What controls the location of the
sea ice edge in the N. Atlantic?
Why do we care?
David S. Battisti
University of Washington
1.
Motivation
2.
Impact of atmosphere on sea ice and ocean
3.
Impact of ocean heat transport (OHT) on sea ice
4.
Feedbacks between the ocean and the atmosphere, mediated
by the sea ice
• Atmosphere -> Sea Ice -> Atmosphere
• Atmosphere -> Ocean -> Sea Ice -> Atmosphere
5.
Will Greenhouse Warming initiate a thermohaline
catastrophe, and subsequently a large rapid cooling of
Europe and beyond?
1. Motivation: Who cares about the
location of the sea ice edge?
• Climate models forced by
increasing Greenhouse Gas
Concentrations project a
slowdown in the ocean
thermohaline circulation
(THC) -- the main deliverer
of heat by the ocean to the N.
Atlantic*.
• Even if the OTC shuts down,
these models project the
large climate changes will
likely be confined to over the
N. Atlantic Ocean.
Temperature Change 20-30 years
after a shutdown of the Ocean
Thermohaline Circulation
(Villenga et al. 2003)
1. Motivation: Who cares about the
location of the sea ice edge?
• If reductions in the ocean
heat transport (OHT) by the
OTC cause the sea ice edge
to migrate significantly
southward, a large rapid
cooling of the N.
Hemisphere would occur.
(see the movie).
Temperature Change:
“Heat Transport” minus “No Heat
Transport”
(Seager et al. 2002)
Winter Surface
Heat Flux Q
Ocean Heat Flux
Convergence D
Release of Heat in
Winter due to
summer Storage
Q-D
In the Greenland-IcelandNorwegian (GIN) Seas,
the heat supplied by the
ocean circulation is
comparable to the energy
absorbed from sunlight.
Also, only a small
change in the ocean heat
flux convergence is
required to melt a
substantial fraction of
the sea ice in the Arctic.
2. What influences location of the sea ice
edge (part 1)? Advection by the wind
The year-to-year variability in the sea ice
edge is mainly driven by atmospheric
variability:
• The leading pattern of atmospheric
variability in N. Hemisphere winter is
the Arctic Oscillation, the AO (aka the
North Atlantic Oscillation, the NAO)
2. What influences location of the sea ice
edge? Advection by the wind
• The winter averaged NAO index is
highly correlated with the leading
pattern of SST and sea ice variability:
Sea Ice anomalies are due to
wind stress changes (advection)
Less
Ice
More
Ice
r= 0.4
r=-0.55
SST anomalies due to
wind induced changes in
turbulent heat fluxes and
mixing/convection
(Visbeck et al. 2003 & refs. therein)
( e.g., Chapman & Walsh 1983; Bitz 1996; Deser et al. 2000)
2. Impact of the atmosphere on the ocean
heat transport (interannual to decadal time scales)
• Changes in ocean thermohaline
circulation (THC) on decadal and
shorter time scales are
predominately due to forcing by
the atmosphere:
– Changes in temperature that are
forced by atmospheric induced
flux anomalies and then
advected/convected/mixed by
mean ocean currents;
– Changes in ocean currents due to
changes in wind stress and
buoyancy flux gradients.
(e.g., Mauritzen and Hakkinen 1999;
Holland et al. 2001; Visbeck et al.
2003 & refs therein).
2. Impact of atmosphere on ocean heat
transport (interannual time scales)
• Changes in ocean currents
due to the NAO:
– Ekman changes enhance the
temperature anomalies due to the
surface heat flux.
–Gyre changes due to wind stress curl changes
also enhances the SST anomalies driven by the
NAO related turbulent heat fluxes:
•E.g., Positive NAO creates turbulent cooling
in the subpolar gyre, anomalous Ekman
currents that cool the Southern subpolar gyre;
and enhanced gyre circulation that further
cools the Labrador Sea.
3. What influences location of the sea ice edge
(part 2)? Variability in ocean heat transport
• On Centennial and longer time
scales, changes in the ocean
currents may be due to internal
ocean variability/instability.
– Example: Dansgaard/Oeschger
events?
•
D/O events & Greenland ‘temperature’:
–Rapid onset ( 10 K in < 30 years!)
–Long-lived (~ 200 - 600 years)
Abrupt warming in Greenland is
consistent with a melt-back of sea
ice in wintertime:
– Temperature (N fractionation)
– delta 18O
– Accumulation
Li et al (2004). See poster 28
(Li et al do not comment on the cause of sea ice retreat)
4. Feedbacks: Atmospheric response to
changes in the location of the sea ice edge
• On interannual time
scales, the atmosphere
forces changes in the sea
ice edge (e.g., through
NAO forcing)
• The response of the
atmosphere to those
changes in sea ice is a
moderate negative
feedback (Magnusdottir et
al. 2004)
•
40 year (1954-94) trends in
Z500 and (2x) Sea Ice
Concentrations
•
The same relationship holds
on interannual time scales.
4. Feedbacks: Atmospheric response to
changes in the location of the sea ice edge
Forcing
Sea Ice
Response
Atmospheric
Feedback
• The response of the atmosphere to the changes in sea ice
is a moderate (50%) negative feedback (Magnusdottir et
al. 2004)
• Most of the response is due to the GIN sea ice changes
4. Feedbacks: (NAO induced) Ocean
Circulation Changes impacting the sea ice edge
• On the interannual time scales, the dynamical response of
the ocean to NAO forcing is to enhance the SST anomalies
driven by the NAO induced surface heat anomalies.
• On longer time scales (e.g, decadal), the subpolar gyre
circulation will be affected by changes in the steric height
in the Labrador Sea.
Lab
Sea
P.E.
Lab Sea
1500db
Temp
1950
2000
(Curry and McCartney 2001)
4. Feedbacks: (NAO induced) Ocean
Circulation Changes impacting the sea ice edge
• In turn, NAO induced changes in the steric height in the Labrador Sea
will lead to changes in the strength of the subpolar gyre:
Lab
Sea
P.E.
Lab
Sea
Temp
Subpolar
Gyre
Transport
NAO
Index
1950
2000
(Curry and McCartney 2001)
4. Feedbacks: (NAO induced) Ocean
Circulation Changes impacting the sea ice edge
•
0.5 yr (surf.
Heat fluxes)
Cooling in Lab
Sea/Increased
Convection
5-10 yrs baroclinic adj.
The baroclinic adjustment time scale of the subpolar
gyre is O(decade). Thus, there there should be a lagged
response in the ocean circulation change to NAOinduced changes in ocean heat content (Visbeck et al.
2003):
Sustained NAO+ forcing
0.5 yr (wind)
1-2 yr (Sverd)
?
Increased
Subpolar Gyre
Decrease sea ice in
GIN Seas (wind)
0.5 yrs increased ocean
heat and salt transport
5. Will Greenhouse Warming initiate a
thermohaline catastrophe, and thus cause a
large rapid cooling of Europe and beyond?
• Increasing CO2 will increase the
Greenhouse Effect and cause warming
0 PW
of the planet.
• If Global warming is responsible for the
positive trend in the NAO due to
atmospheric processes (e.g., Shindell et
al.; Hoerling et al., etc) …
– the feedback loops acting on sea ice in the
40N
N. Atlantic appear to be positive because
the subpolar gyre appears to be responsive
to thermal anomalies in the Labrador Sea.
Hence, increased heat and salt import into
the GINs seas should be expected, along
with the continual retreat of sea ice; hence,
no abrupt European cooling.
90N
Change in Ocean Heat
Transport at the time of
doubling of CO2. (from
CIMP runs; Holland and Bitz
2004)
5. Will Greenhouse Warming initiate a
thermohaline catastrophe, and thus cause a
large rapid cooling of Europe and beyond?
• A fly in the ointment: the GIN sea and the
subpolar gyre are have undergone a freshening
over the past 30 years.
– The equivalent of an extra 0.2m of fresh water per year
in the GIN Seas for 30 years.
• Where is this freshwater coming from, and could
the freshening rate overtake the positive ocean
feedbacks to shutdown the thermohaline
circulation, ice over the North Atlantic and freeze
Europe?
5. Will Greenhouse Warming initiate a
thermohaline catastrophe?
• Where did this freshwater come from?
Plausible explanations:
Increased runoff; increased precip - evap (displaced storm
track), increased advection of sea ice (melt), and increased
freshwater import from the Arctic Ocean via currents.
(Vinje 2001, Peterson et al. 2002, Curry et al. 2003 & refs therein)
5. Will Greenhouse Warming initiate a
thermohaline catastrophe?
• A rapid shutdown of the
thermohaline circulation
requires enormous volumes of
freshwater in a short time.
• For example, Vellinga et al
(2003) place 16 Sv of
freshwater on the N. Atlantic in
one year -- over 200 times the
observed freshening rate and
more than 100 times the annual
average precipitation over the
N. Atlantic.
• Even with such unrealistic*
dollops of freshwater, the sea
ice returns to its normal state
by 20 years and temperature
changes over the land are
modest.
Surface Temperature Change
Ice Line;
T=0, T > 20 yrs
0 < T < 20 yrs
(Villenga et al. 2003)
Summary
Will Greenhouse Warming initiate a thermohaline
catastrophe, and a subsequent abrupt large cooling in parts of
the NH?
• Greenhouse Warming is thought to be responsible for
the upward trend in the NAO/AO which has, in turn,
been shown to be responsible for the reduction of sea
ice in the GIN Seas (IPCC 2001 and refs therein).
• The resultant changes in ocean gyre circulation appear
to be a positive feedback on the ice -- more heat
imported into the GIN seas. Hence, the feedbacks
associated with subpolar gyre changes would enhance
warming in the North Atlantic. Answer: Not likely.
• The fly in the ointment: the (GH-NAO induced?) trend
in the net “P-E plus runoff plus sea ice melt” exceeds
the increase in salt imported by the (GH/NAOinduced) spin-up of the subpolar gyre, for a net
increase in buoyancy in the GIN seas.
Summary (cont):
Would 200 more years of freshening at the observed rate cause a
slowdown the THC and its heat transport into the N. Atlantic?
– If the increase in buoyancy due to the net increase in “P-E
plus runoff plus sea ice melt” exceeds the decrease in
buoyancy due to the GH/NAO spin-up of the subpolar gyre,
a gradual regional cooling (as in G&R). Answer: Not likely.
– The cooling would be superimposed on the large-scale
warming due to (by then) large increases in GH gases.
Could (a) GH warming cause half of Greenland to slide
catastrophically into the north Atlantic, thereby (b) initiating a
THC shutdown and an abrupt large-scale cooling?
– If (a) is true, then (b) should be true -- at least for a few
decades. (but c.f. Heinrich events and other large freshwater
surges that don’t appear to cause abrupt global cooling).