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The Feedback Mechanism Greenland Scotland Ridge World Ocean Nordic Seas + Arctic Ocean Cooling Salt Overflow Entrainment NADW
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Bogi Hansen, Hjálmar Hátún, Regin Kristiansen, Steffen M. Olsen and Svein Østerhus
Iceland
Scot- land
NordicSeas
Faroes
0.8 Sv 3.8 Sv3.8 Sv
Greenland
Østerhus et al., GRL 2005
Thermohaline forcingmaintains the Iceland-Faroe inflow of
heat and salt to the Arctic
IF-inflow40 - 45%
Didn’t we know that already ?
The Feedback Mechanism
GreenlandScotland
Ridge
World Ocean Nordic Seas + Arctic Ocean
CoolingSalt
Overflow
Entrainment
NADW
The Feedback Mechanism implies:
GreenlandScotland
Ridge
World Ocean Nordic Seas + Arctic Ocean
Atlantic inflow
Removal of water from the Nordic Seas maintains the Atlantic inflow through sea-level forcing
Estuarine
Thermoh.But !!
Maybe
• Sea-level forcing maintains the average Atlantic inflow
• Wind stress dominates the variations ?
The IF-inflow4 CTD cruises a year since 1988
ADCPs since 1997
The Standard sectionthrough the IF-inflow
SalinityVelocity (cm/s)
ADCPs
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 20080
1
2
3
4
5
6
Vol
ume
flux
of A
W (S
v)Monthly and annually averaged Atlantic water
transport of the IF-inflow 1997 - 2008
Average: 3.5 Sv
No trend
The flow changes character across the Ridge
It is accelerated and becomes focusedThis requires a local (and very stable) force
Not local wind stress
PVDs at 225 m depth
Iceland
Scot- land
Faroes
Transport
Sea-level drop: SLHW - SLHE
Monthly correlation: R = 0.63
Annual correlation: R = 0.87
Atlantic inflow and sea-level drop across the Ridge
SLHW
SLHE
Bernoulli Geostrophyimplies:
~ 0.2 Sv per cm
+
Iceland-Faroe Atlantic inflow
AtlanticOcean
NordicSeas
Atlantic inflow
IcelandFaroeRidge
IFR-overflow
FBC-overflow
To Arctic Ocean
To Denmark Str.
Remote wind forcing could raise sea-level west of the Ridge
R > 0R < 0
West East
Outflow from Nordic Seas tends to reduce sea-level east of the Ridge
8.5 Sv out implies lowering of Nordic Seas SLH by 30 cm/day
Correlation coefficient between monthlysea-level height and IF-inflow
+
-0.4
Conclusions• Sea-level forcing drives the flow across
the Iceland-Faroe Ridge
• Processes west (upstream) of the Ridge do not dominate the forcing
• Water removal east (downstream) of the Ridge generates the average IF-inflow – and its variations
Thermohaline forcing dominates the outflow
70 %
IF-inflow and FBC-overflow
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 20080
1
2
3
4
5
6
Vol
ume
flux
of A
W (S
v)
FBC-overflow (Sv)
IF-flow (Sv)
Negative correlation
Coupling between IF-inflow and FBC-overflow
U
D
)18()()( SUHhhdtdA
)19(SDHhdtdHA
R<0
R>0
Correlation coefficient between monthlysea-level height and IF-inflow
-0.4
The equation of motion
Sea-levelforcing
Externalforcing(wind)
T and S in the core of the IF-inflow