Depths of the Weddell Sea are warming five times faster than elsewhere


Date:
October 20, 2020
Source:
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine
Research
Summary:
Over the past three decades, the depths of the Antarctic Weddell
Sea have warmed five times faster than the rest of the ocean at
depths exceeding 2,000 meters.



FULL STORY ==========================================================================
Over the past three decades, the depths of the Antarctic Weddell Sea have warmed five times faster than the rest of the ocean at depths exceeding
2,000 metres. This was the main finding of an article just published
by oceanographers from the Alfred Wegener Institute, Helmholtz Centre
for Polar and Marine Research (AWI). In the article, they analyse an unprecedented oceanographic time series from the Weddell Sea and show
that the warming of the polar depths is chiefly due to changed winds
and currents above and in the Southern Ocean. In addition, the experts
warn that the warming of the Weddell Sea could permanently weaken the overturning of tremendous water masses that takes place there -- with far-reaching consequences for global ocean circulation. Their study was
just released on the online portal of the Journal of Climate.


==========================================================================
Over the past several decades, the world's oceans have absorbed more
than 90 percent of the heat trapped in the atmosphere by greenhouse-gas emissions, effectively slowing the rise in air temperatures around
the globe. In this regard, the Southern Ocean is pivotal. Though it
only accounts for 15 percent of the world's oceans in terms of area,
because of the overturning that takes place there, it absorbs roughly three-fourths of the heat.

Until recently, very little was known about what happens to this heat
in the depths of the Southern Ocean, due to the lack of sufficiently
long time series.

In order to trace the development down to the seafloor, researchers relied
on regularly repeated ship-based measurements taken with 'CTD' probes (Conductivity, Temperature and Depth). These probes have now become so
precise that they can measure changes in water temperature down to the
nearest ten- thousandth of a degree Celsius. The data they gather can
also be used to determine the water masses' density and salinity.

For the past 30 years, AWI oceanographers have been taking these
temperature and salinity readings during expeditions to the Weddell Sea
on board the German research icebreaker Polarstern -- always at the
same sites, always from the surface to the seafloor, and always with
extremely high accuracy. By doing so, the researchers have produced the
only time series of its kind on the South Atlantic and the Weddell Sea,
which has now allowed them to precisely reconstruct the warming of the
Weddell Sea and identify potential causes.

Only the water below 700 metres is growing warmer Their findings are surprising. "Our data shows a clear division in the water column of the
Weddell Sea. While the water in the upper 700 metres has hardly warmed
at all, in the deeper regions we're seeing a consistent temperature rise
of 0.0021 to 0.0024 degrees Celsius per year," says Dr Volker Strass,
an AWI oceanographer and the study's first author.



========================================================================== These values may seem minuscule at first glance. But, as Strass
explains, "Since the ocean has roughly 1,000 times the heat capacity
of the atmosphere, these numbers represent an enormous scale of heat absorption. By using the temperature rise to calculate the warming rate
in watts per square metre, you can see that over the past 30 years, at
depths of over 2,000 metres the Weddell Sea has absorbed five times as
much heat as the rest of the ocean on average." Through the formation
of bottom water in the Weddell Sea, this heat is then distributed to
the deep basins of the world's oceans.

Potential effects on global circulation In the Weddell Sea, which
represents the southern extension of the Atlantic Ocean and is roughly ten times the size of the North Sea, tremendous water masses cool down. In
the course of sea-ice formation they take on salt, sink to deeper water
layers as cold and heavy Antarctic Bottom Water, and then spread to the
great ocean basins as a deep-sea current. This overturning is considered
to be an important motor for the global ocean circulation. The warming
of the depths of the Weddell Sea could weaken that motor, since warmer
water has a lower density. Consequently, it is lighter and could fill
higher layers of the water column.

"Our field data already shows a temperature-related loss in density
in the deeper water masses of the Weddell Sea. This change is most
pronounced in the Bottom Water," says co-author and AWI oceanographer
Gerd Rohardt. Whether or not the Antarctic Bottom Water will continue to
fulfil its function as the deepest limb of the global ocean overturning circulation chiefly depends on how the density of the water masses
above it changes. "In order to monitor these developments, we'll need
to continue our regular ship-based readings in the Weddell Sea," says
the researcher.

Tracking down the cause: Winds and currents are transporting more heat
farther south As the cause of the increased heat input in the depths of
the Weddell Sea, the researchers have identified a change in the wind
and current systems over and in the Southern Ocean. "Over the past three decades, the westerlies and with them the Antarctic Circumpolar Current
have not only shifted one to two degrees to the south; they have also intensified. As a result, the diameter of the Weddell Gyre has decreased,
and the flow speed of the water masses has increased. Because of these
two factors, more heat from the Circumpolar Current is transported to the Weddell Sea today than when we first began our measurements," explains
Prof Torsten Kanzow, Head of the AWI's Climate Sciences Division and
another co-author of the study.

Once the heat reaches the depths of the Weddell Sea, the major bottom
water currents distribute it to all ocean basins. "Our time series
confirms the pivotal role of the Southern Ocean and especially the Weddell
Sea in terms of storing heat in the depths of the world's oceans," says
Volker Strass. If the warming of the Weddell Sea continues unchecked,
he explains, it will have far- reaching consequences not only for the
massive ice shelves on the southern coast of the Weddell Sea, which extend
far out into the ocean, and as such, for sea-level rise in the long term,
but also for the conveyor belt of ocean circulation as a whole.


========================================================================== Story Source: Materials provided by Alfred_Wegener_Institute,_Helmholtz_Centre_for_Polar_and
Marine_Research. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Volker H. Strass, Gerd Rohardt, Torsten Kanzow, Mario Hoppema, Olaf
Boebel. Multidecadal Warming and Density Loss in the Deep Weddell
Sea, Antarctica. Journal of Climate, 2020; 33 (22): 9863 DOI:
10.1175/JCLI-D- 20-0271.1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201020105530.htm

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