Scope: The Transport Matrix Method (TMM) is a computationally efficient ``offline'' scheme for simulation of ocean biogeochemical tracers. The TMM essentially encapsulates the circulation of any ocean model, including sub-grid-scale parameterizations, as a sparse matrix, and advection-diffusion as a sequence of sparse matrix-vector products that can be efficiently carried out on modern distributed-memory computers. Simulations carried out with the TMM can potentially be orders of magnitude faster than the underlying ocean circulation model. The flexible architecture of the TMM framework also allows ``mixing and matching'' of circulations and biogeochemical models. The TMM code, various biogeochemical models, and transport matrices from different ocean models are freely available to download from https://github.com/samarkhatiwala/tmm.

The aim of this Special Issue is to bring together under one roof papers using the TMM as the underlying simulation method. This can range from manuscripts documenting various technical aspects of the TMM framework to those describing new biogeochemical models/parameterizations and their application.