Oxygen Supply to the Tropical North East Atlantic Oxygen Minimum Zone

Transcription

1 Oxygen Supply to the Tropical North East Atlantic Oxygen Minimum Zone Tim Fischer, Johannes Hahn Peter Brandt, Richard Greatbatch, Arne Körtzinger, Toste Tanhua, Martin Visbeck Donata Banyte, Marcus Dengler, Gerd Krahmann

2 1. Motivation 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 2

3 1. Motivation Low oxygen in the open ocean O 2 distribution at 600m obtained from the Meteor expedition (Wattenberg, 1939) Pocesses influencing the oxygen concentration in the open ocean Hier Wyrtki bzw. wie werden OMZ gebildet, Rieselgraphik Consumption/Supply einführen Karstensen et al µmol/kg 3

4 1. Motivation Low oxygen in the open ocean O 2 distribution at 400m, in µmol/kg (Karstensen et al. 2008) 4

5 1. Motivation Low oxygen in the open ocean O 2 distribution at 400m, in µmol/kg (Karstensen et al. 2008) Sensitivity of marine organisms to low oxygen (Keeling et al. 2010, After Vaquer Sunyer and Duarte 2008) 5

6 1. Motivation Low oxygen in the open ocean O 2 distribution at 400m, in µmol/kg (Karstensen et al. 2008) Outgassing of supersaturated N 2 O (Suntharalingam 2000) 6

7 1. Motivation Low oxygen in the open ocean O 2 distribution at 400m, in µmol/kg (Karstensen et al. 2008) TNEA OMZ Expanding and intensifying (Stramma et al. 2008) 7

8 1. Motivation OMZs in model simulations State of the art global models not capturing distribution nor levels (300m concentrations, µmol/kg) Observations WOA Oschlies pers. comm (Max Planck Inst. HH) (NCAR) 8

9 1. Motivation OMZs in model simulations State of the art global models not capturing distribution nor levels (300m concentrations, µmol/kg) Observations WOA In order to be able to better prognose future oceanic oxygen developments, It is necessary to understand involved physics and biology, in particular the OMZ response to circulation and ventilation. (One major scientific question of SFB 754: Climate Biogeochemistry Interactions in the Tropical Ocean ) 9

10 1. Motivation Oxygen Minimum Zone (OMZ) in the Tropical North East Atlantic (TNEA) O 2 distribution and equatorial current system (300m 500m depth) 23 W section Brandt et al. (2010) Mean O 2 along 23W Mean zonal currents Brandt et al. (2010) 10

11 1. Motivation Oxygen Minimum Zone (OMZ) in the Tropical North East Atlantic (TNEA) Average oxygen profile depth CW deep oxycline deep OMZ core density CW Central Water AAIW Antarctic Intermediate Water AAIW 11

12 1. Motivation O 2 budget: Processes in density coordinates Source/ Sink Diapycnal advection Diapycnal diffusion Source/sink + isopycnal supply + diapycnal supply = tendency/storage The supply is the difference of fluxes into and out of the volume, i.e. flux divergence 12

13 1. Motivation O 2 budget: Processes in density coordinates Consumption Upwelling Double diffusion and turbulence Source/sink + isopycnal supply + diapycnal supply = tendency/storage The only important source/sink term in the deep ocean is consumption. 13

14 1. Motivation O 2 budget for the TNEA OMZ region (mean profiles of budget terms) aour + R = 0 consumption (Karstensen et al., 2008) residual (total supply) Stationarity assumed Quantify some of the missing supply terms 14

15 2. Diapycnal O 2 Supply 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 15

16 2. Diapycnal O 2 Supply Diapycnal processes Consumption + isopycnal supply + diapycnal supply = tendency/storage ( ) Diapycnal advection + double diffusion + turbulent diapycnal diffusion Estimation method McDougall 1991 Estimation method St.Laurent and Schmitt 1999 F Diapycnal flux Diapycnal supply to a volume (flux convergence) = K c F c oxygen concentration K diapycnal diffusivity F diapycnal flux diapycnal gradient Estimating diapycnal supply requires simultaneous data for K and c 16

17 2. Diapycnal O 2 Supply Measurement programme ( ) Tracer Release Experiment (TRE) Microstructure Profiles (MSS) Acoustic Current Profiles (ADCP) Oxygen Profiles (CTD O 2 ) Analysis box for this study: 6 to 15 N, 30 to 15 W. K c TRE MSS ADCP CTD O 2 SF 5 CF 3 17

18 2. Diapycnal O 2 Supply The 3 methods to estimate diapycnal diffusivity K TRE MSS ADCP Integrative in space and time. Only 1 K value. Groundtruthing. Points in region and time. Vertical structures. Lines in region and time. Horizontal structures. depth K time 18

19 2. Diapycnal O 2 Supply The 3 methods to estimate diapycnal diffusivity K TRE MSS ADCP Integrative in space and time. Only 1 K value. Groundtruthing. Points in region and time. Vertical structures. Lines in region and time. Horizontal structures. All 3 methods work in an overlapping depth range. Double diffusion is negligible here. 19

20 2. Diapycnal O 2 Supply Diapycnal diffusivity K: intermediate results Average dissipation profile depth Focus depth range MSS: K is approximately constant with depth in the focus depth range m 6 5 lg ε 4 3 F = K c = K c K and gradient c are independent in each depth layer. The two properties simplify the merging of the 3 methods. K TRE = (1.2 ± 0.2) m s K from TRE and from MSS/ADCP agree in uncertainty limits K MSS = (1.0 ± 0.2) 10, ADCP 5 2 m s K is substantially stronger than the expected background value 20

21 2. Diapycnal O 2 Supply Concentration : flux : supply 0 m c F = K c F 100 m 200 m Shallow Ox Min. Zonal currents 300 m 400 m Oxycline OMZ core Oxygen concentration Diapycnal flux Diapycnal flux in Diapycnal flux out 21

22 2. Diapycnal O 2 Supply Concentration : flux : supply 0 m c F = K c F 100 m 200 m Shallow Ox Min. Zonal currents 300 m 400 m Oxycline OMZ core Oxygen concentration Diapycnal flux Diapycnal flux in Diapycnal flux out 22

23 2. Diapycnal O 2 Supply Concentration : flux : supply 0 m c F = K c F 100 m 200 m Shallow Ox Min. Zonal currents 300 m 400 m Oxycline OMZ core Oxygen concentration Diapycnal flux Diapycnal flux in Diapycnal flux out 23

24 2. Diapycnal O 2 Supply Concentration : flux : supply 0 m c F = K c F 100 m 200 m Shallow Ox Min. Zonal currents 300 m 400 m Oxycline OMZ core Oxygen concentration Diapycnal flux Diapycnal flux in Diapycnal flux out 24

25 2. Diapycnal O 2 Supply Concentration : flux : supply 0 m c F = K c F 100 m 200 m Shallow Ox Min. Zonal currents 300 m 400 m Oxycline OMZ core Oxygen concentration Diapycnal flux Diapycnal flux in Diapycnal flux out 25

26 2. Diapycnal O 2 Supply O 2 budget aour + O 2,dia + R (1) = 0 consumption (Karstensen et al., 2008) diapycnal supply Isopycnal residual (advective + eddy supply) Stationarity assumed Fischer et al

27 3. Meridional O 2 Supply 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 27

28 3. Meridional O 2 Supply Characteristic section cutting through the OMZ of the TNEA 23 W section Brandt et al. (2010) 28

29 3. Meridional O 2 Supply Zonal mean and mesoscale velocity along 23 W mean, <u> mesoscale, U e + = EKE = ( u' 2 v' 2 ) / 2 29

30 3. Meridional O 2 Supply Zonal mean and mesoscale velocity along 23 W mean / mesoscale mean > mesoscale mean < mesoscale 30

31 3. Meridional O 2 Supply Eddy driven meridional O 2 Flux Two methods (I) Flux gradient parameterization analysis based on repeated ship sections do2 F = Ke dy (II) Correlation method analysis based on mooring time series F = v' O ' 2 SFB 754, proposal 2 nd phase,

32 3. Meridional O 2 Supply 1.Repeated ship sections along 23 W ( ) 23 W section background: O 2 distribution at 400m depth from World Ocean Atlas 2009 (I) Hydrography (CTD/O 2 ) average # cruises = 8 (> 500 profiles in upper 1000m) (II) Velocity (ADCP) average # cruises = 10 (for depth range > 700m) photo: A. Krupke Copyright GEOMAR 32

33 3. Meridional O 2 Supply 2. Climatological data World Ocean Atlas 2009 annual mean hydrography of the Tropical Atlantic O 2 distribution at 400m depth from World Ocean Atlas

34 3. Meridional O 2 Supply Goal: estimate a mean K e profile to estimate eddy driven meridional O 2 flux F = K e do dy 2 34

35 3. Meridional O 2 Supply Basic approach: K e U e ~ L Goal: estimate a mean K e profile mean state mesoscale L ~ characteristic eddy length scale B A B A L ~ U e characteristic eddy velocity 2 U e = EKE = ( u' + v' 2 ) / 2 35

36 3. Meridional O 2 Supply Goal: estimate a mean K e profile 1. Mixing length theory (Armi and Stommel (1983), O Ferrari and Polzin (2005)) K e = ceu el 2' e L e = O σ 2 U e = EKE = ( u' + v' 2 2 ) / 2 c e = Rhines scale (Eden, 2007) K U e e L R L = R U e 2β 2 U e = EKE = ( u' + v' 2 ) / 2 36

37 3. Meridional O 2 Supply mean K e profile Brandt et al. (2010) TNEA: Hahn et al. (subm.) GUTRE: Banyte et al. (2013) NATRE: Ferrari and Polzin (2005) 37

38 3. Meridional O 2 Supply (I) F Eddy Driven Meridional O 2 flux along 23 W = K e do dy 2 northward O 2 flux Brandt et al. (2010) southward O 2 flux northward O 2 flux at 400m 600m 38

39 3. Meridional O 2 Supply 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 39

40 3. Meridional O 2 Supply Eddy driven meridional O 2 Flux Two methods (I) Flux gradient parameterization analysis based on repeated ship sections do2 F = Ke dy (II) Correlation method analysis based on mooring time series F = v' O ' 2 SFB 754, proposal 2 nd phase,

41 3. Meridional O 2 Supply Moored observations along 23 W hydrography (CTD/O 2 ) background: O 2 distribution at 400m depth from World Ocean Atlas 2009 horizontal velocity (ADCP) photo: B. Vogel Moored observations along 23 W (O 2 distribution: update from Brandt et al. (2010)) Copyright Teledyne RD Instruments 41

42 3. Meridional O 2 Supply Oxygen time series along 23 W 300 m 500 m 42

43 3. Meridional O 2 Supply Oxygen and velocity time series at 5 N, 23 W time series correlation: F = v' O ' 2 where v = v + v' O = O + ' (Reynolds decomposition) 2 2 O2 43

44 3. Meridional O 2 Supply Eddy Driven Meridional O 2 flux along 23 W (I) F = K e do dy 2 5 N, 23 W northward O 2 flux Brandt et al. (2010) 8 N, 23 W southward O 2 flux mooring data, 5 N and 8 N (II) F = v' O2' northward O 2 flux at 400m 600m 44

45 3. Meridional O 2 Supply 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 45

46 3. Meridional O 2 Supply Eddy driven meridional O 2 supply along 23 W df dy = d dy K do dy Meridional O 2 flux divergence e 2 convergence of O 2 flux divergence of O 2 flux 450m 600m: O 2 supply due to mesoscale (2.1 μmol kg 1 yr 1 ) above 400m: bands with strong O 2 flux divergence/convergence associated with mean zonal currents 46

47 3. Meridional O 2 Supply O 2 budget aour + O R (1) 2,dia + O 2,y,eddy = 0 + R (2) = 0 consumption (Karstensen et al., 2008) diapycnal supply meridional residual (advective residual + (advective + eddy supply) zonal eddy supply) Fischer et al. (2013) average between 350m 570m Term µmol kg 1 yr 1 % aour aour 4.1 O 2,dia % O 2,y,eddy % R (2) % 47

48 3. Meridional O 2 Supply O 2 budget aour + O 2,dia + O 2,y,eddy + R (3) = t O 2 consumption (Karstensen et al., 2008) diapycnal supply meridional residual (advective + eddy supply zonal eddy supply) O 2 tendency (Brandt et al., 2010) average between 350m 570m Term µmol kg 1 yr 1 % aour aour 4.1 O 2,dia % O 2,y,eddy % R (3) % t O % 48

49 4. Summary and Outlook 1. The Oxygen Minimum Zone of the Tropical North East Atlantic (TNEA OMZ) 2. Diapycnal Oxygen Supply 3. Eddy Driven Meridional Oxygen Supply 3.1 Flux Gradient Parameterization 3.2 Time Series Correlation 3.3 Oxygen Flux Divergence 4. Summary and Outlook 49

50 4. Summary and Outlook Summary Diapycnal diffusivity K larger than expected, constant in m depth Maximum diapycnal supply near OMZ core Above 400m: bands with O 2 flux divergence/convergence associated with mean zonal currents 450m 600m: homogeneous eddy driven meridional O 2 supply OMZ core depth: mainly diapycnal (up to 30%) and meridional eddy supply (>50%) Above OMZ core depth: strong residual supply (associated with mean zonal currents) 50

51 4. Summary and Outlook Summary Main oxygen supply processes in the TNEA OMZ (recent interpretation) CW Sketch ofmain ventilation processes. Advection Meridional Eddy Diffusion Diapycnal Diffusion AAIW 51

52 4. Summary and Outlook Outlook Expand the analyses to 800m depth (AAIW) Main missing term: mean zonal advection Why is there that jump at 300m? Tracer Release Experiment (OSTRE) in SFB phase II ( ): Measure integrative lateral diffusivity K e in OMZ core 52

53 Thank you for your attention BMBF SOPRAN BMBF NORDATLANTIK