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Journal of Tropical Oceanography

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Analysis of the mechanisms underlying the low-frequency variability of the low-salinity tongue in the Southeastern Indian Ocean

PANG Yanran1, 2, SUN Qiwei3, 4, ZHANG Yuhong1, 2, 5, ZHANG Ying1, 2, 5, CHI Jianwei1, 2, 5, DU Yan1, 2, 5   

  1. 1.  State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;

    2.  University of Chinese Academy of Sciences, Beijing 100049, China;

    3.  Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China;

    4.  Department of Ocean Science and Hong Kong Branch of the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Hong Kong University of Science and Technology, Hong Kong 999077, China;

    5.  Guangdong Key Laboratory of Ocean Remote Sensing, South China Sea Institute of Oceanology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China



  • Received:2025-11-17 Revised:2025-12-17 Accepted:2025-12-29
  • Supported by:

    National Natural Science Foundation of China (42090042, 42306026); Chinese Academy of Sciences (183311KYSB20200015); South China Sea Institute of Oceanology (LTO2306, SCSIO202201, SCSIO2023HC07); Key Talents Project of Guangdong Province (2024TQ08A880)

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Abstract: Ocean salinity serves as a key indicator of the global water cycle and exerts important controls on oceanic circulation, sea level, and stratification, thereby playing a critical role in marine thermodynamic and dynamic processes. In recent years, salinity variability in the tropical Indian Ocean, particularly its dynamic mechanisms and climatic effects, has attracted growing scientific interest. Using 31 years of satellite observations, in-situ data sets, and model reanalysis data, this study investigates the decadal variability and formation mechanisms of the low salinity tongue in the South Indian Ocean between the equator and 20ºS. The results indicate that both the volume and mean salinity of the low-salinity tongue exhibit a quasi-12-year oscillation, which is primarily associated with the Interdecadal Pacific Oscillation (IPO). Further analysis reveals that on decadal timescales, variability in the volume of the upper 50m low-salinity tongue is mainly driven by local precipitation. Through anomalous atmospheric circulation, sea surface temperature anomalies in the tropical Pacific lead to multi-year precipitation anomalies in the southeastern Indian Ocean, which subsequently alter the west extension of the surface low-salinity tongue and ultimately govern its volume variability in the upper 50 m. However, in the subsurface layer (50—200 m), variability in the volume and average salinity of the low salinity tongue is dominated by freshwater transport associated with the Indonesian Throughflow (ITF). During negative IPO phases, wind anomalies over the tropical Pacific trigger oceanic wave adjustments, which enhance the ITF salinity transport. This process subsequently leads to an expansion of the low salinity tongue and a decrease in its average salinity in the southeastern Indian Ocean. Based on the three-dimensional variability of the low salinity tongue, this study reveals the relationships between the volume and average salinity of the low salinity tongue at different depths and local freshwater forcing, as well as salinity transport by the ITF, thereby contributing to an improved understanding of how regional water mass changes respond to long-term climate changes.

Key words: Indonesian Throughflow, Southeastern Indian Ocean, Ocean Salinity, Decadal Variability