*4.5. Wave Storm Directionality*

Wind and wave directionality in the external locations is shown in Figure 8. The left panels depict the directionality of wind and wave conditions considering the whole datasets, while the right panels illustrate directionality during selected storm events. The upper panels reveal that the predominant wind conditions in the outer flanks of the archipelago are associated with the prevalence of trade winds, giving rise to relatively mild wind conditions blowing principally from the NNE sector (panel a). However, during severe wave storms, the prevailing wind conditions change significantly, except for the eastern strip (EP3), where the average direction remains from NNE, with a very small directional spread, but the intensity increases notably (panel b). Wind at locations in the northern and western flanks become dominated by intense winds blowing from the WNW-NNW sector. It is interesting to note that winds at locations north of Lanzarote (EP2) display mixed conditions, receiving the influence of the trade winds, intensified during the summer, and the arrival of strong winds from the NW during winter. At the southern location (EP5), wind conditions during wave storm situations become dominated by W-SW intense wind conditions, while point EP4, located in the channel between GC and F, receives the influence of winds with western, northern, and northeastern components.

With respect to the directionality of wave fields reaching the islands at the outer edges and their severity (middle panels), average directional conditions depict a similar pattern to that of the wind, with the predominance of low and moderate wave conditions travelling mainly from the N-NE directional sector, but rolling towards NW when shifting away from the African coast along the northern side and especially on the western coast (panel c). This effect becomes very clear during stormy wave conditions (panel d). In these situations, severe wave storms travel from the NW sector, affecting predominantly the western and northern sides of the archipelago. Due to self-sheltering effects, locations at the eastern side receive much less energy, with waves travelling from the NNE, while southern locations are affected by mild or moderate storms, following a similar pattern to that of the winds, including its propagation through the channels formed between islands.

Regarding the bivariate distribution of wave direction and period (lower panels), the situation is almost similar to that of wave height and directionality. Thus, during stormy conditions (panel f), waves approaching northern and western locations have notably long periods, revealing a swell structure. However, locations sheltered from these conditions (EP3 and EP5) receive smaller and shorter waves, indicating the frequent presence of wind-driven seas travelling from the NNE (EP3) and from WSW (EP5).

In brief, it is worth noting that most frequent and severe storms arrive from the N-NW sector and affect mainly the western and northern sides of the archipelago. The eastern flank is subjected to a strong sheltering effect against these events, so that it is almost exclusively exposed to wave fields generated by the trade winds (N-NNE). The presence of channels between islands allows the propagation of NW-NE wave fields towards the eastern and western edges of the central islands' southern flanks. In the southern areas, during storm conditions, waves approaching from the S-SW sector predominate, although these are usually more moderate than on the rest of the flanks.

Even taking into account the above-mentioned uncertainty associated with the reproducibility of some parameters during extreme events with models, the differences are substantially large and consistent, showing meaningful and realistic changes between the characteristics of the wave fields under general conditions and during extreme episodes.

**Figure 8.** Wind and wave roses at outer locations. Wind roses for the whole dataset (**a**), during stormy conditions (**b**). Wave roses for *Hm*<sup>0</sup> during all the studied period (**c**) and under wave storm conditions (**d**). Wave roses for peak wave period during the whole study interval (**e**) and during storms (**f**).

Results from the analysis of wind and wave directionality at the inner points, located along the southwest, south, and southeast areas of Tenerife, in terms of wind speed, *Hm*0, and *Tp*, are shown in Figure 9. Variations in each pair of values are examined under two different types of situation: on the one hand, under average conditions, or average regime, extracted by using the whole dataset, and, on the other hand, during the extreme events selected in each location. Figure 9a,d show that, in general, the dominant direction of the wind regime is NE. Wind speed reaches relatively low values very often (panel a). In contrast, wind speeds observed during wave storms are substantially higher and directionali-ty exhibit a more complex spatiotemporal pattern. In the southwest locations of the island (IP1), the dominant wind direction during wave storms (panel d) shows a considerable dispersion in the NW-NNW sector, although the intensity of the wind is notable only during periods when the wind flows from the NNW, passing through the G-T channel. At points located on the southern flank, the intensity is notably higher than the global average, but the direction in which the wind flows is the opposite, highlighting the presence of strong wind events from the SW. On the other hand, in the southeast

locations (IP4, IP5), the predominant direction remains around the NW, but with strong winds blowing along the GC-T channel.

**Figure 9.** Wind and wave roses at inner locations. Wind roses for the whole dataset (**a**), during stormy conditions (**d**). Wave roses for *Hm*<sup>0</sup> during all the studied period (**b**) and under wave storm conditions (**e**). Wave roses for peak period during the whole study interval (**c**) and during storms (**f**).

With regard to wave conditions south of Tenerife, it should be noted that, under global conditions (panel b), the southern coasts rarely experience wave conditions with *Hm*<sup>0</sup> values above 2 m. The most western point (IP1) receives waves from the NNW, while waves reaching the southern (IP2, IP3, IP4) and eastern points (IP5) travel predominantly from the NE. Under severe conditions (panel e), the general pattern of wave directionality is practically the same as for wind in these conditions (panel d). However, note that at the westernmost point (IP1), the directional dispersion observed for the wind disappears, with waves travelling almost exclusively from the NNW, through the G-T channel.

The joint distribution of wave direction and peak period reflects an average behavior (panel c) very similar to that observed for wave direction and height. However, it is interesting to note that *Tp* values associated with storms observed at south and southeast areas (panel f) correspond mainly to locally wind-driven seas or young swell waves. In particular, waves reaching point IP5 and IP4 are associated with dominant trade winds and are funneled through the GC-T channel. Wave storms detected in the southwest points (IP1) are often due to wave fields reaching these coasts from distant storms located in the northwest area of the North Atlantic, through the channels between G-T, while those located in the southern border (IP2, IP3) show a much more complex pattern generated by the alternation of storms arriving through G-T and GC-T channels, as well as less frequently from the SSW-S directional sector.
