To check tracer conservation in advect_xz/input.nlfs set-up,
and in general when
- using NonLin FreeSurf
- using or not Adams-Bashforth (used here for Salinity)
- without thermal/salt/fresh-water forcing

From a standard run with input.nlfs parameter files:
> cd advect_xz/tr_run.nlfs

Download (using cvs):
> cvs co -d tmp MITgcm_contrib/jmc_script/extract_StD
> mv tmp/extract_StD .
> rm -rf tmp
> cvs co -d tmp MITgcm_contrib/jmc_script/read_StD.m
> mv tmp/read_StD.m .
> rm -rf tmp

Split multi-variable stats-diags file into variable specific files:
> ./extract_StD dynStDiag.0000000000.txt dynStD std

Plot global stats evolution using matlab:
>> grph_StD_AB

figure-3, top-panel: "Avr T"
evolution of global averaged (potential) Temperature.
 (should be flat if conserving)

figure-4, top-panel: "Avr S"
evolution of global averaged Salinity

Since Adams-Bashforth is used (alphAB= 0.6 = 0.5+abEps) for Salt
with NonLin-FreeSurf, we don't expect to exactly conserve Salt,
but should not drift and exactly conserve a slightly modified
quantity: S_ab = S - alphAB*deltaT*gSnm1

After editing grph_StD_AB.m (commenting out the 2nd spec of alphAB):
>> grph_StD_AB

figure-3 is unchanged but top-panel on figure-4, now plot
the evolution of global averaged S_ab.
 (should be flat if conserving)
Note a problem of conservation at the very 1rst iteration,
but later on the curve is flat. This problem is not seen
after a restart.