observations of deep meridional overturning...
TRANSCRIPT
CLIM 752 Ocean General Circulation
Observations of Deep Meridional Overturning Circulation
Barry A. Klinger
George Mason University
Pacific Atlantic
Why is most of the ocean so cold and dense?
Tomczak and Godfrey, Regional Oceanography, ch 15, after Bainbridge (1980)
Pot. Temp.
Salinity
Oxygen
Tracers hint at deep flows in Atlantic: Relatively salty North Atlantic Deep Water (NADW) moves southward at 2 – 4 km depth Relatively fresh Antarctic Bottom Water (AABW) moves northward near bottom Relatively fresh Antarctic Intermediate Water (AAIW) moves northward at about 1 km depth. Called “Deep Meridional Overturning Circulation” (DMOC)
Pacific Atlantic
Pacific tracers don’t reveal deep flows.
Major changes in water T-S properties occur at surface Therefore deep water properties are “set” (strongly influenced) by surface sources How does the water get down to great depths?
Deep convection winter mixed layer can reach very deep 1000 m in Labrador Sea and nearby (as great as 2000 m) to bottom in center of Nordic Seas Northwest Mediterranean (off coast of France) Weddell Sea in Antarctica
Klinger et al. (1995, Dyn Atmos Oceans) Visbeck et al. (1996, JPO)
Numerical models: (left) 25 m res. into homogeneous fluid, (right) 250 res into stratified fluid
Mode Water: relatively deep winter ML depths (100’s of m)
Hanawa andTalley, in Ocean Circulation and Climate
Coastal overflows shallow shelf region doesn’t allow deep mixed layer winter cooling/brine rejection creates very dense surface water ageostrophic frictional flow “leaks out” down shelf Barents Sea, Antarctic Shelves (such as Ross Sea), Adriatic
dense
dense
• Sill overflows Two basins separated by a ridge (“sill”) Water on one side of the sill is denser Water leaks into lighter basin as ageostrophic frictional flow Nordic Sea – Atlantic via Denmark Strait and Faero Bank Mediterranean – Atlantic via Strait of Gibraltar Mediterranean – Black Sea via Dardanelles Red Sea – Indian Ocean Med water is very dense due to S, but water sinks to 1000 m only surface water at that latitude is light enough to dilute it before it can go to bottom
dense
dense
Sources of North Atlantic Deep Water
Barents Sea
Nordic Seas
Labrador Sea
D
Overflows at sills: 5.6 Sv Overflow water + entrainment: 8.6 Sv Dickson and Brown (1994) Total NADW formation 13 – 20 Sv
2 2
1
.25
North Atl Overflows
Denmark Strait Overflow
Denmark Strait Overflow
Mediterranean Outflow: Follows coast, descends, entrains, grows
Distance from Gibraltar (km)
Baringer and Price (1997, JPO) 1.4 Sv 1.0 Sv .9 Sv
Weddel Sea
Ross Sea
Souces of Antarctic Bottom Water and Circumpolar Deep Water
Water temperature and density changes associated with DMOC Gets denser: surface heat loss/ice formation Signal gets deeper: convection and overflows Gets lighter: surface heat gain at low latitudes (surface heat gain at high latitudes also?) Signal gets deeper: mixing probably caused by breaking internal gravity waves mostly generated by tidal “sloshing” across topography? mesoscale eddies? Convection works WITH gravity: very large mixing (O(1 m2/s)) Other mixing works AGAINST gravity: very small mixing (O(10-4 m2/s)) Relatively small, intense regions where cooling mixed downward Relatively large regions where heating weakly mixed downward
cooling
heating
thermocline
y
z
Important note: There’s another way for flow to return -- will discuss later
Deep water sources in northern N Atl and Antarctic (not in northern N Pac) Where does the water come up? Not as easy to observe Can’t directly observe w for either sinking or rising, but… Water sinks in relatively small locations: •Relatively intense deep flows leaving these sources •Distinct water properties (S, trace chemicals) based on surface conditions Water rises in relatively broad regions (most of the ocean)? Much weaker flows than sinking region Water properties not so distinct
Schmitz (1996), cited in Ocean Circulation and Climate
Schematic of Global Deep Meridional Overturning Circulations
Three-layer Summary of Deep Meridional Overturning Circulation
Schmitz (1996, Rev. Geophys.)
Ganachaud and Wunsch (2000, Nature)
Estimate of Deep Meridional Overturning Circulation
(27.72 deeper than 1 km over most of ocean)
“Direct” Measurement of Ocean Heat Transport (based on hydrography, Ekman transport, tracer distributions)
Ganachaud and Wunsch (2000, Nature)
Observational Estimate of Meridional Overturning
Atlantic Pacific
Indian
Talley, Reid, Robbins (2003, JPO) c.i. = 2 Sv
“The Southern Ocean Limb of the Global Deep Overturning Circulation”
Sloyan and Rintoul (2001, JPO)
Hirst et al. (1996, JPO)
Meridional overturning, Numerical model of near-global ocean GFDL model 21 levels, 1.6 X 2.8 deg (lat X lon) Diffusivity .3 in thermocline, about 1 in abyss (X 10-4 m2/s)
NADW
AABW
CDW
Same model, ρ-coord overturning…