Influence of cloud-radiation effects on the location of the intertropical convergence zone
Final Report Abstract
The intertropical convergence zone (ITCZ) is a central element of Earth’s climate system. Its position marks the tropical band of of convection, ascending air motion and rainfall. Meridional shifts of the ITCZ are a prominent feature of both past and anticipated future climate changes, and trigger manifold other changes. ITCZ shifts do not only imprint on tropical water availability but also affect the frequency of tropical cyclones, the poleward edge of the Hadley circulation and the position of the mid-latitude jet. ITCZ shifts have also been proposed to be potent regulators of hemispheric albedo differences. This richness of phenomena provided the project’s motivation to study how cloud-radiative effects, whose changes under climate change are uncertain and strongly differ between current climate models, impact ITCZ shifts. The project employed the four comprehensive atmosphere general circulation models ECHAM6- TNT, ECHAM6-TTT, LMDz5A and LMDz5B. The models differ markedly in their simulation of clouds so that comparing ITCZ shifts across these four models offered an opportunity to separate robust from non-robust model behavior, to relate robust model behavior to physical mechanisms, and to connect non-robust model behavior to model differences in cloud-radiative effects. The project was be carried out during a 6-month visit to the Laboratoire de Météorologie Dynamique (LMD) in Paris, France in close collaboration with Dr. Sandrine Bony and Dr. Jean-Louis Dufresne (both from LMD) and Prof. Dr. Bjorn Stevens from the Max Planck Institute for Meteorology, Hamburg. The models were employed in idealized aquaplanet setup (no continents) to elucidate the basic responses of clouds and the large-scale atmospheric circulation. A wide range of ITCZ shifts was triggered by hemispherically-asymmetric surface albedo perturbations of different magnitudes. Furthermore, simulations with active cloud-radiative feedback were compared to simulations in which the cloud-radiative feedback was disabled. The project analyzed this set of simulations from two complementary perspectives. First, the firm link between ITCZ shifts and the inter-hemispheric energy transport of the atmosphere was used to study how individual components of the top-of-atmosphere energy budget contribute to the transport and thus ITCZ shifts. The simulations showed that tropical asymmetries from clouds dominate the transport, while extratropical shortwave and longwave asymmetries cancel each other nearly perfectly due to a local thermodynamical compensation between shortwave and longwave irradiances. The results do not only emphasize the importance of tropical cloud changes for ITCZ shifts but also suggests that shortwave-triggered ITCZ shifts are more robustly modelled than longwave-triggered ITCZ shifts, because model differences in extratropical cloud changes imprint on longwave-triggered but not on shortwave-triggered ITCZ shifts. Second, the magnitude of the ITCZ shift in simulations with active and disabled cloud-radiative feedback was compared to further analyze the impact of the cloud-radiative feedback on the ITCZ shift. It was found that the the impact of the cloud-radiative feedback differs strongly in sign and magnitude between the four models, and that the cloud-radiative feedback is responsible for about half of the model spread in the ITCZ shift. Furthermore, much of the model differences in the ITCZ shift could be explained by model differences in the dependence of cloud-radiative properties on the state of the tropical large-scale circulation. This suggests that tuning this dependence to observations offers a way forward to substantially reduce the uncertainty in model estimates of future ITCZ shifts.
