Collapse of Deep-Sea Circulation during an Eocene Hyperthermal Hothouse – A DeepMIP Study with CESM1.2

Abstract. During the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), a rapid injection of greenhouse gases (with isotopically depleted carbon) into the atmosphere led to a ~5 °C global temperature rise, ocean acidification, and perturbation of marine and terrestrial ecosystems. In this study, we carried out a series of DeepMIP climate sensitivity experiments t using the Community Earth System Model CESM1.2 to evaluate how changes in the radiative forcing could have contributed to Eocene hyperthermal events. An atmospheric change from 3xCO₂ relative to pre-industrial levels (PAL) equivalent during the latest Paleocene to 6xCO₂ PAL in response to a carbon input pulse of 1680 PgC resulted in equatorial warming to 36.9 °C consistent with proxy estimates. The lower equator-to-pole temperature gradient in this 6xCO₂ PAL scenario as compared to the pre-industrial experiment with 1x CO₂ PAL is due to the lack of an ice sheet, the increase in greenhouse gases, and a lower cloud optical depth. The climate simulations suggest an intensified hydrological cycle with higher precipitation in the tropics, particularly over the Indian Eocene continent, and in mid-latitude. In contrast, mega-droughts are prominent in the subtropics, particularly in Africa and South America. Topographic effects such as the closure of the Drake Passage and the more southern location of Australia as well as a lower-than-present meridional temperature gradient contribute to a much weaker surface ocean circulation near the Antarctic continent, as compared to the current pronounced Antarctic Circumpolar Current. In response to the increase in greenhouse gas forcing to 6xCO₂ PAL, deep water formation in the Southern Ocean nearly collapsed and changed from a southern-dominated deep-sea ventilation to a weak deep water formation in the North Atlantic Ocean and further to a polar collapse of deep water formation and a shallow haline-mode ventilation in the subtropics at 12xCO₂ PAL. Bipolar convective overturning in the Pacific Ocean is not supported and remains uncertain, but southern component water mass formation in the Pacific Ocean has been simulated with 1x CO₂ PAL. Increased stratification and reduced solubility of dissolved oxygen caused by warming may have contributed to lower abyssal dissolved oxygen concentrations and thus stresses on the marine ecosystem. However, decreased upwelling and productivity may have decreased the apparent oxygen utilization and thus could have increased the oxygen concentration in the twilight zone.