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Causes, Effects & Impacts: Ocean currents

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Ocean currents

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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 IPCC modelling estimated.
  • The Antarctic Circumpolar Current (ACC) (Videos 1 & 2) is so powerful that it moves an area equivalent to the South Island one metre every second. It’s  warming faster than the global ocean as a whole, threatening the world’s largest ice sheet. and disrupting global ocean currents, which in turn affects global weather systems. Recent modelling and observations shows it is slowing down.

“Physical measurements confirm these changes are already well underway. As Antarctica melts, more freshwater flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. From there this water normally spreads northwards to ventilate the far reaches of the deep Indian, Pacific and Atlantic Oceans. But that could all come to an end soon. In our lifetimes.” – England et al, 2023

“We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.”Ditleveson & Ditleveson 2023

“We discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is nearing its tipping point.” van Westen 2024

  • El Niño (Video 5) brings warmer and windier conditions to New Zealand when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for extended periods. Recent research suggests 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 as well as La Niñas; that is, more frequent and intense extreme weather events.

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 IPCC modelling estimated.
  • The Antarctic Circumpolar Current (ACC) (Videos 1 & 2) is so powerful that it moves an area equivalent to the South Island one metre every second. It’s  warming faster than the global ocean as a whole, threatening the world’s largest ice sheet. and disrupting global ocean currents, which in turn affects global weather systems. Recent modelling and observations shows it is slowing down.

“Physical measurements confirm these changes are already well underway. As Antarctica melts, more freshwater flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. From there this water normally spreads northwards to ventilate the far reaches of the deep Indian, Pacific and Atlantic Oceans. But that could all come to an end soon. In our lifetimes.” – England et al, 2023

“We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.”Ditleveson & Ditleveson 2023

“We discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is nearing its tipping point.” van Westen 2024

  • El Niño (Video 5) brings warmer and windier conditions to New Zealand when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for extended periods. Recent research suggests 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 as well as La Niñas; that is, more frequent and intense extreme weather events.
Fig. 1: When Earth is viewed from a Spillhause projection, that is, the oceans surrounded by land, it becomes clear that the Antarctic Circumpolar Current (ACC) is at the heart of the global ocean circulation.

Antarctic Circumpolar Current (ACC) Thermohaline Current

Video 1: Prof. Eric Rignot explains how the ACC is changing, melting marine glaciers and ice sheets along the coast of Antarctica.

The strongest ocean current on Earth, the ACC 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. 2).

Fig. 2: 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

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, and warmer water from temperate latitudes us getting through. 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 2: 10.07 – 12.23 mins).

“If the Antarctic overturning slows down, nutrient-rich seawater will build up on the seafloor, five kilometres below the surface. These nutrients will be lost to marine ecosystems at or near the surface, damaging fisheries.

“Changes in the overturning circulation could also mean more heat gets to the ice, particularly around West Antarctica, the area with the greatest rate of ice mass loss over the past few decades. This would accelerate global sea-level rise.

“An overturning slowdown would also reduce the ocean’s ability to take up carbon dioxide, leaving more greenhouse gas emissions in the atmosphere. And more greenhouse gases means more warming, making matters worse.

“Put simply, a slowing or collapse of the overturning circulation would change our climate and marine environment in profound and potentially irreversible ways.

“The signs of melting around the edges of Antarctica are very clear, with increasingly large volumes of freshwater flowing into the ocean and making nearby waters less salty and therefore less dense. And that’s all that’s needed to slow the overturning circulation. Denser water sinks, lighter water does not….

“…We ran three different experiments, one where conditions remained unchanged from the 1990s; a second forced by projected changes in temperature and wind; and a third run also including projected changes in meltwater from Antarctica and Greenland.

“In this way we could separate the effects of changes in winds and warming, from changes due to ice melt.

“The findings were striking. The model projects the overturning circulation around Antarctica will slow by more than 40% over the next three decades, driven almost entirely by pulses of meltwater.” England et al

Video 2: Is the Southern Ocean about to have its own ‘Day After Tomorrow’ moment? The authors explain how these changes would profoundly alter the ocean’s overturning of heat, freshwater, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come.

Atlantic Meridional Overturning Circulation (AMOC) Thermohaline Current

The AMOC is literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans. 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. Video 3 explains how it works and why it’s so important. Video 4 is a 3D animation of the flow and eddies as the current moves around the Atlantic.

Video 3: How AMOC transports heat and nutrients around the planet, and the implications if it shuts down (August 2023).
Video 4: Animation of the flow and eddies of AMOC.

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 was so large that until recently, the sinking salty water is one of the the world’s largest waterfalls (the largest is around Antarctica, discussed below). 

However, less sea ice is forming in the Arctic every year. The Greenland ice sheet is also melting at a record-breaking pace, along with glaciers and permafrost in the lands surrounding the Arctic Ocean. Together, this is disgorging ever-increasing amounts of freshwater into the ocean. This means that less salt and more freshwater is being added to the Arctic ocean. The result? The mechanism that drives AMOC is disappearing. The current is now weaker than at any time in the past thousand years.

The last time the current shut down was at the end of the last glacial maximum (LGM) leading to abrupt cooling across much of Europe (‘Younger Dryas’) with an unstable climate and wild weather globally for several thousand years. The climate is now warming far faster than at the end of the LGM. In 2019 there is concern amongst key scientists that the current may have already reached tipping point and is shutting down. 

Recent (2023) research indicates it will shut down this century. The tipping point for when this could occur has been calculated (Video 5).

Video 5: The lead author of Physics-based early warning signal shows that AMOC is on tipping course (Science Advances 10) explains the processes and findings.

El Niño-Southern Oscillation (ENSO) and equatorial currents

Video 6: NIWA video explaining how this system affects New Zealand’s weather, including drought in Canterbury.

“ENSO is a cycle of warm El Niño and cool La Niña episodes that happen every few years in the tropical Pacific Ocean. It is the most dramatic year-to-year variation of the Earth’s climate system, affecting agriculture, public health, freshwater availability, power generation, and economic activity “McFadden et al (eds), 2020

Fig. 3: The pink and blue line shows the sea surface temperatures during the 1997 and 2015 El Niño years. By late August 2023, a new El Niño began at a time when sea surface temperatures were already at the highest ever recorded (red line).(Image: NOAA)

ENSO is 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 6).

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 in the last century, whereas La Niña helped cool 2020. El Niño events are projected to increase in frequency and last longer.

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. That leads to a domino effect around 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.

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