Water isotope data from ice cores, particularly δ18O, have long been used in paleoclimatology. Although δ18O has been primarily interpreted as a proxy for local air temperature, isotope-enabled climate models have established that there are many nonlocal and nontemperature-related climatic influences on isotopic signals at coring locations. Moreover, recent observational studies have linked ice core isotopes to nonlocal patterns of climate variability, particularly to midlatitude atmospheric circulation patterns and to variations in tropical climate. Therefore, paleoclimate reconstructions may better utilize ice core isotope proxies by combining them with isotope-enabled climate models. Here we employ a data assimilation-based technique that fuses isotopic proxy information with the dynamical constraints of climate models. Through several idealized and real proxy experiments we assess the spatial and temporal extent to which isotope records can reconstruct surface temperature, 500 hPa geopotential height, and precipitation. We find local reconstruction skill to be most robust across the reconstructions, particularly for temperature and geopotential height, as well as limited nonlocal skill in the tropics. These results are in agreement with long-held views that isotopes in ice cores have clear value as local climate proxies, particularly for temperature and atmospheric circulation. These results also show that in principle nonlocal climate information may also be inferred from ice cores. However, the spatial range of this information is nonuniform and depends on skillful modeling of the proxy data within the reconstruction process.
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We would like to thank David Battisti and David Noone for helpful discussions and Jesse Nusbaumer for running the iCAM5 simulation. This research was supported by the following awards: National Science Foundation grants 1304263, 1503281, and 1504267 as well as the National Oceanic and Atmospheric Administration grant NA14OAR4310176. The authors declare no conflicting financial interests. The ice core isotope data are available from NOAA paleoclimatology: https:// www.ncdc.noaa.gov/data-access/ paleoclimatology-data or upon request from email@example.com. The PSM modeling code is available at https://github.com/sylvia-dee/PRYSM. The modeling output of the ECHAM5-wiso simulation is available upon request from firstname.lastname@example.org, while the modeling output of the iCAM5 simulation is available upon request from email@example.com.
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