Ric Williams (Liverpool) how much heat & CO2 is being sequestered? what are the controlling. mechanisms? what are the wider climate. implications?

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1 So WESTERLIES Physical Controls on the Air Sea Partitioning of CO2 SOUTH AMERICA ANTARCTICA BUOYANCY LOSS The Oce Upp BUOYANCY GAIN (i) Th (ii) O (iii) M Ric Williams (Liverpool) (iv) C EKMAN TRANSPORT EDDIES ACC MODE / INTERMEDIATE WATER Low (i) Th (ii) O (iii) M DEEP WATER (iv) C Con Challenges for the community how much heat & CO2 is being sequestered? what are the controlling BOTTOM WATER Image from Morrison et al., 2015: "Upwelling in the Southern Ocean." Phys. Today. I Wind-driven Antarctic Circumpolar Current (ACC) I transfer Upper & gas lower overturning cells I Buoyancy fluxes also drive circulation air-sea mechanisms? depth what are the wider climate respiration/ regeneration CO 2 biological fallout DIC + N+ Fe - implications? diapycnal mixing south trace metal supply Fe DIC NFe + eddy stirring DIC + N+ Fe north Thanks to Mick Follows (MIT), Jon Lauderdale (MIT), Phil Goodwin (Southampton), Andy Ridgwell (Bristol), Alessandro Tagliabue (Liverpool), and other collaborators.

2 So WESTERLIES Physical Controls on the Air Sea Partitioning of CO2 SOUTH AMERICA ANTARCTICA BUOYANCY LOSS The Oce Upp BUOYANCY GAIN (i) Th (ii) O (iii) M Ric Williams (Liverpool) (iv) C EKMAN TRANSPORT EDDIES ACC MODE / INTERMEDIATE WATER Low (i) Th (ii) O (iii) M DEEP WATER (iv) C Con Lecture overview 1. Mechanisms 2. Frameworks BOTTOM WATER Image from Morrison et al., 2015: "Upwelling in the Southern Ocean." Phys. Today. I Wind-driven Antarctic Circumpolar Current (ACC) I transfer Upper & gas lower overturning cells I Buoyancy fluxes also drive circulation air-sea 3. Effect of residual circulation depth 4. Global implications respiration/ regeneration CO 2 biological fallout DIC + N+ Fe diapycnal mixing south trace metal supply Fe DIC NFe + eddy stirring DIC + N+ Fe north Thanks to Mick Follows (MIT), Jon Lauderdale (MIT), Phil Goodwin (Southampton), Andy Ridgwell (Bristol), Alessandro Tagliabue (Liverpool), and other collaborators.

3 1. Mechanisms carbonate chemistry atmosphere ocean reactions in seawater buffer factor

4 1. Mechanisms air-sea exchange atmosphere ocean Kg air-sea transfer velocity h mixed layer depth air-sea exchange timescale month 1/10 170

5 1. Mechanisms physics (a) physical transport and solubility atmosphere CO 2 CO 2 rate limiting processes on annual timescale: mixed layer warming outgassing physical transfer from interior to surface cooling physical transfer from surface to interior uptake subduction into main thermocline entrainment into winter mixed layer resulting annual air-sea uptake ocean interior fine-scale dynamical processes only important in modifying these processes

6 1. Mechanisms biology (b) physical transport and biology atmosphere mixer layer CO 2 entrainment of carbon-rich waters biological drawdown biological fallout CO2 biological drawdown can respond to fine-scale delivery of nutrients & trace metals affects DIC profile ocean interior respiration and regeneration Prevailing view: biological response is not directly important for the anthropogenic response, but is crucial for glacial-interglacial cycles.

7 1. Mechanisms physics (a) physical transport and solubility atmosphere biology (b) physical transport and biology atmosphere CO 2 CO 2 CO 2 CO2 mixed layer warming outgassing cooling uptake mixer layer biological drawdown physical transfer from interior to surface physical transfer from surface to interior entrainment of carbon-rich waters biological fallout respiration and regeneration ocean interior ocean interior see first order opposing signs in the physical & biological responses

8 2. Frameworks atmosphere preformed & regenerated in mixed layer N N pre mixed layer thermocline N =N pre + N reg in ocean interior efficiency of the biology Ito & Follows (2005)

9 2. Frameworks Dissolved free iron preformed +regenerated +benthic - scavenged preformed Dissolved free iron flux to surface dominated by winter entrainment Tagliabue)et)al.)(2014,)Nature)Geoscience)) benthic Supply of preformed & benthic Fe to the Southern Ocean Tagliabue)et)al.)(2014,)GRL))

10 2. Frameworks Dissolved inorganic carbon, DIC atmosphere DIC C pre = C sat +ΔC dis mixed layer in mixed layer C pre C thermocline = C sat + C dis saturated disequilibrium

11 2. Frameworks Dissolved inorganic carbon, DIC in ocean interior DIC = C pre + C reg DIC C pre atmosphere mixed layer thermocline DIC=C pre + C reg C sat + C dis + C soft + C carb saturated disequilibrium soft tissue regenerated carbonate tissue regenerated

12 3. Residual circulation depth surface heat loss/ density gain eastward winds diapycnal mixing H Ekman transport eddy transport surface heat gain/ density loss eddy stirring upper limb of overturning lower limb of overturning depth air-sea gas transfer respiration/ regeneration DIC + N + Fe - CO 2 biological fallout diapycnal mixing eddy stirring trace metal supply Fe DIC - N - Fe + DIC + N + Fe - south north south north air-sea carbon uptake affected by how far surface waters are away from saturation air-sea carbon anomalies eroded on annual & longer timescales expect opposing preformed & regenerated nutrient responses

13 Consider effect of changes in residual circulation ψres=ψeul+ψeddy,,(ψeddy KGMSi), eastward winds H Ekman transport surface heat gain/ density loss upper limb of overturning eddy transport eddy stirring depth 3. Residual circulation surface heat loss/ density gain diapycnal mixing example: stronger westerly wind stress lower limb of overturning south north Si), generally stronger ψres=ψeul+ψeddy,,(ψeddy KGM generally thicker subtropical thermocline MIT model 2.8ox2.8o, fixed KGM in eddy closure, integrated 5000 years with online biogeochemistry (Lauderdale et al., 2013, Climate Dynamics)

14 3. Residual circulation stronger westerly wind C pre preformed carbon anomaly increases via greater subduction mmol C m-3 C reg regenerated carbon anomaly decreases, as more nutrients are subducted

15 3. Residual circulation stronger westerly wind C pre increases via greater subduction dis increases due to shorter C residence time Wind-driven changes in the Southern Ocean carbon reservoirs sat from T, S & Alk decreases C from warmer thermocline mmol C m-3

16 3. Residual circulation depth air-sea gas transfer respiration/ regeneration DIC + N + Fe - CO 2 biological fallout diapycnal mixing eddy stirring trace metal supply Fe DIC - N - Fe + DIC + N + Fe experiments: variety of winds, GM values, limited buoyancy forcing changes R 2 =0.89 south north Atmospheric CO2 (ppm) 260 stronger residual cell, greater atmospheric CO2 0 Residual circulation (Sv) 30 (Lauderdale et al., 2013, Climate Dynamics) Consistent with paleo argument: enhanced upwelling driving last deglacial (Anderson et al., 2009)

17 3. Residual circulation wind changes & different eddy closures 150. stronger winds weaker winds Atmospheric CO2 Preformed Carbon 125. Atmospheric CO2 Preformed Carbon Regenerated Carbon 100. Carbon anomaly (PgC) Eddy KGM closures: constant Hoffman & Morales Maqueda (2011) Ferreira et al. (2005) Visbeck et al. (1997) opposing sign of regenerated and preformed carbon anomalies L2013 Constant KGM F2005 Variable KGM HMM2011 Variable KGM V1997 Variable KGM L2013 Constant KGM F2005 Variable KGM HMM2011 Variable KGM V1997 Variable KGM atmospheric CO2 increase due to contraction of regenerated carbon pool see Jon Lauderdale poster

18 3. Residual circulation wind changes & different eddy closures 150. Atmospheric 125. CO2 Preformed Carbon Regenerated Carbon 100. southward shift northward shift Atmospheric CO2 Preformed Carbon Carbon anomaly (PgC) Eddy KGM closures: constant Hoffman & Morales Maqueda (2011) Ferreira et al. (2005) Visbeck et al. (1997) opposing sign of regenerated and preformed carbon anomalies L2013 Constant KGM F2005 Variable KGM HMM2011 Variable KGM V1997 Variable KGM L2013 Constant KGM F2005 Variable KGM HMM2011 Variable KGM V1997 Variable KGM atmospheric CO2 increase due to contraction of preformed carbon pool see Jon Lauderdale poster

19 4. Global implications Southern Ocean probably playing a crucial role: sequestering heat sequestering anthropogenic CO2 Ultimately important for global climate change atmosphere (600 PgC) radiative forcing from CO 2 surface warming ocean heat uptake ocean (38000 PgC) ocean CO 2 uptake carbon emissions terrestrial fossil (2000 fuels PgC) (accessible 4000 PgC) T surface temperature anomaly, ( o C) since 1870 IPCC (2013) cumulative carbon emission, Iem (PgC) since 1870

20 em I em global surface temperature change change in cumulative carbon emissions

21 em I em global surface temperature change warming dependence on radiative forcing, R from CO2 change in cumulative carbon emissions radiative forcing R dependence on carbon emissions, Iem

22 em I em global surface temperature change warming dependence on forcing, R from CO2 forcing R dependence on carbon emissions change in cumulative carbon emissions time-varying variables: normalised ocean heat uptake (relative to R) constants: =0.5 to 1.2 K(W m -2 ) -1 climate parameter Goodwin et al. (2015) Nature Geoscience =5.35 Wm -2 = 3500PgC normalised ocean carbon undersaturation (relative to Iem) radiative forcing from CO2 buffered ocean+atmos C inventory

23 how does surface warming vary in time? (a) change in surface warming only due to ocean heat uptake inital response from emissions later response after emissions atmosphere increased radiative forcing from CO 2 radiative forcing from CO 2 (assume unchanged) surface ocean interior ocean surface warming ocean heat uptake increased surface warming reduced ocean heat uptake (b) change in radiative forcing only due to ocean carbon uptake surface warming increases in time due to weakening ocean heat uptake

24 how does radiative forcing from CO2 vary in time? (b) change in radiative forcing only due to ocean carbon uptake inital response from emissions later response after emissions excess CO in atmosphere 2 increased radiative forcing from CO 2 less excess CO in atmosphere 2 reduced radiative forcing from CO 2 ocean CO 2 uptake ocean undersaturated in excess CO 2 ocean CO 2 uptake more excess CO in ocean 2 radiative forcing decreases in time due to ocean carbon uptake

25 test in a coarse-resolution atmosphere-ocean model (GENIE) with coupled circulation & biogeochemistry (a) 21st century surface warming from theory 5 T (K) Δ temperature change T (K) error Δ (b) discrepancy of surface warming (model minus theory) RCP 4.5 RCP 6.0 RCP 8.5 B1 IMAGE A1T MESSAGE B2 MESSAGE A1 AIM A2 ASF A1G MINICAM cumulative carbon emissions, I em (PgC) response from year 2000 to 2100 driven by IPCC scenarios (Goodwin et al., 2015, Nature Geoscience)

26 why the similar transient response? heat & carbon sequestration mainly achieved via ventilation process thermal response anthropogenic carbon response (a) initial response to carbon emissions on decadal timescales atmosphere R radiative forcing atmosphere surface warming N ΔT ocean heat uptake climate response warm excess CO increased from emissions 2 ocean uptake of CO low DIC upper ocean undersaturated 2 depth thermocline heat uptake in deep ocean deep ocean undersaturated ocean S. high latitude equator ocean (b) (b) upper upper ocean ocean equilibrates with with the the atmosphere after after decades decades to to centuries centuries atmosphere high latitude warming N R ΔT cold N. high latitude radiative forcing climate response warmer upper ocean heat uptake ceases atmosphere high DIC excess CO relative to the deep ocean 2 high latitude uptake of CO 2 moderate DIC upper ocean saturated heat uptake in deep ocean deep ocean undersaturated ocean cold ocean high DIC heat & carbon sequestration can though differ via air-sea timescales & biology...

27 6. Challenges surface heat loss/ density gain eastward winds H surface heat gain/ density loss 1. Southern Ocean likely to be crucial in sequestering heat & carbon depth Ekman transport eddy transport eddy stirring upper limb of overturning rate limiting processes unclear, probably: subduction into main thermocline entrainment into winter mixed layer mismatch drives annual air-sea transfer 2. Residual circulation crucial for communication with the rest of ocean: stronger residual circulation increases atmospheric CO2. link to carbon transport is unclear also role of deep cell is unclear 3. Global climate implications from ocean heat & carbon drawdown depth south south air-sea gas transfer respiration/ regeneration DIC + N + Fe - diapycnal mixing CO 2 biological fallout diapycnal mixing eddy stirring lower limb of overturning Partly compensating due to ventilation 1 Mismatch in heat & carbon uptake 0 likely to be important T (K) Δ temperature change trace metal supply (a) 21st century surface warming from theory 5 north Fe DIC - N - Fe + DIC + N + Fe - north RCP 4.5 RCP 6.0 RCP 8.5 B1 IMAGE A1T MESSAGE B2 MESSAGE A1 AIM A2 ASF A1G MINICAM (b) discrepancy cumulative of surface carbon warming emissions, I (model em minus (PgC) theory)