On the effect of the Greenland Scotland Ridge on the dense water formation in the Nordic Seas

Dorotea Iovino

NoClim/ProClim meeting

4-6 September 2006

Fundamental aspect of the circulation in an idealized North Atlantic-Nordic Seas system

Dorotea Iovino and Tor Eldevik

How meridional overturning, sinking and convective activity are influenced by variations in

bottom topography (lateral boundary, “Greenland-Scotland Ridge”)

surface forcing (prescribed SST, wind)

Winton, 1997; Marotzke and Scott, 1999; Park and Brian, 2000; Spall and Pickart, 2001; Nilsson et al., 2003 .…

Sensitivity to boundary topography

csc

s s

new location of sinking

Sensitivity to buoyancy forcing (constant SST north of 60°N)

motionless region

topographic features guide the northward flow

cs

c s

s

Sensitivity to wind stress

cs

s

Ekman transport in the upper layer

western intensification and gyres consistent with the applied wind stress

MOC, location of sinking and convection qualitatively similar to the case without wind below the Ekman layer

Sensitivity to wind stress

Ekman transport in the upper layer

western intensification and gyres consistent with the applied wind stress

MOC, location of sinking and convection qualitatively similar to the case without wind below the Ekman layer

cs

s cs s

angled SST – no wind

Sensitivity to a “GSR” (uniform depth 860m)

c

s

max at 60°N

basic features south of the ridge qualitatively similar to the basin without ridge

~9 Sv south of the ridge, ~4 Sv north of it

sinking essentially located on the eastern boundary at the ridge latitude

no strong influence on the location of convection

s

Conclusions

sinking and convection are generally not collocated and their locations depends on basin geometry and surface forcing

boundary topography double-gyre circulation

continental shelf allows barotropic flow over topography circulation in the northern basin “remotely set” by the SST gradient in the south

effect of wind limited to the upper layer

cyclonic circulation maintained in the “Nordic Seas” even in the absence of wind forcing

the “GSR” limits the northward transports of mass and heat

“tuning” the model topography: different ridge geometries cause differences in the circulation “in and out”

A new paradigm for a convective basin:

Straneo, 2006

Spall, 2004

convection occurs in mostly quiescent interior region (no sinking) - Pickart et al, 2002

surrounded by a boundary current which is the principal

pathway for the import of light fluid and export of

dense fluid from the basin- Lavender et al., 2000

the exchange between the two regions is regulated by boundary current instabilities - eddy fluxes

(proportional to the isopycnal gradient between interior and

boundary current) – Prater, 2002

Does the sill affect dense water formation in the Nordic Seas?

D. Iovino, F. Straneo and M. Spall

Marginal sea WITH SILL and Nordic Seas parameters

How the sill modifies the characteristics of inflow and outflow waters and influence the water mass formation in a semi-enclosed basin subject to a net buoyancy loss

Several assumptions:

no Artic/Barents connection

closed Denmark Strait

flatbottom

simplified topography

spatially uniform forcing

600k

m

140km

D=2200m

T(z)

no salt - Q=200 W m-2

sill-depth from 500 to 1200m

160km

No sill vs. sill

Temperature

Meridional velocity

Temperature

Meridional velocity

colder interior

re-circulationcolder outflow

different b. current structure reduced inflowing heat flux

Effects of the sill: blocking and eddy efficiency

Theoretical argument

)('' 0TTcVTu inin

Temperature of water formed in the basin interior relative to the open-ocean temperature

2110

inin HTT

blocking effect

stability interior/eddies exchange

pin C

QRTuRH

0

2

''2

Deep sill (no net blocking) no effect

Shallow sill different b.c. structure

less heat transport in

different eddy efficiency

colder interior and colder outflow

re-circulation (closed geostrophic contours)

Very shallow sill new inflow/outflow dynamics

Qualitatively good agreement between theoretical arguments and numerical simulations (blocking effect and stability)

Dense water formation does NOT imply sinking: not collocated and not necessarily covarying

Sinking in the boundary current (as result of changes in the current’s baroclinic structure)

May the Nordic Seas (basins with sill) be considered as horizontal transport system?

Conclusions