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1. Last Millennium Reanalysis with an expanded proxy database and seasonal proxy modeling

   https://doi.org/10.5194/cp-15-1251-2019  

   Tardif, Robert ; Hakim, Gregory J. ; Perkins, Walter A. ; Horlick, Kaleb A. ; Erb, Michael P. ; Emile-Geay, Julien ; Anderson, David M. ; Steig, Eric J. ; Noone, David ( January 2019 , Climate of the Past)

   null (Ed.)

   Abstract. The Last Millennium Reanalysis (LMR) utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records and the forward models that map climate variables to proxy measurements. Results based on an updated proxy database and seasonal regression-based forward models are compared to the LMR prototype, which was based on a smaller set of proxy records and simpler proxy models formulated as univariate linear regressions against annual temperature. Validation against various instrumental-era gridded analyses shows that the new reconstructions of surface air temperature and 500 hPa geopotential height are significantly improved (from 10 % to more than 100 %), while improvements in reconstruction of the Palmer Drought Severity Index are more modest. Additional experiments designed to isolate the sources of improvement reveal the importance of the updated proxy records, including coral records for improving tropical reconstructions, and tree-ring density records for temperature reconstructions, particularly in high northern latitudes. Proxy forward models that account for seasonal responses, and dependence on both temperature and moisture for tree-ring width, also contribute to improvements in reconstructed thermodynamic and hydroclimate variables in midlatitudes. The variability of temperature at multidecadal to centennial scales is also shown to be sensitive to the set of assimilated proxies, especially to the inclusion of primarily moisture-sensitive tree-ring-width records. 

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2. Local Regions Associated With Interdecadal Global Temperature Variability in the Last Millennium Reanalysis and CMIP5 Models

   https://doi.org/10.1029/2019JD030426  

   Parsons, L. A. ; Hakim, G. J. ( September 2019 , Journal of Geophysical Research: Atmospheres)

   Despite the importance of interdecadal climate variability, we have a limited understanding of which geographic regions are associated with global temperature variability at these timescales. The instrumental record tends to be too short to develop sample statistics to study interdecadal climate variability, and Coupled Model Intercomparison Project, Phase 5 (CMIP5) climate models tend to disagree about which locations most strongly influence global mean interdecadal temperature variability. Here we use a new paleoclimate data assimilation product, the Last Millennium Reanalysis (LMR), to examine where local variability is associated with global mean temperature variability at interdecadal timescales. The LMR framework uses an ensemble Kalman filter data assimilation approach to combine the latest paleoclimate data and state‐of‐the‐art model data to generate annually resolved field reconstructions of surface temperature, which allow us to explore the timing and dynamics of preinstrumental climate variability in new ways. The LMR consistently shows that the middle‐ to high‐latitude north Pacific and the high‐latitude North Atlantic tend to lead global temperature variability on interdecadal timescales. These findings have important implications for understanding the dynamics of low‐frequency climate variability in the preindustrial era.

    

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3. Northern Hemisphere winter warming and summer monsoon reduction after volcanic eruptions over the last millennium: Winter Warming in the Last Millennium

   https://doi.org/10.1002/2017JD026728  

   Zambri, Brian ; LeGrande, Allegra N. ; Robock, Alan ; Slawinska, Joanna ( August 2017 , Journal of Geophysical Research: Atmospheres)
4. The Tropical Pacific ENSO–Mean State Relationship in Climate Models over the Last Millennium

   https://doi.org/10.1175/JCLI-D-19-0673.1  

   Wyman, D. Allie ; Conroy, Jessica. L. ; Karamperidou, Christina ( September 2020 , Journal of Climate)

   Abstract ENSO and the mean zonal sea surface temperature gradient (dSST) of the tropical Pacific are important drivers of global climate and vary on decadal to centennial time scales. However, the relationship between dSST and ENSO cannot be assessed with the short instrumental record, and is uncertain in proxy data, with intervals of both stronger and weaker ENSO postulated to occur with overall strong dSST in the past. Here we assess the ENSO–dSST relationship during the last millennium using general circulation models (GCMs) participating in phase 3 of the Paleoclimate Modeling Intercomparison Project. Last millennium GCM simulations show diversity in the strength and direction of the ENSO–dSST relationship. Yet, the models that best simulate modern tropical Pacific climate frequently have a more negative ENSO–dSST correlation. Thus, last millennium tropical Pacific climate simulations support the likelihood of enhanced ENSO during decadal to centennial periods of reduced tropical Pacific dSST. However, the alternating directional ENSO–dSST relationship in all model simulations suggests that this relationship is not constant through time and is likely controlled by multiple mechanisms. 

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5. The Role of Internal Variability in ITCZ Changes Over the Last Millennium

   https://doi.org/10.1029/2021GL096487  

   Roldán‐Gómez, P. J. ; González‐Rouco, J. F. ; Melo‐Aguilar, C. ; Smerdon, J. E. ( February 2022 , Geophysical Research Letters)