Figure 1. Posterior long-term mean ocean fluxes from CarbonTracker. The pattern of air-sea exchange of CO2 averaged over the time period indicated, as estimated by CarbonTracker. Negative fluxes (blue colors) represent CO2 uptake by the ocean, whereas positive fluxes (red colors) indicate regions in which the ocean is a net source of CO2 to the atmosphere. Units are gC m-2 yr-1.

Starting with the CT2011 release, two models are used to provide prior estimates of air-sea CO2 flux. The OIF scheme provides one of these flux priors, and the other is an updated version of the Takahashi et al. pCO2 climatology.

2.   Detailed Description
Oceanic uptake of CO2 in CarbonTracker is computed using air-sea differences in partial pressure of CO2 inferred either from ocean inversions (called "OIF" henceforth), or from a compilation of direct measurements of seawater pCO2 (called "pCO2-clim" henceforth). These air-sea partial pressure differences are combined with a gas transfer velocity computed from wind speeds in the atmospheric transport model to compute fluxes of carbon dioxide across the sea surface.

In either method, the first-guess fluxes have no interannual variability (IAV) other than a smooth trend. IAV in oceanic CO2 flux is due to anomalies in surface pCO2, such as those that occur in the tropical eastern Pacific during an El Niño, and to associated variability in winds, ocean circulation, and sea-surface properties. In CarbonTracker, only the surface winds (and hence gas transfer), manifest these interannual anomalies; the remaining IAV of flux must be inferred from atmospheric CO2 signals.

In the following sections we describe the two ocean flux prior models. We then describe the air-sea gas transfer velocity parameterization and discuss detais of the inversion methodology specific to oceanic exchange of CO2. Figures comparing the air-sea flux priors are presented in Box 1 below.

2.1   OIF: the Ocean Inversion Fluxes prior
For the OIF prior, long-term mean air-sea fluxes and the uncertainties associated with them are derived from the ocean interior inversions reported in Jacobson et al. [2007]. These ocean inversion flux estimates are composed of separate preindustrial (natural) and anthropogenic flux inversions based on the methods described in Gloor et al. [2003] and biogeochemical interpretations of Gruber, Sarmiento, and Stocker [1996]. The uptake of anthropogenic CO2 by the ocean is assumed to increase in proportion to atmospheric CO2 levels, consistent with estimates from ocean carbon models.

OIF contemporary pCO2 fields were computed by summing the preindustrial and anthropogenic flux components from inversions using five different configurations of the Princeton/GFDL MOM3 ocean general circulation model [Pacanowski and Gnanadesikan, 1998], then dividing by a gas transfer velocity computed from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA40 reanalysis. There are two small differences in first-guess fluxes in this computation from those reported in Jacobson et al. [2007]. First, the five OIF estimates all used Takahashi et al. [2002] pCO2 estimates to provide high-resolution patterning of flux within inversion regions (the alternative "forward" model patterns were not used). To good approximation, this choice only affects the spatial and temporal distribution of flux within each of the 30 ocean inversion regions, not the magnitude of the estimated flux. Second, wind speed differences between the ERA40 product used in the offline analysis and the ECMWF operational model used in the online CarbonTracker analysis result in small deviations from the OIF estimates.

Other than the smooth trend in anthropogenic flux assumed by the OIF results, interannual variability (IAV) in the first guess ocean flux comes entirely from wind speed effects on the gas transfer velocity. This is because the ocean inversions retrieve only a long-term mean and smooth trend.

2.2  pCO2-Clim: Takahashi et al. [2009] climatology prior
The pCO2-Clim prior is derived from the Takahashi et al. [2009] climatology of seawater pCO2. This climatology was created from about 3 million direct observations of seawater pCO2 around the world between 1970 and 2007. With the exception of measurements in the Bering Sea, these observations were all linearly extrapolated to the corresponding month of the year 2000 by assuming a constant trend of 1.5 μatm yr-1. This set of global monthly measurements corrected to the reference year 2000 was then interpolated onto a regular grid using a modeled surface current field.

The Takahashi et al. [2009] product goes beyond providing this estimate of surface water pCO2. They also compute climatological air-sea exchange of CO2 by using the GLOBALVIEW-CO2 atmospheric carbon dioxide product to infer air-sea ΔpCO2, sea surface properties inferred from ocean climatologies, and winds from atmospheric reanalysis to estimate gas-transfer velocity. Unlike other atmospheric analyses, we have chosen not to use the climatological fluxes as our prior, nor to use the climatological ΔpCO2. Instead, we take only the seawater pCO2 distribution from the Takahashi et al. climatology--our atmospheric model provides both pCO2 in the air at the sea surface and the winds needed to estimate gas transfer. Seawater pCO2 is extrapolated from 2000 to the actual year of the CarbonTracker simulation using the presumed increase of 1.5 μatm yr-1 at every point in the global ocean.

2.3   Gas-transfer velocity and ocean surface properties
Both priors use CO2 solubilities and Schmidt numbers computed from World Ocean Atlas 2009 (WOA09) climatological fields of sea surface temperature [Locarnini et al., 2010] and sea surface salinity [Antonov et al., 2010] fields. Gas transfer velocity in CarbonTracker is parameterized as a quadratic function of wind speed following Wanninkhof [1992], using the formulation for instantaneous winds. Gas exchange is computed every 3 hours using wind speeds from the ECMWF operational model as represented by the TM5 atmospheric transport model.

Air-sea transfer is inhibited by the presence of sea ice, and for this work fluxes are scaled by the daily sea ice fraction in each gridbox provided by the ECMWF forecast data.

Box 1. Comparison of air-sea flux priors

Time series of global-total ocean flux among the two priors and the CT2011 posterior. Global CO2 uptake by the ocean, expressed in PgC yr-1. Positive flux represents a gain of CO2 to the atmosphere, and the negative numbers here indicate that the ocean is a sink of CO2. While both priors manifest similar trends of increasing oceanic uptake of CO2, the OIF prior (in green) has more oceanic uptake and a greater annual cycle than the pCO2-clim prior (in tan). The CT2011 across-model posterior estimate is shown in black for comparison.

Differences in long-term mean ocean fluxes between the two priors. Red indicates areas where the pCO2-clim prior has less oceanic uptake (or more outgassing to the atmosphere) than the OIF prior, and blue represents the opposite. Units are gC m-2 yr-1.