Major Atlantic ocean current system might be approaching critical
threshold

Date:
August 5, 2021
Source:
Potsdam Institute for Climate Impact Research (PIK)
Summary:
A major Atlantic ocean current -- the Atlantic Meridional
Overturning Circulation, or AMOC -- may have been losing stability
in the course of the last century, according to new research. A
potential collapse of this ocean current system could have severe
consequences.



FULL STORY ========================================================================== [Ocean waves (stock | Credit: (c) Noradoa / stock.adobe.com] Ocean waves
(stock image).

Credit: (c) Noradoa / stock.adobe.com [Ocean waves (stock | Credit:
(c) Noradoa / stock.adobe.com] Ocean waves (stock image).

Credit: (c) Noradoa / stock.adobe.com Close The major Atlantic ocean
current, to which also the Gulf stream belongs, may have been losing
stability in the course of the last century. This is shown in a new study published in Nature Climate Change. The Atlantic Meridional Overturning Circulation, or AMOC, transports warm water masses from the tropics
northward at the ocean surface and cold water southward at the ocean
bottom, which is most relevant for the relatively mild temperatures in
Europe. Further, it influences weather systems worldwide. A potential
collapse of this ocean current system could therefore have severe
consequences.


==========================================================================
"The Atlantic Meridional Overturning really is one of our planet's key circulation systems," says the author of the study, Niklas Boers from the Potsdam Institute for Climate Impact Research, Freie Universita"t Berlin
and Exeter University. "We already know from some computer simulations
and from data from Earth's past, so-called paleoclimate proxy records,
that the AMOC can exhibit -- in addition to the currently attained strong
mode -- an alternative, substantially weaker mode of operation. This bi-stability implies that abrupt transitions between the two circulation
modes are in principle possible." Loss of dynamical stability could
ultimately lead to collapse It has been shown previously that the AMOC
is currently at its weakest in more than a 1000 years. However, so far it
has remained an open question whether the observed weakening corresponds
to a change in the mean circulation state, or whether it is associated
with an actual loss of dynamical stability. "The difference is crucial,"
says Niklas Boers, "because the loss of dynamical stability would imply
that the AMOC has approached its critical threshold, beyond which a
substantial and in practice likely irreversible transition to the weak
mode could occur." Long-term observational data of the strength of
the AMOC does unfortunately not exist, but the AMOC leaves so-called fingerprints in sea-surface temperature and salinity patterns of the
Atlantic ocean. "A detailed analysis of these fingerprints in eight
independent indices now suggests that the AMOC weakening during the last century is indeed likely to be associated with a loss of stability,"
says Boers. "The findings support the assessment that the AMOC decline is
not just a fluctuation or a linear response to increasing temperatures
but likely means the approaching of a critical threshold beyond which
the circulation system could collapse." In addition to global warming, freshwater inflow is a factor -- which is also linked to climate change
A number of factors are likely important for the phenomenon -- factors
that add to the direct effect that the warming of the Atlantic ocean has
on its circulation. These include freshwater inflow from the melting of
the Greenland ice sheet, melting sea-ice, increasing precipitation and
river run-off.

Freshwater is lighter than saltwater and reduces the tendency of the
water to sink from the surface to greater depths, which is one of the
drivers of the overturning.

"I wouldn't have expected that the excessive amounts of freshwater added
in the course of the last century would already produce such a response in
the overturning circulation," says Boers. "We urgently need to reconcile
our models with the presented observational evidence to assess how far
from or how close to its critical threshold the AMOC really is." While
the respective relevance of the different factors has to be further investigated, they're all linked to human-caused climate change.

========================================================================== Story Source: Materials provided by Potsdam_Institute_for_Climate_Impact_Research_(PIK).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Niklas Boers. Observation-based early-warning signals for a
collapse of
the Atlantic Meridional Overturning Circulation. Nature Climate
Change, 2021 DOI: 10.1038/s41558-021-01097-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210805115420.htm

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