The Role of Surface Freshwater Flux Boundary Conditions in

Arctic Ocean/Sea-Ice Models

The Role of Surface Freshwater Flux Boundary Conditions in

Arctic Ocean/Sea-Ice Models

EGU General Assembly, Nice, April 2004

Matthias PrangeMatthias Prangeandand

Rüdiger GerdesRüdiger Gerdes

Research Center Ocean Margins, University of BremenResearch Center Ocean Margins, University of Bremen

andand

Alfred Wegener Institute, Bremerhaven,Alfred Wegener Institute, Bremerhaven,

GermanyGermany

Wind S

tress

Wind S

tress

Density G

radients

Density G

radients

≈ ≈ Salinity

Gra

dients

Salinity

Gra

dients

What drives the circulation in the Arctic Ocean? What drives the circulation in the Arctic Ocean?

In spite of its importance, little attention as been paid to freshwater In spite of its importance, little attention as been paid to freshwater forcing in Arctic ocean/sea-ice models.forcing in Arctic ocean/sea-ice models.

Only few models omit salinity restoring to climatological data.Only few models omit salinity restoring to climatological data.

In these models, surface freshwater fluxes are conventionally In these models, surface freshwater fluxes are conventionally treated as treated as virtual salt fluxesvirtual salt fluxes;;

i.e., (P-E+R) is neglected in the kinematic S.B.C.i.e., (P-E+R) is neglected in the kinematic S.B.C.

Present work:Present work:

We use the regional ocean/sea-ice model COSMOS withWe use the regional ocean/sea-ice model COSMOS withopen surfaceopen surface

(i.e., surface freshwater forcing is naturally implemented)(i.e., surface freshwater forcing is naturally implemented)

We compare a COSMOS control run with two modified model We compare a COSMOS control run with two modified model versions, in which a material surface is used along withversions, in which a material surface is used along with

virtual salt fluxesvirtual salt fluxes

Ocean

ww00ww00 = = tt + + (E-P-R)(E-P-R)ww00 = = tt + + (E-P-R)(E-P-R)

tt (E-P-R)(E-P-R)

z = 0z = 0

z = z =

(Non-material) open surface:(Non-material) open surface:

Ocean model MOM 2• Open surface• FCT advective scheme• 19 levels

Ocean model MOM 2• Open surface• FCT advective scheme• 19 levels

CouplingHeat, salt and momentum fluxesfollowing Hibler & Bryan (1987)

CouplingHeat, salt and momentum fluxesfollowing Hibler & Bryan (1987)

Dynam. ice model (Hibler/Harder)• Viscous-plastic rheology• Modified upstream scheme• Snow layer

Dynam. ice model (Hibler/Harder)• Viscous-plastic rheology• Modified upstream scheme• Snow layer

Model grid• Arakawa B• Rotated grid• 1° (c. 100 km)

Model grid• Arakawa B• Rotated grid• 1° (c. 100 km)

Atmosph. forcingTypical year (based on ECMWF 1979-1993) with daily winds (Roeske 2001)

Atmosph. forcingTypical year (based on ECMWF 1979-1993) with daily winds (Roeske 2001)

Model Setup:Model Setup:

Bering Strait inflowMonthly varying,mean values: 0.8 Sv, 32.5 psu

Bering Strait inflowMonthly varying,mean values: 0.8 Sv, 32.5 psu

+ 700 km+ 700 km33/yr ungauged (diffuse) runoff/yr ungauged (diffuse) runoff

Implemented Arctic river runoff:Implemented Arctic river runoff:

Model domain:Model domain:

Model experiments:Model experiments:

Experiment S1:Neglecting surface freshwater volume fluxes

corresponds to using the virtual salt flux

FS = (z1)-1 (-P+E-R) S1

Experiment S1:Neglecting surface freshwater volume fluxes

corresponds to using the virtual salt flux

FS = (z1)-1 (-P+E-R) S1

Experiment SREF:Use a constant reference salinity Sref = 35 psu in the

salt flux boundary condition, i.e.

FS = (z1)-1 (-P+E-R) Sref

Experiment SREF:Use a constant reference salinity Sref = 35 psu in the

salt flux boundary condition, i.e.

FS = (z1)-1 (-P+E-R) Sref

For each experiment the model is integrated 30 years,For each experiment the model is integrated 30 years,starting from the same spin-up run.starting from the same spin-up run.

Experiment CTRL:Control run with open surface

Experiment CTRL:Control run with open surface

Mea

n sa

linity

(0-8

0 m

)

Mea

n sa

linity

(0-8

0 m

)

Experiment CTRL:Experiment CTRL:

Mea

n ve

loci

ty (0

-80

m)

Mea

n ve

loci

ty (0

-80

m)

[cm

/s]

[cm

/s]

Experiment CTRL:Experiment CTRL:

Mea

n sa

linity

diff

eren

ce

Mea

n sa

linity

diff

eren

ce

(0-8

0 m

)

(0-8

0 m

)

Experiment S1 minus CTRL:Experiment S1 minus CTRL:

Mea

n ve

loci

ty d

iffer

ence

Mea

n ve

loci

ty d

iffer

ence

(0-8

0 m

)

(0-8

0 m

)[c

m/s

]

[cm

/s]

Experiment CTRL minus S1:Experiment CTRL minus S1:

Experiment CTRL minus S1:Experiment CTRL minus S1:

Mea

n se

a ic

e th

ickn

ess

Mea

n se

a ic

e th

ickn

ess

diffe

renc

e

diffe

renc

e[m

][m

]

Upstream:Upstream:

ddiidt = Q (dt = Q (i-1i-1 – – ii))

Solution forSolution for

i i (t=0) = 0(t=0) = 0

00 = Q = 1 = Q = 1

Experiment CTRL versus S1:Experiment CTRL versus S1:

dddt = Q (dt = Q (00 – – 11))

Solution forSolution for

i i (t=0) = 0(t=0) = 0

00 = Q = 1 = Q = 1

Experiment CTRL versus S1:Experiment CTRL versus S1:

Mea

n sa

linity

diff

eren

ce

Mea

n sa

linity

diff

eren

ce

(0-8

0 m

)

(0-8

0 m

)

Experiment SREF minus CTRL:Experiment SREF minus CTRL:

Mea

n ve

loci

ty d

iffer

ence

Mea

n ve

loci

ty d

iffer

ence

(0-8

0 m

)

(0-8

0 m

)[c

m/s

]

[cm

/s]

Experiment CTRL minus SREF:Experiment CTRL minus SREF:

Arctic Ocean mean salinity:Arctic Ocean mean salinity:

S1S1

SREFSREF

CTRLCTRL

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

Experiment CTRL

Aagaard &Carmack (1989)

P-EP-E

Riv

er

wate

rR

iver

wate

r

Beri

ng S

tr.

Beri

ng S

tr.

Ice

Ice

Wate

rW

ate

r

Ice

Ice

Wate

rW

ate

r

Ice

Ice

Wate

rW

ate

r

Bare

nts

S.

Bare

nts

S.

Fram

Str

.Fr

am

Str

.

C. A

rchip

el.

C. A

rchip

el.

kmkm33/yr/yr

Reference: 35 psu

Out

put

Inpu

t

Freshwater balance of the Arctic Ocean:Freshwater balance of the Arctic Ocean:

Conclusions:Conclusions:

• Neglecting the volume input of surface freshwater fluxes

leads to significant salinity increases in the upper Arctic

Ocean.

• Introducing a constant reference salinity Sref = 35 psu in

the salt flux b.c. results in hydrographic fields which are

much more similar to those from the control run with open

surface.

• The Canadian Archipelago is an important sink in the

Arctic Ocean freshwater balance:

20% of the Arctic Ocean freshwater input is exported by

ocean currents through the archipelago.

• Neglecting the volume input of surface freshwater fluxes

leads to significant salinity increases in the upper Arctic

Ocean.

• Introducing a constant reference salinity Sref = 35 psu in

the salt flux b.c. results in hydrographic fields which are

much more similar to those from the control run with open

surface.

• The Canadian Archipelago is an important sink in the

Arctic Ocean freshwater balance:

20% of the Arctic Ocean freshwater input is exported by

ocean currents through the archipelago.