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Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Mechanisms of ocean-atmosphere coupling.

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Presentation on theme: "Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Mechanisms of ocean-atmosphere coupling."— Presentation transcript:

1 Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Mechanisms of ocean-atmosphere coupling

2 Part I Ocean and Climate (heat transport and storage)

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

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

5 Seasonal Heat storage Q5

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

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

8 Poleward heat transport at 24ºN Pacific0.76 +/- 0.3 PW Atlantic1.2 +/- 0.3 PW Atlantic+Pacific2 +/- 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 Earth’s budget”. Hall & Bryden, 1982; Bryden et al., NB: 1PW = 10^15 W

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

10 Ha+Ho Ha Ho Wunsch, JCl GERBE approved!

11 Ganachaud & Wunsch, 2003

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

13 “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, Eddy surface air temperature from NCAR reanalysis (January, CI=3K) WARM! COLD! Lieutenant Maury “The Pathfinder of the Seas”

14 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

15 Seager et al. (2002) Q3

16 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 :

17 Ensemble mean model results from the IPCC-AR4 report Q1

18 Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100) Q4

19 Part II Some key oceanic observations

20 World Ocean Atlas surface temperature ºC

21

22 Thermocline

23 World Ocean Atlas Salinity (0-500m) psu

24 The “great oceanic conveyor belt”

25 Temperature No heat exchange, only pressure effects. Salinity. No phase change in the range of observed concentration. The ocean is conservative below the surface (≈100m) layer

26 Conservative nature of the ocean 50km 10km 2km Spatial variations of temperature and salinity are similar on scales from several hundreds of kms to a few kms. Salinity on kg/m3 surface Ferrari & Polzin (2005)

27 Matsumoto, JGR 2007

28 “Circulation” scheme

29 Broecker, 2005 NB: 1 Amazon River ≈ 0.2 Million m3/s Q6

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

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

32 1 yr NB: Same velocity vectors but rotated Moorings in the North Atlantic interior (28N, 70W = MODE) Schmitz (1989) (ask more to Ute and Chris. O.!)

33 Direct ship observations NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot

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

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

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

37 10-yr average sea surface height deviation from geoid Subtropical gyres

38 10-yr average sea surface height deviation from geoid Antarctic Circumpolar Current Subpolar gyres

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

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

41 RAPID – WATCH array at 26N Q2

42 RAPID – WATCH array at 26N 14 millions £

43

44 The movie…

45 Friday’s session


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