EFFECTS OF TROPICAL CYCLONE AND MADDEN-JULIAN OSCILATION (MJO) ON HIGH WAVE OCCURENCES IN THE INNER INDONESIAN SEAS

• Main Author: Ramdhani, Andri
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/72276
• Institution: Institut Teknologi Bandung

In this dissertation, effects of tropical cyclones and Madden Julian Oscillation (MJO) on the occurrence of high (> 2 m) waves in the inner Indonesian seas have been investigated by performing long-term wave simulation using Wavewatch III (WW3) model and cross-calibrated multi-platform (CCMP) surface wind data as input for the period of 1988-2011. The resulted 24-year hindcast data were then explored and analysed using statistical methods. The simulated wave heights have been validated against buoy measurements from Indonesian Sea Watch project and Topex satellite altimetry data. The accuracy of the WW3 model output has been quantified in terms of bias and root-mean square error (RMSE). The bias and RMSE of the simulated significant wave height (SWH) range respectively from 0.12 m to 0.21 m and 0.14 to 0.26 m against buoy data, whereas those values range from 0.02 – 0.06 m and 0.2 – 0.5 m against altimetry data. In general, the pattern of the simulated SWH agrees well with the observed values, especially for wave height > 1.0 m. Although the relative errors are larger for SWH < 1.0 m, particularly in near shore areas, the 24-year hindcast data are valid for further analyses as this research is focused on the occurrence of high (> 2 m) waves. Results of this study shows that the SWH in the inner Indonesian seas is dominated by wind sea rather than by swell, as indicated by high correlation between wind speed and wave heights, as well as wave spectra. The correlation coefficient between local wind speed and SWH in the inner Indonesian seas is around 0.8. This result is consistent with the fact that during Asian winter (December-January-February/DJF) and Australian winter monsoons (June-July- August/JJA), SWH is higher than that during the transitional periods (March- April-May/MAM and September-October-November/SON). Wave height of ? 3 m has higher probability of occurrence during the DJF period in the inner Indonesian seas, such as Java Sea, Flores Sea, Banda Sea, and Arafuru Sea. Because the local wind field over Java Sea is affected primarily by the monsoons, further investigation has been carried out to understand how or when modulation by tropical cyclone and MJO activities could modify the length and duration of the fetch and induce the occurrences of high waves. iv It is found that the tropical cyclones formed in the north of Indonesian seas (north western Pacific Ocean) can affect the development of high waves in the inner Indonesian seas when the following criteria are met: (1) the cyclone develops in July and August, (2) the strength ? 4 (wind speed maximum: 70 m/s), and (3) the track goes to the north towards the East China Sea. For tropical cyclones formed in the south of Indonesian seas, the criteria are: (1) the cyclone develops in DJF period, (2) the strength ? Tropical Storm (TS / wind speed maximum: 32 m/s) and (3) the cyclone track goes to the western coast of Australia (the south eastern tropical Indian Ocean). Results of further analyses also reveal that MJO has an impact on the development of high waves in the inner Indonesian seas when the following criteria are met: (1) the MJO develops in DJF period, (2) MJO is in phase 5, (3) the modulus of MJO (RMM) index ? 2, and (4) the duration of active MJO > 5 days. The effects of such MJO are stronger when at the same time there exists tropical cyclone in the Indian Ocean that moves to the western coast of Australia (the south eastern tropical Indian Ocean). On the other hand, if the MJO develops during the JJA period, it will weaken the development of high waves regardless the existence of tropical cyclones. In general the dissertation concludes that: The high waves occurences in the inner Indonesian seas influenced by monsoon as the main forcing which reinforced by the remote tropical cyclone and MJO on phase 5. This interaction can be used to predict the potential of high waves occurences in the inner Indonesian seas.