Analysis of the Holocene climate variability using a data assimilation method applied to the model LOVECLIM
In order to improve climate projections and to estimate the probability of future abrupt changes, the objective of this research is to investigate the climate of the present interglacial, the Holocene. This period defines the time during which civilization developed and span the lasts ~11,600 years [Wanner et al., 2008]. During the Holocene, abrupt climate changes has occurred such as the 8.2k event and the desertification of the Saharan region. Other well documented climate changes, as the Little Ice Age or the Medieval Warm Period, took place during the period of interest.
Numerous climate records (proxy data) and results from climatic model have detailed these changes throughout the scientific literature. Proxy data provides estimates of past changes based on the climatic signal they record. Results from climatic model can provide information as detailed as the complexity of the model used to study climate change. This second source of information is based on our knowledge of the physical mechanisms governing the current climate and on our estimation of the past climate forcing : the orbital forcing, the total solar irradiance, the greenhouse gases concentrations, etc.
The purpose of this research is to combine, through the data assimilation, these two types of information to provide the best reconstruction of past climate changes. This reconstruction will be used to understand the mechanisms that drive the multi-centennial Holocene climate variability.
Figure 1 : summary of the objectives
This three-dimensional Earth system model of intermediate complexity includes three main components : the atmospheric quasi-geostrophic model (ECBilt), the general circulation model of ocean and sea ice (CLIO) and the model of the continental biosphere (VECODE) [Goosse et al., 2010]. Such a model is used because it provides a satisfactory description of the reality while allowing to perform ensembles simulations of several thousand years.
The proxies selected here represent the surface temperature (air and sea) and are located between 20°N and 90°N. Each proxy has a mean temporal resolution of at least 166 years for the reference period (950 to 450 BP) and during the simulated period. To date, approximately 50 proxies are used. They are mainly coming from pollen data, oceanic core and ice core.
Data assimilation method
This method allows combining directly model results and proxy records in order to have a reconstruction of past climate changes that is consistent with proxy data, model physics and the forcing, taking into account uncertainties on model results and proxy records. This method is based on a particle filter with resampling [Dubinkina et al., 2011] and uses an ensemble of 96 simulations/particles. Every 6 months, the likelihood of each particle is evaluated, and the particles that have the largest likelihood, i.e. the particles whose states are closer to the proxy records, are kept. The kept particles are resampled in order to keep a constant number of particles and avoid degenerative issue, adding a slight perturbation to the ones that are sampled more than once for the following 6 month period.
S. Dubinkina, H. Goosse, Y. Sallaz-damaz, E. Crespin, and M. Crucifix. Testing a particle filter to reconstruct climate changes over past centuries. International Journal of Bifurcation and Chaos 2011.
H. Goosse, V. Brovkin, T. Fichefet, R. Haarsma, P. Huybrechts, J. Jongma, A. Mouchet, F. Selten, P.-Y. Barriat, J.-M. Campin, E. Deleersnijder, E. Driesschaert, H. Goelzer, I. Janssens, M.-F. Loutre, M. A. Morales Maqueda, T. Opsteegh, P.-P. Mathieu, G. Munhoven, E. J. Pettersson, H. Renssen, D. M. Roche, M. Schaeffer, B. Tartinville, A. Timmermann, and S. L. Weber. Description of the earth system model of intermediate complexity LOVECLIM version 1.2. Geoscientific Model Development, 3(2):603–633, 2010.
H. Wanner, J. Beer, J. Butikofer, T. Crowley, U. Cubasch, J. Fluckiger, H. Goosse, M. Grosjean, F. Joos, J. Kaplan, et al. Mid-to Late Holocene climate change: an overview. Quaternary Science Reviews, 27(19- 20):1791–1828, 2008.