Ice, ice baby

We’re about to cross the Drake Passage – an area of ocean between the tip of Chile in South America and the South Shetland Islands near Antarctica.

The Drake Passage has a reputation for being a bit… choppy. In fact, waves can be up to 18m tall here! There are a few reasons for this. 

The Southern Ocean encircles Antarctica; it’s the only ocean that can flow around the globe, uninterrupted by land. We call this current the ‘Antarctic Circumpolar Current’, because it circumnavigates the continent!

The Drake Passage is the narrowest part of this circle of ocean surrounding Antarctica, which is one of the reasons for the rough seas and its fearsome reputation.

The Southern Ocean extends from the edge of the continent to the ‘polar front’, which separates it from the surrounding Pacific, Indian and South Atlantic Oceans.

This Antarctic Polar Front marks the location where there is a strong contrast in the temperature of the sea water: the Southern Ocean water is significantly colder than the waters of the surrounding oceans.

It can also be extremely windy – winds can whip around the continent, again unimpeded by land. (The winds have been fondly nicknamed the ‘roaring 40s’, ‘furious 50s’ and ‘screaming 60s’).

Feeling a bit queasy?

How are your sea legs holding up? We’re about to reach the Southern Ocean and cross the notorious Drake Passage. As we said, it can get pretty rough so we need all the tips we  can get to help us with sea sickness!

Some people are lucky, and never get sea sick – no matter how rough it gets. Others get sick with even just a little bit of motion. Some of our previous ships could be a bit bouncy in rough weather. But, because of its size and shape, most people find RRS Sir David Attenborough to be much more stable in rough weather.

One of the main things we have to look out for is icebergs.

Icebergs are large chunks of ice that break off from ice shelves – a process known as ‘calving’. Ice shelves are the part of a glacier that floats on the water when the glacier gets all the way down to the sea.

This means that the icebergs are made of freshwater, rather than salty sea water.

Perhaps the most important thing to know about icebergs, from a ship navigation point of view, is that most of the mass of an iceberg is below the water, where we can’t see it. This is where the phrase “the tip of the iceberg” comes from!

Really big bergs will also get a formal name made of numbers and letters so that we can keep track of it – like the megaberg ‘A23a’, for example.

Icebergs also come in different shapes. Tabular icebergs have steep sides and a flat top, and still look like a part of the ice shelf they calved from. This is what the A23a iceberg looks like.

Non-tabular icebergs are all sorts of different shapes and sizes – with spires and domes, for example. Some icebergs are also a beautiful blue colour.

This is because the bond between oxygen and hydrogen in the water absorbs light at the red end of the spectrum, and reflects the striking blue colours we see.

Did someone say pancakes?

Sea ice is made when the ocean temperature drops below -1.8°C. It forms each winter before melting away again in the summer. First, the surface waters turn to slush, before solidifying into ice, usually a few metres thick.

Sea ice is different from ice shelves, which are extensions of glaciers, floating on the ocean surface. Ice shelves can be hundreds of metres thick and are made up of freshwater.

There are lots of different types of sea ice that we spot around Antarctica.

Near the coast, sea ice is referred to as ‘fast ice’. Further out to sea, where it can be moved by ocean currents and wind, it is called ‘drift ice’. Large areas of drift ice are called ‘pack ice’.

There’s also a type of sea ice called pancake ice because – you guessed it – it looks like pancakes! These are slushy, circular discs of ice that form as the ocean begins to freeze.

Around Antarctica, the area covered by sea ice expands from a low of around 3 million km² in February to a whopping 20 million km² in October.

How does the SDA break through sea ice? Well, it rises up on to the ice and uses its weight to break through! The ship can break through one metre thick ice at a speed of three knots (5.6km/h).

Sea ice science 

In 2023, we saw record-breaking lows of Antarctic sea ice. It re-grew over the winter – but very slowly – and when the sea ice should have been approaching its maximum winter coverage, there was an area of ice missing that was bigger than Algeria – the world’s 10th largest country.

Is it caused by climate change? There are so many things that affect sea ice in the Antarctic that it’s hard to say, but the sheer scale of 2023’s sea ice low suggests something unusual is going on.

Why is sea ice important? 

• The frozen continent acts as the Earth’s fridge – its bright, white surface reflects the sun’s energy and helps keep the planet cool.
• When sea ice forms it acts as an engine for ocean currents and influences weather patterns.
• Floating ice acts as a buffer to protect ice sheets from destructive waves – helping to slow down sea level rise.
• It’s an important habitat for marine life – for example, emperor penguins breed on sea ice.

Want to learn more about sea ice? Check out our handy guide…

Sea ice and wildlife

When the sea ice melts in the summer, it releases millions of tiny plants and animals into the water. This kickstarts an explosion of life that stretches across the Southern Ocean.

Last year, as part of a project called BIOPOLE, scientists on board RRS Sir David Attenborough collected data from the edge of the melting sea ice to learn more about how this process drives global cycles of carbon and nutrients.