Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation

Abstract

During the two most recent deglaciations, the Southern Hemisphere warmed before Greenland. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge, which is generally assumed to reduce the formation of North Atlantic Deep Water. Yet during deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode. Here we use a three-dimensional ocean circulation model to investigate the impact of Southern Ocean warming and the associated sea-ice retreat on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of deglaciation, in agreement with the available proxy records.

The Global Carbon Cycle During the Last Glacial/Interglacial Transition

“…(Köhler et al, 2004, Smith et al 1999. Thus, initial processes during deglaciation in the Southern Ocean, followed by the 1500 year delayed kick-in of the thermohaline circulation (THC) in the North Atlantic (as revealed in the DEKLIM project CliTrans, Knorr and Lohmann, 2003) are consistent with atmospheric carbon records. In addition, the signifi cant infl uence of the terrestrial biosphere on changes in the isotopic composition of atmospheric pCO 2 during the second half of the termination is supported, and together with the contribution of carbonate compensation, fully explains the observed increase in pCO 2 .…”

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…The fact that both major ocean basins show this deglacial signal suggests that the driver for ocean reorganization lies in the Southern Ocean. This builds upon previous work that suggests the Southern Ocean as the driver behind deglaciation and the resumption of the Atlantic thermohaline circulation [Knorr and Lohmann, 2003]. Of specific interest to this study is the fact that the tropical foraminifer G. menardii was reseeded in the Atlantic Ocean during the last deglaciation [Schott, 1935].…”

“…Of specific interest to this study is the fact that the tropical foraminifer G. menardii was reseeded in the Atlantic Ocean during the last deglaciation [Schott, 1935]. The only mechanism to accomplish this is by transporting organisms around the Cape of Good Hope, which again suggests that changes in the Agulhas current and retroflection may have also played an important role in the deglacial ocean reorganization [Knorr and Lohmann, 2003]. …”

Investigating the Global Impacts of the Agulhas Current

“…First, the warm tropical waters carried by the current stimulate convection of the overlying atmosphere with direct consequences for regional weather systems [Reason and Jagadheesha, 2005]. Second, the shedding of Agulhas rings south of Africa causes buoyancy anomalies in the South Atlantic that stimulate dynamical responses with potential consequences for the Atlantic MOC [Biastoch et al, 2009;Knorr and Lohmann, 2003]. While the significance of these mechanisms is increasingly recognized, their dynamics and sensitivity are not well understood.…”

“…Further, embedding high-resolution Agulhas modules into global models (as seen in Figure 1) in conjunction with atmosphere-ocean simulations will allow for the assessment of impacts of the Agulhas regime on global oceanic and atmospheric circulation [Biastoch et al, 2009;Park and Latif, 2008]. This has important implications for climate studies, and Earth system models of intermediate complexity (EMICs) [Knorr and Lohmann, 2003] are valuable tools for studying the relationships between different climate components and for validating paleoceanographic data.…”