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Scientists from the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences, the Marine Hydrophysical Institute of the Russian Academy of Sciences and MIPT have discovered a new mechanism for the variability of Antarctic water circulation in the depressions of the World Ocean and have shown that even small vortices on the surface can change, and in some cases even stop, bottom currents at a depth of more than 4000 meters. These currents are important from the point of view of climate — they are the ones that carry the coldest waters from Antarctica to the equator. The physical mechanisms influencing this movement are important for understanding how the ocean redistributes heat between low and high latitudes. The results of the study are published in the journal Scientific Reports.

Deep currents, as well as their temporal variability, remain the least studied part of the circulation of the world's oceans: they are invisible from satellites, are not covered by most autonomous stations and are rarely modeled numerically. As a result, most of the research is based on very expensive and time-consuming observations from specialized research vessels. Unlike surface currents, the abyssal circulation (a zone of depths greater than 3000 m) is significantly influenced by the bottom relief. Thus, the key points ensuring the transfer of Antarctic bottom waters between oceanic basins are deep-sea passages, channels, depressions, troughs and fault zones on the ocean floor. Monitoring of currents at these key points provides basic data on deep water exchange between neighboring basins.

"We know less about the topography of the World Ocean floor than the surface of the Moon or some other planets. At the same time, it is he who fully determines the deep-sea currents, which remain the most unexplored link in the oceanic circulation. While the currents in the upper ocean layer are clearly visible from satellites and are calculated fairly accurately using modern computer models, bottom circulation at a depth of three to four kilometers is usually not available for such automatic methods. Expeditions and on-site measurements remain the main method of research. However, the remoteness of the study areas, depths of several kilometers and the large spatial scales of the studied currents make these works extremely difficult, and currents in many abyssal channels and passages have never been measured so far. It is these, so far, secret ocean currents that are the subject of our laboratory's research," Dmitry Frey, a leading researcher at the Laboratory of Hydrological Processes at the Shirshov Institute of Oceanology of the Russian Academy of Sciences, Associate professor of the Department of Thermohydromechanics of the Ocean at MIPT, said about the project.

Scientists conducted research in the Atlantic Ocean, the deep layer of which is filled with cold and dense water with a potential temperature below 2 °C. This water mass occupies more than a third of the total volume of the world's oceans, and its spread to the north is the main component of thermohaline circulation, a type of water exchange that is formed as a result of differences in water density. Thermohaline circulation also depends on the temperature and salinity of water and has an impact on the Earth's climate on a global scale.

Scientists are particularly interested in the Vima Channel, a narrow, long 700—kilometer-long channel at the bottom that connects two large-scale basins of the Southwestern Atlantic: Argentinean and Brazilian. One of the strongest abyssal currents in the world's oceans is observed in the Vima Channel — the flow rate can exceed half a meter per second, which is only twice as slow as the Gulf Stream, and the water flow through the channel is at least 10 times higher than the flow of the Amazon, the most full—flowing river in the world. This flow washed the Vima canal — the process has been going on for tens of millions of years, and now the height of the canal walls reaches 500 meters. Such deep-sea channels are key points controlling the bottom circulation: they determine the ways of spreading bottom waters, affect water exchange between oceanic basins and set the potential temperature in the bottom layer of the World Ocean.

Historically, scientists have gained knowledge about deep circulation mainly from measurements of thermohaline properties (temperature and salinity) of water, rather than from direct measurements of current velocity. However, in narrow passages like the Vim Canal, the flow is significantly accelerated and its structure becomes more informative, which attracts the attention of researchers around the world to it. At the same time, measurements carried out in the channel over the past 40 years have shown that the bottom waters in this area are steadily warming, with a temperature trend of 0.0019 °C per year. Comparable warming trends are observed in the ocean depths of different regions. Earlier measurements showed that the bottom velocity, depending on the location in the channel, reaches 55 cm/s, however, observations made at different times since the early 1980s revealed the presence of variability in both the bottom flow velocity and its potential temperature.

"The Vima Canal is a unique natural laboratory that allows us to study large-scale currents in the abyssal ocean of the World. In this case, using the example of the channel, it was possible to investigate the temporal variability of currents, the mechanisms of which remain poorly understood. Scientists know that periodically the flow in the Vima channel almost completely stops, and periodically, on the contrary, accelerates almost twice. The driving force for the flow of water in the channel is the accumulated potential energy: the Argentine and Brazilian basins, in fact, are communicating vessels filled with water of different densities. Since the water in the abyssal basin of the Argentine basin is colder and, accordingly, heavier, a flow of water is formed through the channel into the Brazilian basin, where the waters in the abyssal basin are warmer and lighter. Thus, the causes of the flow are known, but why is there such a strong variability? There is no answer to this question, and the mechanisms of such changes are extremely difficult to track due to the almost complete lack of data at such depths. The purpose of our work is to get closer to understanding the variability of abyssal currents and to show that processes on the ocean surface have a more significant connection with bottom circulation than previously thought," Dmitry Frey emphasized.

Scientists have proposed a simple explanation for the observed changes in the bottom flow in the Vima channel, including periodic flow stops. The basic idea is that mesoscale vortices (circular movements of water in the upper ocean layer) change the hydrostatic pressure in the entire water column. The hydrostatic pressure at the entrance to the channel differs from the pressure at the exit, and this difference causes deep waters to pass through the channel. It is natural to assume that pressure changes lead to a change in the velocity of the deep current.

"Due to the higher density of waters in the Argentine basin, the pressure at the entrance to the channel is always higher than at the exit — this difference creates a force that drives the water in the channel. In our work, we found that this pressure significantly depends on the processes on the surface. Local changes in sea level, primarily associated with the passage of cyclonic and anticyclonic vortices, lead to pressure changes not only in the upper layer of the ocean, but also in its entire thickness up to the abyssal depths. At the same time, sea level changes are well monitored from space satellites. When comparing satellite data and data from the underwater autonomous observatory installed at a depth of 4,500 m in the Vima Channel, a clear correlation was found between processes on the surface and in the depths of the ocean. The processes on the surface turned out to be strong enough to affect the flow velocity at a depth of more than four kilometers, and previously it was believed that at such a depth this influence is insignificant," concluded Dmitry Frey.

The underwater observatory, which allowed scientists to make such a conclusion, stood at the bottom of the Vima Channel for almost four years. In addition to studying the dynamics of currents, such data make it possible to record the long-term warming trend of abyssal waters. These data are also used to set up modern computer models that allow calculating the entire three-dimensional circulation of the world's oceans. The physical mechanism described in the presented study opens up a discussion about the importance of mesoscale vortices and processes at the ocean-atmosphere boundary in water exchange between deep-sea basins, heat transfer in the ocean and possible ocean reactions to the observed sea level rise in a changing climate.

PHOTO: Geographical location of the Vima Canal © Dmitry Frey, Dmitry Borisov

Information and photos provided by the MIPT Center for Scientific Communication

The information is taken from the portal "Scientific Russia" (https://scientificrussia.ru /)