Drake Passage Oceanic pCO2: Evaluating CMIP5 Coupled Carbon–Climate Models Using in situ Observations

Abstract

Surface water partial pressure of CO 2 (pCO 2 ) variations in Drake Passage are examined using decade-long underway shipboard measurements. North of the Polar Front (PF), the observed pCO 2 shows a seasonal cycle that peaks annually in August and dissolved inorganic carbon (DIC)-forced variations are significant. Just south of the PF, pCO 2 shows a small seasonal cycle that peaks annually in February, reflecting the opposing effects of changes in SST and DIC in the surface waters. At the PF, the wintertime pCO 2 is nearly in equilibrium with the atmosphere, leading to a small sea-to-air CO 2 flux.These observations are used to evaluate eight available Coupled Model Intercomparison Project, phase 5 (CMIP5), Earth system models (ESMs). Six ESMs reproduce the observed annual-mean pCO 2 values averaged over the Drake Passage region. However, the model amplitude of the pCO 2 seasonal cycle exceeds the observed amplitude of the pCO 2 seasonal cycle because of the model biases in SST and surface DIC. North of the PF, deep winter mixed layers play a larger role in pCO 2 variations in the models than they do in observations. Four ESMs show elevated wintertime pCO 2 near the PF, causing a significant sea-to-air CO 2 flux. Wintertime winds in these models are generally stronger than the satellite-derived winds. This not only magnifies the sea-to-air CO 2 flux but also upwells DIC-rich water to the surface and drives strong equatorward Ekman currents. These strong model currents likely advect the upwelled DIC farther equatorward, as strong stratification in the models precludes subduction below the mixed layer.

The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks

“…Despite overestimating the mean carbon sinks, both HadGEM2-ES and MPI-ESM-LR reproduce the seasonal phase of the data. The MPI-ESM-LR simulates carbon uptake in the early spring period that is too strongly biologically mediated, consistent with the anomalously strong late winter mixing (August-September) (Jiang et al, 2014;Sallée et al, 2013b), which causes the required nutrients to upwell to fuel biological production. The GFDL-ESM2G, one of the two models that simulate carbon sinks closest to the data-based estimate, simulates the opposite pCO 2 seasonal phase, which is largely attributed to the bias in absolute values and amplitude of SST seasonal cycle.…”

“…With respect to the annual mean CO 2 uptake in the Southern Ocean, Jiang et al (2014) evaluate a set of CMIP5 models but from different simulations, i.e., with prescribed atmospheric CO 2 concentrations, and over a slightly smaller domain (56-62 • S). Despite these differences, our present findings are very much comparable to the prior study, with the CanESM2 and GFDL-ESM2G models simulating net outgassing and close to neutral CO 2 fluxes.…”

“…This indicates the need to consider the seasonal cycle when evaluating carbon uptake projections in the SO. Jiang et al (2014) also examine the CMIP5 models' simulated pCO 2 seasonal cycle in the Drake Passage as compared to shipboard measurements. They show that the pCO 2 seasonal cycle in this confined domain is representative of the broader circumpolar region.…”

The O2/N2 Ratio and CO2 Airborne Southern Ocean Study

“…However, primary production in the Southern Ocean surface tends to be strongly iron limited; since vertical exchange is the dominant source of iron to the mixed layer (Tagliabue et al 2014), the greater stratification implied by shallower mixed-layer depths should be associated with diminished iron supply-which is not consistent with greater productivity. More work is required to understand these results; however, it is clear that O 2 and CO 2 provide a more powerful constraint together than either gas does alone, possibly enabling better evaluation of divergent Earth system model simulations (Anav et al 2013;Jiang et al 2014).…”

“…Despite its importance, the processes controlling air-sea gas exchange in the Southern Ocean are poorly represented by models. This was highlighted in a recent comparison of models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), wherein the simulated seasonal cycles of air-sea CO 2 exchange with the Southern Ocean were widely divergent and in poor agreement with observational estimates (Anav et al 2013;Jiang et al 2014), suggesting possible model biases in the timing, spatial A recent Southern Ocean airborne campaign collected continuous, discrete, and remote sensing measurements to investigate biogeochemical and physical processes driving air-sea exchange of CO 2 , O 2 , and reactive biogenic gases. …”