Modelling the response of stable water isotopes in Greenland precipitation to orbital configurations of the previous interglacial

The relation between δ 18O of precipitation and temperature has been used in numerous studies to reconstruct past temperatures at ice core sites in Greenland and Antarctica. During the past two decades, it has become clear that the slope between δ 18O and temperature varies in both space and time. Here, we use a general circulation model driven by changes in orbital parameters to investigate the Greenland δ 18O–temperature relation for the previous interglacial, the Eemian. In our analysis, we focus on changes in the moisture source regions, and the results underline the importance of taking the seasonality of climate change into account. The orbitally driven experiments show that continental evaporation over North America increases during summer in the warm parts of the Eemian, while marine evaporation decreases. This likely flattens the Greenland δ 18O response to temperature during summer. Since the main climate change in the experiments occurs during summer this adds to a limited response of δ 18O, which is more strongly tied to temperature during winter than during summer. A south–west to north–east gradient in the δ 18O–temperature slope is also evident for Greenland, with low slopes in the south–west and steeper slopes in the north–east. This probably reflects the proportion of continental moisture and Arctic moisture arriving in Greenland, with more continental moisture in the south–west and less in the north–east, and vice versa for the Arctic moisture

How does sea ice influence δ¹⁸O of Arctic precipitation?

This study investigates how variations in Arctic sea ice and sea surface conditions influence δ18O of present-day Arctic precipitation. This is done using the model isoCAM3, an isotope-equipped version of the National Center for Atmospheric Research Community Atmosphere Model version 3. Four sensitivity experiments and one control simulation are performed with prescribed sea surface temperature (SST) and sea ice. Each of the four experiments simulates the atmospheric and isotopic response to Arctic oceanic conditions for selected years after the beginning of the satellite era in 1979. Changes in sea ice extent and SSTs have different impacts in Greenland and the rest of the Arctic. The simulated changes in central Arctic sea ice do not influence δ18O of Greenland precipitation, only anomalies of Baffin Bay sea ice. However, this does not exclude the fact that simulations based on other sea ice and sea surface temperature distributions might yield changes in the δ18O of precipitation in Greenland. For the Arctic, δ18O of precipitation and water vapour is sensitive to local changes in sea ice and sea surface temperature and the changes in water vapour are surface based. Reduced sea ice extent yields more enriched isotope values, whereas increased sea ice extent yields more depleted isotope values. The distribution of the sea ice and sea surface conditions is found to be essential for the spatial distribution of the simulated changes in δ18O

Solar activity of the past 100 years inferred from 10Be in ice cores – implications for long-term solar activity reconstructions

Differences between 10Be records from Greenland and Antarctica over the last 100 years have led to different conclusions about past changes in solar activity. The reasons for this disagreement remain unresolved. We analyze a seasonally resolved 10Be record from a firn core (NEEM ice core project) in Northwestern Greenland for 1887-2002. By comparing the NEEM data to 10Be data from the NGRIP and Dye3 ice cores, we find that the Dye3 data after 1958 are significantly lower. These low values lead to a normalization problem in solar reconstructions when connecting 10Be variations to modern observations. Excluding these data strongly reduces the differences between solar reconstructions over the last 2000 years based on Greenland and Antarctic 10Be data. Furthermore, 10Be records from polar regions and group sunspot numbers do not support a substantial increase in solar activity for the 1937-1950 period as proposed by previous extensions of the neutron monitor data.This article is protected by copyright. All rights reserved

Assessing the robustness of Antarctic temperature reconstructions over the past 2 millennia using pseudoproxy and data assimilation experiments

The Antarctic temperature changes over the past millennia remain more uncertain than in many other continental regions. This has several origins: (1) the number of high-resolution ice cores is small, in particular on the East Antarctic plateau and in some coastal areas in East Antarctica; (2) the short and spatially sparse instrumental records limit the calibration period for reconstructions and the assessment of the methodologies; (3) the link between isotope records from ice cores and local climate is usually complex and dependent on the spatial scales and timescales investigated. Here, we use climate model results, pseudoproxy experiments and data assimilation experiments to assess the potential for reconstructing the Antarctic temperature over the last 2 millennia based on a new database of stable oxygen isotopes in ice cores compiled in the framework of Antarctica2k (Stenni et al.,). The well-known covariance between δ 18 O and temperature is reproduced in the two isotope-enabled models used (ECHAM5/MPI-OM and ECHAM5-wiso), but is generally weak over the different Antarctic regions, limiting the skill of the reconstructions. Furthermore, the strength of the link displays large variations over the past millennium, further affecting the potential skill of temperature reconstructions based on statistical methods which rely on the assumption that the last decades are a good estimate for longer temperature reconstructions. Using a data assimilation technique allows, in theory, for changes in the δ 18 O-temperature link through time and space to be taken into account. Pseudoproxy experiments confirm the benefits of using data assimilation methods instead of statistical methods that provide reconstructions with unrealistic variances in some Antarctic subregions. They also confirm that the relatively weak link between both variables leads to a limited potential for reconstructing temperature based on δ 18 O. However, the reconstruction skill is higher and more uniform among reconstruction methods when the reconstruction target is the Antarctic as a whole rather than smaller Antarctic subregions. This consistency between the methods at the large scale is also observed when reconstructing temperature based on the real δ 18 O regional composites of Stenni et al. (2017). In this case, temperature reconstructions based on data assimilation confirm the long-term cooling over Antarctica during the last millennium, and the later onset of anthropogenic warming compared with the simulations without data assimilation, which is especially visible in West Antarctica. Data assimilation also allows for models and direct observations to be reconciled by reproducing the east-west contrast in the recent temperature trends. This recent warming pattern is likely mostly driven by internal variability given the large spread of individual Paleoclimate Modelling Intercomparison Project (PMIP)/Coupled Model Intercomparison Project (CMIP) model realizations in simulating it. As in the pseudoproxy framework, the reconstruction methods perform differently at the subregional scale, especially in terms of the variance of the time series produced. While the potential benefits of using a data assimilation method instead of a statistical method have been highlighted in a pseudoproxy framework, the instrumental series are too short to confirm this in a realistic setup

Reconstructed main atmospheric circulation modes during winter in the North Atlantic region covering 1241-1970 CE, based on Greenland ice cores and an isotope enabled climate model simulation

This dataset contains the time series (1241-1970 CE) of the first two principal components (PCs) of reconstructed winter (DJF) Sea Level Pressure (SLP) in the North Atlantic region (20-70N, 90W-40E). The SLP is reconstructed by matching modeled d18O patterns from an isotope enabled climate model simulation to the spatio-temporal variability of the winter d18O from Greenland ice cores, and extracting the model SLP from an ensemble of the best matching years. PC1 of reconstructed SLP corresponds to the North Atlantic Oscillation, and PC2 of reconstructed SLP corresponds to the Eastern Atlantic pattern. Both time series are normalized and centred. The sign of the data series is as shown in Figure 2 of Sjolte et al. (2018) with the corresponding spatial patterns shown in Supplementary Figure S3. For example, positive values of PC1 means positive NAO, while positive values of PC2 means negative pressure anomalies south of Greenland

Seasonal climate reconstructions of sea level pressure and temperature for the North Atlantic region 1241-1970

Seasonal climate reconstructions of sea level pressure and temperature for the North Atlantic region 1241-1970. Details on the reconstruction method and validation of the data can be found in Sjolte et al. (2018, 2020). The data include reconstructed 2m temperature (t2m) (data 01) and sea level pressure (slp) (data 02). The summer reconstruction (May-Oct) is done in two versions, one with, and one without tree-ring data, while there is only one version for winter (DJF). The variables ending on ''tr'' include tree-ring data

A climatology of strong large-scale ocean evaporation events. Part II : Relevance for the deuterium excess signature of the evaporation flux

This paper discusses the relevance of transient events of strong large-scale ocean evaporation (SLOE) for the deuteriumexcess of marine boundary layer vapor d using a theoretical framework that invokes the closure assumption. We argue that during SLOE events, d is essentially determined by the evaporation flux signature. Distinct high d during SLOE with global-mean values in the range of 12‰-23‰ depending on the nonequilibrium fractionation factor αk result from the large air-sea humidity gradients reflected in low relative humidity with respect to sea surface temperature (hs 5 53% ± 9%) that characterize these events. Extratropical cyclones are highlighted as an important driver for the variability of d. On the one hand, they are themselves associated with high hs and low d, especially in areas of cloud formation and precipitation in the warm sector. On the other hand, cyclones are the main driver inducing SLOE events with high d in regions of cold-air advection upstream of their path. The sensitivity of d to its direct climate controls (hs and SST) is analyzed during SLOE for different αk formulations and found to be coherent with d-hs and d-SST slopes determined from available observations. The d-hs relationship exhibits a robust negative correlation as opposed to the d-SST relationship, which shows regional and time-scale-dependent variations in strength and sign that are induced by indirect hs-SST cross-correlation effects. The dynamical features involved in SLOE generation appear to exert a key control on the moisture source properties relevant for d in the extratropics

Modelling stable water isotopes in monsoon precipitation during the previous interglacial

International audienc