The climatology and long-term trend of wave energy resources around Japan were estimated from in situ wave data measured at 25 observation points during 1980 to 2009. The climatological annual mean wave energy around Japan was estimated to be 6.4 kW m−1, and the long-term trend was 0.27 kW m−1/30-yr. Our results indicate that the long-term trend in wave energy varies among areas and seasons. There is a marked increasing trend in wave energy in the southern half of the east coast of Japan. Increased average wave period and height contributed to this trend on the east coast. We also found an increasing trend in surface wind speeds east of Japan. These results suggest that more frequent swells have been generating greater wave energy off Japan's east coast during the last decade.
 Ocean wave energy is a renewable and sustainable marine energy source that is expected to be an important next-generation energy resource, along with solar, wind, and geothermal energy. Although the efficiency of wave energy conversion has been increasing, more technical improvements are required for efficient wave-power generation. In addition, areas with high potential for wave power generation must be identified because wave energy resources are strongly influenced by the terrain and natural conditions. A number of studies have assessed regional and global wave energy resources using wave data obtained from buoys, satellites, and numerical wave models. For instance,Beels et al.  reported wave energy resources in the North Sea. The wave energy along the coast of California was also investigated in a series of studies [Beyene and Wilson, 2006, 2007; Wilson and Beyene, 2007]. Cornett  estimated global wave energy resources based on a numerical wave hindcast, and Cruz  documented the current status and future prospects of ocean wave energy.
 For Japan, with its long ocean shorelines, wave energy is an attractive alternative to fossil fuels. Tabata et al.  estimated a mean wave energy of 6.0 kW m−1 along the Japanese coast, based on observations at 17 sites from 1975–1978. Using coastal wave data from 1970 to 1985, Takahashi and Adachi  estimated the mean wave energy on the Japanese coast to be about 7 kW m−1. In addition, Maeda and Kinoshita  estimated the annual mean wave energy around Japan as 10 kW m−1based on visible offshore wave observation data. These studies focused on the climatological annual mean and the seasonal cycle of the wave energy. However, their analysis periods were not long enough to assess the climatology because estimates based on short-term observations could be influenced by interannual and/or decadal climate variability.
 Here we employ in situ wave data measured at 25 observation points around Japan during 1980–2009 to report the climatology and long-term trend of wave energy. We analyze both the full 30-year period and the recent wave conditions around Japan. We focus on the wave energy in three regions: the east coast of Japan, the Japan Sea coast, and the whole coastal area around Japan. The long-term trends in the wave energy are also examined using reanalysis data of surface winds. The rest of this paper is structured as follows.Section 2 describes the data, the results are presented in Section 3, and Section 4 contains the summary and discussion.
 To estimate wave energy around Japan, we use observed in situ wave data compiled by the Nationwide Ocean Wave Information Network for Ports and Harbors (NOWPHAS; http://nowphas.mlit.go.jp/index_eng.html). The NOWPHAS archives wave parameters at observation stations around Japan every 2 hours and currently operates 75 wave observation stations. Here we use the significant wave height and wave period measured at 25 observation points (Figure 1a) from 1980 to 2009, during which time a large quantity of data were observed. We focus on the whole coastal area around Japan and on two regions containing many observation stations: the Japan Sea coast and the east coast of Japan.
 Wave energy E is approximately estimated by E = 0.5 × Hs2 × Ts, where Hs and Ts denote the significant wave height and wave period every 2 hours, respectively [e.g., Tabata et al., 1980]. The monthly means for wave energy, significant wave height, and wave period are calculated from the 2-hourly data. The monthly mean value is considered ‘missing data’ if the data coverage ratio is less than 80% during the month. The color shading inFigure 1bshows data coverage ratios greater than 80%; non-shaded areas indicate missing data. Although each observation station has some data gaps, the observations generally cover the whole period of 1980–2009. Selection of the threshold for the data coverage ratio may change the estimated linear trend. We changed the criterion for the data coverage ratio from 10% to 90% at 10% intervals to test whether the linear trend and the statistical significance changed. The results did not change, suggesting that our estimates of the linear trend inSection 3 are robust.
 To examine the long-term trend of the wave energy around Japan, we also investigate changes in surface winds during 1980–2009, using wind data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis [Kalnay et al., 1996] and Japanese 25-year Reanalysis (JRA-25) [Onogi et al., 2007]. For convenience, the seasons we refer to are those for the Northern Hemisphere.
3.1. Climatology and Interannual Variability of Wave Energy Around Japan
Table 1 describes the climatology of wave energy around Japan. The annual mean of the wave energy around Japan is estimated to be 6.4 kW m−1. This is generally consistent with estimates by Tabata et al.  and Takahashi and Adachi . The seasonal cycle of the wave energy for the whole sea around Japan is characterized by a maximum in winter (9.6 kW m−1) and a minimum in summer (3.3 kW m−1). This is explained by the strong seasonality of the wave energy on the Japan Sea coast (15.4 kW m−1 in winter and 1.3 kW m−1 in summer) induced by the monsoonal flows (i.e., strong northwesterly flow) in winter and the low winds associated with the Pacific high in summer.
Table 1. Seasonal Mean (kW m−1) and Linear Trend (kW m−1/30-yr) of Wave Energy Around Japan During 1980–2009a
The mean and linear trends are estimated by averaging the observation stations. The numbers of the observation points in each region are in parentheses (see also Figure 1a).
Japan Sea coast (1–9)
East coast (17–25)
Entire coast of Japan (1–25)
 The seasonal cycle of wave energy has a lower amplitude on the east coast of Japan than on the Japan Sea coast (Table 1). This is because the fetch length on the east coast is limited by the northwesterly flow in winter, whereas waves develop as propagating swells from the Pacific in summer. A bimodal peak in the wave energy on the east coast in spring and autumn (Table 1) can be attributed to the occurrence of travelling low pressure regions in these seasons.
 The wave energy at Habu (obs. point 16) is high in summer but low in winter. This is consistent with the seasonal cycle of significant wave height off Hiratsuka on the southern coast of Japan [Sasaki et al., 2005]. The annual mean, seasonal cycle, and spatial characteristics of the wave energy around Japan estimated in this study are generally consistent with those reported by Tabata et al.  and Takahashi and Adachi .
 A green bar in Figure 2a shows the amplitude of the interannual variability of wave energy in summer. The interannual variability of the summertime wave energy is low along the Japan Sea coast compared to the east coast. In contrast, the southern coast including Nakagusuku (obs. points 12, 14, and 16) has relatively large interannual variability in summer (Figure 2a). This variability would be induced by western North Pacific typhoon activity associated with El Niño events [Sasaki et al., 2005; Sasaki and Hibiya, 2007]. In winter, interannual variability of the wave energy is large on the Japan Sea coast (Figure 2d) owing to the interannual variability of significant wave height rather than that of the wave period (Figures 2e and 2f). This indicates that interannual variability of the wintertime wave energy on the Japan Sea coast is induced by changes in the wind waves. Interestingly, the interannual variability of the wave period on the east coast is greater than that in other regions (Figure 2f), while the interannual variability of significant wave height on the east coast is comparable to that in other regions (Figure 2e). This suggests that the interannual variability of the wintertime wave energy on the east coast is dominated by changes in the wave period.
3.2. Long-Term Trend of Wave Energy Around Japan
Table 1 also lists the seasonal linear trend of the wave energy around Japan during 1980–2009. The linear trend for the whole coastal area around Japan is estimated to be 0.27 kW m−1/30-yr, representing only 4.2% of the annual mean wave energy. However, the long-term trend varies greatly by area and season. The increasing wave energy around Japan is mostly due to an increasing trend for the east coast (0.62 kW m−1/30-yr;Table 1) and Okinawa area (0.4 kW m−1/30-yr). Here, we focus on the east coast because the long-term trend for many observation stations there is statistically significant. On the east coast, the increasing trend in the wave energy is greater than 0.90 kW m−1/30-yr in winter, spring, and autumn, whereas the summertime trend is negative (Table 1). The increasing trend is also statistically significant for the observation stations on the southern half of the east coast (obs. points 19–21; see Figure 2d). In addition, the significant wave height and wave period are increasing on the east coast (Figures 2e and 2f), suggesting that more frequent and higher swells have been generating greater wave energy on the east coast in recent decades. Figure 3 shows changes in the joint distribution of significant wave height and wave period for observation stations on the east coast of Japan. Waves with longer wave period (greater than 8 s) are increasing, whereas those with a shorter wave period (less than 8 s) are decreasing.
 Wave energy is decreasing in summer along the east coast of Japan (Table 1), although this decrease is statistically significant only at Kajima (obs. point 17). This is consistent with a decreasing trend of significant wave height (Figure 2b) but inconsistent with the increasing trend of the wave period (Figure 2c). This indicates that wind waves are decreasing in the basin in summer.
3.3. Changes in Surface Winds Around Japan
 Ocean surface waves are mostly determined by surface winds. In this section, we investigate changes in the surface winds to examine the long-term trend of the wave energy on the east coast of Japan.Figures 4a and 4b show the linear trends of surface winds around Japan in summer. Although the magnitude of the trends is different between the two reanalysis datasets, there is a decreasing trend in the surface wind speed east of Japan in summer. This is consistent with the decreasing trend in the significant wave height on the east coast in summer (Figure 2b).
Figures 4c and 4dshow the linear trends of surface winds around Japan in winter. The linear trend of the surface wind speed for the NCEP/NCAR reanalysis is consistent with that for the JRA-25 reanalysis, although the magnitude of the NCEP/NCAR trend is slightly greater than that of the JRA-25 reanalysis trend. Furthermore, there is an increasing trend in the surface wind speed east and north of Japan. This result supports our finding of increasing wave energy on the east coast of Japan.
4. Summary and Discussion
 We estimated the 30-yr climatology and long-term trend of wave energy around Japan based on in situ wave data measured at 25 observation stations during 1980–2009. We estimated the annual mean wave energy around Japan to be 6.4 kW m−1, with an increasing trend of 0.27 kW m−1/30-yr. Although the increasing trend was only 4.2% relative to the annual mean, the long-term trends varied among the areas and seasons. We found a marked increasing trend in the wave energy on the east coast of Japan. The increasing trend is attributed to the increased occurrence of swells during recent decades, which is generally consistent with the increasing trend in surface wind speed east of Japan.
 There are marked long-term trends in the wave energy at several observation stations. For instance, Onahama and Hitachinaka on the east coast observed increasing trends of 1.49 and 2.17 kW m−1/30-yr, respectively, which reached about 20% of the annual mean. This suggests that these areas are potentially suitable for continuous wave energy development from a wave climate viewpoint. However, to assess potential locations for wave energy development, many other factors should also be considered, such as the distance to shore and the water depth for the wave-power device. Statistics regarding the extremes of waves are also needed to assess the durability of a wave-power device. For more detailed assessments of potential locations for wave-power generation, numerical wave simulations and further continuous wave observations are needed. In addition, the impact of wave-power generation on the marine ecosystem should be evaluated by numerical experiments and/or testing of the wave-power generation system. Wave power is environmentally friendly in terms of its low carbon dioxide emissions, but might have negative side-effects on the marine environment. Many previous studies of the wave climate have focused on changes in the mean and maximum of significant wave height [e.g.,Sterl and Caires, 2005]. In the future, more attention should also be focused on changes in the wave period.
 We thank two anonymous reviewers for their helpful and constructive comments and suggestions.
 The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.