| dc.description.abstract | We investigated the variation of wave climate around Taiwan Waters and over the Tropical Northwestern Pacific. The global sea surface wind fields from NCEP reanalysis data were used to drive SWAN wave model to re-construct the historical records of wave from 1948 to 2008. The reconstruct wave data was compared and verified with observation from the Central Weather Bureau (CWB) , Water Resources Agency (WRA) and the NDBC/NOAA.
The discussion of the trend of long-term wave climate change was categorized into two parts. In order to clarify the oscillations and trend of wave climate in different time scale, the first part was focused on the oscillations of wave heights, wave energy, wave steepness, and wave direction in the Taiwan Waters. The second part is the analysis of extreme wave events and its statistical characteristics.
The results of the first part showed that the oscillations consisted of three different time-scale oscillations in Taiwan wave climate variations. There were seasonal oscillation, interannual oscillation affected by ENSO, and interdecadal oscillation. In the seasonal oscillation, the wave energy in winter was about 3~3.6 times than in summer. The ratio in east coast was higher than in west coast. The influence of ENSO was significant in interannual oscillation of wave energy. Wave height and wave energy became large in La Niña years. The time of the occurrences winter extreme wave events in La Niña years was several weeks late, which was in January. Wave height and wave energy weakened in El Niño years. The occurrence of winter extreme wave events in El Niño years was in November. In addition, there was no phase lag between wave climate oscillation in the seaaround Taiwan and Southern Oscillation Index (SOI). But there existed one year phase lag in the Northwest Pacific. The interdecadal oscillation of wave climate correlated medially with Pacific Decadal Oscillation (PDO). A decrease trend of wave height was in the sea area nearby Taiwan, which reduced 0.3 cm per year. In winter, the decrease trend of wave height and wave energy was due to decreasing the intensity of Siberia Heighs and associated winter monsoon. It is noted that increase trend is obvious in summer which might indicate the increasing strength of typhoon in this region.
The occurrence probability analysis of extreme wave events in the sea area nearby Taiwan was discussed in the second part. In recent 60 years, most extreme wave events happened in two periods: 1967-1974 and 2000-2008. The larger wave height and longer duration of extreme wave events occurred in the latter than in the former. The extreme wave events in summer (winter) had the increase (decrease) trend in the sea area nearby Taiwan. All the extreme wave events in summer were caused by typhoon. After 1985, the number of extreme wave event caused by typhoon was more than caused by winter monsoon. The difference increased year by year. But the total number of extreme wave event in one year did not have any significant change.
In addition, the statistical distribution of wave extreme value had large difference between El Niño years and La Niña years. The wave height extreme value in La Niña years was about double in El Niño years. The wave energy extreme value was about five times. This result can be provided for the design of coastal engineering.
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