Machine dependence and reproducibility for coupled climate simulations: the HadGEM3-GC3.1 CMIP Preindustrial simulation

When the same weather or climate simulation is run on different high-performance computing (HPC) platforms, model outputs may not be identical for a given initial condition. While the role of HPC platforms in delivering better climate projections is to some extent discussed in the literature, attention is mainly focused on scalability and performance rather than on the impact of machine-dependent processes on the numerical solution. Here we investigate the behaviour of the Preindustrial (PI) simulation prepared by the UK Met Office for the forthcoming CMIP6 (Coupled Model Intercomparison Project Phase 6) under different computing environments. Discrepancies between the means of key climate variables were analysed at different timescales, from decadal to centennial. We found that for the two simulations to be statistically indistinguishable, a 200-year averaging period must be used for the analysis of the results. Thus, constant-forcing climate simulations using the HadGEM3-GC3.1 model are reproducible on different HPC platforms provided that a sufficiently long duration of simulation is used. In regions where El Niño–Southern Oscillation (ENSO) teleconnection patterns were detected, we found large sea surface temperature and sea ice concentration differences on centennial timescales. This indicates that a 100-year constant-forcing climate simulation may not be long enough to adequately capture the internal variability of the HadGEM3-GC3.1 model, despite this being the minimum simulation length recommended by CMIP6 protocols for many MIP (Model Intercomparison Project) experiments. On the basis of our findings, we recommend a minimum simulation length of 200 years whenever possible

Sea ice led to poleward-shifted winds at the Last Glacial Maximum: the influence of state dependency on CMIP5 and PMIP3 models

Latitudinal shifts in the Southern Ocean westerly wind jet could drive changes in the glacial to interglacial ocean CO2 inventory. However, whilst CMIP5 model results feature consistent future-warming jet shifts, there is considerable disagreement in deglacial-warming jet shifts. We find here that the dependence of pre-industrial (PI) to Last Glacial Maximum (LGM) jet shifts on PI jet position, or state dependency, explains less of the shifts in jet simulated by the models for the LGM compared with future-warming scenarios. State dependence is also weaker for intensity changes, compared to latitudinal shifts in the jet. Winter sea ice was considerably more extensive during the LGM. Changes in surface heat fluxes, due to this sea ice change, probably had a large impact on the jet. Models that both simulate realistically large expansions in sea ice and feature PI jets which are south of 50° S show an increase in wind speed around 55° S and can show a poleward shift in the jet between the PI and the LGM. However, models with the PI jet positioned equatorwards of around 47° S do not show this response: the sea ice edge is too far from the jet for it to respond. In models with accurately positioned PI jets, a +1° difference in the latitude of the sea ice edge tends to be associated with a −0.85° shift in the 850 hPa jet. However, it seems that around 5° of expansion of LGM sea ice is necessary to hold the jet in its PI position. Since the Gersonde et al. (2005) data support an expansion of more than 5°, this result suggests that a slight poleward shift and intensification was the most likely jet change between the PI and the LGM. Without the effect of sea ice, models simulate poleward-shifted westerlies in warming climates and equatorward-shifted westerlies in colder climates. However, the feedback of sea ice counters and reverses the equatorward trend in cooler climates so that the LGM winds were more likely to have also been shifted slightly poleward

Simulating the 128-ka Antarctic climate response to Northern Hemisphere ice sheet melting using the isotope-enabled HadCM3

Warmer than present Antarctic and Southern Ocean temperatures during the last interglacial, approximately 128,000 years ago, have been attributed to changes in north‐south ocean heat transport, causing opposing hemispheric temperature anomalies. We investigate the magnitude of Antarctic warming and Antarctic ice core isotopic enrichment in response to Northern Hemisphere meltwater input during the early last interglacial. A 1,600‐year HadCM3 simulation driven by 0.25 Sv of meltwater input reproduces 50–60% of the peak Southern Ocean summer sea surface temperature anomaly, sea ice retreat, and ice core isotope enrichment. We also find a robust increase in the proportion of cold season precipitation during the last interglacial, leading to lower isotopic values at the Antarctic ice core sites. These results suggest that a HadCM3 simulation including 0.25 Sv for 3,000–4,000 years would reconcile the last interglacial observations, providing a potential solution for the last interglacial missing heat problem

Holocene dynamics of the Southern Hemisphere westerly winds and possible links to CO2 outgassing

The Southern Hemisphere westerly winds (SHW) play an important role in regulating the capacity of the Southern Ocean carbon sink. They modulate upwelling of carbon-rich deep water and, with sea ice, determine the ocean surface area available for air–sea gas exchange. Some models indicate that the current strengthening and poleward shift of these winds will weaken the carbon sink. If correct, centennial- to millennial-scale reconstructions of the SHW intensity should be linked with past changes in atmospheric CO2, temperature and sea ice. Here we present a 12,300-year reconstruction of wind strength based on three independent proxies that track inputs of sea-salt aerosols and minerogenic particles accumulating in lake sediments on sub-Antarctic Macquarie Island. Between about 12.1 thousand years ago (ka) and 11.2 ka, and since about 7 ka, the wind intensities were above their long-term mean and corresponded with increasing atmospheric CO2. Conversely, from about 11.2 to 7.2 ka, the wind intensities were below their long-term mean and corresponded with decreasing atmospheric CO2. These observations are consistent with model inferences of enhanced SHW contributing to the long-term outgassing of CO2 from the Southern Ocean

Dansgaard-Oeschger events in climate models: review and baseline Marine Isotope Stage 3 (MIS3) protocol

Dansgaard-Oeschger (D-O) events, millennial-scale climate oscillations between stadial and interstadial conditions (of up to 10-15°C in amplitude at high northern latitudes), occurred throughout the Marine Isotope Stage 3 (MIS3; 27.8-59.4ka) period. The climate modelling community up to now has not been able to answer the question of whether our climate models are too stable to simulate D-O events. To address this, this paper lays the ground-work for a MIS3 D-O protocol for general circulation models which are used in the International Panel for Climate Change (IPCC) assessments. We review the following: D-O terminology, community progress on simulating D-O events in these IPCC-class models (processes and published examples), and evidence about the boundary conditions under which D-O events occur. We find that no model exhibits D-O-like behaviour under pre-industrial conditions. Some, but not all, models exhibit D-O-like oscillations under MIS3 and/or full glacial conditions. Greenhouse gases and ice sheet configurations are crucial. However most models have not run simulations of long enough duration to be sure which models show D-O-like behaviour, under either MIS3 or full glacial states. We propose a MIS3 baseline protocol at 34ka, which features low obliquity values, medium to low MIS3 greenhouse gas values, and the intermediate ice sheet configuration, which our review suggests are most conducive to D-O-like behaviour in models. We also provide a protocol for a second freshwater (Heinrich-event-preconditioned) experiment, since previous work suggests that this variant may be helpful in preconditioning a state in models which is conducive to D-O events. This review provides modelling groups investigating MIS3 D-O oscillations with a common framework, which is aimed at (1) maximising the chance of the occurrence of D-O-like events in the simulations, (2) allowing more precise model-data evaluation, and (3) providing an adequate central point for modellers to explore model stability.Evan J. Gowan is funded by an international postdoctoral fellowship from the Japan Society for the Promotion of Science. Bette Otto-Bliesner acknowledges funding by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under cooperative agreement no. 1852977. Xu Zhang acknowledges funding from NSFC (no. 42075047). Matthias Prange and Ute Merkel acknowledge support from the PalMod project (http://www.palmod.de, last access: 14 April 2023; FKZ 01LP1916C), funded by the German Federal Ministry of Education and Research (BMBF). Kira Rehfeld and Nils Weitzel acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), project no. 395588486, and the PalMod project (https://www.palmod.de/, last access: 13 October 2022), subproject no. 01LP1926C. Chuncheng Guo acknowledges funding from the Research Council of Norway under grant no. 325333 (ABRUPT).Peer reviewe

Large-scale features of Last Interglacial climate: results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4)

The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka. New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization

The Changing Landscape for Stroke\ua0Prevention in AF: Findings From the GLORIA-AF Registry Phase 2

Background GLORIA-AF (Global Registry on Long-Term Oral Antithrombotic Treatment in Patients with Atrial Fibrillation) is a prospective, global registry program describing antithrombotic treatment patterns in patients with newly diagnosed nonvalvular atrial fibrillation at risk of stroke. Phase 2 began when dabigatran, the first non\u2013vitamin K antagonist oral anticoagulant (NOAC), became available. Objectives This study sought to describe phase 2 baseline data and compare these with the pre-NOAC era collected during phase 1. Methods During phase 2, 15,641 consenting patients were enrolled (November 2011 to December 2014); 15,092 were eligible. This pre-specified cross-sectional analysis describes eligible patients\u2019 baseline characteristics. Atrial fibrillation disease characteristics, medical outcomes, and concomitant diseases and medications were collected. Data were analyzed using descriptive statistics. Results Of the total patients, 45.5% were female; median age was 71 (interquartile range: 64, 78) years. Patients were from Europe (47.1%), North America (22.5%), Asia (20.3%), Latin America (6.0%), and the Middle East/Africa (4.0%). Most had high stroke risk (CHA2DS2-VASc [Congestive heart failure, Hypertension, Age  6575 years, Diabetes mellitus, previous Stroke, Vascular disease, Age 65 to 74 years, Sex category] score  652; 86.1%); 13.9% had moderate risk (CHA2DS2-VASc = 1). Overall, 79.9% received oral anticoagulants, of whom 47.6% received NOAC and 32.3% vitamin K antagonists (VKA); 12.1% received antiplatelet agents; 7.8% received no antithrombotic treatment. For comparison, the proportion of phase 1 patients (of N = 1,063 all eligible) prescribed VKA was 32.8%, acetylsalicylic acid 41.7%, and no therapy 20.2%. In Europe in phase 2, treatment with NOAC was more common than VKA (52.3% and 37.8%, respectively); 6.0% of patients received antiplatelet treatment; and 3.8% received no antithrombotic treatment. In North America, 52.1%, 26.2%, and 14.0% of patients received NOAC, VKA, and antiplatelet drugs, respectively; 7.5% received no antithrombotic treatment. NOAC use was less common in Asia (27.7%), where 27.5% of patients received VKA, 25.0% antiplatelet drugs, and 19.8% no antithrombotic treatment. Conclusions The baseline data from GLORIA-AF phase 2 demonstrate that in newly diagnosed nonvalvular atrial fibrillation patients, NOAC have been highly adopted into practice, becoming more frequently prescribed than VKA in Europe and North America. Worldwide, however, a large proportion of patients remain undertreated, particularly in Asia and North America. (Global Registry on Long-Term Oral Antithrombotic Treatment in Patients With Atrial Fibrillation [GLORIA-AF]; NCT01468701

Analysing the timing of peak warming and minimum winter sea-ice extent in the Southern Ocean during MIS 5e

The peak of the Last Interglacial, Marine Isotope Stage (MIS) 5e (130–116 ka), provides a valuable ‘process analogue’ for validating the climatic feedbacks and forcings likely active under future anthropogenic warming. Reconstructing exact timings of MIS 5e peak warming and minimum winter sea-ice extent (WSIE) throughout the Southern Ocean (SO) will help to identify the interactions and feedbacks within the ice-ocean system. Here we present a new MIS 5e marine sediment record from the SW Atlantic sector together with 28 published core records (chronologies standardised to the LR04 δ18O benthic stack; Lisiecki and Raymo, 2005) to investigate the timing and sequence of minimum WSIE and peak warming across the SO. Sea-surface temperatures (SSTs) peaked earliest in the Indian (20oE–150oE) and Atlantic (70oW–20oE) sectors, at 128.7 ± 0.8 ka and 127.4 ± 1.1 ka respectively, followed by the Pacific sector (150oE–70oW) at 124.9 ± 3.6 ka. The interval of minimum WSIE for all three sectors occurred within the period from 129–125 ka, consistent with the ∼128 ka sea salt flux minimum in Antarctic ice cores. Minimum WSIE appears to have coincided with peak July insolation at 55 oS, suggesting it could be linked with the mildest winters. The reduced WSIE during MIS 5e would have likely reduced the production of deep- and bottom water masses, inhibiting storage of CO2 in the abyssal ocean and lowering nutrient availability in SO surface waters. Examining a wide spatial range of proxy records for MIS 5e is a critical step forward in understanding climatic interactions and processes that will be active under warmer global temperatures

Isochronous information in a Greenland ice sheet radio-echo sounding dataset

The evaluation of ice sheet models is one of the pressing problems in the study of ice sheets dynamics. Here we examine the question of how much isochronous information is contained within the publicly available CReSIS Greenland airborne radio-echo soundings dataset. We identify regions containing isochronous reflectors using ARESP algorithms [Sime et al., 2011]. We find that isochronous reflectors are present within 36% of the CReSIS RES englacial data by location, and 41% by total number of data. Between 1000 and 3000 m in depth, isochronous reflectors are present along more than 50% of the dataset flight path. Lower volumes of cold glacial period ice also correspond with more isochronous reflectors. We find good agreement between ARESP and continuity index [Karlsson et al., 2012] results, providing confidence in these findings. Ice structure datasets, based on data identified here, will be of use in evaluating ice sheet simulations and the assessment of past rates of snow accumulation

Reconstructing Antarctic winter sea-ice extent during Marine Isotope Stage 5e

Environmental conditions during Marine Isotope Stage (MIS) 5e (130-116 ka) represent an important "process analogue"for understanding the climatic responses to present and future anthropogenic warming. The response of Antarctic sea ice to global warming is particularly uncertain due to the short length of the observational record. Reconstructing Antarctic winter sea-ice extent during MIS 5e therefore provides insights into the temporal and spatial patterns of sea-ice change under a warmer-than-present climate. This study presents new MIS 5e records from nine marine sediment cores located south of the Antarctic Polar Front between 55 and 70°S. Winter sea-ice extent and sea-surface temperatures are reconstructed using marine diatom assemblages and a modern analogue technique transfer function, and changes in these environmental variables between the three Southern Ocean sectors are investigated. The Atlantic and East Indian sector records show much more variable MIS 5e winter sea-ice extent and sea-surface temperatures than the Pacific sector records. High variability in the Atlantic sector winter sea-ice extent is attributed to high glacial meltwater flux in the Weddell Sea, indicated by increased abundances of the diatom species Eucampia antarctica and Fragilariopsis cylindrus. The high variability in the East Indian sector winter sea-ice extent is conversely believed to result from large latitudinal migrations of the flow bands of the Antarctic Circumpolar Current, inferred from latitudinal shifts in the sea-surface temperature isotherms. Overall, these findings suggest that Pacific sector winter sea ice displays a low sensitivity to warmer climates. The different variability and sensitivity of Antarctic winter sea-ice extent in the three Southern Ocean sectors during MIS 5e may have significant implications for the Southern Hemisphere climatic system under future warming.</p