A mechanistic account of increasing seasonal variations in the rate of ocean uptake of anthropogenic carbon

Abstract. A three-dimensional circulation model that includes a representation of anthropogenic carbon as a passive tracer is forced with climatological buoyancy and momentum fluxes. This simulation is then used to compute offline the anthropogenic ΔpCO₂ (defined as the difference between the atmospheric CO₂ and its seawater partial pressure) trends over three decades between the years 1970 and 2000. It is shown that the mean increasing trends in ΔpCO₂ reflects an increase of the seasonal amplitude of ΔpCO₂. In particular, the ocean uptake of anthropogenic CO₂ is decreasing (negative trends in ΔpCO₂) in boreal (austral) summer in the Northern (Southern) Hemisphere in the subtropical gyres between 20° N (S) and 40° N (S). In our simulation, the increased amplitude of the seasonal trends of the ΔpCO₂ is mainly explained by the seasonal sea surface temperature (SST) acting on the anthropogenic increase of the dissolved inorganic carbon (DIC). It is also shown that the seasonality of the anthropogenic DIC has very little effect on the decadal trends. Finally, an observing system for pCO₂ that is biased towards summer measurements may be underestimating uptake of anthropogenic CO₂ by about 0.6 PgC yr⁻¹ globally over the period of the WOCE survey in the mid-1990s according to our simulations. This bias associated with summer measurements should be expected to grow larger in time and underscores the need for surface CO₂ measurements that resolve the seasonal cycle throughout much of the extratropical oceans.