Week 6: Currents in the Southern Ocean
The main current in the Southern Ocean is the Antarctic Circumpolar Current (ACC), which flows from west to east, all the way around Antarctica. The ACC is the biggest current in the world and can transport up to 150 billion litres per second of water and reach speeds in narrow channels of up to 1 metre per second. This is equivalent to 150 times the water contained in all the rivers in the world, or the same as the water contained in 75,000 Olympic swimming pools going past every second! The volume of water that is transported by the ACC is so large, not because it flows fast, but because it reaches from the ocean surface down 4,000 metres to the sea floor, and stretches from just south of New Zealand to most of the way to Antarctica.
Iceberg floating in the sea
Icebergs are moved around by a combination of wind and ocean currents, generally the biggest icebergs follow the ocean currents, while smaller icebergs are moved by the wind.
Closer to the coast of Antarctica, the ocean currents are more complex. Within the Ross Sea, where this voyage went, the currents form two gyres. Each gyre forms a loop of current that connects back on itself. These currents do not transport as much water as the ACC, because they are neither as broad nor as deep.
Southern Ocean fronts
Regions of water in the ocean are organised by temperature and salinity, the boundaries between them are called fronts. In the Southern Ocean there are two main fronts the Subantarctic Front (SAF) and Antarctic Polar Front (APF).
The Southern Ocean is in one of the windiest parts of the planet, and its currents are driven mainly by the wind. The wind that blows over the surface of the ocean passes its energy to the water molecules in the ocean, giving the water more kinetic energy. Friction between the sea water and the bottom of the ocean cause the movement of the water or current to slow down. It is the balance between these two forces that stop the currents from getting faster and faster.
Useful link
For more information see the NIWA Publication – Squeezing information from an elusive ocean: surface currents from satellite imagery.
IPY blogs week 6
Wind and ocean currents
Of particular interest to the mariner (seafarer) is the surface current that is caused by the reaction of the wind and the friction created with the surface of the sea, which moves the sea in the same direction as the wind. Around some of the seamounts, we observed that the entire ice mass was drifting at approximately 0.5 knots.
It is of great importance for the officer manoeuvring the ship to understand the surface currents, as they control the direction of drift of the ship when it is stationary. Seafarers have mastered the use of surface currents for generations, transporting their ships from port to port in the shortest available time by using the most favourable winds and currents.
Written by Brent Whyte
See the video: Ocean currents and iceberg movements.
Forty knot wind
A steady 40 knot wind blowing and the friction caused with the sea surface created these 4.5 metre waves on 12 February 2008 in the Ross Sea.
Deploying the DTIS camera
Tangaroa deploying the DTIS camera system while in an open area within heavy ice. Ice and ship drifting together at 0.5 knots.
Where should we look for Antarctic toothfish larvae?
Of the relatively large population of Antarctic toothfish in the Ross Sea region, no Antarctic toothfish eggs or larvae have ever been collected. To find out where the eggs and larvae may be found, NIWA scientists Dr Graham Rickard and Dr Mike Williams have developed a mathematical model that simulates the likely movement of toothfish eggs and larvae by releasing ‘floats’ into areas 88.1C and 88.1G on the map (where we believe the toothfish spawn). The model suggested that most eggs and larvae would be moved by the currents to the east and then south down towards the Antarctic continent. We are currently surveying this complex seamount area and hope that we are able to find some of the elusive toothfish larvae there.
Written by Stu Hanchet
See the video: Ross Sea currents.
Predicted circulation patterns
Predicted circulation showing modelled locations of the Antarctic Circumpolar Current, Antarctic Coastal Countercurrent and Ross Sea gyres.
Predicted location of floats
Predicted location of floats after 24 months. The red floats released in the north and the green floats released in the south.
Currents and icebergs
The Antarctic coastal counter current travels east to west across the face of the Ross Ice Shelf and then northwards along the western coast of mainland Antarctica where it then moves west along the coastline. This current moves huge icebergs slowly northwards, like B15, which broke off during 2000 and was approximately 150 km by 50 km, which equates to around 1 trillion tonnes of ice. When this iceberg got stuck around McMurdo Sound, it caused havoc, blocking off access to open water for the penguin colonies. Over the next few years, this berg broke into smaller pieces – the last small piece named B15J, which we have seen during our voyage, is being moved slowly north by the coastal current.
Written by John Mitchell
B-15J
A small part of B-15J seen off the Drygalski ice tongue in February 2008, having only travelled 120 nautical miles in 8 years.
Iceberg Graveyard
Remnants of some of the massive bergs grounded along the coast of Antarctica, northwest of Cape Adare, after being transported out of the Ross Sea by the prevailing currents. Photo taken from 20,000 ft.
Ocean currents, ice berg scars
In water depths down to 400 metres the multibeam echo-sounder shows numerous linear features criss-crossing the ocean floor. These scars are varied in size – some are straight, others are bent and ‘wiggly’. Scattered amongst these scars are small depressions or holes. All of these are the traces of icebergs. After breaking off, the icebergs are driven northwards by wind and ocean currents. At shallow depths, they touch the seafloor, leaving scars, and if they become grounded, they can also leave depressions where their huge weight presses onto the seafloor. As sea levels have risen by approximately 120 metres since the last ice age, some of these old scars now are too deep for icebergs to reach and are preserved on the seafloor.
Written by Arne Pallentin
Iceberg scars on the seafloor
A 3D perspective view, with six times vertical exaggeration, of a series of iceberg scars in the seabed off Cape Adare.
Straight and curved scars
Sun illuminated image of a series of iceberg scars in the seabed showing how some are straight, others curved and some very deeply incised. One can also see how new scars cross or obliterate older ones.
Currents and seamounts
When large-scale currents, such as the Antarctic Circumpolar Current travelling at around 0.5 knots, meet obstacles such as a seamount or ridge on the ocean floor, the water must go around, over or through gaps between obstacles. When moving water passes through gaps, it has to accelerate, just like a river speeds up when it is constricted by a gorge. In the case of seamounts, speeds can increase up to 2 knots along the edges. On the Scott seamount chain, we have seen ripples in the seabed sediment and the winnowing away of the finer sediments leaving behind a coarse pavement of rock or shell, both of which indicate higher current speeds on the edges of the seamounts.
Written by John Mitchell
Ripple effect
Muddy seabed at 1,300 metres. The ripples show the effect of currents on fine sediments.
Evidence of higher current speed
Gravel pavement at 1,200 m where the currents have winnowed out the fines leaving behind the coarser, heavier material. (Photo DTIS)
Useful link
Ships travelling to Antarctica face an extra challenge – they must be able to contend with sea ice. Learn more about Antarctic sea ice, including what it is and why it is important.
