Direct estimates of turbulent mixing in the Indonesian Seas and its impact on the water-mass transformation

Taira Nagai, University of Tokyo, Tokyo, Japan, Toshiyuki Hibiya, Univ Tokyo, Tokyo, Japan and Fadli Syamsudin, Indonesia Agency for Assessment and Application of Technology, Jakarta, Indonesia
The Indonesian Archipelago is characterized as a region which has the warmest SSTs in the world’s oceans playing a key role in the global climate system. Previous studies numerically showed that SST patterns in the Indonesian Seas are highly influenced by tidal mixing. The other role of tidal mixing is to change the properties of the Indonesian Throughflow (ITF). The modified ITF waters affect the basin-scale ocean current system. The tidal mixing in the Indonesian Seas is, therefore, crucial for the predictions of the large-scale oceanic/atmospheric circulation. Although tidal mixing in this region has been investigated using numerical models and/or simple parameterizations, there have been much limited estimates based on direct microstructure measurements.

In this study, to make direct estimates of tidal mixing in the Indonesian Seas, we conducted the first extensive microstructure measurements (120 casts of Vertical Microstructure Profilers) covering the whole Indonesian Seas in the winter and summer of 2019.First, observed results showed a large spatial variation in turbulent dissipation rates; vigorous turbulent mixing (ε ∼ 10-5 W/kg) was detected in the narrow straits (e.g., Lombok, Sape, Manipa, and Halmahera straits) which are thought to be major internal tide generation regions, whereas, far from the generation cites, weak turbulent mixing comparable to open-ocean mixing (ε < 10-10 W/kg) was observed. Second, observed temperature-salinity profiles provided evidence of the significant water-mass transformation in the Indonesian Seas; water-mass transformation occurs quite locally in the mixing hotspots of the narrow straits. We therefore conclude that, in the Indonesian Seas, the narrow straits play indispensable role in generating the strong turbulent mixing as well as controlling the water-mass properties of the ITF. A detailed discussion on the generation processes of the strong turbulent mixing in the narrow straits will be made at the presentation.