[Vendée Globe 2024 – #5] – They’re there, the Vendée Globe skippers, in those famous southern seas! Even if they skilfully get around some of the lows, they are encountering sea conditions that are often Dantesque and their nerves are being sorely tested. The Southern Ocean, which encircles the Antarctic continent, is the most feared by sailors, and for good reason. It reigns supreme over global ocean circulation and the climate machine.
As a scientific partner of the Vendée Globe, océans connectés will be providing scientific insights into the oceans during this 2024 race. Each week, we’ll be highlighting the ocean features crossed by the skippers and the highlights of some of their commitment to science.
By Carole Saout-Grit
Cover photo: Antarctica seen from space © NASA
The only point of connection between the world’s major ocean basins
While the northern hemisphere is predominantly continental, the southern hemisphere is mainly oceanic, with 80% of its surface area covered by sea. A single continent, Antarctica, covers the regions around the South Pole, and the Southern Ocean surrounds it without interruption.
Majestic, immensely vast and rich in a biodiversity that is unique in the world, the Southern Ocean accounts for almost a third of the planet’s ocean surface. It extends laterally from west to east for more than 2,000 km around the Antarctic continent and opens onto the 3 largest basins: the Atlantic, the Indian and the Pacific. It is the only point of connection between them, making it a key element in the regulation of the Earth’s climate.
It is subject to the most violent winds, the Roaring 40s and the Howling 50s, which activate the most powerful marine current on the planet: the Antarctic Circumpolar Current (ACC). Under the effect of these intense winds and the very cold climatic conditions at the South Pole, the temperature and density properties of the ACC are relatively homogenous over the vertical, so that it extends to a depth of 3,000 metres. It is an annular current with the capacity to move a volume of more than 140 million m3 of water per second, which is the equivalent of 140 times all the rivers on Earth combined.
The most important ocean for the climate
Scientists are well aware that the North Atlantic is the real engine of the climate machine. The warm waters of the North Atlantic Drift, an extension of the Gulf Stream, disappear into the deep North Atlantic at the level of the subpolar gyre, allowing heat to be redistributed from the tropics to the poles and keeping the climate in balance.
What oceanographers didn’t realise until a few decades ago was that a second climate engine exists… at the South Pole! Isolated and remote, the Southern Ocean has long been little known because it has been little explored. But the millions of pieces of data collected over the last two decades have gradually come to document it and reveal all its dimensions.
Southern Ocean eddies – map produced by AVISO © CNES/CLS 2014
This ocean, whose waters are partly icy and subject to the strongest winds, is the crossroads of the world’s oceans, thanks in particular to its annular geometry. Around Antarctica, the waters of the Indian and Pacific Oceans, but above all those of the North Atlantic, are literally stirred up by the ACC. It is this great current that modifies and redistributes the water masses of the world’s three major ocean basins at the greatest depths. It is in the Ross and Weddell Seas and off the coast of Adélie Land that most of the coldest deep water on the planet is formed. The waters there are swallowed up and gradually cover the bottom of the Atlantic, Pacific and Indian basins.
Global ocean circulation. Source : Kuhlbrodt et al, 2007
An ocean under close surveillance
Too far from the coasts, too violent, too vast, the Southern Ocean remained absent from scientific expeditions for a long time until the 1990s, when climatologists and oceanographers realised the importance of this ocean and decided to expand their knowledge of the links between atmosphere and climate.
Major scientific advances were made possible when the oceanographic vessel Marion Dufresne was launched 29 years ago. The international WOCE (1990-1997) and SURVOSTRAL (since 1992) programmes gave scientists the first 3D view of a global ocean circulation centred around the Southern Ocean. Finally, it was the Argo network and the programmes dedicated to the Southern Ocean – in particular BONUS-GOODHOPE in 2008 – that revealed the very high spatial and temporal variability of this immense ocean.
The results now confirm that almost three-quarters of the increase in global heat due to global warming is absorbed by the Southern Ocean and injected into its abysses. They also show that while a quarter of the carbon emitted into the atmosphere by human activities is absorbed by the world’s oceans, the majority is absorbed in the Southern Ocean.
Iceberg in the Southern Ocean @ Sabrina Speich – ENS Paris
The Southern Ocean therefore reigns as the master of ocean currents and acts as a driving force in the climate machine, moderating at the same time the climate disruption caused by human activities. It may be doing us a favour, but it is already suffering greatly from this excess heat: around Antarctica, the ocean depths are warming more rapidly than elsewhere; the waters are becoming more acidic, endangering some biodiversity; the expanses of pack ice are changing with the seasons; and the Antarctic polar ice cap is melting under the effect of these warm waters, becoming unstable and threatening coastal regions with a worrying rise in water levels.
There is now an urgent need to give scientists the means to continue their research efforts and gain a better understanding of this vast and still little-known ocean. It is also essential to protect the Southern Ocean in order to preserve its biodiversity and enable it to continue to play its major role in balancing the climate.