Earth and Planetary Science

Water Mass Vertical Mixing in The Sulawesi Sea and Makassar Strait During the Second Transition Monsoon

Shofia Karima

Master’s Program in Earth Sciences, Faculty of Earth Sciences and Technology, Bandung Institute of Technology, Bandung 40132, Indonesia
Research Center for Geological Disaster, National Research and Innovation Agency, Bandung 40293, Indonesia

Nining Sari Ningsih

Environmental and Applied Oceanography Research Group, Faculty of Earth Science and Technology, Bandung Institute of Technology, Bandung 40132, Indonesia

Anastasia Rita Tisiana

Agency for Marine and Fisheries Research and Human Resources, Ministry of Marine Affairs and Fisheries, Jakarta 14430, Indonesia

Gandhi Napitupulu

Environmental and Applied Oceanography Research Group, Faculty of Earth Science and Technology, Bandung Institute of Technology, Bandung 40132, Indonesia

DOI: https://doi.org/10.36956/eps.v4i1.2061

Received: 26 February 2025 | Revised: 22 March 2025 | Accepted: 26 March 2025 | Published Online: 3 April 2025

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Abstract

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.

Keywords: Richardson Number; Vertical Diffusivity; Sulawesi Sea; Vertical Mixing

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