Fast Offline Ocean Tracer Transport Model for the Community Earth System Model

To obtain the steady-state solution for oceanic tracers takes 1000's of model years using a traditional prognostic model. But tracer simulations can be computed efficiently using an offline tracer-transport model formulated in terms of a sparse matrix tracer transport operator. We have developed an implicit solution method for obtaining equilibrium tracer fields for the cases where the transport operator and tracer source function can be approximated as being steady or cyclostationary. We present sample simulations of natural radiocarbon and ideal age using a steady (time-averaged) and periodic (seasonally-cycling) transport operator constructed from the ocean component of the Community Earth System Model (CESM). The offline model can also be used to perform efficient transient-tracer simulations, in which the source can vary with time. We compare simulations using the steady and seasonally-cycling model to simulations using the full time-dependent model. Our simulations help characterize the ventilation characteristics of CESM's ocean component, and comparisons with observations of natural radiocarbon and CFC-11 help identify model deficiencies. The computational efficiency of the offline model and implicit solver also make it ideal for optimizing/calibrating parameters in CESM's Biogeochemical Elemental Cycling (BEC) module.