Southern Ocean: An ocean like no other

Hyderabad: The Southern Ocean, also called the Antarctic Ocean (or even the Austral ocean), is like no other and best described in superlatives.

Storing heat and carbon

The world’s oceans take up more than 90% of the excess heat generated by the burning of fossil fuels and a third of the additional carbon dioxide. It is the primary storage of heat and carbon for the planet. The Southern Ocean connects all major ocean basins, except the Arctic. The link is the Antarctic Circumpolar Current (ACC) – the largest ocean current on the planet. It carries more than 100 times the flow of all the rivers on Earth and transports enough water to fill Lake Ontario in just a few hours. A combination of strong winds and a nearly uninterrupted passage around Antarctica give the ACC its strong flows and speed.

Mixing global currents

The Roaring Forties, Furious Fifties and Screaming Sixties are all popular names for the strong westerly winds that blow, nearly uninterrupted, across the Southern Ocean, creating equally impressive waves. This results in a massively energetic – and hard to measure – ocean surface. Here, warm subtropical water is mixed south, deep cool water from the North Atlantic rises back up toward the surface and colder polar water masses mix northward and sink back down. This complex interplay is guided by the wind and by the shape of the seafloor. The ocean plays a crucial role in the global climate system by bringing relatively warm Circumpolar Deep Water into contact with the ice fringing Antarctica.

Annual thaw and freeze of sea ice

The annual cycle of sea ice growing and melting around Antarctica is one of the defining rhythms of our planet and an important facet of the Southern Ocean. In contrast to the clear and dramatic changes in the north, the rhythm of Antarctic sea ice has followed less obvious patterns. In the face of a warming ocean, it was actually slowly expanding northward until around 2016, when it suddenly started to contract.

Looking at the annual cycle of Antarctic sea ice, one might think it simply grows and melts in place as things get cold and warmer through the year. But in truth, much of the sea ice production happens in polynya – sea ice factories near the coast where cold and fast Antarctic winds both create and blow away new sea ice as fast as it appears. This process brings us back to global ocean circulation. When the new ice grows, the salt from the freezing seawater gets squeezed out and mixes with the seawater below, creating colder and saltier seawater that sinks to the seafloor and drains northward.

Not all ice shelves respond the same

Computer simulations have shown how the ice shelves at Antarctica’s fringe have waxed and waned over past millennia. Because these floating extensions of the ice sheet interact directly with the ocean, they make the ice sheet sensitive to climate. Ocean warming and changes in the source of the water coming into contact with an ice shelf can cause it – and in turn the whole ice sheet – to change. But not all ice shelves will respond to warming in the same way. Some ocean cavities are cold and slowly evolving. Others are actually described as hot – in polar terms – because of their interaction with Circumpolar Deep Water. The latter is changing rapidly right now.

Past and future Southern Ocean

At times in the Earth’s past, the Southern Ocean didn’t even exist. Continents and ocean basins were in different positions and the climate system operated very differently. The Southern Ocean has been a stable component of a climate system and subject to relatively benign glacial oscillations. But glacial cycles play out over tens of thousands of years.

Life in sub-zero ocean

Antarctica seems an inhospitable and almost barren environment of ice and snow, speckled with occasional seabirds and seals. But diving beneath the surface reveals an ocean bursting with life and complex ecosystems, from single-celled algae and tiny spineless creatures to the well-known top predators: penguins, seals and whales. The Southern Ocean is home to more than 9,000 known marine species.

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