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Causes, Effects & Impacts: Ocean currents

Circumpolar surface temperatures image: Earth live (click for live updates)

Ocean currents

Summary

  • Just like us, the Earth has a circulatory system. Instead of blood, oceanic currents transport heat, oxygen, carbon dioxide and nutrients around the planet.
  • The world’s oceans have absorbed around 93% of global warming and heating up 40% faster than the IPCC estimated in 2013.
  • The Great Oceanic Conveyor Belt or AMOC is literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans. It’s now weaker than at any time in the past thousand years. The gyres that feed this conveyor belt in the Labrador Sea are now failing.

“A conveyor belt of ocean water that loops the planet and regulates global temperatures could be heading for a tipping point.”National Geographic

  • El Niño brings warmer and windier conditions to New Zealand when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for an extended period of time. Recent research suggests that the increasing loss of sea-ice around Antarctica is leading to more warming in the eastern equatorial Pacific, which is where ENSO patterns form. This could result in more frequent and stronger El Niños.
  • The Antarctic Circumpolar Current (ACC) is so powerful it moves an equivalent area of the South Island one metre every second. It’s also warming faster than the global ocean as a whole, threatening the world’s largest ice sheet.
  • Ocean currents played a significant role in how the current Ice Age started, and why we still are in an Ice Age. They’re now changing due to warming temperatures and feedback effects.

Summary

  • Just like us, the Earth has a circulatory system. Instead of blood, oceanic currents transport heat, oxygen, carbon dioxide and nutrients around the planet.
  • The world’s oceans have absorbed around 93% of global warming and heating up 40% faster than the IPCC estimated in 2013.
  • The Great Oceanic Conveyor Belt or AMOC is literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans. It’s now weaker than at any time in the past thousand years. The gyres that feed this conveyor belt in the Labrador Sea are now failing.

“A conveyor belt of ocean water that loops the planet and regulates global temperatures could be heading for a tipping point.”National Geographic

  • El Niño brings warmer and windier conditions to New Zealand when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for an extended period of time. Recent research suggests that the increasing loss of sea-ice around Antarctica is leading to more warming in the eastern equatorial Pacific, which is where ENSO patterns form. This could result in more frequent and stronger El Niños.
  • The Antarctic Circumpolar Current (ACC) is so powerful it moves an equivalent area of the South Island one metre every second. It’s also warming faster than the global ocean as a whole, threatening the world’s largest ice sheet.
  • Ocean currents played a significant role in how the current Ice Age started, and why we still are in an Ice Age. They’re now changing due to warming temperatures and feedback effects.

The Great Oceanic Conveyor Belt / Thermohaline Current / AMOC

Literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans, the AMOC (Atlantic Meridional Overturning Circulation) is such a key part of that conveyor belt that the two names are often used to describe the same current. The process begins in the Labrador Sea. A major part of this current includes the Gulf Stream, which keeps Europe warmer than the east coast of the United States. Below is a simple explanation from National Geographic. Videos 1 and 2 explain how it works and why it’s so important.

The current, which moves nearly 20 million cubic metres of water per second, is driven in part by the formation of Arctic sea ice each year. When ocean water freezes, it leaves salt behind, making the surrounding water denser and heavier, so it sinks. The scale of sea ice formation is so large that this sinking salty water is the world’s largest waterfall. However,  less sea ice is forming in the Arctic every year. The Greenland ice sheet is also melting, along with glaciers and permafrost in the lands surrounding the Arctic Ocean. Together, this is disgorging ever-increasing amounts of freshwater into the ocean, so less salt and more freshwater is being added to the Arctic ocean. The result? A key part of the mechanism that drives AMOC is disappearing. The current is now weaker than at any time in the past thousand years.

Video 1: 13 mins; explains how one of the key oceanic currents transports heat and nutrients around the planet, and the implications if it shuts down.

“We are 50 to 100 years ahead of schedule with the slowdown of this ocean circulation pattern relative to what the models predict. Why might that be true? Well one of the things that the models also aren’t capturing is the rate at which we’re losing ice from the ice sheets: the West Antarctic Ice Sheet and the Greenland Ice Sheet.”  – Dr. Michael Mann, Pennsylvania State University (Video 2).

Video 2: 6 minutes; less detailed but covers the key points. The cold ‘blue’ spot is in the Labrador Sea.

This current abruptly shut down when Earth warmed quickly at the end of the last glacial maximum, leading to equally abrupt cooling much of Europe (‘Younger Dryas’) with an unstable climate and wild weather globally for several thousand years.

Today, the climate is warming far faster than at the end of the last glacial maximum, so there is concern amongst key scientists that the current may have already reached tipping point and is shutting down.

“Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.”Boers 2021

El Niño-Southern Oscillation (ENSO) and equatorial currents

While 2020 tied with 2016 as the warmest year on record for average global surface temperatures, there is one very big difference. Warming in 2016 was boosted by one of the largest El Niño events in the last century. Whereas La Niña helped cool 2020. And El Niño events are protected to increase in frequency and last longer.

Probably best known for its episodic impact on seasonal weather rather than as oceanic currents, this highlights the interlinked relationship between the ocean and weather, especially for small island nations like New Zealand (Video 3). The Pacific basin covers one third of the planet, so changes in this area also have profound effects on east Asia and the western areas of the Americas. And there’s also a domino effect on other parts of the planet.

Why and how El Niño and its reverse current, La Niña flip back and forth is still unclear. Recent research suggests that the increasing loss of sea-ice around Antarctica is leading to more warming in the eastern equatorial Pacific, which is where ENSO patterns form.

Video 3: NIWA video explaining how this system affects New Zealand’s weather, including drought in Canterbury.

Antarctic Circumpolar Current (ACC)

The strongest ocean current on Earth, it encircles Antarctica and extends from the surface to the bottom of the ocean. It carries an estimated 165 million to 182 million cubic metres of water every second (a unit called a ‘Sverdrup’) from west to east, more than 100 times the flow of all the rivers on Earth, or the equivalent of pushing the entire South Island of New Zealand one metre every second. It helps to act as a planetary thermostat, keeping Antarctica cool (Fig. 1).

Fig. 1: Satellite view over Antarctica reveals a frozen continent surrounded by icy waters. The sea ice extent is in light blue. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. The Antarctic Circumpolar Current (ACC) maintains this boundary. The two black lines indicate the long-term position of the southern and northern front of the ACC. (Image: The Conversation)

“The ACC also plays a part in the Meridional (or Global) Overturning Circulation, which brings deep waters formed in the North Atlantic southward into the Southern Ocean. Once there it becomes known as Circumpolar Deep Water, and is carried around Antarctica by the ACC. It slowly rises toward the surface south of the Polar Front.

“Once it surfaces, some of the water flows northward again and sinks north of the Subarctic Front. The remaining part flows toward Antarctica where it is transformed into the densest water in the ocean, sinking to the sea floor and flowing northward in the abyss as Antarctic Bottom Water. These pathways are the main way that the oceans absorb heat and carbon dioxide and sequester it in the deep ocean.”  – Phillips et al

Video 3: Prof. Eric Rignot explains how the ACC is changing, melting marine glaciers and ice sheets along the coast of Antarctica.

Like the Arctic Ocean, the Southern Ocean (where the current flows) has become warmer (we are now experiencing marine heat waves). The warmest water is deep and it’s undercutting the marine ice sheets and ice shelves around Antarctica that hold back  ~30 million cubic kilometres of ice. As the climate warms, the deeper warmer waters of the ACC are being by pushed closer to the coast by the strengthening Westerly winds. This is accelerating the rate at which the water is eroding the base of the marine ice sheets, which in turn is accelerating the rate at which they’re melting and collapsing (Video 4: 10.07 – 12.23 mins).

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