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In-Depth Information
model-data correction to the Arctic observations,
projects that by the end of this century, surface
waters with annual average Ω
c
< 1 will be found
over much of the Arctic (Feely
et al.
2009 ). These
waters would be chemically corrosive to calcite and
all other forms of CaCO
3
. The main mechanism
explaining why undersaturation will occur gener-
ally sooner in the Arctic Ocean than the Southern
Ocean is the enhanced freshwater input in the Arctic
from climate change. In short, enhanced ice melt
and increased precipitation dramatically reduce
Arctic surface [CO
3
2-
] ( Steinacher
et al.
2009 ). This
i nding is consistent with freshwater dilution sup-
pressing [CO
3
2-
] as suggested by Salisbury
et al.
(2008), who illustrates large reductions in Ω with
declining salinity near river mouths and the gener-
ally more acidic pH of river discharge plumes.
Indeed, some low-salinity, near-coastal surface
waters in the Arctic are already undersaturated
with Ω
a
< 1 (Yamamoto-Kawai
et al.
2009 ). Let us
return to these freshwater dilution effects later,
when discussing acidii cation in the context of cli-
mate change (Section 3.6.6) and the coastal ocean
(Section 3.6.7) after considering natural variability
and subsurface changes.
spring-summer bloom. Secondary factors that also
contribute to seasonal variability include winter-
time cooling and enhanced wintertime mixing with
CO
2
-rich deep waters, both of which lower surface
[CO
3
2-
] while raising surface
p
CO
2
. Thus as levels of
atmospheric CO
2
continue to increase and [CO
3
2-
]
generally decreases, high-latitude undersaturation
(Ω
a
< 1) will be reached i rst during winter, and as
years progress, surface waters will remain under-
saturated during an increasing number of months
per year. Summer conditions will be the most resist-
ant to the advancing undersaturation.
For the Southern Ocean, McNeil and Matear
(2008) combined observational estimates of the
annual cycle with the future trend from a model
and found that those surface waters start to become
undersaturated with respect to aragonite in winter
when atmospheric CO
2
reaches about 450 ppmv.
That undersaturation happens about 100 ppmv
sooner than for annual average conditions, which
translates to a 30-yr advance for winter undersatu-
ration under the IS92a scenario.
In the Arctic Ocean, seasonal data for the carbon-
ate system are extremely sparse. However, data
were collected during spring and summer cruises in
2002 and 2004 over the Chukchi Sea shelf and slope
as well as into the Canada Basin during the Shelf-
Basin Interactions project. The seasonal amplitude of
surface [CO
3
2-
] in that data reaches up to ±12 μmol kg
-1
( Bates
et al.
2009): as found elsewhere, the maxi-
mum in surface [CO
3
2-
] is attained in summer. In
contrast, subsurface waters overlying the shelf
exhibit a similar magnitude of change but reversed
phasing. That is, subsurface [CO
3
2-
] reaches its mini-
mum in summer due to intense remineralization of
organic matter produced from high primary pro-
ductivity in overlying waters. Elsewhere in the high
Arctic, the annual cycle has not been assessed owing
to a lack of seasonal observations.
3.6.4
Seasonal and interannual variability
So far, the focus has been on annual-mean surface
trends, but surface [CO
3
2-
] also varies seasonally and
interannually, along with natural variations in
p
CO
2
and related biogeochemical variables, such as nutri-
ents and
C
T
. The OCMIP study suggests that inter-
annual variability in surface-ocean [CO
3
2-
] is small
everywhere when compared with the magnitude of
the anthropogenic transient (trend in annual means).
It also suggests that seasonal variability is small at
low latitudes, but that at high latitudes the average
amplitude of the annual cycle can reach up to ±15
μmol kg
-1
. Similar or higher seasonal amplitudes
have been observed in the subarctic Pacii c (Feely
et
al.
1988), the Bering Sea (Merico
et al.
2006 ), and the
Norwegian Sea (Findlay
et al.
2008 ). Comparable
amplitudes are also found in the Southern Ocean,
based on seasonal variations in [CO
3
2-
] derived from
carbonate system data (McNeil and Matear 2008).
In all these cases, [CO
3
2-
] is highest during summer,
when
p
CO
2
is driven downward mainly by the
3.6.5 Future changes in interior ocean
chemistry
Penetration of anthropogenic CO
2
into the deep
ocean also reduces subsurface [CO
3
2-
] and pH. Feely
et al.
(2004) use observations to demonstrate that the
industrial-era invasion of anthropogenic CO
2
has
already caused the ASH to shoal. Figure 3.5
