CLOUD FEEDBACKS MAINTAIN MERIDIONAL TEMPERATURE GRADIENT DURING EARLY CRETACEOUS GREENHOUSE CLIMATE

Paleoceanographic data indicate strongly reduced meridional temperature gradients and severe perturbations of the climate system and the global carbon cycle (Oceanic Anoxic Events, OAE) during the Early Cretaceous greenhouse climate. With a state-of-the-art climate model we quantify atmospheric processes that modify the merdional temperature gradient and therefore determine the large-scale atmospheric and oceanic circulation changes influencing or even causing OAEs. We employ the Kiel Climate Model, a coupled atmosphere-ocean-sea ice general circulation model, under Early Aptian (120 Myrs) boundary conditions. We perform equilibrium integrations with different atmospheric CO₂ levels and a pre-industrial reference run to distinguish between CO₂ induced changes and those solely caused by the modified continental configuration. Comparison of simulated upper ocean temperatures with a compilation of OAE 1a TEX₈₆ data shows highest agreement at pCO₂ levels of 1200 ppm. Corresponding steady state global mean surface air temperatures are elevated by 10 °C compared to a pre-industrial simulation. The surface warming is mainly radiatively driven by the higher atmospheric greenhouse gas levels (~70% of the warming) and surface albedo changes (~30% of the warming). Cloud feedbacks show little influence on global mean temperatures, but their spatial heterogeneity significantly changes tropospheric temperature gradients and the atmospheric circulation. Weaker tropical deep convection reduces low-latitude cloud cover and leads to a net cloud-induced warming in the tropics. The absence of polar ice caps prevents high-latitude atmospheric subsidence resulting in thicker, low-level clouds that reflect incoming shortwave radiation and consequently cool the surface. These cloud radiative feedbacks contribute to maintain mid-latitude meridional temperature gradients similar to today. Our simulations support recent studies showing a previous overestimation of polar warmth during past greenhouse climates.