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OCEAN SCIENCE
Arctic sea ice heated from below
Beneath the fresh and cold surface water in the Arctic Ocean resides more saline and warmer water of Atlantic
origin. Pan-Arctic measurements of turbulent mixing suggest that tidal mixing is bringing up substantial amounts of
heat in some areas.
Camille Lique
T
he Arctic Ocean has been undergoing
rapid transformation over the past few
decades. Among the features of this
changing environment, the most striking
is the fast shrinking of the sea-ice cover.
Atmospheric warming in the Arctic region
is probably the main driver of this sea-ice
decline1, yet the ocean might also contribute
significantly. Indeed, the layer of Atlantic
water that is found at intermediate depths
in the Arctic Ocean contains a large amount
of heat that, if brought to the ocean surface,
could melt all of the Arctic sea ice within
a few years. Writing in Nature Geoscience,
Rippeth and colleagues2 report that tidegenerated turbulent mixing can locally be
large enough to bring significant amounts
of heat upward to the surface layer of the
Arctic Ocean.
Salty water with relatively warm
temperatures of 2 to 3 °C is transported from
the North Atlantic Ocean to the Arctic Ocean
through Fram Strait and the Barents Sea. This
water mass brings a large amount of heat to
the Arctic Ocean. As the Atlantic Water enters
the Arctic Ocean, it descends beneath the
fresh surface layer, the halocline. The surface
water originates largely from river runoff.
It is colder and contains so much less salt
that its overall density is lower. The sea ice is
thereby insulated from the heat contained in
the Atlantic water by this surface layer. The
Atlantic water circulates anti-clockwise within
the intermediate layer of the Eurasian and
Canadian Basins, following the slope of the
seafloor. Eventually, it is exported back into
the North Atlantic Ocean as cold water near
the freezing point.
Based on simple heat budget
considerations, the Atlantic Water must
undergo substantial cooling along its transit
within the Arctic Basin. The loss of heat
occurs mainly through upward transfer.
Although crucial for understanding the role
of the ocean in the stability of sea-ice cover,
our knowledge of the upward heat flux from
the Atlantic Water layer and its controlling
mechanisms remains incomplete.
Rippeth and colleagues2 address this
question with the help of the most extensive
Solar radiation
Sea ice
Cold fresh
halocline
Tide-generated
mixing and
associated
upward heat flux
Warm
Atlantic
water
Figure 1 | Layers in Arctic waters. In the Arctic Ocean, a cold, fresh water layer insulates the sea ice from
the more saline, warmer water of Atlantic origin. Vertical mixing and resulting upward heat flux could
warm the fresh upper layer, and lead to melting sea ice. Rippeth and colleagues2 present measurements
of turbulent mixing that suggest the tides generate locally substantial upward heat flux, whereas the
presence or absence of sea ice seems to have little influence on turbulent mixing rates.
172
pan-Arctic survey of microstructure
measurements of turbulent mixing to date.
They infer large vertical heat fluxes, of up to
50 W m–2, in some locations. Their estimates
of turbulent vertical mixing vary widely in
space: vertical mixing tends to intensify over
regions with steeper bathymetry, but it seems
to be insensitive to presence or absence of
sea ice. Based on these findings, Rippeth and
colleagues suggest that the spatial variations
of vertical mixing are mostly driven by
variations in tidal dissipation, which can be
locally important.
These results improve our understanding
of the mechanisms that can transfer heat
from warm Atlantic water to the overlying
surface layer. Tidal flow over topography, as
well as wind blowing at the ocean surface,
can act to generate internal waves at density
interfaces within the ocean. As they propagate
and eventually break, these internal waves
are the main source for vertical mixing in
the ocean. In the Arctic, however, sea ice
forms a barrier between the atmosphere and
the ocean, impeding the injection of wind
energy to the ocean surface. Moreover, most
of the Arctic basin lies poleward of the critical
latitude beyond which the free propagation of
tide-generated internal waves is inhibited. The
amount of mixing at a distance from strong
tide generation is therefore expected to be
limited, in line with the findings by Rippeth
and colleagues. As a result, the Arctic Basin
is remarkably quiet compared with the rest of
the world’s oceans, as shown by earlier direct
observations of the turbulent mixing 3.
However, as the sea-ice pack has been
retreating further and for longer each year
over the past few decades, the amount of
energy input to the ocean from the wind
forcing has increased. This effect may have
led to a seasonal increase in internal wavemixing, with the possible consequence of
destabilizing the stratification of the water
column4. At the same time, competing effects
may also have worked in favour of a more
stable stratification: the Arctic hydrological
cycle has intensified, and increases in river
discharge, Greenland ice melting and
precipitation over the Arctic Basin have all
added more and more fresh water to the
NATURE GEOSCIENCE | VOL 8 | MARCH 2015 | www.nature.com/naturegeoscience
© 2015 Macmillan Publishers Limited. All rights reserved
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very large fraction of the total heat lost by the
Atlantic water mass during its entire transit
through the Arctic Basin. The remaining
heat is lost over a large area in the interior of
the Arctic Basin through very small vertical
heat fluxes6, with very limited effect for the
sea-ice pack.
The study by Rippeth and colleagues2
identifies the important role of tides,
compared with winds and sea ice, in
controlling vertical mixing and associated
heat fluxes in the Arctic basin. However, if
we are to fully understand the influence of
the ocean on current and future sea-ice loss,
other processes such as the absorption of solar
radiation in the surface layers of the Arctic
Ocean will also have to be considered.
❐
Arctic Ocean. Combined with the warming
of the surface ocean, these trends enhance
the stratification within the top layers of the
ocean, and are consequently expected to
reduce the amount of vertical mixing 5.
Regionally intensified levels of vertical
mixing, as observed by Rippeth and
colleagues, result in large upward heat fluxes
from the Atlantic layer. The largest dissipation
rates are found at the entrance of the Arctic
Ocean, north of Fram Strait, where the
Atlantic Water layer is at its warmest and
can be in direct contact with the atmosphere
during years with low sea-ice cover. Rippeth
and colleagues2 infer vertical heat flux up to
50 W m–2 in this region. Although this heat
loss occurs over a small region, it represents a
Camille Lique is at the Department of Earth
Sciences, University of Oxford, South Parks Road,
Oxford OX1 3AN, UK.
e-mail: [email protected]
References
1. Perovich, D. K. & Richter-Menge, J. A. Annu. Rev. Mar. Sci.
1, 417–441 (2009).
2. Rippeth, T. P. et al. Nature Geosci. 8, 191–194 (2015).
3. Rainville, L. & Winsor, P. Geophys. Res. Lett.
35, L08606 (2008).
4. Rainville, L., Lee, C. M. & Woodgate, R. Oceanography
24, 136–145 (2011).
5. Guthrie, J. D., Morison, J. H. & Fer, I. J. Geophys. Res. Oceans
118, 3966–3977 (2013).
6. Timmermans, M‑L., Toole, J., Krishfield, R. & Winsor, P.
J. Geophys. Res. 113, C00A02 (2008).
Published online: 16 February 2015
CARBON SEQUESTRATION
The ocean is a remarkable sink for the
increasing amount of carbon dioxide in
the atmosphere. Carbon dioxide does not
just diffuse into the ocean, it also reacts
abiotically with seawater, producing
bicarbonate and carbonate and thereby
allowing more CO2 to diffuse into the ocean.
All in all, as a result of these reactions
the oceans take up roughly a quarter of
anthropogenic CO2 emissions globally, and
nearly all of this is converted to bicarbonate
and carbonate.
In the Southern Ocean, biological
production and decomposition also play
an important role in regulating the CO2
exchange between the ocean and the
atmosphere. Although the Southern
Ocean became a net CO2 sink following
the industrial revolution, CO2 fluxes in this
region are strongly seasonal. The Southern
Ocean is a carbon dioxide sink in summer,
when organisms use the dissolved inorganic
carbon in the surrounding water — CO2,
bicarbonate, and carbonate — to grow. In
winter, when organic material decomposes
and releases CO2 in the process, carbon
dioxide is emitted back to the atmosphere.
In the 1950s, Roger Revelle and
Hans Suess noticed that the efficiency of
ocean uptake of atmospheric CO2 can be
quantified by a number that came to be
known as the Revelle factor: a ratio relating
changes in seawater CO2 concentrations
to changes in seawater concentrations of
total dissolved inorganic carbon. But the
equilibrium of the reaction between CO2
concentration and dissolved inorganic
carbon changes with pH. As oceans take
up more CO2, they become more acidic
© STEVE BLOOM IMAGES / ALAMY
Biology’s growing role
and the production of dissolved inorganic
carbon becomes less favourable. Over time,
the Revelle ratio increases, because a larger
increase in the concentration of dissolved
CO2 is required to create a given amount of
dissolved inorganic carbon. As a result, the
oceans become less efficient at taking up CO2.
Yet, as the oceans’ chemical capacity to
soak up CO2 diminishes, biological activity
could play an increasingly important role
in regulating CO2 uptake. Judith Hauck and
Christoph Voelker simulated a scenario of
large CO2 emissions and climate change
in the twenty-first century with a coupled
ocean–ecosystem model (Geophys. Res. Lett.
http://doi.org/zzc; 2014). Their approach
allowed them to distinguish how CO2 uptake
NATURE GEOSCIENCE | VOL 8 | MARCH 2015 | www.nature.com/naturegeoscience
© 2015 Macmillan Publishers Limited. All rights reserved
is likely to change in the future in response
to changes in the ocean’s chemical capacity
to take up CO2 independent of the effects
of changes in temperature, circulation, or
resource availability.
It is no surprise that ocean uptake
of CO2 would increase alongside rising
anthropogenic emissions of CO2 over the
twenty-first century. But as the ocean’s
ability to efficiently take up CO2 diminished,
the strength of the seasonal cycle in CO2
uptake increased throughout the Southern
Ocean. Late in the twenty-first century, the
biological uptake of dissolved inorganic
carbon caused a much larger decline in
the amount of CO2 in the Southern Ocean
surface waters than it did early in the
century, seawater CO2 concentrations
became more sensitive to changes in
dissolved inorganic carbon concentrations:
the change in the Revelle ratio means that
a given decline in the levels of dissolved
inorganic carbon will result in larger uptake
of atmospheric CO2 by the oceans. In total,
CO2 uptake from biological activity increased
by roughly 2.5 times over the course of the
twenty-first century, even though changes in
biological activity were small.
With some effort, we can avoid the
large increases in atmospheric CO2
concentrations described in this highemissions scenario. But even if emissions
grow at a slower pace, CO2 may eventually
reach concentrations that shift the ocean
system into a new chemical state, where
marine organisms would play an increasingly
important role in controlling CO2 uptake.
JONATHAN E. HICKMAN
173