Role of CO<sub>2</sub> and Southern Ocean winds in glacial abrupt climate change

Abstract

Abstract. The study of Greenland ice cores revealed two decades ago the abrupt character of glacial millennial-scale climate variability. Several triggering mechanisms have been proposed and confronted against growing proxy-data evidence. Although the implication of North Atlantic deep water (NADW) formation reorganisations in glacial abrupt climate change seems robust nowadays, the final cause of these reorganisations remains unclear. Here, the role of CO 2 and Southern Ocean winds is investigated using a coupled model of intermediate complexity in an experimental setup designed such that the climate system resides close to a threshold found in previous studies. An initial abrupt surface air temperature (SAT) increase over the North Atlantic by 4 K in less than a decade, followed by a more gradual warming greater than 10 K on centennial timescales, is simulated in response to increasing atmospheric CO 2 levels and/or enhancing southern westerlies. The simulated peak warming shows a similar pattern and amplitude over Greenland as registered in ice core records of Dansgaard-Oeschger (D/O) events. This is accompanied by a strong Atlantic meridional overturning circulation (AMOC) intensification. The AMOC strengthening is found to be caused by a northward shift of NADW formation sites into the Nordic Seas as a result of a northward retreat of the sea-ice front in response to higher temperatures. This leads to enhanced heat loss to the atmosphere as well as reduced freshwater fluxes via reduced seaice import into the region. In this way, a new mechanism that is consistent with proxy data is identified by which abrupt climate change can be promoted.

Southern westerlies in LGM and future (RCP4.5) climates

“…In particular, Toggweiler et al (2006) and Banderas et al (2012) suggest that an equatorward position of the southern jet stream prevents CO 2 stored in the deep ocean from reaching the atmosphere at the LGM, but that as soon as the warming starts and the jet stream shifts poleward, degassing of this CO 2 to the atmosphere can start, therefore providing a positive feedback to the initial warming. Many other factors, in particular involving marine biology and increased dust fluxes (e.g.…”

On the Abruptness of Bølling–Allerød Warming

“…(iii) TCC is unlikely to be the only mechanism responsible for the whole BA warming. It is necessary to investigate the potential coupling effects between TCC and other important AMOC-related feedback mechanisms including ice sheets (e.g., Zhu et al 2014), sea ice (e.g., the ''sea ice switch'' mechanism; see Gildor et al 2014;Gildor and Tziperman 2003;Ashkenazy et al 2013), atmospheric circulation (e.g., Banderas et al 2012), the greenhouse effect (e.g., Zhang et al 2014), and salt feedback (e.g., Knorr and Lohmann 2007). (iv) Our two-dimensional simulation does not resolve baroclinic instability, which may trigger TCC (Killworth 1979).…”

“…This dramatic surface warming in North Atlantic exposes a huge basin-scale heat reservoir to the atmosphere and thus may directly contribute to the abrupt BA warming. These TCC events may further contribute to the BA warming by strengthening the AMOC, which causes more northward heat transport by decadal time scales (e.g., Banderas et al 2012;Hogg et al 2013;Buizert et al 2014).…”

A new approach for simulating the paleo-evolution of the Northern Hemisphere ice sheets

“…Panels (g-i) show the same fields as in (d-f) for the millennial component of the spatial forcing generated from the combination of the Is and the St climatic states simulated by CLIMBER-3α (Banderas et al, 2012. All variables have been corrected by elevation assuming a linear vertical atmospheric profile (see Sect.…”