values (+0.058–0.060 ◦C/year) are observed along the northeastern coast of the sea (Russian coast), the minimum (less than + 0.044 ◦C/year)—in the center of the Western Black Sea (in the area of the western cyclonic gyre) and at the northwestern shelf of the sea [35]. Roughly the same picture of the distribution of SST trends over the sea area for the period 1981–2015 was presented in [39]. In the northeastern part of the sea from 1983–2015, the SST trends were recorded within + 0.075–0.084 ◦C/year [40].

In the present study, we are interested in the SST variability during summertime, this is why we calculated the interannual variability of SST for the summer season (June, July, August) only for a 40-years-long time period (1981–2020) and for the northeastern part of the Black Sea (Figure 5). We found that, on average, in the 1980s, the SST was around 22 ◦C, in the 1990s—23 ◦C, in the 2000s there was a sharp rise of the SST from 23 ◦C to 26 ◦C in 2010, and in the 2010s the SST stabilized around 25 ◦C. We have to note that features of interannual variability of the SST are similar to those characteristics of the air temperature (see Figure 2).

**Figure 5.** Interannual variability of the sea surface temperature in the northeastern Black Sea during summer from 1981–2020 based on the MERRA-2 M2TMNXOCN V5.12.4 Model.

> We also repeated the same analysis for May and September (Figures 6 and 7) to understand in what direction the summer hydrological season is expanding. May shows a slowly warming, but only in 2012, 2013. and 2018 the SST was over 18 ◦C, and in the past decade, the SST varied from 14.76 ◦C in 2011 to 18.76 ◦C in 2018. Thus, the SST in May became much more unstable in comparison with the previous three decades and the SST remains too cold to open the swimming season (Figure 6). Similar to the case with the air temperature, September is already becoming a fully-fledged tourist month suitable for comfortable swimming because the SST during the past two decades has varied between 21.75 ◦C in 2003 and 24.87 ◦C in 2020 with a steady tendency to increase (Figure 7). September and October of 2020 were exceptionally warm, and it was possible to swim in the coastal zone of Russia till the end of October.

> We also calculated the relationship between the air temperature and SST for the region under investigation and the same time period in the form of the scatter plot shown in Figure 8. It shows a relationship between the monthly mean 2m air temperature and the monthly mean SST. It is interesting that in the middle of the graph, i.e., between 5 and 20 ◦C of the air temperature, there is a hysteresis curve where there are different values of SST depending on the direction of change of the air temperature during a change of seasons. This is an interesting and evident result which is explained by the fact that in spring the air temperature is rising much faster than warming of the sea surface, and in autumn with a decrease in the air temperature, SST remains high for a longer period of time. Thus, at the same values of the air temperature, SST is higher in autumn than in spring.

**Figure 6.** Interannual variability of the sea surface temperature in the northeastern Black Sea in May from 1981–2020 based on the MERRA-2 M2TMNXOCN V5.12.4 Model.

**Figure 7.** Interannual variability of the sea surface temperature in the northeastern Black Sea in September from 1981–2020 based on the MERRA-2 M2TMNXOCN V5.12.4 Model.

**Figure 8.** Relationship between the monthly mean 2−meter air temperature and the monthly mean SST for 1981–2020.

### *3.3. Heavy Rains and River Plumes*

Heavy rains have become a serious problem for the coastal zone of the Krasnodar Krai of the Russian Federation and the Black Sea coast of Turkey. They lead to a significant increase in the water level in rivers, flooding of villages and even large cities, damage to urban and tourist infrastructure, roads and railways, bridges, beaches, and the washout of garbage and sewage into rivers. This leads to environmental problems and even human casualties.

For example, on 8 August 2002, the city of Novorossiysk and the nearby resort area Shirokaya Balka were flooded as a result of heavy rain, where, in less than a day there fell an equivalent of six-month precipitation (Figure 9). The tourist infrastructure was destroyed in Shirokaya Balka. According to official figures, almost 20,000 people were affected. More than 100 people died in the flooded area [41]. On 6–7 July 2012, according to the Hydrometeorological Center of Russia, the equivalent of five-month precipitation (275 mm) fell overnight in Krymsk, Novorossiysk, and Gelendzhik. One hundred and seventy-one people died in the flood, which damaged nearly 13,000 homes and affected nearly 30,000 more. The flooding was part of the aftermath of a severe storm that hit the Krasnodar Krai, which resulted in precipitation of the equivalent of almost six months in two days, and it was the largest in the last 70 years. Heavy rains caused dangerous flooding on rivers, including a catastrophic flood on the Adagum River near the city of Krymsk. The total damage is estimated at RUB 20 billion (USD 0.3 billion). Such torrential rains occur annually, for example: 7–8 September 2018, when torrential rains hit the entire coastal zone from Adler to the Kerch Strait; 24–25 October 2018, when torrential rains destroyed roads, railways, and bridges in the Tuapse region; 24 June 2019, when torrential rains caused landslides and damaged roads and tourist infrastructure in the mountain resort area of Krasnaya Polyana near Adler; 29 June and 16 July 2019, when torrential rains hit the city of Sochi [42].

**Figure 9.** (**a**) Flooded streets and embankment in the center of Novorossiysk on 8 August 2002; (**b**) damaged tourist infrastructure in Shirokaya Balka on 8 August 2002.

The summer of 2020 was rather dry, however the summer of 2021 again witnessed catastrophic rains and flooding. On 17 and 18 June 2021, heavy rains hit the coast of the Krasnodar Krai and of Crimea. In Yalta, the monthly norm of precipitation fell on the first night, in Kerch—2 norms of precipitation. Houses, streets, roads, the federal highway "Tavrida" turned out to be flooded. The hailstorm broke through the windows of cars and roofs of houses in some districts of the Krasnodar Krai. Following this, on June 18 in Yalta, another three monthly precipitation rates fell. For three weeks, the forces of the Ministry of the Russian Federation for Civil Defence, Emergencies, and Elimination of Consequences of Natural Disasters were engaged in dismantling debris, clearing streets, pumping out water, restoring housing, flooded and damaged infrastructure, and cleaning streets. The summer vacation was completely ruined, and the beaches were closed for three weeks. People returned tickets, canceled hotel reservations, and returned home.

On 26 June 2021, heavy rains in Gelendzhik and Anapa (50 mm for one hour) flooded the centers of cities, as well as damaged embankments and beaches. On 4 and 5 July 2021, the cities of Adler, Sochi, Lazarevskoye, Tuapse, Lermontovo, Dzhubga, Novorossiysk, and Anapa were flooded again as a result of heavy rains and the storm surge that occurred along the whole Black Sea coast of Russia from Adler to Kerch (Figure 1). In the Khosta Region of Sochi, one monthly norm of precipitation fell during the day, in the Bakhchisaray Region of the Crimea—2–3 norms of precipitation. More than 300 houses, roads, streets, embankments, beaches, gas and water supply, and hundreds of cars were flooded and broken. The only road leading from Dzhubga to Sochi along the entire Black Sea coast was closed for several days due to landslides and destructions by mudflows in several places. Eight people died, and several dozens were injured.

More heavy rains followed on 23 July 2021 (floods in Sochi and Khosta), 8 August 2021 (floods in Sochi, Tuapse, Novorossiysk, and Kerch), 10 August 2021 (flood in Novorossiysk, 4.5 mm during 1 h, Anapa, Vityazevo), 12–13 August 2021 (floods in Novorossiysk, Anapa, Kerch—2–4 monthly precipitation norm or 40–200 mm during 1 day), 15–16 August 2021 (floods in Gelendzhik, Novorossiysk (100 mm), Anapa (219 mm), Kerch (176 mm), 2000 houses were flooded. Water in Anapa was present till 20 August, 500 houses were still flooded, as well as a dozen power electricity stations which were switched off); 18–19 August 2021 (floods in Anapa—monthly norm).

We calculated monthly average precipitation over the northeastern Black Sea for June– August from 2000 to 2020 and found that since 2007 there was a steady rise of precipitation from 39.2 mm/month in 2007 to 86.8 in 2014 and 88.3 mm/month in 2017 (Figure 10). A peak in 2002 was partially related to the catastrophic rains on 21–22 June and 8 August in the area of Novorossiysk. A relative minimum of 2020 (58.3 mm/month) was related to the drought which was observed from summer 2019 till December 2020.

**Figure 10.** Interannual variability of atmospheric precipitation (mm/month) in the northeastern Black Sea in summer from 2000–2020 based on the GPM\_3IMERGM v06 Model.

Looking at Figure 10, we cannot say that there are changes in heavy rains because: (1) the graph shows average values of total precipitation (mm/month) for three summer months; (2) the shown parameter is average monthly precipitation measured in mm/month but not in mm/hr which should be taken from records at meteorological stations; (3) the values are averaged over a large area. Zolina et al. [43,44], Zolina and Bulygina [45], based on records from meteorological stations, showed that in Europe, including the Black Sea coast of Russia, the structure of precipitation has changed to more abundant rainfalls. During 1950–2008, the occurrence of the association of heavy rainfall with longer WP (wet periods defined as consecutive days with significant precipitation > 1 mm/day) was increasing by 3–4% per decade. The occurrence of intense precipitation associated with longer WPs increased from 40% in the 1950s–1960s to 55% in the 1990s–2000s. Heavy rainfalls associated with longer WPs intensified over Europe with upward trends of 2–3% per decade in Western Europe and >5% per decade in European Russia, implying actual changes from 4 to 9 mm/day over the 60-year period. The changing character of WPs (short rain events have been regrouped into prolonged wet spells) may significantly alter the frequency and strength of floods. The highest values of the thresholds for absolute extreme precipitation (more than 26 mm/day) are observed in the North Caucasian Federal District (including the Caucasian Black Sea coast), which is characterized by a complex orography and the presence of mountain systems [45]. They found that for all seasons of the year, practically throughout the entire territory of Russia, there is a steady tendency towards an increase in the values of extreme precipitation up to 8% per decade (values for 1966–2012), which corresponds to an increase in the absolute values of extreme precipitation by about 4 mm/day over the last 50 years in winter and about 7 mm/day in summer.

Besides damage to coastal infrastructure, numerous rains lead to the cooling of seawater in the coastal zone and may delay the establishment of the comfortable conditions for swimming in the tourist season. Such a case was observed in June 2021, when seawater was colder than usual by about 5 ◦C, while the air temperature was quite typical for this month. The other problem related to heavy rains and river plumes concerns seawater

quality, which is very dirty, with a very high concentration of suspended matter and all kinds of pollutants gathered on land. Usually, after heavy rains, people do not swim in the sea for 1–2 days, in case of repeated heavy rains, and beaches can be officially closed for swimming for a week or more. This was the case in Yalta in the second half of June 2021.

River runoff forms river plumes in the coastal zone of the sea (Figure 11), which, depending on the speed and direction of the wind, the velocity, and direction of the coastal current, the magnitude of the runoff, and the difference in density with seawater, can have different forms and spread in different directions from the river mouth, as well as participate in the mesoscale and sub-mesoscale circulation of coastal waters [42,46–50]. River plumes significantly affect the quality of seawater and the sanitary and epidemiological situation on the beaches of the resort area of the Krasnodar Krai. The situation is exacerbated by the fact that sewerage systems are in poor condition in cities, or there are no such systems at all in most small villages along rivers and coastal areas. After heavy rains, wastewater very often reaches the sea and poses a serious threat to human health.

**Figure 11.** Joint river plumes (light green colors) along the Russian coast of the Black Sea from Novorossiysk to Adler and the border with Abkhazia on 7 July 2021 (Terra MODIS True Color Corrected Reflectance, https://wvs.earthdata.nasa.gov/ api/v1/snapshot?REQUEST=GetSnapshot&LAYERS=MODIS\_Terra\_CorrectedReflectance\_TrueColor,Coastlines\_15m, Reference\_Features\_15m&CRS=EPSG:4326&TIME=2021-07-07&WRAP=DAY,X,X&BBOX=43.335571,37.41211,44.84436, 40.327148&FORMAT=image/jpeg&WIDTH=1327&HEIGHT=687&AUTOSCALE=TRUE&ts=1626260065393, accessed on 14 July 2021).

> Figure 11 shows a satellite view of the coastal zone of the northeastern Black Sea on 7 July 2021 after heavy rains occurred along the whole coast on 4–5 July. Light green colors show numerous river plumes generated at the mouths of small rivers located along the coast. Heavy rains generate huge river runoff which results in merging of separate river plumes into a single turbid water mass which is characterized by a high concentration of suspended matter and all kinds of pollution. River plumes propagate northwestward

along the coast with a general current. Such a situation persists for several days until the flood flow of rivers stops and coastal waters mix with those around them.
