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 CO2 on land-based 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 CO2, 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 CO2 (270–400 ppm). At low CO2, 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 CO2, 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 CO2, 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 CO2. We suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing, following a gradual downward trend of CO2.
Highlights
Since the first quantitative reconstructions of ocean temperatures were made (Emiliani 1955), investigators have pondered how changes in climate state may have altered deep ocean circulation during the ice age cycles of the past1 3 Vol.:(0123456789)E
The simulations use the coupled ocean–atmosphere–icebiogeochemistry model CM2Mc.v2. This includes: a finite volume atmospheric model, similar to that used in the GFDL ESM2 models (Dunne et al 2012); MOM5, a non-Boussinesq ocean model with a fully-nonlinear equation of state, parameterizations for mesoscale (Griffies 1998) and submesoscale (Fox-Kemper et al 2008) turbulence, and parameterizations for vertical mixing due to boundary layer turbulence (Large et al 1994) and to the breaking of internal tides above rough topography (Simmons et al 2004); a sea-ice module; a static land module; and a coupler to exchange fluxes between the components. It differs from the model described by Galbraith et al (2011) due to the following small changes: the codebase was updated to the Geophysical Fluid Dynamics Laboratory Siena version; the albedo of snow-covered sea ice was increased to a more realistic value of 0.85 from 0.80; the bottom drag was increased from 0.001 to 0.002; the background vertical mixing was reduced from 0.1 to 0.05 cm2 s−1; additional cross-land mixing in Indonesia was included to account for unresolved flows between islands
Our results show that trends in volume contributions do not reflect a see-saw between northern and southern overturning, since the Atlantic Meridional Overturning Circulation (AMOC) does not weaken at high C O2 despite the shrinking North Atlantic Deep Water (NADW) fraction, and since the AMOC and NADW fraction respond differently to the inclusion of a large Laurentide ice sheet
Summary
Since the first quantitative reconstructions of ocean temperatures were made (Emiliani 1955), investigators have pondered how changes in climate state may have altered deep ocean circulation during the ice age cycles of the past1 3 Vol.:(0123456789)E. Numerous mechanisms have been championed as key drivers behind glacial-interglacial changes in ocean state, including changes in southern westerly winds (Toggweiler et al 2006; Anderson et al 2009), sea ice dynamics (Stephens and Keeling 2000; Schmittner 2003; Shin et al 2003; Bouttes et al 2010; Ferrari et al 2014), ice sheet topography (Pausata et al 2011; Gong et al 2015), tidal mixing (Wunsch 2003; Schmittner et al 2015), and the nonlinear dependence of density on temperature (Winton 1997; Sigman et al 2004; De Boer et al 2007) Such physical oceanographic changes may have contributed to changes in atmospheric C O2 concentrations—hereafter referred to as “CO2”—measured in ice core records. The close correlation of atmospheric C O2 with ice core records of Antarctic temperature over the past 800,000 years, combined with proxy evidence of accompanying changes in the Southern Ocean, has inspired many authors to link changes in Southern Ocean circulation to oceanic CO2 sequestration (Paillard and Parrenin 2004; Fischer et al 2010; Sigman et al 2010; Skinner et al 2010; Jaccard et al 2013; Ferrari et al 2014; Watson et al 2015)
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