*3.5. Coastal Upwelling*

Coastal upwelling is a natural hydrodynamic phenomenon, which is caused by favorable alongshore (the coast should be on the left in the Northern Hemisphere) and offshore winds. It leads to the upwelling of relatively cold waters from deep layers right in the coastal zone with a temperature minimum near the shore. This is a large-scale process, which in the coastal zones of North-West and South-West Africa, and North-West and South-West America are thousands of km long, a hundred km wide, and in some places are observed all year round with seasonal variability of its intensity at northern and southern limits of the upwelling zone. In the Black Sea, there are no permanent upwelling zones, upwelling can arise sporadically at all coastal zones when favorable winds start to blow [62–66]. The main problem for beach tourism concerns the fact that during the upwelling event, the SST may drop by 10–15 ◦C from about 25 ◦C, which is a typical and comfortable temperature at the coasts of the Black Sea, and last from several days to several weeks. As a result, people cannot enter water even at an air temperature of 30 ◦C. The most known analog of this problem is the western coast of Portugal, where during summer coastal upwelling occurs every year, and it is the main cause why, at these coasts of Portugal, there are no resort areas while at the same latitudes in Spain, along the

Mediterranean coast, there are dozens of well-known resort areas and beach tourism is very popular.

Silvestrova [65] reported that from 1979 to 2016 120 cases of coastal upwelling were registered near Gelendzhik, which is one of the most important resort areas on the Black Sea coastal zone of Russia. In 51 cases, the SST drop was greater than 5 ◦C. According to this research, strong upwelling near Gelendzhik occurs every year with no evident tendency to increase or decrease in the frequency of appearance. In 1987, eight upwelling events were recorded, in 1995, 1996, 2004, 2009, and 2013—5–6 events. Stanichnaya and Stanichny [66] investigated the occurrence of coastal upwelling in the summer season between 15 May and 15 September. They showed that the average time of one upwelling event is of 5–7 days, while in some cases it can last 14–15 days. For the coastal zone of Crimea, from 1997 to 2011, the number of upwelling events varies from 5 to 12 per year (summer), the number of strong upwelling events with an SST drop of more than 5 ◦C—from 1 to 5 events, and the total duration—from 20 (2002) to 70 days (2011). Minimal SST values reached 8–9 ◦C, and the maximum difference with the surrounding waters was 15–16 ◦C. For instance, only during summer 2003, coastal upwelling occurred on 22–23 May, 7–11 June, 13–20 June, 22–25 June, 27 June–2 July, 7–21 July, 22–25 July, 13–16 August, and the SST near the shoreline varied from 10 to 17.5 ◦C instead of the typical 20–24 ◦C in-between the upwelling events [66].

Two examples of these upwelling events that have occurred along the coast of the Crimea and the Krasnodar Krai right during the summer tourist season are shown in Figure 16. On 30 July 2017, a strong upwelling was observed between Sevastopol and Yalta, where the SST was as low as 11–12 ◦C, while in the surrounding waters it was as high as 24–25 ◦C (Figure 16a). On 2 July 2018, the upwelling was detected along the Caucasian coast from Abkhazia in the south to Novorossiysk in the north (Figure 16b). Then, from 2 to 10 July it propagated from south to north reaching the area between Anapa and the Kerch Strait. The SST along the shore was 20–21 ◦C, while in the offshore waters it was 27 ◦C.

**Figure 16.** (**a**) Coastal upwelling (shown in SST) at the southern coast of the Crimea on 30 July 2017 (VIIRS-NPP, 10:18 GMT); (**b**) coastal upwelling (shown in SST) along the coast of the Krasnodar Krai on 2 July 2018 (MODIS-Aqua, 10:55 GMT) (courtesy by D.M. Soloviev, Marine Hydrophysical Institute).
