# Long-Term Analysis of Wave Climate and Shoreline Change along the Gulf of California
## 5 Discussion  
The results from the long-term wave climate analysis between 1980 and 2019 reveal a spatial variability in the multi-year mean values and long-term trends of \(H_{99}\), \(H_{s}\), \(T_{s}\) and \(P\) along the GC, with the exception of the multi-year mean of \(H_{99}\), which is practically the same throughout the Gulf (see Figure 5a). The multi-year mean values of \(H_{s}\), \(T_{s}\) and \(P\) gradually decreased northward (see Figure 5b-d). This is consistent with the spatial variability of the mean wind intensity, which also gradually decreased to the north, as observed by [47] using QuikScat wind data. Moreover, the long-term trends of \(H_{99}\), \(H_{s}\) and \(P\) increased in all the Gulf (see Figure 6a,b,d), whereas the long-term trend of \(T_{s}\) decreased in both the lower and middle Gulf and increased in the upper Gulf (Figure 6c). The rate of increase was greater for \(H_{99}\) (Figure 6a) compared to \(H_{s}\) (Figure 6b), which is also consistent with the global trend of greater rates for extreme events compared to mean conditions [13]. According to [48], an increase in wave heights is primarily driven by increases in surface wind energy, so the long-term upward trend of \(H_{99}\) and \(H_{s}\) is thus probably related to a long-term upward trend in wind intensity. Contrary to the general idea that not only the wave heights but also the wave periods increase, this study found a long-term downward trend for \(T_{s}\) in the lower and middle parts of the Gulf. This issue needs further investigation to elucidate the causes and consequences of steeper waves. The long-term trend of \(P\) indicated that waves were getting more energetic despite the downward trend given by \(T_{s}\). In agreement with [1], this study shows that \(P\) is a better indicator of the long-term behavior of wave  
Figure 6: Variability of annual means of \(H_{99}\), \(H_{s}\), \(T_{s}\) and \(P\) at C1, C2 and C3 during the 40-year period (1980–2019): (**a**) \(H_{99}\); (**b**) \(H_{s}\); (**c**) \(T_{s}\); (**d**) \(P\). climate than other parameters, such as the wave height taken alone, because \(P\) includes information on wave heights and periods (see Equation (2)) and can also represent the accumulated wave energy over periods of time. The results of this study suggest that climate change is modifying the long-term climate waves in mean and extreme conditions along the GC, where the trends are significant and comparable with the trends of other closed or semi-enclosed basins around the globe (i.e., Red Sea [49], Black Sea [50] and Persian Gulf [46]). On the other hand, the long-term shoreline analysis revealed that 2283.6 km (67.8% of the total coast) of the coastline is suffering erosion at a mean rate of \(-\)4.2 m per year, while the remaining 1087.4 km (32.2% of the total coast) is under accretion at a mean rate of 1.9 m per year. The most eroded coastal strips are located on the northern and southern coasts of Sonora, on the northern coast of B.C., and along the coast of Sinlaoa, whereas the most accretional coastal strips are located along the coasts of B.C.S. and southern B.C. The distribution of the erosion and accretion points along the GC coast and the dominance of erosion processes can be explained by natural processes, human activities and climate change. Figure 7 shows the tracks of tropical cyclones (hurricanes and depressions) that passed through the GC region between 1980 and 2019. In this period, a total of 63 cyclones occurred in the GC region, 44 of which made landfall at the GC coast, i.e., 1.1 cyclones per year on average impacted the GC coast. Of the 44 tropical cyclones that made landfall at the GC coast, 11 did so in more than one state. The numbers of times tropical cyclones made landfall in the GC region are shown in Table 4: Sinlaoa and B.C.S. are the states hit by the highest numbers of tropical cyclones. This observation coincides with [51], who defined these two states as tropical cyclone hotspots for the period 1970-2010.