Reconstruction of the Southern Hemisphere climate over the past millennium taking into account explicitly the links between climate and tree growth with process-based dendroclimatic models
Authors: Rezsöhazy Jeanne , Goosse Hugues , Guiot Joël , Klein François , Dalaiden Quentin 
Published: March 2021

&lt;p&gt;&lt;span&gt;Trees are one of the main archives to reconstruct past climate variability at the interannual scale. The links between tree-ring proxies and climate have usually been estimated on the basis of statistical approaches, assuming linear and stationary relationships. Yet, both assumptions can be inadequate. &lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;his issue can be overcome by the use of process-based dendroclimatic models.&lt;/span&gt; &lt;span&gt;For example,&lt;/span&gt;&lt;span&gt; MAIDEN (Modeling and Analysis In DENdroecology) &lt;/span&gt;&lt;span&gt;is a &lt;/span&gt;&lt;span&gt;mechanistic&lt;/span&gt;&lt;span&gt; tree-growth model that&lt;/span&gt;&lt;span&gt; simulates tree-ring growth starting from &lt;/span&gt;&lt;span&gt;surface air &lt;/span&gt;&lt;span&gt;temperature, precipitation &lt;/span&gt;&lt;span&gt;and CO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;&amp;#160;&lt;/span&gt; &lt;span&gt;daily inputs. &lt;/span&gt;&lt;span&gt;VS-Lite &lt;/span&gt;&lt;span&gt;(Vaganov-Shashkin Lite) &lt;/span&gt;&lt;span&gt;is a &lt;/span&gt;&lt;span&gt;simpl&lt;/span&gt;&lt;span&gt;ified&lt;/span&gt; &lt;span&gt;model &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;is more generally used in global applications and&lt;/span&gt;&lt;span&gt; simulates &lt;/span&gt;&lt;span&gt;unitless &lt;/span&gt;&lt;span&gt;tree-growth indexes with mont&lt;/span&gt;&lt;span&gt;h&lt;/span&gt;&lt;span&gt;ly temperature and precipitation as inputs&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;In this study, &lt;/span&gt;&lt;span&gt;we provide a climate reconstruction of &lt;/span&gt;&lt;span&gt;continental temperature, precipitation and w&lt;/span&gt;&lt;span&gt;inds &lt;/span&gt;&lt;span&gt;in the mid to high latitudes of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;Southern Hemisphere&lt;/span&gt; &lt;span&gt;over the last millennium &lt;/span&gt;&lt;span&gt;that takes&lt;/span&gt;&lt;span&gt; into account explicitly the links between climate and tree growth with process-based dendroclimatic models. To this end, &lt;/span&gt;&lt;span&gt;an offline&lt;/span&gt;&lt;span&gt; d&lt;/span&gt;&lt;span&gt;ata assimilation procedure is used to combine the information from the physics of the &lt;/span&gt;&lt;span&gt;climate &lt;/span&gt;&lt;span&gt;system, &lt;/span&gt;&lt;span&gt;as &lt;/span&gt;&lt;span&gt;included in climate models, and &lt;/span&gt;&lt;span&gt;paleoclimate records, in particular tree-ring width and ice cores records (snow accumulation and &lt;/span&gt;&lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O)&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Those records are &lt;/span&gt;&lt;span&gt;the best &lt;/span&gt;&lt;span&gt;available &lt;/span&gt;&lt;span&gt;continental proxies in the Southern Hemisphere &lt;/span&gt;&lt;span&gt;continents &lt;/span&gt;&lt;span&gt;(South America, Australia, Tasmania, New Zealand and Antarctica) for reconstructing the past climate. &lt;/span&gt;&lt;span&gt;MAIDEN and VS-Lite are used &lt;/span&gt;&lt;span&gt;here &lt;/span&gt;&lt;span&gt;as proxy system models, also referred to as observation operators in data assimilation framework, to make the link between climate model outputs and indirect climate observations from tree-rings. &lt;/span&gt;&lt;span&gt;The ice cores &lt;/span&gt;&lt;span&gt;records&lt;/span&gt;&lt;span&gt; are directly compared to the outputs of the climate model. &lt;/span&gt;&lt;span&gt;More specifically&lt;/span&gt;&lt;span&gt;, we evaluate the benefit&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; of using tree-growth model&lt;/span&gt;&lt;span&gt;s&lt;/span&gt; &lt;span&gt;such as VS-Lite and MAIDEN for reconstructing past climate with &lt;/span&gt;&lt;span&gt;data assimilation compared to the commonly used linear regression.&lt;/span&gt;&lt;/p&gt;