Ocean circulation Arnaud Czaja 1 Ocean and Climate

Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics

Part I Ocean and Climate (heat transport and storage)

Net energy loss at top-of-the atmosphere = Poleward energy transport + Ha Imbalance between and = energy (heat) storage Ho

Poleward heat transport and storage are small… Energy exchanged at top-of-atmosphere : Planetary albedo Solar constant

Seasonal Heat storage Q 5

Heat transport: a long history of measurements… Northward heat transport Ha+Ho Ha Ho Equator Pole Bjerknes’ (1964) monograph. Data from Sverdrup (1957) & Houghton (1954)

Northward heat transport Ha+Ho Ha Ho 10 N 30 N 50 N 70 N Vonder Haar & Oort, JPO 1973. GERBE approved!

NB: 1 PW = 10^15 W Pacific Poleward heat transport at 24ºN 0. 76 +/- 0. 3 PW Atlantic 1. 2 +/- 0. 3 PW Atlantic+Pacific 2 +/- 0. 4 PW “Across the same latitude, Ha is 1. 7 PW. The ocean therefore can be considered to be more important than the atmosphere at this latitude in maintaining the Earth’s budget”. Hall & Bryden, 1982; Bryden et al. , 1991.

GERBE approved! (ask more to Chris D. !) Trenberth & Caron, 2001

GERBE approved! Ha+Ho Ho Ha Wunsch, JCl. 2005.

Ganachaud & Wunsch, 2003

Sometimes effects of heat storage and transport are hard to disentangle • Is the Gulf Stream responsible for “mild” European winters?

WARM! COLD! Eddy surface air temperature from NCAR reanalysis (January, CI=3 K) “Every West wind that blows crosses the Gulf Stream on its way to Europe, and carries with it a portion of this heat to temper there the Northern winds of winter. It is the influence of this stream upon climate that makes Erin the “Emerald Isle of the Sea”, and that clothes the shores of Albion in evergreen robes; while in the same latitude, on this side, the coasts of Labrador are fast bound in fetters of ice. ” Maury, 1855. Lieutenant Maury “The Pathfinder of the Seas”

Model set-up (Seager et al. , 2002) • Full Atmospheric model • Ocean only represented as a motionless “slab” of 50 m thickness, with a specified “qflux” to represent the transport of energy by ocean currents Atmosphere

Q 3 Seager et al. (2002)

Heat storage and Climate change The surface warming due to +4 Wm-2 (anthropogenic forcing) is not limited to the mixed layer… How thick is the layer is a key question to answer to predict accurately the timescale of the warming. NB: You are welcome to download and run the model : http: //sp. ph. ic. ac. uk/~arnaud Ho = 50 m Ho = 150 m Ho = 500 m

Ensemble mean model results Q 1 from the IPCC-AR 4 report

Strength of ocean overturning at 30 N (A 1 B Scenario + constant after yr 2100) Q 4

Part II Some key oceanic observations

World Ocean Atlas surface temperature ºC

Thermocline

World Ocean Atlas Salinity (0 -500 m) psu

The “great oceanic conveyor belt”

The ocean is conservative below the surface (≈100 m) layer • Temperature Not changed by absorption/emission of photons. • Salinity. No phase change in the range of observed concentration.

Salinity on 1027. 6 kg/m 3 surface Conservative nature of the ocean Spatial variations of temperature and salinity are similar on scales from several hundreds of kms to a few kms. 50 km Ferrari & Polzin (2005) 10 km 2 km

Matsumoto, JGR 2007

“Circulation” scheme

“Circulation” scheme Two “sources” of deep water: NADW: North Atlantic Deep Water AABW: Antarctic Bottom Water Williams & Follows (2009)

In – situ velocity measurements Amplitude of time variability Depth Location of “long” (~2 yr) currentmeters From Wunsch (1997, 1999) NB: Energy at period < 1 day was removed

Moorings in the North Atlantic interior (28 N, 70 W = MODE) 1 yr Schmitz (1989) (ask more to Ute and Chris. O. !) NB: Same velocity vectors but rotated

Direct ship observations NB: 1 m/s = 3. 6 kmh = 2. 2 mph = 1. 9 knot

Surface currents measured from Space “Geostrophic balance” Time mean sea surface height Standard deviation of sea surface height

Momentum balance Rotation rate f/2 East to west acceleration f. V East to west deceleration NB: f = 2 Ω sinθ up North East

Geostrophic balance! Rotation rate f/2 High Pressure f. V Low Pressure East to west acceleration East to west deceleration up North East

10 -yr average sea surface height deviation from geoid Subtropical gyres

10 -yr average sea surface height deviation from geoid Subpolar gyres Antarctic Circumpolar Current

ARGO floats (since yr 2000) T/S/P profiles every 10 days Coverage by lifetime Coverage by depths

All in-situ observations can be interpolated dynamically using numerical ocean models Overturning Streamfunction (Atlantic only) From Wunsch (2000)

RAPID – WATCH array at 26 N Q 2

14 m illi on s£ RAPID – WATCH array at 26 N

The movie…

Part III Key physics

Because T is conserved by fluid motion the temperature structure simply reflects transport by waves and mean currents Upward heat Sea surface transport = Downward heat transport Zo No internal heat source/sink Z X, Y Ocean bottom

This simply happens when warm water goes up or cold water goes down Upward heat Sea surface transport = Downward heat transport Zo No internal heat source/sink Z X, Y Ocean bottom

This happens when warm water goes down or cold water goes up… Upward heat Sea surface transport = Downward heat transport Zo No internal heat source/sink Z X, Y Ocean bottom

Requires mechanical forcing (winds/tides)! Upward heat Sea surface transport = Downward heat transport Zo No internal heat source/sink Z X, Y Ocean bottom

“Historical” view Sea surface Zo Z X, Y Ocean bottom

“Historical” view “Conveyor-belt” upwelling/downwelling Sea surface Zo Z X, Y Ocean bottom

Q 6 Broecker, 2005 NB: 1 Amazon River ≈ 0. 2 Million m 3/s

“Historical” view “Conveyor-belt” upwelling/downwelling = C W z x, y “Small scale” wave breaking Sea surface Zo Z X, Y Ocean bottom Q 7

Internal waves • Waves inducing displacement of density surfaces whose restoring mechanism is gravity. • Frequency of linear wave is between the Coriolis frequency f (T~10 h in midlatitudes) and the buoyancy frequency N (T=10 mn in upper ocean; 100 mn in deep ocean)

“Small scale” wave breaking strength (Naveira-Garabato, 2006)

Numerical model results Conveyor belt strength -2ºX 2º horizontal resolution (Sv) -Single basin 2/3 K slope -No wind -Surface heating-cooling -Small scale wave breaking parameterised by a constant diffusivity coefficient K (cm²/s) From Vallis (2000)

“Historical” view “Conveyor-belt” upwelling/downwelling = z Small scale wave breaking Sea surface Zo Z X, Y x, y Ocean bottom

“Historical” A very view bold statement! z x, y -Is the ocean circulation “Conveyor-belt” driven by tides? Small scale = wave breaking upwelling -Can hurricanes drive the Sea surface “conveyor belt”? Zo Z X, Y Ocean bottom

10, 000 km “Historical” view “Conveyor-belt” upwelling = ≈km x, y Small scale wave breaking Sea surface Zo Z X, Y z Ocean bottom

In-situ observations are dominated by a “meso-scale” (≈100 km) KE spectra (surface) Infrared based surface temperature

Alternative paradigm Zo Z X, Y Ocean bottom

Alternative paradigm “Meso-scale” waves upwelling/downwelling Zo Z X, Y Ocean bottom

Alternative paradigm “Meso-scale” waves upwelling/downwelling = Wind forced “pumping” Zo Z X, Y Ocean bottom

Momentum balance Rotation rate f/2 East to west acceleration f. V East to west deceleration up North East

Ekman balance! Rotation rate f/2 Windstress f. V East to west acceleration East to west deceleration up North East

Wind forced pumping Westerly winds (≈ 45º latitude) Trade winds (≈10º latitude) X Sea surface Ekman layer Upwelling Downwelling Upwelling

Alternative paradigm “Meso-scale” waves upwelling/downwelling = Wind forced “pumping” Zo Z X, Y Ocean bottom

Lab experiments -Rotating tank -Pump warm fluid down from a more slowly rotating disk Depth of warm lens Wind strength From Marshall (2003)

Results from realistic coupled models NB: >0 means upward Gnanadesikan et al. (2007) • Upper ocean: 0 -2500 m, w. T by the resolved flow is downward and balanced by upward heat flux due to eddy advection. • Abyssal ocean: below 2500 m, very weak but positive upward heat transport by the resolved flow, opposed by downward diffusive heat transport.

Friday’s session