Oceanic vortices transport significant amounts of heat toward Antarctica not only at the surface but also at depths of about 1,000 m. Although vortex energy at such depths is roughly five times lower than at the surface, they transport about half as much heat. In some regions, deep‑water heat transport can be comparable to or even exceed surface transport. Furthermore, about 60% of the heat carried by deep-sea vortices toward high latitudes comes from the Indian Ocean sector. These are the conclusions of scientists from the Second Institute of Oceanography of the Ministry of Natural Resources of China, the University of East Anglia in the UK, the South China Sea Institute of Oceanology of the Chinese Academy of Sciences, the Australian National University, and the Guangdong Southern Marine Science and Engineering Laboratory.
The Southern Ocean is a vast ring of water surrounding Antarctica. Through it flows the Antarctic Circumpolar Current – the only current that continuously circles the entire planet. Its powerful streams move predominantly from west to east and hinder the penetration of warmer water from the north toward Antarctica. However, this barrier can be overcome by mesoscale vortices – huge rotating water masses tens to hundreds of kilometers in size. They entrain water and transport it across the main current, and can extend to depths of more than 1,500 meters.
Until now, the role of such vortices at great depths remained poorly studied. Satellites allow detailed observation of the ocean surface but cannot see what happens at kilometer‑scale depths. The researchers therefore used data from the international Argo system which is a network of autonomous oceanographic floats. For most of their roughly ten‑day working cycle, they drift with the current at a depth of about 1,000 meters, then dive deeper and rise to the surface, measuring temperature, salinity, and other water characteristics. The scientists analyzed more than 528,000 current velocity measurements at depths of 800‑1,200 meters collected in the Southern Ocean from 2001 to 2020. Their analysis involved tracking water movements through probe data, and heat transfer was estimated using whirl patterns based on temperature readings.
It turned out that at a depth of about one kilometer, vortices work particularly efficiently. Between 40° and 60° south latitude, they transport about half as much heat as surface vortex, despite having only one‑fifth of their kinetic energy. The reason is that at the surface, the powerful Antarctic Circumpolar Current more strongly impedes southward water movement. At depth, the main flow weakens, so vortices can more easily transport warm water across it. The scientists consider their estimates to be rather conservative, as the operational characteristics of Argo floats may lead to some underestimation of water movement.
To separately assess the influence of vortices, the scientists performed computer experiments. They first calculated water movement under the action of the mean current alone, and then added vortices. In the first case, water moved mainly along the Antarctic Circumpolar Current and spread only weakly poleward. With vortices included, warm subtropical waters penetrated more than 10° of latitude further south, specifically, over 1,100 km. In addition, vortices roughly doubled the volume of water crossing the Antarctic Circumpolar Current and roughly doubled the speed of this process: the amount of water that without the vortex took 1,000 days to transport was moved in less than 500 days with vortices.
The main source of this deep transport turned out to be the Indian Ocean sector of the Southern Ocean. It accounts for about 60% of the heat transported by deep-sea cyclones at 1,000 meters depth, even though this sector occupies only about one‑third of the space around Antarctica. In one model calculation, its share was 63%, while the Pacific sector accounted for 27% and the Atlantic sector for 10%. In the Indian sector, temperature anomalies within deep-sea vortices at depths of 700 ‑1,000 meters exceed 1.5 °C and are noticeably higher than in other parts of the Southern Ocean.
These results are important for assessing future changes in the Antarctic region. Warm water transported by vortices can rise from depth and influence sea‑ice distribution and the melting of ice shelves from below. However, current climate models do not always reproduce deep-sea vortices and their temperature structure with sufficient accuracy. More precise accounting of this heat transport hidden from satellite detection, especially from the Indian Ocean sector, will help better assess the heat balance of the Southern Ocean and the associated climatic changes in Antarctica.
