Freshwater flux in POP

[karthik23]

By default, POP uses virtual salt fluxes, meaning that the
global-mean ocean volume cannot change. When you add anomalies
to the PREC_F field, the model will convert that to a virtual
salt flux. Thus, even though you may see local sea surface
height changes, its global-mean must not (cannot) change.
Instead, depending on the sign of the anomaly, you will
see a change in the global-mean salt content. This would mean that
even though we set lfw_as_salt_flx as 'true', we may still see some local changes in sea-level height when you add anomalies to the precipitation field.