Evaluating satellite ocean color-derived export production in the Southern Ocean using atmospheric O 2 /N 2 data Cindy Nevison University of Colorado Mati Kahru, Ralph Keeling, Manfredi Manizza, B. Greg Mitchell Scripps Institution of Oceanography Matt Charette, Kanchan Maiti Woods Hole Oceanographic Institution Acknowledgements: NASA NNX08AB48G, Michael Bender, Nicolas Cassar, Ray Langenfelds, N 2 O and CFC-12 data from AGAGE, NOAA CCGG and CSIRO

Outline 1)Export Production from Ocean Color 2) Atmospheric O 2 /N 2 Data 3) Decomposition of O 2 /N 2 seasonal cycle into Component Signals (land, thermal, ventilation, production) 4) Atmospheric Transport Model Uncertainty – APO Transcom Experiment

Improved Chl-a Algorithm for Southern Ocean B.G. Mitchell et al.

NPP estimated from Ocean Color Modified Behrenfeld and Falkowski = f(Surface T, Chl-a, PAR) Palmer LTER data using SPGANT algorithm (Kahru and Mitchell)

Export Production = ef*NPP export fraction (ef) = f(NPP, SST) Southern Ocean in situ data compiled from the literature (Maiti et al.)

Export Production from Ocean Color Atmospheric O 2 /N 2 Monitoring Stations SIO (R. Keeling), Princeton (M. Bender, N. Cassar) Global South of 50  S 18 Pg C/yr 1.6 Pg C/yr (Unmodified Laws model for ef) 12 Pg C/yr 1.0 Pg C/yr (Laws scaled to Schlitzer, 2002)

Atmospheric O 2 /N 2 : Decreasing Trend and Seasonal Cycles Bender et al., 2005 per meg

Atmospheric Potential Oxygen: Barrow, Alaska Remove Land Signal from O 2 /N 2 using CO 2 data O 2 /N 2 APO

What causes Seasonal Cycles in APO? How are they linked to carbon export production? Adapted from Keeling et al., 1993 C org 1 to sediment Base of Seasonal Thermocline Euphotic Zone Deep Ocean Main Thermocline C org 20 C org 2 C org 10 Production 100 Consumption 80 Atmosphere O 2 Flux 15 O 2 Flux 5 O 2 Flux 3 O 2 Flux 1 Wintertime Ventilation O 2 Flux 8 Thermocline Ventilation O 2 Flux 4 Deep Water Ventilation O 2 Flux 3 Consumption 10 Consumption 8 Consumption 1 O 2 Flux 1 Export Production

APO Seasonal Cycle at Cape Grim, Tasmania Compare to MATCH:NCEP transport model run with satellite-derived export flux assume 1.4 mole O 2 released per mole C exported APO observed = APO thermal + APO net_biological = APO thermal + APO deep_ventilation + APO surface_production

Thermally Corrected APO Seasonal Cycle at CGO Thermal cycle estimated based on Ar/N 2 data or QS T /Cp (need ATM)

APO Seasonal Cycle at Cape Grim, Tasmania Compare to MATCH:NCEP transport model run with satellite-derived export flux APO observed = APO thermal + APO net_biological = APO thermal + APO surface_production + APO deep_ventilation

Estimation of APO ventilation signal based on atmospheric N 2 O data AGAGE data from Cape Grim, Tasmania

Correlated Changes in Atmospheric N 2 O and APO during Coastal Upwelling Events at Trinidad Head Lueker et al., 2003 APO N2ON2O N 2 O is produced during subsurface remineralization, but not affected by euphotic zone primary production Grey Band is Upwelling Event

N 2 O and O 2 are Anticorrelated in Ocean Depth Profiles Southern Ocean data from Brian Popp and Tom Trull  N 2 O/AOU ratio is well defined

The  N 2 O/AOU molar ratio in the subsurface ocean is reflected in observed  N 2 O/  APO atmospheric variations during coastal upwelling events Lueker et al., 2003

Decomposition of APO and N 2 O Seasonal Cycles at Cape Grim, Tasmania N 2 O APO thermal + stratospheric + ventilation thermal + ventilation + production

Compare Decomposed APO Seasonal Cycle at Cape Grim to MIT Ocean Model Results M. Manizza

N 2 O Seasonal Cycles in Individual Years at Macquarie Island (CSIRO Data)

Separating Stratospheric and Oceanic N 2 O Signals at Macquarie Island

Decomposition of APO and N 2 O Seasonal Cycles at Macquarie Island N 2 O APO thermal + ventilation thermal + ventilation + production

Decomposition of APO Seasonal Cycle at Palmer Station, Antarctica 30 per meg * * =  O 2 /N (moles O 2 /moles dry air) But what is N? Some fraction of 1.77 x total moles dry air

Atmospheric Transport Model (ATM) Uncertainty Evaluation of oceanic O 2 fluxes based on observed APO seasonal cycles or latitudinal gradient, e.g., Naegler et al. [2007] Conclusion: These evaluations are difficult because shortcomings in ATMs and problems of data representativity cannot be distinguished from uncertainties in the O 2 fluxes.

APO Transcom Experiment (Blaine, 2005) 9 atmospheric transport models forced with Garcia and Keeling [2001] monthly oceanic O 2 and N 2 flux anomalies. Below: APO seasonal cycles at Cape Grim, Tasmania

APO Transcom Seasonal Amplitude vs. Vertical Gradient at four Southern Hemisphere Stations Summer Winter Red dotted line is observed APO amplitude

APO Transcom Vertical Profiles near Cape Grim, Tasmania vs. CSIRO Aircraft Data Summer Winter Data courtesy of Ray Langenfelds

Conclusions 1) Satellite ocean color data: uncertainties compounded in the sequence from Chl-a to NPP to Export Production. 2) Partitioning of net biological APO signal into surface production and deep ventilation: Atmospheric N 2 O-based method, MIT ocean model and satellite ocean color data converge on similar answer. 3) Can now compare surface production signal in APO directly to export production from ocean color, but some major uncertainties: 1) Need for atmospheric transport model as intermediary 2) N 2 O cycle decomposition – thermal and stratospheric signals

Supplementary Slides

Southern Ocean CO 2 Sink and Biological Pump Takahashi et al., 2009 CO 2 Flux Export Production from Ocean Color Satellite 14 º -50 º S sink = 1.05 Pg C/yr

Southern Ocean as Source and Sink of CO 2. Note scale difference Lovenduski et al., 2007

Components of net biological signal in APO: Three Possible Views observed – thermal = net biological = production + ventilation MIT Ocean Model Results from M. Manizza

APO at Cape Grim, Tasmania Remove Land Signal from O 2 /N 2 using CO 2 data O 2 /N 2 APO