MOSCOW, October 12. /TASS/. A research team featuring two institutes of the Russian Academy of Sciences and the Moscow Institute of Physics and Technology (MIPT) completed a large-scale study of a complex natural phenomenon in the northern seas: very powerful internal waves in the Kara Gate Strait. The scientists linked this phenomenon to sea tides, press service of the Russian Ministry of Science and Higher Education said.
"Russian scientists have found out how internal waves emerge in the strait between the Barents and the Kara Seas," the press service said. "The waves are strong enough to shift water layers vertically by almost 30 meters. Their combination helps explain why the waves appear regularly in different parts of the strait and how they spread in different directions."
Seawater is heterogeneous: lighter, warmer or desalinated waters may lie above denser ones, the scientists continued. Boundaries between such layers can swing the sea surface. This is how internal waves arise. The emerging waves may be tens of meters, while at the same time the surface may remain almost calm. When destroyed, these waves enhance the water mixing and participate in the transfer of heat and nutrients between the depth and surface layers.
One of the areas where internal waves are particularly common is the Kara Gate Strait between Novaya Zemlya and the Vaygach Island. Here, strong tidal currents pass over a complex underwater terrain: shallow hills are next to deep troughs. Previously, specialists associated the origin of short internal waves in the strait with the internal tide transformation - a slow oscillation of water layers caused by tidal forces. However, satellite images have shown a more complex picture: numerous groups of waves were diverging in different directions.
During the MIPT Floating University Expedition of the Russian Ministry of Science and Higher Education on board the Dalniye Zelentsy scientific research vessel in July 2023, scientists measured the water temperature and salinity at different depths. By using frequent measurements, scientists could trace the rapid displacement of water layers. Over eight hours of observations, they recorded more than 30 wave-like vibrations, some of which reached almost 30 meters.
The off-board data was compared with satellite images and tidal currents calculations, taking into account the updated bottom relief. The satellite images have shown traces of internal waves: the currents associated with them change the small ripples on the sea surface, creating specific stripes. The joint analysis helped to connect the observed waves with conditions of their origin. Having compared off-board measurements, satellite imagery, and calculations of tidal currents, the researchers identified three mechanisms for the formation of such waves.
Wave formation mechanisms
The first mechanism operates when the current passes over an underwater elevation. The water rises and falls, disturbing boundaries between layers of different densities. Conditions are especially favorable when the flow velocity is close to the internal wave's propagation velocity. When the current changes, the developed disturbances may separate from the obstacle to go further in the form of groups of short waves. In the strait's some shallow areas, a suitable regime may occur up to four times a day, the researchers said.
The second mechanism is related to the internal tide. Its extended wave gradually changes shape, becomes steeper and generates a sequence of shorter waves. Observations have shown that both mechanisms can operate in the same area, and their contribution varies during a tidal cycle. Results of data comparison indicate that during high tides, the role of the internal tide transformation increases, and when they weaken, grows the role of wave formation in the stream above the bottom.
Researchers associate the third mechanism with the sea front - the boundary between waters with different properties. Tidal currents move this boundary across the strait. According to observations, its position has changed by about two to three kilometers within less than a day. The authors suggest that such movements are capable of generating separate groups of internal waves. This explanation is consistent with satellite images in which waves are propagating from the frontal zone into different directions.
The scientists noted separately the role of tide's daily component. The calculations have shown that the associated currents in the Kara Gate can be comparable in speed to the currents of the main semi-diurnal tide. Therefore, in understanding the origin of waves, it is important to take into account the combination of tides with different periods.
Water masses and sea ice
"Hydrological conditions in the Kara Gate are changing constantly: the current may be strengthening or weakening, and the water masses boundaries are shifting. This affects the formation of internal waves, the mixing of different layers, and even the sea ice. In winter, when practically the entire Arctic is ice-trapped, intense currents along with the tide can completely destroy sea ice: we can often see this on satellite images, but it is very complicated to get to those places in winter," said Ilya Kopyshov of the Marine Polar Research Laboratory at the Marine Hydrophysical Institute of the Russian Academy of Sciences, the study's first author.
The obtained results help in tracing how the tide energy transforms into short internal waves, and further on it may be spent on mixing the water. This is necessary for a more accurate description of heat exchange in the Arctic seas to refine climate models. According to the authors, a similar combination of mechanisms may operate in other Arctic straits with strong tides and pronounced separation of water into layers.
The analysis of the internal waves' formation mechanisms was supported by a grant from the Russian Science Foundation. The work featured specialists of the Moscow Institute of Physics and Technology, the Marine Hydrophysical Institute of the Russian Academy of Sciences (Sevastopol), and the Shirshov Institute of Oceanology of the Russian Academy of Sciences. The results are published in the Journal of Geophysical Research: Oceans.