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Ocean circulation and coupling with the atmosphere Arnaud Czaja 1. Ocean heat storage & transport 2. Key observations 3. Ocean heat uptake and global warming.

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Presentation on theme: "Ocean circulation and coupling with the atmosphere Arnaud Czaja 1. Ocean heat storage & transport 2. Key observations 3. Ocean heat uptake and global warming."— Presentation transcript:

1 Ocean circulation and coupling with the atmosphere Arnaud Czaja 1. Ocean heat storage & transport 2. Key observations 3. Ocean heat uptake and global warming 4. Mechanisms of ocean-atmosphere coupling

2 Part I Ocean heat storage and transport

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 Q4

6 Trenberth & Caron, 2001

7 Ganachaud & Wunsch, 2003

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

9 “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”

10 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

11 Seager et al. (2002) Q3

12 Part II Some key oceanic observations

13 World Ocean Atlas surface temperature ºC

14

15 Thermocline

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

17 The “great oceanic conveyor belt”

18 Matsumoto, JGR 2007

19 “Circulation” scheme

20 Broecker, 2005 NB: 1 Amazon River ≈ 0.2 Million m3/s Q5

21 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

22 1 yr NB: Same velocity vectors but rotated Moorings in the North Atlantic interior (28N, 70W = MODE) Schmitz (1989)

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

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

25 10-yr average sea surface height deviation from geoid Subtropical gyres

26 10-yr average sea surface height deviation from geoid Antarctic Circumpolar Current Subpolar gyres

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

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

29 RAPID – WATCH array at 26N Q2

30 RAPID – WATCH array at 26N 14 millions £

31 Part III Ocean heat uptake and anthropogenic forcing of climate change

32 Heat storage and Climate change The surface warming due to +4Wm-2 (anthropogenic forcing) is not limited to the mixed layer. Heat exchanges between the mixed layer and deeper layers control the timescale of the surface warming.

33 Anthropogenic forcing Net surface ocean heating Upper ocean cooling via diabatic processes Upper ocean cooling via mass exchange with deep ocean Weak vertical ocean heat transport

34 Anthropogenic forcing Net surface ocean heating Upper ocean cooling via diabatic processes Upper ocean cooling via mass exchange with deep ocean Large vertical ocean heat transport

35 The Environmental Physics Climate Model Tropics Extra Tropics Ocean Atmosphere T A1 Heat content (J)

36 Upper (0-750m) ocean heat content vs TOA imbalance: observations Wong et al (2006)

37 Mechanisms of heat exchange between upper and deep layers Wind driven circulation pumping down of warm subtropical waters; upwelling of cold, high latitude waters. Buoyancy driven circulations sinking of dense water and upwelling of light water (= overturning circulations + eddy driven + convection). Mixing isopycnal diffusion and breaking internal gravity waves. Q1

38 Ocean heat uptake in wind driven gyres Global downward ocean heat transport driven by winds. Strength: Levitus (1988) Williams & Follows (2012)

39 Buoyancy driven circulations and ocean heat uptake : Total temperature change in the 10 th decade after 2XCO2 (idealised ocean basin) Temperature change due to change in ocean currents Temperature change in absence of change in ocean currents. Xie and Vallis (2011) Cooling

40 Interior mixing & ocean heat uptake Osborne (1998) Upward heat flux Downward heat flux Vertical heat flux (Wm-2) Equator North Pole South Pole deeper

41 Motions in the ocean are not isotropic: “neutral” surfaces In the simplest case of a waterworld at rest, a fluid parcel does work against the buoyancy force when displaced upward or downward. Motions along z=cst are energetically neutral. Solid Earth where Z=0 Z=h Reference density

42 Motions in the ocean are not isotropic: “neutral” surfaces In the real ocean, neutral surfaces take the shape of a bowl due to the distortion of spheres by the seafloor topography, surface heating, cooling and winds. Neutral surfaces in the Atlantic WOCE A16 NB: These surfaces can be approximated as surfaces of constant density (“isopycnals”). Neutrally energetic displacements

43 The movie…


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