Impact of Southern Ocean surface conditions on deep ocean circulation during the LGM: a model analysis

Abstract

Abstract. Changes in water mass distribution are considered to be a significant contributor to the atmospheric CO2 concentration drop to around 186 ppm recorded during the Last Glacial Maximum (LGM). Yet simulating a glacial Atlantic Meridional Overturning Circulation (AMOC) in agreement with paleotracer data remains a challenge, with most models from previous Paleoclimate Modelling Intercomparison Project (PMIP) phases showing a tendency to simulate a strong and deep North Atlantic Deep Water (NADW) instead of the shoaling inferred from proxy records of water mass distribution. Conversely, the simulated Antarctic Bottom Water (AABW) is often reduced compared to its pre-industrial volume, and the Atlantic Ocean stratification is underestimated with respect to paleoproxy data. Inadequate representation of surface conditions, driving deep convection around Antarctica, may explain inaccurately simulated bottom water properties in the Southern Ocean. We investigate here the impact of a range of surface conditions in the Southern Ocean in the iLOVECLIM model using nine simulations obtained with different LGM boundary conditions associated with the ice sheet reconstruction (e.g., changes of elevation, bathymetry, and land–sea mask) and/or modeling choices related to sea-ice export, formation of salty brines, and freshwater input. Based on model–data comparison of sea-surface temperatures and sea ice, we find that only simulations with a cold Southern Ocean and a quite extensive sea-ice cover show an improved agreement with proxy records of sea ice, despite systematic model biases in the seasonal and regional patterns. We then show that the only simulation which does not display a much deeper NADW is obtained by parameterizing the sinking of brines along Antarctica, a modeling choice reducing the open-ocean convection in the Southern Ocean. These results highlight the importance of the representation of convection processes, which have a large impact on the water mass properties, while the choice of boundary conditions appears secondary for the model resolution and variables considered in this study.

Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice-sheet–climate coupled model

“…These so-called meltwater pulses suggest largescale ice sheet instabilities, but the contribution of the different ice sheets to these events remains debated (e.g. Liu et al, 2016).…”

Antarctic sea ice over the past 130,000 years, Part 1: A review of what proxy records tell us

“…LGM mean WSI surface of ~33 x 10 6 km 2 when an equidistant projection system and a LGM Antarctic ice cap of 17 x 10 6 km 2 are used (Lhardy et al, 2021). This value for WSI extent is slightly lower than previously published (~39 x 10 6 km 2 ), using a polar stereographic projection system and a modern Antarctic ice cap for the LGM (Gersonde et al, 2005;Roche et al, 2012).…”

“…However, it is long known that CLIMAP reconstructions over-estimated glacial SSI extent (Burckle et al, 1982). Based on the few control points and the modern relationship between sea ice and SST, the current understanding is that the SSI extent was 2-3 times greater (i.e., 8-12 x 10 6 km 2 ) during the LGM when compared to today (Gersonde et al, 2005;Lhardy et al, 2021;Green et al, 2022). An important implication of this change is that the seasonal cycle of sea-ice expansion and melt was substantially greater during the LGM as compared to today with potential implications for SO and global circulation through the export of brines to the abyssal waters (Shin et al, 2003;Bouttes et al, 2010;Lhardy et al, 2021;Green et al, 2022).…”

“…Gray field represents the modern Polar front Zone (Orsi et al, 1995). For the LGM map, the thick dark blue line indicates regions where marine data suggest the presence of SSI while the thin dark blue line indicates regions where SSI is inferred to be south of core sites where SSI was not identified and applying the modern relationship between SSI and SST to LGM SSTs (Lhardy et al, 2021). Location of marine sediment core PS1768-8 shown as a blue dot and EDML ice core shown as a white circle on the modern map.…”

Sea ice changes in the southwest Pacific sector of the Southern Ocean during the last 140 000 years

“…Past Antarctic sea ice coverage has been estimated primarily through diatom-based reconstructions, with most work focusing on the Last Glacial Maximum (LGM), specifically the EPILOG time slice as outlined in Mix et al (2001), corresponding to 23 to 19 thousand years before present (ka, calibrated backwards from 1950). During the LGM, these reconstructions suggest that winter sea ice expanded by 7-10 • latitude (depending on the sector of the Southern Ocean), which corresponds to substantial expansion of total winter sea ice coverage compared to modern observations (Gersonde et al, 2005;Benz et al, 2016;Lhardy et al, 2021). Currently, only a handful of studies provide quantitative sea ice coverage estimates back to the penultimate glaciation, Marine Isotope Stage (MIS) 6 (∼ 194 to 135 ka) (Gersonde and Zielinksi, 2000;Crosta et al, 2004;Schneider-Mor et al, 2012;Esper and Gersonde 2014a;Ghadi et al, 2020).…”