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

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 delta pCO(2) (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 delta pCO(2) reflects an increase of the seasonal amplitude of delta pCO(2). In particular, the ocean uptake of anthropogenic CO2 is decreasing (negative trends in delta pCO(2)) in boreal (austral) summer in the Northern (Southern) Hemisphere in the subtropical gyres between 20A degrees N (S) and 40A degrees N (S). In our simulation, the increased amplitude of the seasonal trends of the delta pCO(2) 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 pCO(2) 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.