As Ocean Currents Slow due to Global Heating, Winter Weather Will Grow More Extreme

Topographic map of the Nordic Seas and subpolar basins with surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures. Photo: R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP.

A study conducted by Jianjun Yin, an associate professor at the University of Arizona’s Department of Geosciences, and Ming Zhao, a physical scientist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, revealed this seemingly paradoxical phenomenon will soon begin to affect the continental U.S. in powerful ways.

Their research involved analyzing the Atlantic portion of the global oceanic circulatory “conveyor belt” known as the Atlantic Meridional Overturning Circulation (AMOC).

All parts of the global current system churn and move based on differences in the water’s temperature and saltiness. As recent studies have determined, the AMOC – which includes the Gulf Stream as part of its feeding currents – is slowing because of climate crisis driven ocean heating. That heating is little by little altering the thermodynamic flow of warmer currents in the southern part of the AMOC which normally flow with considerably power northwards and eastwards towards the European and UK land masses.

As the researchers noted, warm water from the AMOC normally travels north in the upper Atlantic Ocean, where the surface currents release 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 northward in the ocean," Yin said. "The magnitude is on the order of 1 petawatts, or 10 to the 15 power watts. 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 as the climate warms, so does the ocean surface. 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," Yin said. "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."

Yin said the study was motivated by the extreme cold weather Texas experienced in February.

"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.

According to recent observational studies, the AMOC has weakened in past decades. Climate models project it will get even weaker in response to increased greenhouse gases in the atmosphere.

"But there is uncertainty about the magnitude of the weakening because, at this point, we don't know exactly how much the Greenland ice sheet will melt," Yin said. "How much it melts depends on the greenhouse gas emissions."

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."

While the paper does not explore it, this secondary effect of the climate crisis will likely contribute to even more extreme weather events during the late spring through early autumn hurricane season. With colder weather during the winter, the same heavier more moisture-filled cloud cover which will help contribute to potentially more disastrous blizzard-filled storms inland from America’s east coast then is likely to fuel energetically charged thunderstorms, tropical storms, and hurricanes as they move northwards as the northern latitudes grow seasonally warmer.

 “Influence of the Atlantic meridional overturning circulation on the U.S. extreme cold weather,” by Jianjun Yin and Ming Zhao, the paper describing the extreme winter weather phenomenon caused by slowing of the AMOC, was published in the October 13, 2021,