Towards higher resolution, global-ocean, tracer simulations Doctoral Thesis: Zouhair Lachkar Contributors: J.-C. Dutay, P. Delecluse, J. C. Orr LSCE/IPSL, Laboratoire des Sciences du Climat et de l’Environnement CEA-CNRS-UVSQ Gif-sur-Yvette, France

Background: Anthropogenic CO2 (1990): Coarse-resolution ocean models (OCMIP-2) Large differences Generally underpredict storage Eddy resolving global model (1/10°) 1st to give adequate Southern Extratropical CFC-11 inventory model data range of OCMIP models What horizontal resolution is needed to capture large-scale distributions? Sasai et al., (2004, GRL)

Ocean spatiotemporal scales governing surface variability Chelton et al. (2001). Satellite altimetry and earth sciences, Chap. on Satellite altimetry, 57–64. Academic Press, NY.

Thesis work of Zouhair Lachkar: Part 1 Evaluate: eddy-induced changes in simulated tracer distributions (model vs. model) any improvement due to explicitly including eddies (model vs. data)? Understand how eddies affect transient tracer uptake & storage Part 2 Explore role of mesoscale eddies in AAIW ventilation (with CFC-11 as a diagnostic tool) Part 3 Study role of eddies & temporal variability of meridional transport of heat, CFC-11, and anthropogenic CO2

Approach 2 versions of same model ORCA-LIM (Madec et al., 1998; Barnier et al., 2006) (ORCA 05, ½° cos   ½°) mean res.  33 km (ORCA 2, 2° cos   2°) mean res.  130 km eddying non-eddying Eddy Kinetic Energy Eddy Kinetic Energy Tracers Atmospheric levels (normalized) Tracer Air-sea equilibration time CFC-11  1 month CO2  1 yr Bomb C14  10 yr 14C CFC-11 3 transient tracers: - CFC-11 - Anthropogenic CO2 - Bomb 14C CO2 Broecker and Peng, 1974 1950 Year 2000

Increasing resolution reduces CFC-11 uptake & storage non-eddying eddying data Inventory Cumulative flux Eddy-induced changes (model vs. model) Global Atlantic Pacific Indian South of 20°s flux - 22% -33,6% -9,8% -23,5% -28% inv - 22% -22,8% -10,8% -36,6% -31% Global inventory (106 mol) – 1994 (model vs. data) non-eddying eddying Sasai et al (2004) GLODAP (data) 648 506 510 540 Decrease of global uptake by 22% (28% in SO) Largest flux decrease  Atlantic (>33%) Largest inventory decrease  Indian (> 36%) Eddying model generally agrees with data

Unlike explictlyGM alone brings little improvement, but not enough… CFC-11 inventory non-eddying non-eddying non-eddying + GM eddying eddying + GM  non-eddying + GM  eddying + GM CFC-11 Zonal Integral (Mmol degree-1)

Increased resolution reduces CO2 uptake & storage non-eddying eddying data Cumulative flux Inventory Eddy-induced changes (model vs. model) Global Atlantic Pacific Indian South of 20°s Global inventory (PgC) - 1994 flux - 18% -21,3% -12,5% -23,8% -25% inv - 18% -17,9% -14,3% -24,8% -26% Global inventory (Pg C) – 1994 (model vs. data) non-eddying eddying Sabine et al (2004) GLODAP 120 98 106 Decrease of total uptake by 18% (25% in SO) Largest air-sea flux decrease  Indian (>23%) Largest inventory decrease  Indian (>24%) Eddying model agrees with data

How do eddies affect Southern Ocean uptake? Mixed Layer Thermocline 60°S 40°S + = Eulerian mean circulation Eddy induced flow Residual mean circulation 30 Sv -10 Sv 20 Sv Eddy-induced slow-down of the SO upper meridional circulation 4-year trajectories of set of particles launched at z=250 m, y=60°S, 110°E<x<120°E depth (m) Zonal mean CFC-11 saturation in the Atlantic 0 m non-eddying strong upwelling rapid northward transport 100 m 250 m non-eddying eddying 0 m eddying weak upwelling slow northward transport 100 m 250 m

Effect of increasing horizontal resolution section I9S – CFC-11 Including mesoscale eddies: reduces excessive penetration of CFC-11 in Southern Ocean  yields a better agreement with data

Isopycnal meridional streamfunction How do eddies affect AAIW ventilation? LCDWAAIW UCDWAAIW SAMW AAIW UCDW LCDW NADW Maximum of mixed layer depth eddying Isopycnal meridional streamfunction non-eddying 30 Sv CFC-11 inventory (27-27.4 layer) non-eddying eddying data Including eddies leads to: - increase of UCDWAAIW: +5 Sv - decrease of LCDWAAIW (ML): -15 Sv - reduction of excessive intermediate ocean ventilation (AAIW, - 42%)

Mesoscale eddies: Decrease southern extratropical uptake (improve data-model agreement) CFC-11: -28%; Anthro. CO2: -25%; Bomb C-14: negligible Increase residence time of tracer-rich waters at the surface Increase surface water saturation of rapid equilibrating tracers (CFC-11, anthropogenic CO2) Reduce upwelling (LCDWAAIW) Decrease formation & ventilation of AAIW modify distribution of AAIW source regions (from circumpolar distribution along ACC to localized in the Pacific Ocean) decrease vertical penetration of CFC-11 and CO2 (improve data-model agreement) Need better parameterization or explicit representation of mesoscale eddies to properly simulate southern extratropical uptake & storage of CFC-11 & anthropogenic CO2

Future work: towards eddy-resolving transient tracer simulations EGU 2006 General Assembly Future work: towards eddy-resolving transient tracer simulations ORCA2 (2° cos   2°) mean res.  130 km ORCA05 (½° cos   ½°) mean res.  33 km ORCA025 (¼° cos   ¼°) mean res.  17km Surface current speed (5-day mean, Jan 1-5) CFC-11 Inventory (1965)  From European-led DRAKKAR project on high-resolution ocean modelling

Global 14C distribution is insensitive to horizontal resolution non-eddying eddying data Cumulative flux Inventory Eddy-induced changes (model vs. model) Global Atlantic Pacific Indian South of 20°s flux - 4,6% -4,7% -4% -6,1% -8% inv - 4,6% -5,7% -1,8% -7,3% -7% Global inventory (1028 atoms 14C) – 1994 (model vs. data) non-eddying eddying GLODAP Broeker et al 1995 3,07 2,94 3,13 3,29 Small decrease of uptake & inventory (<5%) Good agreement with observation