Thanks, I tried that. The output from the timestep that failed looks similar to the output from previous timesteps to me.
There is ocean output in files like /work/n02/n02/radiam24/cylc-run/u-ck857/work/21021101T0000Z/coupled/output*_0004.nc; only a few of these (e.g. output*_0004.nc and output*_0005.nc) seem to contain non-zero values.
Do you have any other ideas about where to look/what to try?
Thanks.
Best wishes,
Rachel