non-closure of salinity diagnostics when using the sea-ice module

[tigerchoo]

Hello. I am interested in salinity processes in the Antarctic Ocean and am currently using ROMS with the sea-ice module activated (Budgell version).

I have found that the daily tracer diagnostic outputs (for salinity budget analysis) do not match the actual salinity changes, with the largest discrepancies occurring at the upper layers.

In addition, this mismatch shows a clear seasonal signal, near 0 in summer, and intensifying since April (in the western Ross Sea), which suggests a potential link to ice-related processes.

At this stage, I do not have any clue for understanding or explanation.


Anyone has encountered a similar issue or has insights?

Any kind of reply would be helpful to me.

[sdurski]

Salinity is not conserved when using the ice module. The sea ice module only approximates the flux of salt or freshwater that should occur when ice forms or melts. Sea ice is represented as a slab that sits 'on top' of the ocean, so the (liquid) ocean volume is not reduced when sea ice forms and not increased when sea ice melts. (The sea ice also doesn't factor into the pressure gradients in any way). It's important to approximate the change in surface conditions associated with a brine rejection or freshwater flux to get freeze up and melt reasonably well. So a salt/freshwater flux is calculated based on the surface salinity and the rate of sea ice production/melt. With this approximation, in a 1D water column, you should get close to conservation over a full freeze-melt cycle. But as soon as you allow lateral advection where ice might say form over fresher water, then melt over saltier water, you will lose closure of your salinity budget.

Here's a paper that explains my attempts to grapple with this.
https://doi.org/10.1016/j.ocemod.2020.101696