Assimilating the Greenland ice sheet stratigraphy for reconstructing accumulation rates

The thickness of englacial isochrones is the combined product of accumulation and dynamical thinning by the flow of ice.Dated ice stratigraphy data provides access to this archive , which in principle holds the potential for improving simulations and reconstructions  of the Greenland ice sheet. However, the combined effects of accumulation and dynamical thinning are convoluted and spatially heterogeneous, making it difficult to separate their respective contributions to the observed thickness of isochrones. Here we simulate isochrones explicitly using the Englacial Layer Simulation Architecture (ELSA) coupled with a fully transient thermomechanical ice sheet model. This enables us to link accumulation rates to the ice stratigraphy, including a physically consistent representation of dynamical thinning. By ensemble data assimilation techniques, the linear model response to changes in past local accumulation are estimated, enabling the inversion of the model and reconstruction of accumulation rates from stratigraphy. An iterative approach is chosen to account for the nonlinear response of ice flow to anomalous accumulation. The result is an optimized simulation of the Greenland ice sheet with an englacial stratigraphy matching the observations, forced by the reconstructed accumulation. Because the model is fully transient including ice dynamics, our approach also constrains ice sheet stability and sea level contribution. Here we present preliminary findings from inversions of idealized ice sheet stratigraphy, including some encouraging insights, physical and methodological limitations and challenges yet to be overcome.