Comment on cp-2021-189

doi:10.5194/cp-2021-189-rc1

Abstract

Currently available data assimilation-based reconstructions of past climate variations have only used statistical proxy system models to make the link between the climate model outputs and the indirect observations from tree rings. However, the linearity and stationarity assumptions of the statistical approach may have limitations. In this study, we incorporate the process-based dendroclimatic model MAIDEN into a data assimilation procedure, using as a test case the reconstruction of near-surface air temperature, precipitation and winds in the mid-latitudes of the Southern Hemisphere over the past 400 years. We compare our results with a data assimilation approach including a linear regression as a proxy system model for tree-ring width proxies. Overall, when compared to instrumental data, the reconstructions using MAIDEN as a proxy system model offer a skill equivalent to the experiment using the regression model. However, knowing the advantages that a process-based model can bring and the improvements that can still be made with MAIDEN, those results are promising.

Highlights

Indirect observations of climate from proxies such as tree-ring width or isotopic content in ice cores and coral inform on past climate variability beyond the instrumental era (Jones et al, 2009)
It is important to highlight that these correlations have been computed between the integrated fields and are not the average of the correlations represented on the map, which may 330 result in slight differences
5 Conclusions 505 In this study, we have included for the first time a dendroclimatic process-based model into a data assimilation procedure to reconstruct the past climate variability of the Southern Hemisphere

Summary

Introduction

Indirect observations of climate from proxies such as tree-ring width (hereafter TRW) or isotopic content in ice cores and coral inform on past climate variability beyond the instrumental era (Jones et al, 2009). Process-based dendroclimatic PSMs are able to account for the complexity of the relationship between climate and tree-ring proxy data by explicitly simulating the biological processes governing the climate dependency of tree-ring growth. The goal is to evaluate if and how a complex process-based model of tree growth like MAIDEN can contribute to the improvement of large-scale DA-based reconstructions of past climate variability compared to the regression model classically used so far. 4.2 to identify the contribution of tree-ring width proxies in our data assimilation framework using a regression- or a process-based dendroclimatic PSM as 75 well as to quantify the impact of the uncertainty of the records. To match the seasonality of tree growth in the Southern Hemisphere, all the analysis in this study are performed on a July-June year

Data assimilation

Particle filter

Ice core data

Proxy System Models

Linear regression

Observation error

Experiment design and diagnostics

Results and discussion

Conclusions