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 c sc s s new location of sinking

Sensitivity to buoyancy forcing (constant SST north of 60°N) motionless region topographic features guide the northward flow c s c s s

Sensitivity to wind stress c s 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 c s s c ss 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 flat bottom  simplified topography  spatially uniform forcing 600km 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-circulation colder outflow different b. current structure  reduced inflowing heat flux

Effects of the sill: blocking and eddy efficiency

Theoretical argument Temperature of water formed in the basin interior relative to the open-ocean temperature blocking effect stability  interior/eddies exchange  

 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