Evaluation of the GECCO2 ocean synthesis: transports of volume, heat and freshwater in the Atlantic

We present results from a new synthesis (GECCO2) which covers the years 1948 to 2011 employing a similar configuration of the Massachusetts Institute of Technology general circulation model as the previous 50-year (1952 to 2001) GECCO synthesis. In GECCO2, the resolution was increased; it now includes the Arctic Ocean and a dynamic/thermodynamic sea ice model. The synthesis uses the adjoint method to bring the model into consistency with available hydrographic and satellite data as well as prior estimates of surface fluxes. In comparison to GECCO, GECCO2 provides a better agreement with the assimilated data, however the estimated flux adjustments remain similar to GECCO. Global heat content changes are in agreement with recent observational estimates and the estimate of the global heat flux is close to a radiative forcing estimate. Both show a clear effect of the radiative forcing from volcanic eruptions and a weak relation to ENSO events. In contrast to GECCO, the importance of the Denmark Strait overflow for the variability of the Atlantic Meridional Overturning Circulation (AMOC) is replaced in GECCO2 by water mass transformation in the subpolar gyre, which is shown to be part of the thermohaline circulation if the overturning is defined as a function of density. Heat and freshwater transport estimates in the Atlantic are more consistent with previous estimates than the unconstrained run. Decomposing heat and freshwater transports into overturning and gyre components by averaging on density coordinates demonstrates that in these coordinates the contribution from the gyre circulation largely disappears for heat transport and is reduced for the freshwater transport.