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

Robert Tardif,David M Anderson,Eric J Steig,Julien Emile-Geay,Walter A Perkins,Gregory J Hakim,David Noone,Michael P Erb,Kaleb A Horlick

doi:10.5194/cp-15-1251-2019

Abstract

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.

Highlights

Reconstructions of Earth’s past climate, covering periods prior to instrumental data sets, are key to understanding the causes of natural climate variability
Paleoclimate data assimilation (PDA)-generated climate field reconstructions have been used to investigate climate variability prior to the instrumental era (Goosse et al, 2006, 2010; Widmann et al, 2010; Bhend et al, 2012; Steiger et al, 2014; Matsikaris et al, 2015; Franke et al, 2017; Okazaki and Yoshimura, 2017; Steiger et al, 2018). Within this general PDA framework, a flexible PDA system is being developed for the Last Millennium Reanalysis (LMR) project for the Published by Copernicus Publications on behalf of the European Geosciences Union
We examine the impact on LMR reconstructions of improvements to these two key components: (1) an updated and expanded proxy database, primarily composed of records from PAGES 2k Consortium (2017), and assessment of the additional records described in Anderson et al (2019); (2) more realistic PSMs in which seasonality and, for tree-ring-width proxies, temperature and moisture sensitivity are taken into account

Summary

Introduction

Reconstructions of Earth’s past climate, covering periods prior to instrumental data sets, are key to understanding the causes of natural climate variability. PDA-generated climate field reconstructions have been used to investigate climate variability prior to the instrumental era (Goosse et al, 2006, 2010; Widmann et al, 2010; Bhend et al, 2012; Steiger et al, 2014; Matsikaris et al, 2015; Franke et al, 2017; Okazaki and Yoshimura, 2017; Steiger et al, 2018). Within this general PDA framework, a flexible PDA system is being developed for the Last Millennium Reanalysis (LMR) project for the Published by Copernicus Publications on behalf of the European Geosciences Union

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