Why a 15 Percent Slow-Down in North Atlantic Ocean Circulation is Seriously Bad News
“We
know somewhere out there is a tipping point where this current system
is likely to break down. We still don’t know how far away or close
to this tipping point we might be. … This is uncharted territory.”
*****
23
April, 2018
The
North Atlantic ocean circulation (often called AMOC or the Great
Ocean Conveyor) is
now the weakest its been in sixteen centuries.
Increasing
melt from Greenland due to human-forced warming of the atmosphere
through the deep ocean is freshening the ocean surface of the far
North Atlantic. To the south, higher ocean temperatures are
increasing surface salt content through greater rates of
evaporation. Fresh water prevents ocean water from sinking in
the north and rising salt content generates increased sinking in the
south. As a result, the rate at which waters move from the Equator
toward the Pole is slowing down. Since the mid 20th Century, this
critical ocean circulation has
reduced in strength by 15 percent on decadal time-sales.
(Deep
water formation in the North Atlantic is driven by the sinking of
cold, salty water. Over recent years, this formation, which drives
larger ocean circulation and atmospheric weather patterns, has been
weakening due to increasing fresh water flows coming from a melting
Greenland. Image source: Commons and the NASA
Earth Observatory.)
Movement
of warm Equatorial waters northward and their subsequent overturning
and sinking in the North Atlantic drives a number of key weather and
climate features. The first is that it tends to keep Europe warm
during winter and to moderate European temperatures during summer.
The second impact is that a fast moving current off the U.S. East
Coast pulls water away from the shore keeping sea levels lower. The
third is that warm water in the North Atlantic during winter time
tends to keep the regional jet stream relatively flat. And the fourth
is that a more rapid circulation keeps the ocean more highly
oxygenated — allowing it to support more life.
A
slowing down of ocean circulation in the North Atlantic therefore
means that Europe will tend to cool during winter even as it heats up
during summer. Sea level rise will accelerate faster for the U.S.
East Coast relative to the rest of the world due to a slowing Gulf
Stream combined with the effects of melting land glaciers and thermal
ocean expansion. The North Atlantic jet stream will tend to become
wavier — with deep troughs tending to form over Eastern North
America and through parts of Europe. These trough zones will tend to
generate far more intense fall and winter weather. Finally, a slowing
ocean circulation will tend to increase the number of low-oxygen dead
zones.
(Cool
pool formation near Greenland juxtaposed by a warming and slowing of
the Gulf Stream as it is forced southward is an early indication of
ocean circulation slow-down. During recent years, this phenomena —
which is related to larger human-forced climate change — has become
a prevalent feature of North Atlantic Ocean climate and weather
patterns. An indicator that climate change and ocean system changes
for this region are already under way. Image source: Earth
Nullschool.)
A
15 percent slow down in ocean circulation is
not yet a catastrophic event.
It is, however, enough to produce odd weather and climate signals. We
have tended to see higher rates of sea level rise off the U.S. East
Coast, we have tended to see more extreme winter weather across the
North Atlantic basin. The
long term trend for increasing ocean dead zones is well established.
And European weather has become more and more extreme — with hot
summers and severe winters.
With
rates of Greenland melt increasing, there is a risk that the historic
observed North Atlantic circulation weakening will increase further
and more radically — producing still more profound results than we
see today. In the event of large melt outflows coming from Greenland
during abnormally warm summers or due to warming deep water melting
glaciers from below — a possibility that rises with each 0.1 C of
global temperature increase — we could see a very rapid weakening
of ocean circulation above and beyond
that which has already been recorded.
(Like
Antarctica, Greenland features a number of below sea level locations
directly beneath its largest ice masses. This feature makes Greenland
more vulnerable to rapid ice loss and large melt outflows. Image
source: NASA
JPL.)
If
such a tipping point event is breached — and there is increased
risk for it as global temperatures enter a range of 1.5 to 2.5 C
above 1880s averages during the 2020s through the 2040s — then we
can expect far more profound weather and climate disruptions than
those we have already experienced.
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