  A novel split-explicit (SE) external mode solver for the Finite volumE Sea
ice-Ocean Model (FESOM2) is presented. It is compared with the semi-implicit
(SI) solver currently used in FESOM2. The SE solver utilises a dissipative
asynchronous (forward-backward) time-stepping scheme. Its implementation with
Arbitrary Lagrangian-Eulerian (ALE) vertical coordinates like Z-star and
Z-tilde is explored. The comparisons are performed through multiple test cases
involving idealised and realistic global simulations. The SE solver
demonstrates lower phase errors and dissipation, but maintain a simulated mean
ocean state very similar to the SI solver. The SE solver is also shown to
possess better run-time performance and parallel scalability across all tested
workloads.
