Earth and Planetary Science

The transformation of water masses along the Indonesian Throughflow (ITF) pathway is evident from the disappearance of Western North Pacific Water (WNPW) south of the Makassar Strait, despite its prior presence in the Sulawesi Sea. One of the primary mechanisms driving this transformation is vertical mixing. This study investigates the characteristics of vertical mixing along the Sulawesi Sea–Makassar Strait route during the second transition season (September–November), using vertical diffusivity (Kz) as a key indicator. The temperature, density, and current velocity data were obtained from the Transport, Internal Waves, and Mixing in the Indonesian Throughflow Regions (TIMIT) cruise in October 2015. The results show spatial variability of vertical mixing both horizontally and vertically. Horizontally, the strongest vertical mixing was observed in the Makassar Strait (Kz = 8.5 × 10⁻³ m² /s and Ri = 2.0−2.2), consistent with values reported for the first transition season but exceeding those typical of the southeast monsoon. Vertically, mixing was most intense in the deep layer (Kz = 9.2 × 10⁻³ m²/s), followed by the homogeneous layer (Kz = 3.9 × 10⁻³ m²/s), while the weakest is in the thermocline layer (Kz = 1.2 × 10⁻³ m²/s). These patterns are influenced by the complex interaction of current dynamics and bathymetric features such as sills. The findings highlight the southern Makassar Strait (Transect 4) as a hotspot of vertical mixing and water mass transformation, playing a critical role in shaping ITF structure and the downstream transport of thermohaline properties.