Northern ocean-atmosphere carbon exchange study

Model output from each of the NOCES modelling groups, as well as contributions from non-European modelling groups, was provided in a standard predefined format based on netCDF and GDT. That standard output was consolidated into one standard hierarchy of NOCES/OCMIP-3 model output files, separated by group and simulation, and archived at IPSL. That archive has been made available to NOCES participants that have requested access, and it will be made publicly available in 2006, following further quality control and analysis. It is probable that OCMIP members and other modellers will continue to exploit this rich NOCES/OCMIP-3 model output archive for many years. The coordinating group LSCE will also promote and facilitate its use by others, namely via a new automated model output diagnostic facility that is under development. The NOCES model output archive provides as an essential benchmark for community, and is another important legacy of NOCES.

A protocol has been developed for use by all ocean modellers involved in this project to make standard simulations of inter-annual to decadal variability. This protocol stipulates i) forcing with NCEP reanalysed atmospheric data (1948-2001), ii) model code, and iii) output routines. The protocol document has been published on the NOCES project web site (http://www.ipsl.jussieu.fr/projets/NOCES). This protocol is also likely to be adopted by those taking on similar ocean simulations in the future. By doing so, modellers will be able to compare their new results to those obtained in this international comparison.

Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms-such as corals and some plankton-will have difficulty maintaining their external calcium carbonate skeletons. For this result we used 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a -business-as-usual- scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become under-saturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this under-saturation could extend throughout the entire Southern Ocean and into the sub-arctic Pacific Ocean. When live pteropods were exposed to our predicted level of under-saturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously. This work is described in an article published in "Nature" in 2005: Orr et al. [2005] Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, Nature, 437, doi:10.1038/nature04095, 681-686.

NOCES has been the first to make simulations in ocean carbon-cycle models 'forced' by atmospheric reanalysis data (heat and water fluxes, winds and sea ice) over a long enough period (1948-2001) to begin to start studying variability with periods of significantly more than 10 years (decadal) [Rodgers et al. 2004; Wetzel et al., 2005; Raynaud et al., 2005]. Prior to NOCES, such "forced" simulations have been made in only two different ocean carbon cycle models, and those were made for at most twenty years (1979-1999). With the longer simulations proposed here, NOCES has been able to study the effect of shifts in climate on the carbon cycle, in particular that which occurred in the mid- to late-1970s, which was the largest decadal shift in the last 50 years [Rodgers et al., 2004]. NOCES has used advanced statistical methods to distinguish decadal modes of variability from inter-annual modes and noise [Raynaud et al, 2005].

Serendipitous discovery of a large pre-industrial, north-to-south inter-hemispheric ocean transport (~0.8 Pg C yr-1) due to iron limited primary productivity in the Southern Ocean. This finding resolves a long-standing enigma as to why all other ocean models (none of which featured iron limitation) could not simulate such a large inter-hemispheiric ocean transport, as theorized based on supposed pre-industrial atmospheric CO2 gradients. For instance, this was studied and reported on extensively in the EU GOSAC project (part of OCMIP-2). These new NOCES results are described in detail in the NOCES-funded Ph.D. thesis of Patrick Wetzel (MPIM, Partner 6): Wetzel [2004] Inter-annual and decadal variability in the air-sea exchange of CO2 a model study, Ph.D. thesis, University of Hamburg, MPIM, Germany.

Prior to the NOCES project there existed a large conflict between estimates for interannual variability for air-sea CO2 fluxes in the North Atlantic. Inverse atmospheric models predicted very large variability; ocean models predicted small variability. Moreover, a databased study agreed with the large variability predicted by atmospheric models. That data based study used data from 1 station in the subtropical gyre of the North Atlantic, considering it to be representative, and extrapolated across the entire basin. Analysis of results from the NOCES simulations revealed that the subtropical gyre is not representative of the entire North Atlantic. The subpolar gyre and the inter-gyre region (the transition area between subpolar and subtropical gyres) also contribute with multipolar anomalies at multiple frequencies: these tend to cancel one another in terms of the basin-wide air-sea CO2 flux. That is, high and low anomalies extend throughout the North Atlantic and partially cancel one another, thereby damping total basin-wide air-sea CO2 flux. Thus it is not reasonable to assume that variability in the subtropical gyre, namely at BATS, is representative of that in the intergyre and subpolar gyre, neither at interannual nor decadal timescales. The former atmospheric inverse model results have also been proved wrong by (1) comparison to observed seasonal variability, which is much lower and (2) a new state-of-the-art atmospheric inverse model that has much higher resolution and likewise simulates much lower varibility. In conclusion, the high air-sea CO2 flux variability over the North Atlantic predicted by the older inverse approaches are clearly wrong due to inadequate spatial resolution and thus "leakage" of high variability from adjacent terrestrial grid cells.

Partners UPMC and LSCE of the NOCES project used an ocean general circulation model was used to test if sea surface C-14 behaves as a thermocline proxy in the eastern equatorial Pacific. For this study, these NOCES partners included C-14 as a passive tracer in the OPA-ORCA2 circulation model, which they forced with reanalysis fluxes during 1948 to 1999. Model results were compared to a previously reported coral record of Delta C-14 collected in the Galapagos Islands. The model reproduces the abrupt increase in the seasonally minima of Delta C-14 that was obsered in 1976-1977. In contrast to previous proposals that have attempted to explain this shift, these NOCES results indicate that increase is associated with neither a shift of thermocline depth over the NINO3 region, nor a change in the relative proportion of Northern vs. Southern source waters. Rather, it is due to a decrease in the Sub-Antarctic Mode Water (SAMW) component of the up-welling water via a decrease in entrainment of water from below the base of the directly ventilated thermocline.

To investigate potential temporal lags, LSCE first performed a basic lag-correlation analysis between the North Atlantic Oscillation (NAO), the major driver of climate variability in the Atlantic, and the air-sea CO2 flux. That analysis revealed the complex spatiotemporal NAO structure that cannot be resolved with traditional methods, such as EOF analysis. Thus LSCE used a method known as Multi-channel Singular Spectrum Analysis (MSSA) to analyse interannual-to-decadal variability of climate and biogeochemical variables. MSSA accounts, simultaneously, for temporal as well as spatial variability among a suite of variables. Delays in the air-sea CO2 flux relative to climate forcing were identified, and had maximum correlation when the flux lags the forcing by 1 and 3 years. The 1-year lag may be due to the 1-year equilibration time required for perturbations in atmospheric CO2 to mix throughout the mixed layer [Broecker and Peng, 1974]. Lags of up to 3 years have been documented previously as being linked to interior ocean dynamics [Hakkinen, 1999; Gulev et al., 2003]. One cause may be export production linked with circulation-driven control factors. Evidence already exists that lateral advection partially controls air-sea CO2 fluxes in both the subtropical and sub-polar gyres based on a Lagrangian model of mixed layer DIC content [Follows and Williams, 2004]. Lateral advection from higher latitude mode waters retains a signature of thermo-cline nutrient anomalies that is delivered to the subtropics [Palter et al., 2005]. For sub-polar air-sea CO2 fluxes, lags may derive from advection within spatially heterogeneous tracer fields that stem in part from competition between entrainment and net carbon export production. More quantitative, process-oriented studies are needed to better resolve the mechanisms responsible for the lags in the air-sea CO2 flux relative to the climate forcing.

Global ocean carbon models and available syntheses of the oceanic CO2 flux suggest that the North Atlantic sub-polar gyre (50°N-70°N, 80°W-10°W) is a region of increasing uptake of CO2 from the atmosphere, with the oceanic partial pressure of CO2 (pCO2) increasing more slowly than the atmospheric CO2 over time. The UEA analysis of available CO2 data shows that, on the contrary, seawater pCO2 has increased faster than the atmosphere in recent decades, especially in summer, resulting in a decrease in uptake from the atmosphere. A decrease in the biological productivity of the region may be the underlying cause of this trend. From the observed trend UEA estimated a significant decrease in the annual carbon uptake in this region.

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