The Indian Ocean as a region encompasses a considerable population of the earth, which live in surrounding lands. Such a population belongs to various countries in Asia, Africa and Australia with different future perspectives, development plans and priorities. However, one of the fundamental issues for all of those nations is access to the supply of energy. As well as the continental coasts, islands such as Maldives, Sri Lanka, Madagascar, Reunion, Mauritius, etc. in the Indian Ocean are the home to a large population and destination to various activities. Hence, providing the energy demand is of great importance and somehow challenging, especially considering the high number of inhabiting populations.
Since not all of the countries in the Indian Ocean region own their fossil fuels, and taking into account the disadvantages of such resources especially in their global warming contribution, finding alternatives to supply the energy is vital. Considering the global tendency to the usage of renewable energies and especially the ocean resources in the areas adjacent to water bodies, the Indian Ocean can be considered as a potential source of energy for the population living in surrounding areas. The previous studies have shown that parts of the Indian Ocean wave climate are swell dominated (e.g., [1
]), which considering the advantages of utilizing the wave energy, such as predictability, higher density and lower visual and environmental impacts [2
], provides a more stable supply of energy. However, due mainly to the lack of vast information about the climate regime, the Indian Ocean has been less studied for that purpose.
The previous studies on wave energy resources assessment in the Indian Ocean have mostly focused on regional scales, and most of them have been performed in the Northern Indian Ocean by India (covering the Arabian Sea and the Bay of Bengal) and Iran (covering including the Persian Gulf and the Gulf of Oman), as well as other studies on the Red Sea. In India a 10-yearly simulated wave dataset has been used to estimate the wave energy potential in nearshore areas [3
]. Aboobacker [4
] also assessed the wave energy potential in the eastern Bay of Bengal and Malacca Strait and showed that the wave power is affected by the southwest monsoon and the swells propagate from the Southern Indian Ocean. In the Persian Gulf, Etemad-Shahidi et al. [5
] assessed the available wave energy based on the modeled wave dataset using localized ERA-40 wind dataset [6
], while later, Kamranzad et al. [7
] investigated the short-term variation in wave energy resources in terms of monthly and seasonal variations. Then, Kamranzad et al. [8
] assessed the impact of climate change on wave energy there using scenarios obtained from the Coupled Model Intercomparison Project (CMIP) Phase 3, to evaluate the long-term changes, as well, and after that, they developed the Optimum Hotspot Identifier (OHI) in order to specify the most appropriate location in the Persian Gulf for wave energy extraction based on the amount of energy, frequency of extractable energy and stability of energy [9
]. In addition, based on the stability of the wind and wave climate in intra-annual scales, Kamranzad [10
] suggested a zone classification for the Persian Gulf. Recently, Mahmoodi et al. [11
] used ERA-5 reanalysis dataset to re-estimate the wave energy resources in the Persian Gulf. In addition, Langodan et al. [12
] used an 18-yearly simulated wave dataset to investigate the wind and wave energy potential in the Red Sea. They concluded that there was no potential site for wave energy extraction considering its potential and the efficiency and available converters. Later, Aboobacker et al. [13
] used 32-yearly simulated wave dataset to assess the inter- and intra-annual variability of wave power in the Red Sea and showed that the highest mean annual wave power reaches around 4.5 kW/m, while in nearshore areas in does not exceed 2 kW/m.
Moreover, there are few studies on wave energy assessment in the Southern Indian Ocean. Hammar et al. [14
] used the altimetry dataset to investigate the wave energy potential in the Western Indian Ocean. They showed that the areas with higher potential are southeast Africa, south and east of Madagascar, and Reunion and Mauritius. Lavidas and Venugopal [15
] used 18-years of the simulated wave using SWAN to estimate the wave energy potential and its variability in South Africa and represented promising conditions for offshore wave applications regarding the low easily accessible depths and short coast distances. Kamranzad and Mori [1
] assessed the impact of climate change on the sustainability of wave energy in southeast Africa and indicated higher values and stability in Reunion, Mauritius, and southern Madagascar.
Besides, as well as the available resources, the sustainability of wave energy is required to be taken into consideration [16
]. Considering the impact of climate change, the supply of energy needs to be as stable as possible and the suitable location for extraction of energy must be selected based on that for sustainable future development. However, none of the above-mentioned studies have considered the impact of climate change and the sustainability of the resources in the long term in defining suitable locations using the updated set of climate data, i.e., CMIP5 [17
Therefore, in this study, the impact of climate change on the available wave energy in the Western Indian Ocean as a supply of energy for its surrounding coasts will be discussed, in order to introduce the most suitable areas for providing part of the energy demand. For this purpose, two 25-yearly periods of historical and future projections of wave climate using a high-resolution CMIP5 global climate model will be utilized and the sustainability of the resources will be evaluated in various regions. In addition, the intra-annual variation of wave energy will be assessed to consider both short-term variation and long-term change. Finally, the total and exploitable storages of the potential of wave energy will be evaluated, and their change due to climate change will be discussed. Various criteria will be assessed to define the stability of wave energy in both short and long-term. Section 2
describes the dataset and method and Section 3
discusses the spatio-temporal assessment in various time scales using different criteria. Finally, Section 4
provides a summary of the different criteria discussed in the previous sections.
4. Summary and Conclusions
Wave power was assessed spatio-temporally in the Western Indian Ocean for both historical and future projections based on the wave simulation using the MRI-AGCM3.2S model. The analysis was performed based on intra-annual variation (monthly, seasonal and annual) and long-term changes (absolute and relative changes). The spatio-temporal distribution of the wave power showed that the maximum mean wave power in the domain reaches around 150 kW/m in the Southern Indian Ocean during JJA and SON. The Northern Indian Ocean and its marginal seas contain lower values of wave power comparing to the Southern parts. Although, wave power reaches the peak in the Horn of Africa and Oman continental shelf especially during the southwest monsoon (JJA). The Northern Indian Ocean becomes relatively calm during DJF and MAM. Regarding future change, the Southern Indian Ocean will be subject to a considerable increase of wave power up to 15 kW/m according to the future projection, while the Northern Indian Ocean will experience a slight decrease.
Considering that wave energy extraction is carried out in the areas adjacent to the land, the analysis was continued for the nearshore areas. The results showed that although generally, the wave power is considerably higher in the Southern Indian Ocean, the nearshore areas of the Northern Indian Ocean are exposed to nearly similar wave power, as well. This indicates a highly interesting potential for wave energy exploitation. The largest future change in wave power will occur in western coasts of India, Maldives, and south to the west of Sri Lanka, as a reduction during JJA. The largest increase in wave power in the Northern Indian Ocean occurs in Oman continental shelf during SON. The Southern Indian Ocean shows a more stable wave power climate in terms of future changes.
The results also represent that the Southern Indian Ocean contains lower monthly variability comparing to the Northern Indian Ocean. Moreover, monthly variability will not change, except for the western parts of the Gulf of Oman near the Strait of Hormuz with higher and west of India with lower variability in the future. The ratio of mean wave power to the monthly variability index also illustrates a higher potential and stability in the Southern Indian Ocean. Furthermore, assessment of long-term changes in the available wave power using climate stability index showed that the northern coasts of Gulf of Oman, Horn of Africa and parts of Sri Lanka in the Northern Indian Ocean and southeast Africa, south of Madagascar and east of Reunion and Mauritius islands in the Southern Indian Ocean are the areas with the least change in the wave power in both short and long-term.
As well as the wave power, the total and exploitable storages of the wave energy and their change according to the future projection were assessed and the results indicated that in the Southern Indian Ocean, the exploitable storage is almost equal to the total storage of wave energy except for northwest of Madagascar. In the Northern Indian Ocean, the lowest ratio of exploitable to total storage of wave energy can be found in the marginal seas (which experience the most intensive changes in the future, as well) and east of India. In addition, the nearshore areas of the Northern Indian Ocean will experience the reduction of the ratio of exploitable to total storage of wave energy in the future.
As a summary to the above-mentioned investigations and considering the different criteria such as the amount of wave power, the ratio of wave power to monthly variability, climate stability, exploitable storage of energy and its stability, it can be concluded that south of Sri Lanka, Horn of Africa, southeast Africa, south of Madagascar and Reunion and Mauritius islands are the most suitable areas for extraction of wave energy. Further assessment is required on a regional scale using wave downscaling in order to provide the dataset with higher accuracy in suitable areas and discuss the most appropriate locations for being considered as the potential spots for the wave energy harvesting.