Baltic Sea salinity steadily declines

[kieranricardo]

The Baltic Sea surface salinity steadily declines in CM3 0.25 degree and will eventually reach 0 (probably ~70 years) crashing the model. Currently the CM3/OM3 grid is missing hand edits to widen and deepen the channel, adding these bathymetry hand edits should fix this problem: ACCESS-NRI/access-om3-configs#802.

[MartinDix]

Original CM2 0.25 degree Baltic average SSS

Fixes described at COSIMA/access-om2#158 (comment)

[MartinDix]

Rate of change of Baltic total salt mass in CM3

C.f. ACCESS-Community-Hub/access-om3-paper-1#44 (comment) which shows a mean restoring flux of ~80 000 kg/s in OM3. If that was significantly reduced by widening the channel it should resolve the problem in CM3 too.

[MartinDix]

[kieranricardo]

Some obvs for comparison (1.5m 2015-2019 avg) from (https://www.researchgate.net/publication/348974649_Diversity_and_Physiological_Tolerance_of_Native_and_Invasive_JellyfishCtenophores_along_the_Extreme_Salinity_Gradient_of_the_Baltic_Sea):

[aekiss]

As far as I know we don't have output data from an IAF OM2-025 run with the updated bathymetry, but we can at least use TOPO5 (025deg_jra55_iaf_omip_straits_topo5_cycle1) in this plot from here as a benchmark. This uses the latest OM2-025 bathymetry (same as CM2-025) and is an IAF run.

Note that the earlier topos (e.g. OMIP) have positive restoring of roughly +60,000 kg/s, consistent in sign and rough magnitude with the -60,000 kg/s salt loss in @MartinDix's plot from CM3 above.

This suggests that if we can tweak the topo to get restoring of about -30,000 kg/s in an IAF run (as in TOPO5) that may be good enough (though zero would presumably be better).

[access-hive-bot]

This issue has been mentioned on ACCESS Hive Community Forum. There might be relevant details there:

https://forum.access-hive.org.au/t/cosima-twg-meeting-minutes-2025/4067/22

[aekiss]

IAF runs with OM2 and OM3 (both without Baltic fixes) are compared to TOPO5 here ACCESS-Community-Hub/access-om3-paper-1#44 (comment)

[kieranricardo]

The Baltic topo fixes seem to be working well - nice work @ezhilsabareesh8 !