Authors: C Bonfils N de NobletDucoudré J Guiot P Bartlein
Publish Date: 2004/05/29
Volume: 23, Issue: 1, Pages: 79-98
Abstract
We propose a new approach for comparing midHolocene climates from 18 PMIP simulations with climate reconstructions of winter and growing season temperatures and the annual water budget inferred from European pollen and lakelevel data A cluster analysis is used to extract patterns of multivariate climate response from the reconstructions these are then compared to the patterns simulated by models According to paleodata summers during midHolocene were warmerthanpresent in the north and coolerthanpresent in the south while winters were colderthanpresent in the southwest but milderthanpresent in the northeast Whereas warmer summers and colder winters may easily be explained as a direct response to the amplified seasonal cycle of insolation during the midHolocene the other recorded responses are less straightforward to explain We have identified from the models that correctly simulate the recorded climate change two important atmospheric and hydrological processes that can compensate for direct insolation effects First a strongerthanpresent airflow from southwestern Europe that veers to the north over Eastern Europe in winter can consistently explain the reconstructed changes in this season’s temperatures and water budget Second the increased winter soil moisture allows a shift of the partitioning of net radiative energy towards latent rather than sensible heat fluxes thereby decreasing surface temperature during the following summer season Our approach therefore solves one of the recurring problems in modeldata comparisons that arises when a model simulates the correct response but in the wrong location as a consequence for instance of model resolution and topographyWe would like to thank warmly JeanYves Peterschmitt for helping in post processing data Sylvie Joussaume Dominique Jolly Sandy Harrison for the constructive comments about this work and Rachid Cheddadi for making his data available to us We gratefully acknowledge Pascale Braconnot for helping us with the cluster method We would like to thank our three reviewers for the substantial revisions they have suggested from the initial version of this work The computer time and the computing facilities were provided by the Commissariat á l’Energie Atomique Drawings have been performed using VCS the software developed at PCMDI This work was supported by EEC under PMIP Contract ENV4CT950075 and involved in ECHO project PI P Braconnot funded by the French national program PNEDC P Bartlein was supported by US NSF grant ATM9910638 At UC Berkeley C Bonfils was partially supported by National Science Foundation grant ATM9987457So for T s = 25 °C eT s = 3166 Pa we can assume reasonably that e r = 07 · eT s = 2216 Pa r a = 20 s/m r c = 430 m/s In this case Δβ = 0018165ΔS which means that a 20 W/m2 increase in incident solar radiation can be offset by an increase 036 no unit in moisture availability β ranging between 0 and 1 For a field capacity in this soil of 300 mm an increase of 036 for β corresponds to a 108 mm increase in soil moisture
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