WOMBATlite: funny behaviour of tracers in near-zero thickness bottom cells

[dougiesquire]

There is some funny behaviour in the near-zero thickness cells at the bottom of the ocean in particularly shallow regions. The following plot shows hourly bottom-cell concentrations for tracers exhibiting the funny behaviour (@ 1902-11-30 23:30:00, created using this configuration).

The funny behaviour (high no3, adic, dic, alk and fe; negative o2) is seen in particularly shallow regions where the cell thickness of the bottom-cell (and indeed a number of overlying cells) is near-zero (order 1mm). The lowest non-near-zero cell looks okay. The following plot is the same as above, but for the deepest cell with a thickness > 2cm

This issue is related to the bottom fluxes from sediment remineralisation (applied here when using MOM6) and does not occur if bottom flux terms (btf) are set to zero. However, there’s nothing obviously going wrong with the bottom fluxes or the bottom layer thicknesses. The following plots show bottom-cell o2 concentration, o2 bottom flux (= 172. / 16. * det_sed_remin) and bottom-cell layer thickness at a location where the oxygen concentrations go negative when bottom fluxes (btf) are included (xh=32.5, yh=75.3).

I’m considering this low priority given that the issue is limited to very thin cells at the bottom of the ocean, but would value any comments or suggestions anyone may have.