https://phys.org/news/2019-04-north-atlantic-hole-impacts-jet.html
[links in online article]
April 15, 2019
North Atlantic warming hole impacts jet stream
by Pennsylvania State University
The North Atlantic warming hole (NAWH), a region of reduced warming
located in the North Atlantic Ocean, significantly affects the North
Atlantic jet stream in climate simulations of the future, according to a
team of researchers.
Sea surface temperatures (SST) are projected to increase in most of the
world's oceans as the result of global climate change. However, within
an area of rotating ocean currents just south of Greenland an anomaly
exists where colder sea-surface temperatures were documented in both
global climate-model projections and in observations.
"It's called a hole because there is a lack of warming," said Melissa
Gervais, assistant professor of meteorology and atmospheric science,
Penn State, who used the Community Earth System model (CESM) to
investigate the impact of the NAWH on atmospheric circulation and
midlatitude jets. "We found that this region of the ocean is a really
important place for forcing the jet stream that goes across the North
Atlantic Ocean."
The researchers published their findings in the Journal of Climate.
Development of the NAWH is linked to a slowdown of the Atlantic
Meridional Overturning Circulation, a large system of ocean currents
that carry warm water from the tropics northwards into the North
Atlantic, and is thought to be caused by an influx of fresh water coming
from melting Arctic sea ice.
Previous research by Gervais and her team demonstrated that this
increase in fresh water to the ocean changes circulation patterns and
leads to surface cooling.
"With more Arctic sea ice melting, more fresh water flows into the
Labrador Sea, which leads to a reduction in deep convection," said
Gervais who also is an Institute for CyberScience co-hire. "That changes
the ocean circulation, allowing it to cool in that region south of
Greenland."
This cooling pattern, relative to global average SST increase, is
predicted to become greater and more apparent relative to the internal
ocean variability as the 21st century progresses.
"These changes in SST patterns occur as the result of changes in ocean
circulation and could have a significant impact on atmospheric
circulation and the North Atlantic storm track in the future," said Gervais.
Jet streams, high altitude currents of wind flowing above the Earth,
transport air masses and drive weather patterns. The relationship
between climate change and jet streams is complex and understanding the
potential impact of climate change on jet streams is crucial for
understanding changes in weather patterns and storm tracks.
"With climate change we have some ideas about how the jets are going to
change. In general, we expect to see a poleward shift and eastward
elongation of the jet," said Gervais. "Right now, it's sort of a tug of
war between impacts of the tropics and impacts of the arctic. So those
two things are competing to shift where the jet is located."
Most climate models seem to agree that the Pacific jet stream is going
to shift poleward but there is a lot of variability in predictions for
the Atlantic, said Gervais.
To investigate how the development of the NAWH impacts the jet stream,
the team conducted a series of large-ensemble, atmospheric model
experiments in the CESM with prescribed SST and sea ice levels over
three different time periods.
"We ran three simulations," said Gervais. "One with current warming-hole
conditions; one where the ocean temperature was increased to fill in the
warming hole; and one where its size was twice as deep, to simulate more
freshwater from melting ice sheets."
Their results indicate that the NAWH plays an important role in
midlatitude atmospheric circulation changes in the model's future
climate simulations.
"We found that it's really quite important for that region," said
Gervais. "The NAWH seems to be elongating the jet even further and
shifting it a little bit north. Instead of just thinking about how the
tropics and arctic amplification are influencing the jet, we now also
need to think about how this warming hole is going to influence the jet.
These local changes in the North Atlantic jet are of a similar magnitude
to the full climate-change response in the region, indicating that the
North Atlantic warming hole could be an important additional factor in
the tug of war on midlatitude circulation, that has received little
attention."
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