Does atmospheric CO2 seasonality play an important role in governing the air-sea flux of CO2?

The amplitude, phase, and form of the seasonal cycle of atmospheric CO₂ concentrations varies on many time and space scales (Peters et al., 2007). Intra-annual CO₂ variation is primarily driven by seasonal uptake and release of CO₂ by the terrestrial biosphere (Machta et al., 1977; Buchwitz et al., 2007), with a small (Cadule et al., 2010; Heimann et al., 1998), but potentially changing (Gorgues et al., 2010) contribution from the ocean. Variability in the magnitude, spatial distribution, and seasonal drivers of terrestrial net pri- mary productivity (NPP) will be induced by, amongst other factors, anthropogenic CO₂ release (Keeling et al., 1996), land-use change (Zimov et al., 1999) and planetary orbital atm variability, and will lead to changes in CO^atm₂ seasonality. Despite CO^atm₂ seasonality being a dynamic and prominent feature of the Earth System, its potential to drive changes in the air-sea flux of CO₂ has not previously (to the best of my knowledge) been explored. It is important that we investigate the impact of CO^atm₂ seasonality change, and the potential for carbon-cycle feedbacks to operate through the modification of the CO^atm₂ seasonal cycle, because the decision had been made to prescribe CO^atm₂ concentrations (rather than emissions) within model simulations for the fifth IPCC climate assessment (Taylor et al., 2009). In this study I undertake ocean-model simulations within which different magnitude CO^atm₂ seasonal cycles are prescribed. These simulations allow me to examine the effect of a change in CO^atm₂ seasonal cycle magnitude on the air-sea CO₂ flux. I then use an offline model to isolate the drivers of the identified air-sea CO₂ flux change, and propose mechanisms by which this change may come about. Three mechanisms are identified by which co-variability of the seasonal cycles in atmospheric CO₂ concentration, and seasonality in sea-ice extent, wind-speed and ocean temperature, could potentially lead to changes in the air-sea flux of CO₂ at mid-to-high latitudes. The sea-ice driven mechanism responds to an increase in CO^atm₂ seasonality by pumping CO₂ into the ocean, the wind-speed and solubility-driven mechanisms, by releasing CO₂ from the ocean (in a relative sense). The relative importance of the mechanisms will be determined by, amongst other variables, the seasonal extent of sea-ice. To capture the described feedbacks within earth system models, CO^atm₂ concentrations must be allowed to evolve freely, forced only by anthropogenic emissions rather than prescribed CO^atm₂ concentrations; however, time-integrated ocean simulations imply that the cumulative net air-sea flux could be at most equivalent to a few ppm CO^atm₂. The findings presented here suggest that, at least under pre-industrial conditions, the prescription of CO^atm₂concentrations rather than emissions within simulations will have little impact on the marine anthropogenic CO₂ sink. [ABSTRACT FROM AUTHOR]