Showing papers by "Anais Orsi published in 2023"

A 2000-year temperature reconstruction on the East Antarctic plateau from argon–nitrogen and water stable isotopes in the Aurora Basin North ice core

Abstract: . The temperature of the Earth is

Phase 4 of PAGES 2k: Hydroclimate of the Common Era

Abstract: The climate of the past two thousand years (2k) provides context for current and future changes, and as such is vital for developing our understanding of the modern climate system. Building on previous phases of the PAGES 2k network, Phase 4 of the PAGES 2k Network paves the way for a new level of understanding of the global water cycle, including enhanced science-policy integration. Previous PAGES 2k network phases emphasised temperature reconstructions, fundamentally improving our understanding of global climate changes over the Common Era. These reconstructions received widespread recognition and were featured in the Summary for Policymakers of the IPCC’s Sixth Assessment Report. Integration of this data with state-of-the-art Earth systems models, proxy system models and data assimilation yielded a more comprehensive understanding of the associated physical drivers and climate dynamics.  Phase 4 challenges our community to turn its focus towards hydroclimate. Our aim is to reconstruct hydroclimatic variability over the Common Era, from local to global spatial scales, at sub-annual to multi-centennial time scales, developing a process-level understanding of past hydroclimate events and variability. Our multi-faceted approach includes (1) developing new hydroclimate syntheses that are well-suited for data-model comparisons, (2) improving the interoperability and scope of existing data and model products, and (3) facilitating the translation of our science into evidence-based policy outcomes. In this presentation, we report on our activities and progress to date, particularly highlighting the early stages of our data synthesis efforts.

Warm Temperature Anomalies Associated with Snowfall in Antarctica

Abstract: Antarctica, the coldest and driest continent, is home to the largest ice sheet. A common feature of polar regions is the warming associated with snowfall, as moist oceanic air and cloud cover contribute to increase the surface temperature. Consequently, the ice accumulated onto the ice sheet is deposited under unusually warm conditions. Here we use the polar-oriented atmospheric model MAR to study the statistical difference between average and snowfall-weighted temperatures. Most of Antarctica experiences a warming scaling with snowfall, although with strongest warming at sites with usually low accumulation. Heavier snowfalls in winter contribute to cool the snowfall-weighted temperature, but this effect is overwritten by the warming associated with atmospheric perturbations responsible for snowfall, which particularly contrast with the extremely cold conditions in winter. Disturbance in apparent annual temperature cycle and interannual variability may have major implications for water isotopes, which are deposited with snowfall and commonly used for paleo-temperature reconstructions.

The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to Intertropical Convergence Zone (ITCZ) movement through the last deglaciation

Abstract: Abstract. Here we present a newly developed ice core gas-phase proxy that directly samples 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 remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs. A list of suggested future studies is provided.