Variability in High Wave Energy Events Around the Southern African Coast

Abstract

The wave energy flux along the southern African coastline often reaches extreme levels, seriously impacting coastal communities, infrastructure, as well as near‐coast and offshore marine operations. Understanding the drivers behind past high wave energy events and their frequency is key to forecasting future events. Using in‐situ wave buoy data recorded by the Council for Scientific and Industrial Research (CSIR), the ERA5 and CACWR global wave model products are evaluated. The better‐performing ERA5 product is used to assess long‐term variability and trends around the coastline between 1979 and 2020. There are significant increasing trends in offshore flux for all seasons, with spring having the strongest coastal trends. Substantial interannual variability exists in wave energy flux and direction. Possible drivers, the Southern Annular Mode (SAM), El Niño Southern Oscillation (ENSO), and the semi‐annual oscillation (SAO), were assessed to find that SAM appears to show the strongest relationship overall. Negative (positive) SAM is associated with above (below) average and westerly (easterly) anomalies in both flux and direction. ENSO directly impacts the summer wave climate while the SAO indirectly impacts the wave energy flux over spring and winter. During many seasons, more than one climate mode is active. It is found the strongest significant near‐coast positive westerly anomalies occurred under negative SAM and negative SAO, with more intense offshore anomalies under El Niño, whereas the strongest significant negative east‐northeasterly anomalies occurred under a combination of La Niña with positive SAM and SAO phases.

Wave Climate Variability along the Coastlines of Senegal over the Last Four Decades

“…Knowledge of these remote connections is interesting as it would indicate that lower-than-average wave energy conditions on the coastline would occur when the two modes, NAO and EA, are concomitantly endowed with their highest positive phase index. Similar observations characterized by several climatic modes active simultaneously have already been reported in the Atlantic (e.g., [16,32,33]). When it is possible to predict these modes on a seasonal scale or even further on a climate change scale, then an idea will be made of the future wave regimes on the Senegalese Coast (and in the region).…”

“…The Hs increase observed in this study is partly explained by the upsurge of South Atlantic storminess associated with SAM. Several recent studies have also relayed an increase in South Atlantic storminess (e.g., [15,16,72]), and other research has associated this upsurge with the strong positive trend observed on the SAM index (e.g., [7,15,34,42]). Similarly, Bertin et al [14] found an increase in Hs in the North Atlantic Ocean over 1900-2008, a period during which the NAO index showed a very weak trend.…”

“…Yet, a good knowledge of wave climate is crucial for decision-making and to reach resilient and sustainable coastal management, particularly in the context of increased coastal risk related to climate change [5,6,[8][9][10][11]. The increase in significant wave heights (hereafter, Hs) since the second half of the 20th century, as well as changes in other wave parameters (mean periods (T m02 ) with peaks (T p ) and directions (DIR)) reported at high and medium latitudes north (e.g., [12][13][14]) and south [15,16], raises questions about the situation for Senegal. More locally, Dahunsi et al [17] reported an increase in the extreme values of Hs (99th percentile) and the mean wave period (T m02 ) in the southern part of the West African coast (Gulf of Guinea, hereafter GG).…”

“…The wave climate variability in the Atlantic Ocean has been explained by the North Atlantic Oscillation (NAO) [12,14,[26][27][28][29][30][31][32][33], the main mode of mid-latitude climate variability in the Northern Hemisphere. Additionally, the Southern Annular Mode (SAM) [15,16,[34][35][36][37], the main mode of mid-latitude variability in the Southern Hemisphere, has also been linked to wave climate variability. Although little investigated, other modes were shown to explain one part of wave climate variability in the eastern tropical Atlantic [31][32][33][38][39][40][41].…”