Didit Ashaf
Characterising the wave climate of the Pantura coast of northern Java is essential for coastal flood-risk assessment and infrastructure design, yet directionally separated, spatially resolved analyses across this densely populated corridor remain limited. This study uses 46 years (1980–2025) of WAVERYS reanalysis data to characterise the directional, seasonal, and interannual wave climate at Jakarta and Semarang, and across five offshore regions spanning the full Pantura coast. Extreme wave conditions were estimated using a Peak-Over-Threshold Generalised Pareto Distribution (POT–GPD) framework, applied separately to Easterly and North-Westerly wave populations corresponding to the two monsoon-driven regimes. Results reveal two complementary spatial and seasonal gradients, with mean significant wave height increases from west to east and is highest during the East Monsoon (June–August), while the upper tail of the wave distribution is governed by North-Westerly events that intensify eastward and reach 100-year return levels of 3.5–4.0 m in the eastern domain. ENSO and the Indian Ocean Dipole modulate the North-West Monsoon wave signal, with neutral-to-negative phases amplifying wave energy and positive phases suppressing it. A media-based inventory of 49 coastal flood events at Jakarta and Semarang (2015–2025) confirms a first-order association between North-West Monsoon wave activity and flood occurrence but reveals contrasting compound-hazard pathways at the two cities. Jakarta flooding is linked to climate-modulated wave forcing interacting with rainfall and river discharge, whereas Semarang's primary May–June flood peak, occurring when offshore wave energy is near or below the annual average, could be more related to the seasonal sea-level anomalies and spring tides amplified by rapid land subsidence. These findings provide the first directionally separated, spatially resolved extreme-wave characterisation across the full Pantura corridor and offer practical guidance for spatially differentiated coastal infrastructure standards, monsoon-separated engineering design, and climate-informed pre-monsoon preparedness.