Water Mass Characteristics and Stratification Stability of the Flores Sea During the First Transition Season of 2023

Abstract

The Flores Sea plays a critical role within the Indonesian Throughflow (ITF) system, functioning as a key transitional pathway where Pacific-derived water masses undergo significant transformation prior to entering the Banda Sea. Despite its importance, scientific understanding of the relationship between water-mass characteristics and water-column stability during the first transition season in this region remains relatively limited. This study was conducted to characterize the water masses present in the Flores Sea and to evaluate water-column stability using conductivity-temperature-depth (CTD) observational data collected from four sampling stations during the Jala Citra III-2023 "Flores" Expedition, carried out between April and May. Vertical profiles of temperature, salinity, and density were systematically analyzed to identify distinct water masses through the construction of temperature-salinity (T-S) diagrams, while water-column stability was assessed by calculating the Brunt-Väisälä frequency at each station. The results revealed a clearly defined vertical hydrographic structure, consisting of a surface mixed layer, a thermocline layer extending from approximately 40 to 300 meters in depth, and a deep layer situated below 300 meters. Analysis of the T-S diagrams identified three major water masses contributing to the hydrographic composition of the Flores Sea: North Pacific Subtropical Water (NPSW), South Pacific Intermediate Water (SPIW), and Antarctic Intermediate Water (AAIW). Furthermore, the highest Brunt-Väisälä frequency squared value, reaching 2.3 × 10⁻² s⁻², was observed at approximately 116 meters depth, indicating that the maximum stratification stability occurred within the thermocline layer. Overall, these findings demonstrate that combining water-mass characterization with Brunt-Väisälä frequency analysis offers a more comprehensive and integrated understanding of water-mass transformation processes and stratification dynamics along the Indonesian Throughflow pathway.