SICOPOLIS-AD v2: tangent linear and adjoint modeling framework for ice sheet modeling enabled by automatic differentiation tool Tapenade

Abstract

SImulation COde for POLythermal Ice Sheets (SICOPOLIS) is an open-source, 3D dynamic/thermodynamic model that simulates the evolution of large ice sheets and ice caps. SICOPOLIS has been developed continuously and applied to problems of past, present, and future glaciation of Greenland, Antarctica, and others. It uses the finite differences discretization on a staggered Arakawa C grid and employs the shallow ice and shallow shelf approximations, making it suitable for paleoclimatic simulations. We present a new framework for generating derivative code, i.e., tangent linear, adjoint, or Hessian models, of SICOPOLIS. These derivative operators are powerful computational engines to efficiently compute comprehensive gradients or sensitivities of scalar-valued model output, including least-squares model-data misfits or important quantities of interest, to high-dimensional model inputs (such as model initial conditions, parameter fields, or boundary conditions). The new version 2 (SICOPOLIS-AD v2) framework is based on the source-to-source automatic differentiation (AD) tool Tapenade which has recently been open-sourced. The switch from a previous AD tool (OpenAD) used in SICOPOLIS-AD version 1 to Tapenade overcomes several limitations outlined here. The framework is integrated with the SICOPOLIS model's main trunk and is freely available.