Transient CO2 experiment using the ARPEGE/OPAICE non flux corrected coupled model

A transient CO2 experiment using the ARPEGE/ OPAICE coupled general circulation model with no flux correction has been performed at CERFACS. Despite a warm initial climate drift, the main features of the observed climate are reasonably captured in the control simulation. In particular, the simulated oceanic circulation is satisfactory. The transient CO2 simulation shows a global warming of 1.6 degrees C in surface air temperature at the time of CO2 doubling. This value and the main geographical patterns of climate change are in agreement with previous studies using either flux corrected or non flux corrected models. The absence of flux correction does not prevent the too strong atmospheric water fluxes to produce an unrealistic freshening of the upper layer of the Northern North Atlantic which leads to a reduction of the thermohaline circulation in the control simulation. As a consequence, the weakening of the thermohaline circulation in response to the anthropogenic radiative forcing is less than expected.