Download 2) Model policy

OCVR : Ocean Carbon Variational Reanalyzer - System
The ocean plays a crucial role as it contributes to an uptake of about a quarter to a third of the
anthropogenic emissions with significant year to year variations (Sabine et al., 2004). For the
CARBONES CCDAS, we have developed a statistical model to estimate air-sea fluxes from satellite,
in-situ measurements and model outputs. The fluxes rely deeply on the sea surface carbon partial
pressure estimated in a first step by the OCVR system described below.
1) Model description
Importance of the ocean surface carbon dioxide partial pressure (PCO2sw)
The air-sea CO2 flux is typically controlled by two terms embedded in the formula : F = (k α) . ΔpCO2
where k is the piston velocity, α is the solubility (Weiss, 1974) and ΔpCO2 the difference between the
pCO2 in surface seawater and that in the overlying air. ΔpCO2 represents the thermodynamic driving
potential for the exchange flux at the sea-air interface. Uncertainties in the air-sea CO2 flux come not
only from the gas exchange coefficient, but also from the ΔpCO2. The uncertainties mainly come from
the poorly constrained estimates of the sea surface pCO2. Indeed the seasonal and geographical
variation of surface water pCO2 is much greater (from 150 to 750 uatm) than that of the atmosphere,
which varies by 20 uatm to around 370 uatm in remote uncontaminated marine air (Feely and al.,
2001).
OCVR-System : an innovative tool to improve pCO2sw estimation
Oceanic Variables
Climatology
pC02 database
Atmosphere
state
Raw
data
Assimilation
module
Systematic
errors
Network
MLP-CLIM
OCVR-System
pCO2sw =
Particular Event + Trend + Seasonal
Figure 1: OCVR architecture used for a global ocean pCO2sw reanalysis from 1989 to 2009 at 2°
resolution.
Ocean pCO2 time series are one of the most valuable tools to observe trends of carbon fluxes. These
analyses are limited by the coverage of measurements (less than 5% at 2° and monthly resolution over
the last 20 years). The rapid development of satellite measurements which provide very large volumes
of data (weekly) and high resolution (less than 1°) is an alternative to this problem. However the
fluxes can only be obtained with indirect methods based either on numerical modelling, or from robust
algorithms using observable drivers. The system OCVR belongs to this latter family.
OCVR is a neural network framework developed by CLIMMOD within the CARBONES-EU FP7
project (see Fig. 1). As input variables, it uses observations from satellites (Surface Chlorophyll, Sea
Surface Temperature...), in-situ and model outputs (Temperature, Salinity, Mixed Layer Depth,…)
which control to first-order the surface ocean pCO2. A variational data assimilation scheme efficiently
incorporates new sets of pCO2 observations (trend and seasonal adjustments), and takes into account
extreme events like El Nino. The system then uses supplied atmospheric CO2 concentration to
calculate air-sea flux according to a selectable exchange parameterization (e.g. Wanninkhof 1992,
Nightingale 2000, Takahashi 2009). The results obtained with the OCVR-system are illustrated as
global maps (see Fig. 2) and ocean time series (see Fig. 3).
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Figure 2 : Global pCO2sw maps from OCVR. January simulations for 1990, 2000 and 2009.
2) Model policy
The model has been developed by ClimMod partner and it is not freely available to the public. Any
request on the model should be sent to:
[email protected]
[email protected]
3) References
Feely, R.A., C.L. Sabine, T. Takahashi, and R. Wanninkhof (2001) : Uptake and storage of carbon
dioxide in the oceans: The global CO2 survey. Oceanography, 14(4), 18–32.
Nightingale, P.D., Malin, G., Law, C.S., Watson, A.J., Liss, P.S., Liddicoat, M.I., Boutin, J., UpsillGoddard, R.C., 2000. In situ evaluation of air-sea gas exchange parameterizations using novel
conservative and volatile tracers. Glob. Biogeochem Cycles 14, 373–387.
Sabine, CL, RA Feely, N. Gruber, R. M. Key, K. Lee, J. L. Bullister, R. Wanninkhof, C. S. Wong,
D.W.R. Wallace, B. Tilbrook, FJ Millero, T.-H. Peng, A. Kozyr, T. Ono and AF Rios (2004): The
oceanic sink for anthropogenic CO2. Science, 305 (5682), 367-371.
Taro Takahashi, Stewart C. Sutherland, Rik Wanninkhof, Colm Sweeney, Richard A. Feely, David W.
Chipman, Burke Hales, Gernot Friederich, Francisco Chavez, Christopher Sabine, Andrew Watson,
Dorothee C.E. Bakker, Ute Schuster, Nicolas Metzl, Hisayuki Yoshikawa-Inoue, Masao Ishii, Takashi
Midorikawa, Yukihiro Nojiri, Arne Körtzinger, Tobias Steinhoff, et al. Corrigendum to
“Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the
global oceans” [Deep Sea Res. II 56 (2009) 554–577].
Wanninkhof, R., 1992. Relationship between wind speed and gas exchange. J. Geophys. Res. 97,
7373–7382.