The Role of Surface Freshwater Flux Boundary Conditions in Arctic Ocean/Sea-Ice Models EGU General...
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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.