Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Abstract

Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO 2 on landbased ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO 2 , orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO 2 (270-400 ppm). At low CO 2 , abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth's rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO 2 , AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a 'sweet spot' of low CO 2 , low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard-Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative 'glacial' state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO 2 . We suggest that this enhanced orbital sensitivity contributed to the developm...

The devil's in the disequilibrium: multi-component analysis of dissolved carbon and oxygen changes under a broad range of forcings in a general circulation model

“…A total of 44 simulations were analyzed with varying radiative forcing (RF), obliquity, precession, ice sheets (PI, pre-industrial; LGM, Last Glacial Maximum reconstruction; LGM*, topography of LGM ice sheets but with PI albedo), and with and without iron fertilization. Runs 1-40 are described by Galbraith and de Lavergne (2018 is approximately the inverse of this. The ventilation rate of the global ocean is roughly correlated with the AABW fraction, with rapid ventilation (small average ideal age) when the AABW fraction is high (Fig.…”

“…The global climate model (GCM) used in this study is CM2Mc, the Geophysical Fluid Dynamics Laboratory's Climate Model version 2 but at lower resolution (3 • ), described in more detail by Galbraith et al (2011) and modified as described by Galbraith and de Lavergne (2018). This includes the Modular Ocean Model version 5, a sea ice module, static land, and static ice sheets, and a module of Biogeochemistry with Light, Iron, Nutrients and Gases (BLINGv1.5; Galbraith et al, 2010).…”

“…The model runs analyzed here are part of the same suite of simulations discussed by Galbraith and de Lavergne (2018). A control run was conducted with a radiative forcing equivalent to 270 ppm atmospheric pCO 2 and the Earth's obliquity and precession set to modern values (23.4 and 102.9 • , respectively).…”

“…Differences in the general ocean state among the model simulations are described in detail by Galbraith and de Lavergne (2018). We provide here a few key points, important for interpreting the dissolved gas simulations.…”

“…We make use of a large number of equilibrium simulations, conducted over a wide range of radiative, orbital, and ice sheet boundary conditions, as a "library" of contrasting ocean circulations in order to test the response of disequilibrium carbon storage to physically plausible changes in ocean circulation. The basic physical aspects of these simulations were described by Galbraith and de Lavergne (2018). We supplement these with a smaller number of iron fertilization experiments to examine the additional impact of circulationindependent ecosystem changes.…”