Slower ocean circulation as the result of climate change could intensify extreme cold weather in the U.S., according to new UArizona research.
Throughout Earth’s oceans runs a conveyor belt of water. Its churning is powered by differences in the water’s temperature and saltiness, and weather patterns around the world are regulated by its activity.
Now, a new science publication shows that the Atlantic portion of this worldwide conveyor belt called the Atlantic Meridional Overturning Circulation, or AMOC, critically influences winter weather in the United States. As AMOC currently slows down, the U.S. will experience more extreme cold winter weather.
How does AMOC work?
Warm water travels north in the upper Atlantic Ocean and releases heat into the atmosphere at high latitudes. As the water cools, it becomes denser, which causes it to sink into the deep ocean where it flows back south.
This circulation transports an enormous amount of heat – on the order of 1 petawatts, or 10 to the 15 power watts- northward in the ocean. Right now, the energy consumption by the entire world is about 20 terawatts, or 10 to the 12 power watts. So, 1 petawatt is enough to run about 50 civilizations.
But ocean surfaces are currently warming. At the same time, the Greenland ice sheet experiences melting, which dumps more freshwater into the ocean. Both warming and freshening of the water can reduce surface water density and inhibit the sinking of the water, slowing the AMOC. If the AMOC slows, so does the northward heat transport.
This is important because the equator receives more energy from the sun than the poles. Both the atmosphere and ocean work to transport energy from low latitudes to high latitudes. If the ocean can’t transport as much heat northward, then the atmosphere must instead transport more heat through more extreme weather processes at mid-latitudes. When the atmosphere moves heat northward, cold air is displaced from the poles and pushed to lower latitudes, reaching places as far south as the U.S. southern border.
Think of it as two highways connecting two big cities. If one is shut down, the other one gets more traffic. In the atmosphere, the traffic is the daily weather. So, if the ocean heat transport slows or shuts down, the weather becomes more extreme.
The study was motivated by the extreme cold weather Texas experienced in February. Here some amazing pictures from the Big Freeze!
“In Houston, the daily temperature dropped to 40 degrees Fahrenheit below the normal,” Yin said. “That’s the typical range of a summer/winter temperature difference. It made Texas feel like the Arctic. This kind of extreme winter weather happened several times in the U.S. during recent years, so the scientific community has been working to understand the mechanism behind these extreme events.”
The crisis in Texas caused widespread and catastrophic power outages, and the National Oceanic and Atmospheric Administration estimated that socioeconomic damages totaled $20 billion. Yin was curious about the role the ocean played in the extreme weather event.
Yin and Zhao used a state-of-the-art, high-resolution global climate model to measure the influence of the AMOC on U.S. extreme cold weather.
They ran the model twice, first looking at today’s climate with a functioning AMOC. They then adjusted the model by inputting enough freshwater into the high-latitude North Atlantic to shut down the AMOC. The difference revealed the role of the AMOC in extreme cold weather. They found that without the AMOC and its northward heat transport, extremely cold winter weather intensifies in the U.S.
The researchers also didn’t take into account in their model the effects of human-caused global warming, but that’s an area of interest for the future, Yin said.
“We basically just turn off the AMOC (in the model) to look at the response by extreme weather,” he said. “Next, we want to factor in the greenhouse gases and look at the combined effects of the AMOC slowdown and global warming on extreme cold weather.” [Arizona EDU, Nature]