Using data assimilation to investigate the causes of Southern Hemisphere high latitude cooling from 10 to 8 ka BP

Abstract

Paleoclimate records show an atmospheric and oceanic cooling in the high latitudes of the Southern Hemisphere from 10 to 8 ka BP. In order to study the causes of this cooling, simulations covering the early Holocene period have been performed with the climate model of intermediate complexity LOVECLIM constrained to follow the signal recorded in climate proxies using a data assimilation method based on a particle filtering. The selected proxies represent oceanic and atmospheric surface temperature in the Southern Hemisphere derived from terrestrial, marine and glaciological records. Using our modeling framework, two mechanisms potentially explaining the 10–8 ka BP cooling pattern are investigated. The first hypothesis is a change in atmospheric circulation. The state obtained by data assimilation displays a modification of the meridional atmospheric circulation around Antarctica, producing a 0.6 °C drop in atmospheric temperatures over Antarctica from 10 to 8 ka BP without congruent cooling of the atmospheric and sea-surface temperature in the Southern Ocean. The second hypothesis is a cooling of the sea surface temperature in the Southern Ocean, simulated here as the response to a higher West Antarctic Ice Sheet melting rate. Using data assimilation, we constrain the fresh water flux to increase by 100 mSv from 10 to 8 ka BP. This perturbation leads to an oceanic cooling of 0.5 °C and a strengthening of Southern Hemisphere westerlies (+6%). However, the observed cooling in Antarctic and the Southern Ocean proxy records can only be reconciled with the combination of a modified atmospheric circulation and an enhanced freshwater flux

Sea Surface Temperatures in the Indian Sub‐Antarctic Southern Ocean for the Last Four Interglacial Periods

“…This freshwater forcing increased the buoyancy of SO surface waters and reduced Antarctic Bottom water (AABW) formation, which subsequently cooled the SO and warmed the northern Atlantic through the bipolar seesaw (Bakker et al., 2017; Holden et al., 2010). A similar mechanism was proposed to explain the cooling of SO surface waters from 10–8 ka (Mathiot et al., 2013) and supports the early Holocene cooling observed in core DCR‐1PC. However, another modeling study calls upon the resumption of upwelling of cold NADW in the SO due to the melting of Laurentide Ice Sheet, therefore counter‐acting the bipolar seesaw (Renssen et al., 2010).…”

Climate variability features of the last interglacial in the East Antarctic EPICA Dome C ice core

“…We also note that the maximum level of variance occurs at the beginning of the EDC plateau following MIS 5e early optimum, coinciding with the sea level high stand due to reduced ice volumes in Greenland ice sheet [ Dutton and Lambeck, 2012;NEEM-communitymembers, 2013] and strong presumptions of significant Antarctic ice volume loss [Bradley et al, 2012[Bradley et al, , 2013Holden et al, 2010;O'Leary et al, 2013]. Increased freshwater fluxes from both hemispheres are known to potentially influence AMOC [Galaasen et al, 2014;Oppo et al, 2006;Swingedouw et al, 2009], with implications for Antarctic sea ice extent and climate [Mathiot et al, 2013].…”

Sea ice diatom contributions to Holocene nutrient utilization in East Antarctica

“…The later decline in δ 30 Si diat is concomitant with a documented Antarctic cooling (up to 1°C) between circa 10,000 and 8000 cal years B.P. [Mathiot et al, 2013]. In the region of East Antarctica, this is reflected as sea ice expansion and glacial advances, as suggested by higher relative abundances of F. curta, and greater surface water stratification, as demonstrated by higher abundances of Chaetoceros resting spores (CRS) [Crosta et al, 2008;Denis et al, 2009a].…”