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Physical oceanography

Ocean circulation keeping it cool in Antarctica

Source: Armour, Kyle C., et al. “Southern Ocean warming delayed by circumpolar upwelling and equatorward transport.” Nature Geoscience (2016). doi:10.1038/ngeo2731

Global warming is ocean warming

We know that greenhouse gases trapped in our atmosphere are causing the Earth to heat up. In fact, scientists have found that 90% of the energy trapped in the climate system is going into the ocean. The ocean has a huge heat capacity, sucking up extra heat like a sponge, stopping the atmosphere from heating up rapidly and keeping our climate liveable. But heating up the ocean comes with a cost; as water heats it expands, causing the sea level to rise, threatening coastal communities around the globe. Although a warmer ocean means a more comfortable atmosphere for humans and land plants and animals, it may not be beneficial for ocean organisms. Just this year, 90% of the Great Barrier Reef was impacted by coral bleaching because of unusually hot water off the coast of Australia.

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More than 90% of the energy trapped in Earth’s climate system by greenhouse gases has gone into the ocean in the past 50 years. IPCC (2013).

 

Cooling off in the South

Although the ocean is warming almost everywhere, it isn’t like one big bathtub and is warming up much faster in some places than others. An odd pattern has been puzzling scientists for years: in the Arctic, the ocean has warmed very rapidly compared to the rest of the globe, but in the Antarctic the ocean has barely warmed at all, and has even cooled in some regions. This was a surprise to scientists, because climate models predict that the oceans surrounding Antarctica will heat up rapidly in the future. This unexpected result prompted a group of scientists to dig deeper into ocean observations and models to figure out why Antarctica hasn’t warmed up.
The researchers started by looking at exactly where heat is being transferred into  the Southern Ocean, which wraps around Antarctica. They found that there is a lot of heat going into the ocean in the area close to Antarctica, in the same areas where the ocean has cooled down. This confusing result is because the atmosphere above these areas is much warmer, so  heat moves into the ocean so that the water and air become more balanced. The opposite happens further north: where the ocean has warmed much faster than the atmosphere, heat escapes out of the ocean to the air.

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Figure 1 from Armour et al. 2016. The top panel shows the trend in sea surface temperature, the second panel shows the trend in surface heat flux, with positive numbers meaning that heat is moving into the ocean. The third and fourth panels show how much heat is stored at different latitudes, with cooling near Antarctica on the left and warming further north.

 

The Ocean conveyer belt

The slow rate of warming in Antarctica can be explained by looking at the ocean circulation. In the Southern Ocean, strong winds blow water at the surface north, away from Antarctica. This water is replaced by centuries-old water from the deep ocean, which flows south and up to the surface, like a conveyor belt. The old water is very cold and hasn’t been exposed to the extra heat trapped on Earth since humans began burning fossil fuels. Below the surface of the ocean, water  has cooled further south and has warmed strongly further north. This pattern matches what we would expect if heat is going into the ocean further south then being carried north by the ocean currents to be stored. Meanwhile, close to Antarctica cold water keeps being replenished by water rising from below, so the surface never has enough time to heat up.

schematic.001

The global ocean overturning circulation, showing cold deep water coming up to the surface near Antarctica where the surface is cooling (in the black box). The heat going into the ocean in this box is carried north in surface currents, piling heat up further north causing rapid warming. Adapted from Talley (2013).

Using an ocean model, researchers conducting this study  wanted to see if this pattern was because of changes in ocean circulation, or if the background ocean circulation could cause this to happen. They compared heat to a passive tracer, which is just like adding dye to the ocean to see where it goes. They found that if they put the tracer in the ocean near Antarctica and followed where it went, it ended up looking a lot like the pattern of heat storage in the ocean. This means that it’s likely that the ocean is just carrying the extra heat away from Antarctica naturally.

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Figure 3 from Armour et al. 2016 showing the similarities between ocean heat around Antarctica (left) and a passive tracer, like dye (right).

What goes down, must come up

Just because Antarctic waters haven’t warmed up yet doesn’t mean they won’t warm up in the future. The cold, deep water that is rising to the surface in Antarctica begins its journey  in the Northern hemisphere where the ocean surface is rapidly warming. This warm signal will be carried through the deep ocean, and eventually, in hundreds or even thousands of years, will come up to the surface around Antarctica. By figuring out why Antarctic waters haven’t warmed, this study helps us better predict how the ocean near Antarctica will change in the future, and how this will affect sea level changes and local marine life.

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