Insolation-Induced Differences in the Southern and Deep Oceans Between the Interglacials before and after the Mid-Brunhes Transition

Abstract:

The interglacials after 430 ka BP are characterized by warmer climates and higher atmospheric CO2 concentrations than the interglacials which occurred before. The cause of this climatic transition (the so-called Mid-Brunhes Event, MBE) is not yet fully understood. Based on climate model simulations, our results show that, in response to insolation changes only, feedbacks between sea ice, temperature, evaporation and salinity caused vigorous pre-MBE Antarctic Bottom Water (AABW) formation and Southern Ocean ventilation (Yin, 2013, Insolation-induced mid-Brunhes transition in Southern Ocean ventilation and deep-ocean temperature. Nature, 494, 222-225). Our results also show that strong Westerlies increased the pre-MBE overturning in the Southern Ocean via an increased latitudinal insolation gradient created by changes in eccentricity during austral winter and in obliquity during austral summer. The enhanced AABW formation leads to a cooler deep ocean during the older interglacials. These insolation-induced differences in the deep-sea temperature and in the Southern Ocean ventilation between the more recent interglacials and the older ones were not expected, because there is no visible systematic difference in the astronomical parameters between the interglacials before and after 430 ka ago. The apparent MBE (i.e. the difference in the interglacial intensity before and after 430 ka BP) appears resulting from the complex response of the climate system to the different combinations of the astronomical parameters prevailing before and after 430 ka BP. Preliminary results from a carbon cycle model shed light on the possible role of the Southern Ocean in the magnitude change of the interglacial atmospheric CO₂ concentration around 430 ka.