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Abstract. Here we present a newly developed ice core gas-phase proxy that directlysamples a component of the large-scale atmospheric circulation:synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). The 86Krxs may therefore reflect the time-averaged synoptic pressure variability over several years (site “storminess”), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate 86Krxs using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p<0.01) between site average 86Krxs and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of 86Krxs require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and 86Krxs at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño–Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remainspeculative. Current scientific understanding of firn air transport andtrapping is insufficient to explain all the observed variations in86Krxs. A list of suggested future studies is provided. 

Abstract. The penultimate deglaciation (PDG, ∼138–128 thousand years before present, hereafter ka) is the transition fromthe penultimate glacial maximum (PGM)to the Last Interglacial (LIG, ∼129–116 ka).The LIG stands out as one of the warmest interglacials of the last 800 000 years (hereafter kyr),with high-latitude temperature warmer than today and global sea level likely higher by at least 6 m.Considering the transient nature of the Earth system,the LIG climate and ice-sheet evolution were certainly influenced by the changesoccurring during the penultimate deglaciation.It is thus importantto investigate, with coupled atmosphere–ocean general circulation models (AOGCMs),the climate and environmental response to the large changesin boundary conditions(i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice-sheet geometry and associated meltwater fluxes) occurring during the penultimate deglaciation. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocolto perform transient simulations of the last deglaciation (19–11 ka; although the protocol covers 26–0 ka).Similar to the last deglaciation, the disintegration of continental ice sheets during the penultimate deglaciation led to significant changesin the oceanic circulation during Heinrich Stadial 11 (∼136–129 ka).However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the Past Global Changes (PAGES)-PMIP working group on Quaternary interglacials (QUIGS), we propose a protocol to perform transient simulations of the penultimate deglaciationunder the auspices of PMIP4.This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice sheets as well as freshwater input from the disintegration ofcontinental ice sheets.This experiment is designed for AOGCMs to assessthe coupled response of the climate system to all forcings.Additional sensitivity experiments are proposed to evaluate the response to each forcing.Finally, a selection of paleo-records representing different parts of the climate system is presented, providing an appropriatebenchmark for upcoming model–data comparisons across the penultimate deglaciation. 

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.