bogi hansen, hjálmar hátún, regin kristiansen, steffen m. olsen and svein Østerhus iceland...

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