Marine Heatwaves in the Java Sea, Indonesia: Characteristics and Atmospheric Onset Drivers

The Java Sea, a critical marine region within the Indonesian Maritime Continent (IMC), is increasingly experiencing marine heatwaves (MHWs). Despite its ecological and economic importance, the mechanisms driving MHWs in this region remain poorly understood. This study investigates the characteristics, the onset atmospheric drivers, and trends of MHWs in the Java Sea using 0.25° spatial resolution of Optimum Interpolation Sea Surface Temperature (OISST) dataset from 1982 to 2021. Results show a significant increase in MHW frequency and duration. Composite analysis reveals that enhanced net surface heat flux is the key factor in MHW onset, driven primarily by reduced latent heat flux, complemented by increased solar radiation. Weakened wind speeds suppress latent heat loss to the atmosphere, while decreased cloud cover enhances solar radiation input, amplifying SST anomalies. On intraseasonal timescales, the Madden-Julian Oscillation (MJO) modulates MHW occurrence, with Phases 2 and 3 exhibiting the highest frequency of events among the active phases. Furthermore, in the interannual variability, MHWs are found to be more frequent and intense during El Niño events, underscoring the strong influence of large-scale climate variability. The extreme MHW event of 2015, among the most severe on record, was characterized by prolonged SST anomalies exceeding climatological thresholds, coinciding with a strong El Niño and dominated by increased shortwave radiation contributions to net surface heat flux.