Non-tidal sheared turbulent mixing observed in the Halmahera sea

## Our Take: Redefining Turbulent Mixing in the Indonesian Throughflow

Recent research published in *[Science Advances]*(https://www.science.org/doi/10.1126/sciadv.adi1343) presents a significant recalibration of our understanding of turbulent mixing within the Indonesian Throughflow (ITF), a critical component of Earth’s climate system. The ITF, a massive current channeling water between the Pacific and Indian Oceans, fundamentally influences global heat distribution and water mass properties. Traditionally, studies of mixing within this region have prioritized the role of tidal currents. However, this new study, utilizing high-resolution moored current meter profiles in the Halmahera Sea, demonstrates that continuous-spectrum super-tidal Internal Gravity Waves (IGWs) are the dominant driver of vertical shear and, consequently, turbulent mixing below 500 meters, accounting for over 80% of the total shear. This finding challenges established paradigms and underscores the complexity of processes shaping ocean dynamics. The implications extend beyond this specific region; understanding IGW-driven mixing has relevance for other dynamically complex ocean areas, such as the Agulhas Current system (https://www.mdpi.com/2076-3417/14/2/38), where similar wave-driven processes are likely at play.

The study's meticulous analysis highlights a layered contribution to vertical shear. While IGWs dominate at depth, a more balanced contribution from the subtidal ITF, IGWs, and tidal currents is observed in the upper 220 meters. Critically, the researchers also evaluate the performance of two commonly used turbulence parameterization schemes—the Gregg-Henyey-Polzin (GHP) and MacKinnon-Gregg (MG) models—against their observations. The GHP scheme, when calibrated with an effective Coriolis parameter accounting for equatorial waveguide effects, provides a reasonable approximation of dissipation in the upper 300 meters. However, it falters at depth due to the deviation of IGWs from the expected Garrett-Munk spectrum, a long-standing challenge in ocean modeling. Conversely, the MG scheme performs well below 300 meters but struggles in the intermediate zone where ITF shear and stratification are strongest. This discrepancy underscores the need for more sophisticated turbulence parameterizations capable of accurately representing the multifaceted nature of mixing processes influenced by IGWs and subtidal flows.

The magnitude of the observed vertical diffusivity—two to three orders of magnitude higher than in open oceans—is perhaps the most striking finding. This amplified mixing directly impacts the efficiency of heat sequestration and water mass transformation within the ITF, reinforcing its pivotal role in the global climate system. The Halmahera Sea, a relatively understudied region, now emerges as a critical laboratory for investigating these fundamental oceanographic processes. The reliance on moored current meter profiles—a time-consuming and resource-intensive observational technique—underscores the value of this dataset and provides a valuable benchmark for validating and improving ocean models. Furthermore, the study’s insights into the limitations of existing turbulence parameterizations emphasize the ongoing need for refined models that can accurately represent the complex interplay between different scales of motion within the ocean. This is particularly relevant as climate models strive to project future changes in ocean circulation and heat uptake.

Looking forward, a critical question arises: how will changes in IGW propagation, potentially influenced by shifting wind patterns or alterations in ocean stratification due to climate change, impact the ITF’s mixing efficiency and, consequently, global climate? Further research employing advanced observational techniques, such as high-frequency radar and satellite altimetry, coupled with improved numerical models, will be essential to address this question and to better quantify the role of IGWs in the broader ocean climate system. Understanding and accurately representing IGW-driven mixing represents a frontier in oceanography, and this research provides a vital step toward that goal, urging a shift in focus toward wave-driven processes in dynamic ocean regions.