Space-geodetic and water level gauge constraints on continental uplift and tilting over North America: regional convergence of the ICE-6G_C (VM5a/VM6) models

We present a series of analyses of the regional convergence of the iterative methodology that has been developed for use in the construction of global models of the glacial isostatic adjustment process. Our specific focus is upon the North American component of such models which embodied the largest concentration of grounded land ice at the Last Glacial Maximum. We show that, although the introduction of the VM6 viscosity structure helps the global ICE-6G_C (VM5a) model to improve the fit to relative sea level data from the region of forebulge collapse along the U.S. East coast, it also leads to a significant misfit to the totality of the available space-geodetic observations, which the original ICE-6G_C (VM5a) model was able to fit with high accuracy. This raises the issue of the convergence of the iterative methodology being employed in the process of model construction. We demonstrate through detailed further analysis that a fully converged solution which reconciles all available data from the continent, including additional data on the time dependent de-levelling of the Great Lakes region, is obtained through modest further modifications of both the viscosity structure of the model and the North American component of the surface mass loading history.