observations of deep meridional overturning...

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…