I’ve emailed this issue to @MartinDix, but I thought I’d post it here as well in case it’s relevant to others.
In my setup of the Last Glacial Maximum (LGM) climate with ACCESS-ESM (present in /home/561/hs2549/access-esm1.5/lgm), I’ve made several changes to the boundary conditions — including GHGs, orbital parameters, ice sheets, topography, and a global salinity increase of 1 PSU from the PI value. These were all implemented around year 11 in the attached time series plot; Orange is LGM, blue is PI.
However, the issue is that I’m seeing a rapid decrease in globally averaged salinity during the run. I am plotting the SALT_GLOBAL_AVE from the ocean-scalar-1monthly-ym_*.nc files.
Could you please let me know how the global salinity budget is calculated in the model?
I’ve also attached sea surface salinity plots, which show an increase in the north — mainly due to topography changes — and some decreases in other regions. However, since total salinity should be conserved, I’m concerned something may be off. I’ve checked that there is no external freshwater input added in the setup.
I’m seeing a rapid decrease in globally averaged salinity during the run
Thanks for sharing your simulation issue @HIMADRI_SAINI. Very curious!
This is a lot of changes all at once. To refine our understanding, have you tried doing these perturbations (where practical) one at a time to see what is driving the very large change in the North of your domain? As you’ve shown it’s a very regional response, that makes me think it could be one aspect of the forcings changes you’ve done.
Do any of your ice sheet changes affect the freshwater budget?
I’ve actually done a step-by-step implementation of boundary condition changes in a separate simulation for the climate around 49,000 years ago. In that case, I found that the large increase in sea surface salinity (SSS) in the north starts with the orbital changes but becomes more pronounced with the topographic modifications. So I agree — some of the regional responses are likely tied to specific forcings.
However, my main concern here is not with the regional changes in SSS but with the drift in global mean salinity, which I had assumed would be conserved. I’ve checked that no external freshwater forcing is applied in the setup, so I’m trying to understand whether some aspect of the ice sheet or topography changes might be unintentionally affecting the global salinity or freshwater budget — which is exactly what I’m hoping to clarify here.
What is happening with the global water volume (its just called total_mass_seawater in MOM5) ? If this is increasing then declining salinity would be a symptom.
Is their sea-ice in your configuration? If the volume of sea-ice is increasing, then that would remove some salt from the ocean.
There is sea-ice in my configuration, which should have been expanding given the very cold nature of the LGM climate (I’ll double-check to confirm). For the LGM, the global ocean volume should be reduced due to the extensive ice sheet formation. I suspect there might be a missing link or setting that connects the imposed topography and ice sheets with a corresponding reduction in global ocean volume.
So the LGM run should have initialised with a smaller global water volume, but in fact has a larger water volume? I assume you made a new land-mask and ocean bathymetry to go with the topography and land-mask change? I am definitely out of my depth, but the sea-level diagnostic might help understand what has happened there:
For the increasing volume, I guess identify where the water is coming from. Have a look at the MOM fields for runoff, precip and sea-ice melt see if there is anything obvious going on. Precip fields are lprec and fprec, runoff fields are runof and licefw (possibly zero due to the configuration), sea-ice melt water is wfimelt and wfiform.
There is a workbook here on a global water balance, as I understand it there isn’t a version for esm1.5 but, as always, we would appreciate a contribution of one
for reporting, not for the issue per se!)
