The ocean and abrupt climate change

Transcription

1 The ocean and abrupt climate change Marisa Montoya Dpto. Astrofísica y Ciencias de la Atmósfera UCM, Madrid UB, Barcelona 23/11/2005

2 Outline (1) Introduction to the ocean s large-scale circulation (2) The role of the ocean in abrupt climate change during the last glacial period (3) Sensitivity and impacts under global warming

3 (1) Introduction to the large-scale ocean circulation

4 The role of the ocean in climate Ocean: 71% Earth s surface; Largest water (97%), heat, carbon reservoir; Exchanges water, heat, momentum, CO 2 with atmosphere; Has a circulation which transports heat, salt and dissolved substances; The oceanic circulation varies in time, forced by climatic changes, and vice versa. Its interaction with the atmosphere determines climate variability on timescales from hours to millennia.

5 Large-scale ocean circulation Rahmstorf (2002)

6 Vertical stratification Atlantic Ocean ρ = ρ (T, S, p) Schlitzer (2000)

7 North Atlantic deep water (NADW) formation Labrador Sea GIN Seas Marshall and Schott, RoG (1997) m 1Sv = 10 6 m 3 s -1 Hansen et al. Science (2004)

8 Large-scale ocean circulation Cold water route Rintoul (1991) Warm water route Gordon (1986) Rahmstorf (2002)

9 Streamfunction of the meridional overturning circulation (MOC) v = x u + y v + z w = 0 y v + z z v = ψ w = + ψ y w = 0 Amplitude (NADW) = 15 +/- 2 Sv (Ganachaud & Wunsch Nature 2000) CLIMBER-3α Global Ocean Atlantic Ocean Montoya et al. (2005)

10 AAIW NADW CDW AABW Schlitzer (2000)

11 (1) Introduction to the large-scale ocean circulation What drives the MOC?

12 Large-scale ocean circulation Cold water route Rintoul (1991) Warm water route Gordon (1986) Rahmstorf (2002)

13 Hadley cell

14 Mean annual net heat flux SOC (Wm -2 ) H ao = SWR + DLWR - ULWR + H s -L e Josey et al. J. Clim. (1999)

15 Sandstroem s theorem (1908,1916) Unstable Rayleigh- Benard convection ~ atmosphere Wunsch and Ferrari (2004) after Defant (1961) MOC is not driven by surface buoyancy fluxes Jeffreys (1925): Sandstroem s theorem fails if turbulent mixing carries heat below cold source

16 Drivers of the MOC: i) diapycnal mixing w z T = k v zz T 1-D advectiondiffusion 30 Sv, w uniform k v = 1 cm 2 s -1 Missing mixing? Brazil basin Garret, Nature (2003) Polzin et al. Science (1997)

17 Schlitzer (2000)

18 Drivers of the MOC: ii) wind-driven upwelling in the Southern Ocean Mean annual wind-stress SOC (Nm -2 ) M M w E x y τ y = f τ x = f 1 r r = τ ρf ρv g dx 1 = f p x dx = 0 Josey et al. J. Clim. (1999) Drake Passage Effect (Toggweiler & Samuels 1995, 1998)

19 Drivers of the MOC: ii) wind-driven upwelling in the Southern Ocean M M w E x y τ y = f τ x = f 1 r r = τ ρf Toggweiler and Samuels, Deep Sea Res. (1995)

20 Schlitzer (2000)

21 Upwelling in the Southern Ocean in an eddy resolving model Doos and Coward (1997)

22 (1) Introduction to the large-scale ocean circulation Role in climate

23 Mean annual net heat flux SOC (Wm -2 ) H ao = SWR + DLWR - ULWR + H s -L e Josey et al. J. Clim. (1999)

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25 Volume transport ~ 15 Sv (10 6 m 3 s -1 ) Heat transport ~ 1 PW (10 15 W)

26 (1) Introduction to the large-scale ocean circulation Hysteresis and bistability

27 Hysteresis and bistability of the thermohaline circulation Atlantic Ocean box-model (Stommel 1961; Rahmstorf 1996)

28 Atlantic Ocean freshwater transport BR: Baumgartner & Reichel (1975) A: Schiller (1995) Atlantic G: Schiller (1995) Global Overturning: Schiller (1995) Rahmstorf, Clim. Dyn, (1996)

29 (2) Past abrupt climate change linked to changes in the MOC

30 ICE CORES Ice core locations (NCDC, NOAA) Ice cap

31 North GRIP camp Drilling GISP2 North GRIP ice-core Ice-core close-up

32 GREENLAND ice-record temperature GLACIAL HOLOCENE 8.2 kbp event Younger Dryas 1-20 : Dansgaard-Oeschger events in Greenland H1-H5: Heinrich events (marine)

33 Heinrich events in marine sediments Non-Heinrich Heinrich Bond et al. 1992

34 Heinrich events Low Greenland temperatures High lithic fraction Low NADW Cold & fresh North Atlantic surface Cortijo et al NADW collapses due to massive freshwater input though icebergs

35 GISP2 δ 18 O/temperature (Grootes et al 1993) Subtropical North Atlantic (Bermuda Rise) SSTs (Sachs and Lehman 1999)

36 Warm DO phase Antiphase Cold DO phase Rahmstorf (2002) after Voelker (2002)

37 CLIMBER-2 (PIK)

38 Atlantic overturning circulation CLIMBER-2 Present Glacial Ganopolski et al., Nature 1998

39 Ganopolski and Rahmstorf 2001 Blunier and Brook 2001 Greenland Antarctica

40 Stadial (glacial) mode

41 Interstadial (warm) mode

42 Stadial (glacial) mode

43 Dansgaard-Oeschger Events in Greenland Model Temperature Time since beginning of warming (yr) (Ganopolski & Rahmstorf, Nature 2001)

44 The bipolar see-saw warm stadial mode warmest DO phase stadial mode Heinrich stadial mode Ganopolski and Rahmstorf 2001

45 Model Data (δ 13 C) Rahmstorf (2001) Alley et al. 1999

46 (3) Sensitivity and impacts under global warming

47 IPCC TAR (Houghton et al. 2001)

48 TAR Global temperature projections IPCC TAR (Houghton et al. 2001)

49 Amplitude Atlantic MOC (Sv) IPCC TAR (Houghton et al. 2001)

50 CMIP (Coupled Model Intercomparison Project) (i) Water hosing experiment (Stouffer et al. J. Clim. 2005): 0.1 Sv during 100 yr over the Atlantic Ocean (50-70 o N) 4 x CO 2 Δ P-E+R = 0.14 Sv (dominant terms over next 100 yr) + Greenland melting < 0.07 Sv (Church et al. 2001) (ii) 1% CO2/yr x 140 yr (Gregory et al., GRL, 2005)

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52 3 CLIMBER-3α description POTSDAM2 atmosphere model: (Petoukhov et al., 2000, Ganopolski et al. 2001) 2.5-dim. statistical-dynamical model 7.5 x 22.5 horizontal resolution COUPLER MOM-3 ocean model: 3.75 x3.75, 24 variably spaced vertical levels (25 m to 500 m) Isopycnal mixing (10 7 cm²/s) Interactive buoyancy fluxes Interactive wind stress anomalies ISIS sea ice model (Fichefet and Morales-Maqueda 1997) Dynamic and thermodynamic sea-ice with elasto-visco plastic rheology Low background value of vertical diffusivity ( cm²/s) Hasumi-Suginohara-Ledwell s parametrization Gent-Mc Williams eddy-induced velocity, KPP mixing of tracers, Low-diffusive tracers advection scheme (Prather, 1986) Montoya et al. Climate Dynamics (2005)

53 Universal vertical structure in CLIMBER-2 performance: ~ 10 5 model-years/day Petoukhov et al. Climate Dyn. 2000

54 Resolution: Atmosphere: 7.5 o x 22.5 o Ocean: 3.75 o Montoya et al (2005)

55 DJF & JJA zonally averaged surface air temperature and precipitation Montoya et al (2005)

56 CMIP (Coupled Model Intercomparison Project) (i) Water hosing experiment (Stouffer et al. J. Clim. 2005): 0.1 Sv during 100 yr over the Atlantic Ocean (50-70N) 4 x CO 2 Δ P-E+R = 0.14 Sv (dominant terms over next 100 yr) + Greenland melting < 0.07 Sv (Church et al. 2001) (ii) 1% CO2/yr (Gregory et al., GRL, 2005)

57 Amplitude of the MOC (Sv) v = 0.4 cm2 s -1 v = 0.1 cm2 s -1 Ganachaud & Wunsch, Nature (2000): 15 +/- 2 Sv

58 Evolution of THC intensity

59 Ensemble mean AMOC (Sv) control run (yr 1-100) yr

60 a) control run (yr 1-100) Atlantic heat transport Ganachaud & Wunsch, Nature (2000): 1.2 +/- PW b) yr

61 ΔSAT ( o C) yr minus yr1-100 ctrl

62 Δ SST (deg C) Δ SSS (psu) Δ Sea-ice cover

63 Zonally averaged temperature in the Atlantic ( o C) Zonally averaged salinity in the Atlantic (psu)

64 CMIP (Coupled Model Intercomparison Project) (i) Two water hosing experiments (Stouffer et al. J. Clim. 2005) 0.1 Sv during 100 yr over the Atlantic Ocean (50-70N) 4 x CO 2 D P-E+R = 0.14 Sv (dominant terms over next 100 yr) + Greenland melting < 0.07 Sv (Church et al. 2001) 1.0 Sv during 100 yr (e.g. ~ deglaciation, Clarke et al. 2003) (ii) 1% CO2/yr (Gregory et al., GRL, 2005)

65 1% CO2/yr (Gregory et al., GRL, 2005) (i) CONTROL integration (ii) TRANSIENT integration (1%/yr CO 2 increase x 140 yr)

66 (i) CONTROL integration Correlation (AMOC, σ AMOC) = 0.27

67 (ii) TRANSIENT run (1% CO2 increase/yr) Correlation (AMOC, Δ AMOC) = -0.74

68 (i) CONTROL integration (ii) TRANSIENT integration (1%/yr CO 2 increase x 140 yr) (iii) CRAD_TH20: control CO2; transient freshwater flux (only effect of freshwater changes only) (iv) TRAD_CH20: transient CO2 changes; control freshwater flux (effect of temperature only

69 Fractions of THC change relative to 1%CO2/yr (Due to freshwater flux change) (Due to temperature change) (+ implies a reduction)

70 Manabe and Stouffer, Tellus (1995)

71 (3) Sensitivity and impacts under global warming Impact (of a THC collapse) on sea-level

72 Steric sea-level rise IPCC Third Assessment Report (Houghton et al. 2001)

73 Steric sea-level change under a collapse of the Atlantic overturning Knutti and Stocker, J. Clim. (2000)

74 Sea Level (cm) Topex-Poseidon satellite data, NOAA

75 Freshwater onto North Atlantic deep water formation region (52-80 o N,48-15 o W) FWF = Sv Surface winds: prescribed Levermann et al. Clim. Dyn. (2004)

76 Atlantic overturning ON Atlantic overturning OFF

77 Time series of sea level difference for different regions in the ocean.

78 Patterns of sea level difference (m) (DWF off on) Effect on surface circulation far away from DWF regions Levermann et al. Climate Dynamics (2005)

79 Conclusions The large-scale ocean circulation plays a key role in climate. It is driven by mixing and winds but reacts to changes in buoyancy forcing. Models suggest multi-stability due to the positive salinity advection feedback. Models & paleodata suggest indicate a role of the MOC in past abrupt climate change. Models suggest a decrease of the MOC intensity during the 21 st century but no model shows a collapse. Collapse: low probability, high impact scenario (e.g. regional sea-level increase up to 50 cm).