How surface freshwater flux works in salinity budget?

Dear all,

I am trying to understand how surface freshwater flux forces the salinity change in NEMO. I attach several salinity budget terms and associated surface freshwater fluxes in Baffin Bay in winter time.

Question1: Why is “saltflx” negative with ice formation?
According to ice production term “iiceprod”, it is forming ice and the salinity flux “saltflx” is negative. Is it because the ocean loses salt contents since it gives mass to ice when freezing?

Question2: How does surface freshwater flux work in salinity budget?
The surface forcing (strd_cdt) in salinity budget is calculated from “saltflx” thus being negative as well. However, ice formation should result in ocean salt increase as brine rejection. The vertical advection term (strd_advz) at the first layer is actually positive with similar spatial pattern as strd_cdt, with vertical diffusion term (strd_zdf) seems to compensate it. The net effect, strd_advz(k=1)+strd_zdf(k=1)+strd_cdt, tends to be positive in ice formation sites which is reasonable for freezing. However, in the vertical, the salt seems not be forced by relatively small and negative strd_cdt, but by large and positive vertical advection strd_advz. The way it works is not familiar to me. Could anybody explain why it works in this way and how does NEMO relate vertical advection and diffusion to surface freshwater forcing?

Many Thanks,

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