"explosive" isopycnal diffusion

[mouchet]

The following behavior is obtained with configuration OFF_MY_TRC & NEMO v3.4

I used several tracers without sms and sbc (i.e. E-P forced to 0). Results are reported for tracers 1 and 6.
Tracer 1 is initialized homogeneously (1 everywhere), tracer 6 has initial vertical and horizontal gradients.

Activating the isopycnal diffusion results in a rapid divergence of concentrations.

The results presented hereafter are obtained every 5 years.
The situation is bad from the start for tracer 6, and degrades severely for tracer 1 after 15 years. Activating the correction of negative values does not help.

Ch. Ethé obtained exactly the same behavior on a different computer with his own version of the model.

Such behavior does not happen with horizontal diffusion (though conservation is not obtained, see #1134).

Many thanks for helping resolve this.
Anne

curie71:EXP00> grep drift ocean.output-TESTIS01

tracer nb : 1 name : H2Ot mean : 0.1000000000E+01 min : 0.9999999996E+00 max : 0.1000000010E+01 drift : 0.2666078212E-09 %
tracer nb : 6 name : ASEL mean : 0.3470195362E+02 min : -0.5063662861E+01 max : 0.1075321689E+04 drift : -0.1702945242E-03 %

tracer nb : 1 name : H2Ot mean : 0.1000000000E+01 min : 0.9999972291E+00 max : 0.1000072516E+01 drift : 0.5318454538E-09 %
tracer nb : 6 name : ASEL mean : 0.3470156440E+02 min : -0.2963918080E+06 max : 0.7757576421E+07 drift : -0.1291890798E-02 %

tracer nb : 1 name : H2Ot mean : 0.1000000000E+01 min : 0.9793435102E+00 max : 0.1540584520E+01 drift : -0.1919518417E-08 %
tracer nb : 6 name : ASEL mean : 0.3179551460E+02 min : -0.2209776402E+10 max : 0.5783029664E+11 drift : -0.8375589448E+01 %

tracer nb : 1 name : H2Ot mean : 0.9999997975E+00 min : -0.1529883352E+03 max : 0.4030905900E+04 drift : -0.2025247416E-04 %
tracer nb : 6 name : ASEL mean : -0.2163199347E+05 min : -0.1647326323E+14 max : 0.4311086329E+15 drift : -0.6243642309E+05 %

tracer nb : 1 name : H2Ot mean : 0.9984901557E+00 min : -0.1147938827E+07 max : 0.3004181861E+08 drift : -0.1509844287E+00 %
tracer nb : 6 name : ASEL mean : -0.1615190996E+09 min : -0.1228035546E+18 max : 0.3213793881E+19 drift : -0.4654460120E+09 %

tracer nb : 1 name : H2Ot mean : -0.1025546557E+02 min : -0.8557569272E+10 max : 0.2239533200E+12 drift : -0.1125546557E+04 %
tracer nb : 6 name : ASEL mean : -0.1204079821E+13 min : -0.9154660386E+21 max : 0.2395793153E+23 drift : -0.3469769406E+13 %

tracer nb : 1 name : H2Ot mean : -0.8390533729E+05 min : -0.6379427751E+14 max : 0.1669509155E+16 drift : -0.8390633729E+07 %
tracer nb : 6 name : ASEL mean : -0.8976077174E+16 min : -0.6824542420E+25 max : 0.1785996565E+27 drift : -0.2586615724E+17 %

tracer nb : 1 name : H2Ot mean : -0.6254981977E+09 min : -0.4755684369E+18 max : 0.1244572225E+20 drift : -0.6254981987E+11 %
tracer nb : 6 name : ASEL mean : -0.6691413647E+20 min : -0.5087504864E+29 max : 0.1331410320E+31 drift : -0.1928249437E+21 %

tracer nb : 1 name : H2Ot mean : -0.4662913545E+13 min : -0.3545229243E+22 max : 0.9277936686E+23 drift : -0.4662913545E+15 %
tracer nb : 6 name : ASEL mean : -0.4988261100E+24 min : -0.3792592111E+33 max : 0.9925290318E+34 drift : -0.1437455844E+25 %

tracer nb : 1 name : H2Ot mean : -0.3476071197E+17 min : -0.2642868915E+26 max : 0.6916441443E+27 drift : -0.3476071197E+19 %
tracer nb : 6 name : ASEL mean : -0.3718608669E+28 min : -0.2827270991E+37 max : 0.7399025409E+38 drift : -0.1071582994E+29 %

curie71:EXP00>