Ocean circulation Arnaud Czaja 1. Ocean and Climate 2
Ocean circulationArnaud Czaja1. Ocean and Climate 2. Key observations 3. Key physics Part I Ocean and Climate (heat transport and storage) = energy (heat) storage
Net energy loss at top-of-the atmosphere = + Poleward energy transport Ha Ho Imbalance between and = energy (heat) storage Poleward heat transport and storage are small
Energy exchanged at top-of-atmosphere : Planetary albedo Solar constant Seasonal Heat storage Q5 Heat transport: a long history of measurements
Ha+Ho Ha Northward heat transport Ho Equator Pole Bjerknes (1964) monograph. Data from Sverdrup (1957) & Houghton (1954) Ha+Ho Ha Ho Vonder Haar & Oort, JPO 1973. GERBE approved!
Northward heat transport Ho 70N 10N 30N 50N Vonder Haar & Oort, JPO 1973. GERBE approved! Poleward heat transport at 24N Pacific 0.76 +/- 0.3 PW Atlantic
NB: 1PW = 10^15 W Poleward heat transport at 24N Pacific 0.76 +/- 0.3 PW Atlantic 1.2 +/- 0.3 PW Atlantic+Pacific 2 +/- 0.4 PW Across the same latitude, Ha is 1.7PW. The ocean therefore can be considered to be more important than the atmosphere at this latitude in maintaining the Earths 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 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=3K) 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 50m thickness, with a specified q-flux to represent the transport of energy by ocean currents Atmosphere Q3 Seager et al. (2002) Heat storage and Climate change
The surface warming due to +4Wm-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. Ho = 50m Ho = 150m Ho = 500m NB: You are welcome to download and run the model : Ensemble mean model results from the IPCC-AR4 report
Q1 Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100)
Q4 Part II Some key oceanic observations World Ocean Atlas surface temperature Thermocline World Ocean Atlas Salinity (0-500m)
psu The great oceanic conveyor belt The ocean is conservative below the surface (100m) layer
Temperature Not changed by absorption/emission of photons. Salinity. No phase change in the range of observed concentration. Conservative nature of the ocean
Salinity on kg/m3 surface Spatial variations of temperature and salinity are similar on scales from several hundreds of kms to a few kms. 10km 50km 2km Ferrari & Polzin (2005) Half time of C14 is 5,730 years Matsumoto, JGR 2007 Circulation scheme Circulation scheme Two sources of deep water: NADW: North Atlantic
AABW: Antarctic Bottom Water Williams & Follows (2009) In situ velocity measurements
Amplitude of time variability Location of long (~2yr) currentmeters Depth NB: Energy at period < 1 day was removed From Wunsch (1997, 1999) Moorings in the North Atlantic interior
(28N, 70W = MODE) (ask more to Ute and Chris. O.!) 1 yr NB: Same velocity vectors but rotated Schmitz (1989) Direct ship observations NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot Surface currents measured from Space
Geostrophic balance Time mean sea surface height Standard deviation of sea surface height Momentum balance f V NB: f = 2 sin Rotation rate f/2 East to west
acceleration f V East to west deceleration up North NB:f = 2 sin East Geostrophic balance! f V Rotation rate f/2 High Low East to west
Pressure Low Pressure East to west acceleration f V 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) Coverage by lifetime Coverage by depths
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 26N Q2 RAPID WATCH array at 26N 14 millions The movie Part III Key physics Because T is conserved by fluid motion the temperature structure simply reflects transport by waves and mean currents Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z X, Y Ocean bottom = This simply happens when warm water goes up or cold water goes down
Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z X, Y Ocean bottom = This happens when warm water goes down or cold water goes up
Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z X, Y Ocean bottom = Requires mechanical forcing (winds/tides)! Downward heat Upward heat
transport Upward heat transport = Sea surface Zo No internal heat source/sink Z X, Y Ocean bottom Historicalview Sea surface Zo Z X, Y Ocean bottom upwelling/downwelling
Historicalview Conveyor-belt upwelling/downwelling Sea surface Zo Z X, Y Ocean bottom Q6 Broecker, 2005 NB: 1 Amazon River 0.2 Million m3/s upwelling/downwelling
Historicalview C W z x,y Small scale wave breaking Conveyor-belt upwelling/downwelling = Sea surface Zo Z X, Y Ocean bottom Q7 Internal waves Waves inducing displacement of density surfaces whose restoring mechanism is gravity. Frequency of linear wave is between the Coriolis frequency f (T~10h in midlatitudes) and the buoyancy frequency N (T=10mn in upper ocean; 100mn in deep ocean) Small scale wave breaking strength
(Naveira-Garabato, 2006) Numerical model results
Conveyor belt strength -2X2 horizontal resolution -Single basin -No wind -Surface heating-cooling -Small scale wave breaking parameterised by a constant diffusivity coefficient K 2/3 K slope (Sv) (cm/s) From Vallis (2000) upwelling/downwelling
Historicalview z x,y Small scale wave breaking Conveyor-belt upwelling/downwelling = Sea surface Zo Z X, Y Ocean bottom Historical view = A very bold statement! -Is the ocean circulation
driven by tides? -Can hurricanes drive the conveyor belt? z x,y Small scale wave breaking Conveyor-belt upwelling = Sea surface Zo Z X, Y Ocean bottom Historical view = 10,000km km Small scale Conveyor-belt
z x,y Small scale wave breaking Conveyor-belt upwelling = Sea surface Zo Z X, Y Ocean bottom In-situ observations are dominated by a meso-scale (100km)
KE spectra (surface) Infrared based surface temperature Alternativeparadigm Zo Z X, Y Ocean bottom upwelling/downwelling
Alternativeparadigm Meso-scale waves upwelling/downwelling Zo Z X, Y Ocean bottom upwelling/downwelling
Alternativeparadigm = Wind forced pumping Meso-scale waves upwelling/downwelling Zo Z X, Y Ocean bottom Momentum balance f V Rotation rate f/2 East to west acceleration
deceleration up North East Ekman balance! f V Rotation rate f/2 East to west Windstress
acceleration f V East to west deceleration up North East Westerly winds ( 45 latitude)
Wind forced pumping Westerly winds ( 45 latitude) Trade winds (10 latitude) X Sea surface Ekman layer Upwelling Upwelling Downwelling upwelling/downwelling
Alternativeparadigm = Wind forced pumping Meso-scale waves upwelling/downwelling 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
Upper ocean: m, wT by the resolved flow is downward and balanced by upward heat flux due to eddy advection. Abyssal ocean: below 2500m, very weak but positive upward heat transport by the resolved flow, opposed by downward diffusive heat transport. NB: >0 means upward Gnanadesikan et al. (2007) Fridays session