The Makassar Strait Throughflow (MST) accounts for approximately 77% of the total volume transport of the Indonesian Throughflow (ITF), thereby significantly influences mass and heat exchanges between the Indo-Pacific basin and global climate.
"Freshwater plug" hypothesis about the seasonal variability of MST was extensively cited in studies. However, there has long been a dearth of quantitative research elucidating both the existence and specific contribution of salinity effect.
Recently, an international team led by Prof. HU Shijian from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) quantified the contribution of salinity effect on the seasonal variability of MST with long-term observations and high-resolution numerical simulations, and they found that the freshwater forcing and salinity effect dominate the seasonal change of MST.
The study was published in Geophysical Research Letters on Oct. 31.
Given that the MST is controlled by the pressure gradient along the Makassar Strait, researchers focused on the role of salinity change in the meridional gradient of steric height in the Makassar Strait.
They compared the in situ observations from Arlindo and monitoring ITF (MITF) and high-resolution reanalysis, and found that the MST is a southward current with significant seasonality. In boreal winter, a weak northward current is present in the upper layer of the southern Makassar Strait, which is subject to the northward dynamic height gradient influenced by both the halosteric and thermosteric components. Quantitatively speaking, the halosteric dynamic height closely associated with the seasonal cycle of salinity contributes (69.6±11.7) % to the total seasonal variability of the sea surface dynamic height gradient within the Makassar Strait.
In boreal winter, the northwest monsoon leads to an increasing in freshwater transport through the Karimata Strait and increasing of local precipitation in the Java Sea, further causes a decline of salinity and an anomalous northward pressure gradient in the Makassar Strait, and hence resulting in a weakened throughflow. The opposite occurs in boreal summer. This might be the first time clarify the quantitative contribution of freshwater plug.
"This quantitative study not only confirms the importance of freshwater plug and salinity effect mechanism in the seasonality of MST, but also deepen our understanding of the ITF dynamics," said Prof. HU.
Fig. 1 Seasonal variability of rainfall (colored), sea surface current field (black arrows), wind field (white arrows), and salinity in the Indonesian seas.
Fig. 2 Seasonal cycles (a, b, c, d) of the absolute dynamic topography (ADT), sea surface dynamic height (DH), thermosteric dynamic height and thermosteric dynamic height, and (e) the differences between the southern (4° S-2.8 °S) and northern (2.8° S-2 °N) parts of the Makassar Strait.
Lu, X., S. Hu, C. Guan, M. Li, J. Sprintall, and F. Wang. (2023). Quantifying the Contribution of Salinity Effect to the Seasonal Variability of the Makassar Strait Throughflow, Geophysical Research Letters, 50 (21), doi:10.1029/2023gl105991.
(Text by HU Shijian)
Media Contact:
ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)