Abstract
Ocean geochemical tracers such as radiocarbon, protactinium and thorium
isotopes, and noble gases are widely used to constrain a range of
physical and biogeochemical processes in the ocean. However their
routine simulation in global ocean circulation and climate models is
hindered by the computational expense of integrating them to a steady
state. Here, a new approach to this long-standing “spin-up’ problem is
introduced to efficiently compute equilibrium distributions of such
tracers in seasonally-forced models. Based on “Anderson Acceleration’,
a sequence acceleration technique developed in the 1960s to solve
nonlinear integral equations, the new method is entirely “black box’
and offers significant speed-up over conventional direct time
integration. Moreover, it requires no preconditioning, ensures tracer
conservation and is fully consistent with the numerical time-stepping
scheme of the underlying model. It thus circumvents some of the
drawbacks of other schemes such as matrix-free Newton Krylov that have
been proposed to address this problem. An implementation specifically
tailored for the batch HPC systems on which ocean and climate models are
typically run is described, and the method illustrated by applying it to
a variety of geochemical tracer problems. The new method, which provides
speed-ups by over an order of magnitude, should make simulations of such
tracers more feasible and enable their inclusion in climate change
assessments such as IPCC.