In MOM 2 and previous versions, Fortran 77 was required. The minimum requirement for MOM 3 is Fortran 90. Much has been learned from experimenting with parallelization in MOM 2. From this experience, a distributed memory paradigm has been adopted with communication calls to exchange data between processors. The domain decomposition is limited to one dimensional (latitude only) which means that there must be more latitude rows than processors. Both SHMEM and MPI message passing protocols are supported through the GFDL message passing interface (http://www.gfdl.gov/ vb). Parallel I/O is supported through the GFDL parallel I/O interface (http://www.gfdl.gov/ vb).