Extremes of residual water levels in the West of Java Sea, Indonesia

This paper discusses extreme magnitudes of residual water levels and their probabilistic behaviors. The study area is situated in the west of Java Sea, NW of Java Island. The discussion relies on recent two decades' Sea Surface Height (SSH) from two offshore points of observation in the west of Java Sea and Water Level (WL) from a tide station in Tanjung Priok port of Jakarta. The dataset of SSH is obtained by satellite altimetry on 10-day basis observation, while WL is recorded on hourly basis. Residuals are extracted from SSH and WL by removing their mean values (i.e. Mean Sea Surface, Mean Sea Level) and the tide. They range within the order of sub-meters. These count already for at least one-third of the tidal amplitude in the region. To these residuals, their mean and extreme patterns are explored. The chances of positive residuals are at least tripled with respect to the negative ones. The nearshore point, i.e. Tanjung Priok, experiences a greater magnitude of residuals than those in the offshore area. Here, we apply Generalized Extreme Value (GEV) and Generalized Pareto Distribution (GPD) methods respectively for Block Maxima/Minima (BM) and Peak Over Threshold (POT) samplings of residuals. It is found that, the extreme positive and negative residuals at 100-year return period are within less than ±1 m in Tanjung Priok and just less than half-meter in the offshore area. While higher residuals are observed during the East monsoon in the offshore area, Tanjung Priok experiences higher residuals during the West monsoon. This is thought due to the prevailing wave fields, which are governed by the monsoonal winds.