EGU meeting 8 April 2013 Vienna

FBC Overflow

Bogi Hansen HAV

image:AGU 2003

The densest overflow from the Nordic Seas has become warmer and more saline since 1995

Bogi Hansen, Karin M. H. Larsen, Hjálmar Hátún, Regin Kristiansen, Ebba Mortensen,

Svein Østerhus, Detlef Quadfasel

Greenland

Iceland

Shet-land

Faroes

FBC-overflow: 2 Sv

entrained water: 2 Sv+

Faroe Bank ChannelSill depth: 840 m

With entrainment, the FBC-overflow feeds ~ 25% of NADW

FBC - overflow

ADCPin

flotation

Monitored since November 1995

Dinner

Volume transport(monthly averaged “kinematic” overflow)

Average: 2.15 Sv

Change: 0.18 ± 0.15 Sv = 8% ± 7%

Potential temperature at OWSM

OWSM

Warming

Warming

FBC sill: 840 m

Bottom temperature at sill

1996 1998 2000 2002 2004 2006 2008 2010 2012

-0.6

-0.5

-0.4

-0.3

0

0.5

1.0

1.5

2.0

2.5

Bot

tom

tem

pera

ture

(°C

)

Vol

ume

tran

spor

t (S

v)

Bottom temperature

Volume transport

Bottom temperature decreases with increasing volume transport

Correlation coefficient: -0.49

Bernoulli aspiration

Weak flow

Strong flow

OWSM

800 m

1000 m

Section N

800 m

900 m

Bottom temperature

1996 1998 2000 2002 2004 2006 2008 2010 2012

-0.6

-0.5

-0.4

-0.3FBC sill 840 m

OWSM

Section N

FBC bottom temperatureadjusted for transport variations

FBC bottom temperature has increased since ~ 2000

1996 1998 2000 2002 2004 2006 2008 2010 2012

-0.6

-0.5

-0.4

-0.3

0

0.5

1.0

1.5

2.0

2.5

Bot

tom

tem

pera

ture

(°C

)

Vol

ume

tran

spor

t (S

v)

Uncertainty

~ 0.1ºC

Δσθ ≈ -0.004 kg/m3

Salinity at fixed temperatureat section S

Salinity at -0.45ºC

Salinity at 1ºC

ΔS ≈ 0.005

Δσθ ≈ +0.004 kg/m3

Section SΔσθ ≈ +0.025 kg/m3

Faroe Bank Channel overflow

• Volume transport has not decreased

• probably, it has increased

• The overflow has become warmer, but also more saline

• The density has not decreased

• probably, it has increased

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreements n.308299 NACLIM www.naclim.eu

and n.212643 THOR www.eu-thor.eu