Science Bite: The ocean connects Antarctica’s two types of ice
So, what are we actually doing out here? Even by Antarctic standards, McMurdo Sound is in a very special location – it's the most southern point on the planet where there is open water. In summer, that is. Come winter, McMurdo Sound is filled with sea ice that is locked to the land on both coasts and pressed up against the 'ice shelf' to the south.
Stretching away from here, nearly 1,000 km to the south and about 1,000 km to the East, lies the great Ross Ice Shelf – a floating extension of Antarctica's land-based glaciers. On meeting the coastline, these ice streams continue flowing out over the ocean, melding together to form massive flat slabs of ice that float out over the ocean. The Ross Ice Shelf is the largest of these, covering an area of ocean around twice the size of New Zealand. And it is typically between 500 and 700 m thick at the point that it leaves the land. The ice gets thinner as it stretches northwards, partly because it's being melted away from beneath by the ocean.
The resulting ice shelf 'cavities' make for highly unique regions of the global ocean, because they are permanently covered by a thick lid of ice. That means that they're not subject to weather or atmospheric influences – leaving only tides and melt-freeze processes as the drivers of variability and mixing. We want to study that ocean because how it interacts with the ice is critically important for understanding the future of Antarctica. But it's really hard to get through all that ice to directly measure or sample the ocean.
Instead, we're using the sea ice at the immediate front of the ice shelf as a stable platform to study the water that's been circulating within the ice shelf cavity – perhaps for as long as ten years – catching it as it flows northwards towards the open ocean. This way, we only need to get through two or three metres of ice instead of several hundred. When we do, we find water that is 'supercooled', which means that it's colder than its own freezing temperature. It's cold enough that it could have frozen, but it's actually still liquid. Which makes for some really interesting and complex processes.
One of these processes is the formation of 'platelet ice' layers beneath the sea ice. This begins as tiny 'seeds' of ice – known as frazil – that are carried along with the water as it leaves the ice shelf cavity. These ice crystals grow fairly rapidly in the supercooled water to become flat discs of ice called platelets. Once the platelets get big enough – perhaps in the range 2-10 cm across – they float upwards to accumulate, in their billions, as a delicate three-dimensional 'lattice' immediately beneath the sea ice.
This special type of ice represents a direct connection between Antarctica's two main types of ice – glacial ice and sea ice. A connection that is facilitated by the flow of the ocean from ice shelf to sea ice. (Incidentally, there is an equivalent and equally important flow in the other direction – I'll tell you about 'Polynyas' in another post). And the phenomenon of platelet ice only occurs where ice shelf and sea ice physically meet. Such as here in McMurdo Sound.
And now, thanks to a world-leading design and engineering effort from Dr Craig Stewart (my colleague at NIWA), we have the capability to quantitatively study this unique structure and habitat. Truly a world first!
Sent from Iridium Mail & Web.
Stretching away from here, nearly 1,000 km to the south and about 1,000 km to the East, lies the great Ross Ice Shelf – a floating extension of Antarctica's land-based glaciers. On meeting the coastline, these ice streams continue flowing out over the ocean, melding together to form massive flat slabs of ice that float out over the ocean. The Ross Ice Shelf is the largest of these, covering an area of ocean around twice the size of New Zealand. And it is typically between 500 and 700 m thick at the point that it leaves the land. The ice gets thinner as it stretches northwards, partly because it's being melted away from beneath by the ocean.
The resulting ice shelf 'cavities' make for highly unique regions of the global ocean, because they are permanently covered by a thick lid of ice. That means that they're not subject to weather or atmospheric influences – leaving only tides and melt-freeze processes as the drivers of variability and mixing. We want to study that ocean because how it interacts with the ice is critically important for understanding the future of Antarctica. But it's really hard to get through all that ice to directly measure or sample the ocean.
Instead, we're using the sea ice at the immediate front of the ice shelf as a stable platform to study the water that's been circulating within the ice shelf cavity – perhaps for as long as ten years – catching it as it flows northwards towards the open ocean. This way, we only need to get through two or three metres of ice instead of several hundred. When we do, we find water that is 'supercooled', which means that it's colder than its own freezing temperature. It's cold enough that it could have frozen, but it's actually still liquid. Which makes for some really interesting and complex processes.
One of these processes is the formation of 'platelet ice' layers beneath the sea ice. This begins as tiny 'seeds' of ice – known as frazil – that are carried along with the water as it leaves the ice shelf cavity. These ice crystals grow fairly rapidly in the supercooled water to become flat discs of ice called platelets. Once the platelets get big enough – perhaps in the range 2-10 cm across – they float upwards to accumulate, in their billions, as a delicate three-dimensional 'lattice' immediately beneath the sea ice.
This special type of ice represents a direct connection between Antarctica's two main types of ice – glacial ice and sea ice. A connection that is facilitated by the flow of the ocean from ice shelf to sea ice. (Incidentally, there is an equivalent and equally important flow in the other direction – I'll tell you about 'Polynyas' in another post). And the phenomenon of platelet ice only occurs where ice shelf and sea ice physically meet. Such as here in McMurdo Sound.
And now, thanks to a world-leading design and engineering effort from Dr Craig Stewart (my colleague at NIWA), we have the capability to quantitatively study this unique structure and habitat. Truly a world first!
Sent from Iridium Mail & Web.
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