A model intercomparison of the tropical response to doubling CO2 in aquaplanet simulations

Abstract

The present-day Earth’s climate has warmer Northern Hemisphere (NH). This hemispheric asymmetry is expected to be amplified in response to increasing CO2. It is of question whether the tropical precipitation consistently shifts toward the even warmer NH. We employ four different climate models (AM2, AM3, HiRAM, and CAM5) that are coupled to an aquaplanet slab ocean. In simulations, a northward ocean heat transport is prescribed to mimic the present-day climate state of a warmer NH. This reference state is then perturbed by a doubling of CO2 to explore the response of tropical precipitation to a uniform radiative forcing. Even though the forcing is uniform in space, the hemispherically asymmetric response of cloud radiative forcing results in the cross-equatorial atmospheric energy transport change, which induce the tropical precipitation shift. Yet, the sign of cross-equatorial atmospheric energy transport is not robust across models, causing a large spread in the response of tropical precipitation. Furthermore, even in the case of little changes in the atmospheric energy transport, there is a significant shift in the tropical precipitation. It is shown that the total gross moist stability (δ∆m) changes, which has been often neglected in previous studies, are critical for understanding the response of tropical precipitation to uniform CO2 forcing. Large uncertainties in δ∆m, due to the dependence of the vertical structure of moist static energy on convection schemes and cloud modeling, calls for their improvement to better project tropical precipitation in the future.