Changing ocean currents are driving extreme winter weather

Date:
October 20, 2021
Source:
University of Arizona
Summary:
Slower ocean circulation as the result of climate change could
intensify extreme cold weather in the U.S., according to new
research.



FULL STORY ========================================================================== 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.


==========================================================================
A pair of researchers studied the Atlantic portion of this worldwide
conveyor belt called the Atlantic Meridional Overturning Circulation,
or AMOC, and found that winter weather in the United States critically
depends on this conveyor belt-like system. As the AMOC slows because of
climate change, the U.S. will experience more extreme cold winter weather.

The study, published in the journal Communications Earth & Environment was
led by Jianjun Yin, an associate professor in the University of Arizona Department of Geosciences and co-authored by Ming Zhao, a physical
scientist at the National Oceanic and Atmospheric Administration's
Geophysical Fluid Dynamics Laboratory.

AMOC works like this: 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 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." ========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Mikayla Mace Kelley. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Jianjun Yin, Ming Zhao. Influence of the Atlantic meridional
overturning
circulation on the U.S. extreme cold weather. Communications Earth &
Environment, 2021; 2 (1) DOI: 10.1038/s43247-021-00290-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211020140042.htm

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