While examining the zos variable from ACCESS-ESM1-5, I noticed that the values are negative across the entire domain. In contrast, when I checked the zos variable from ACCESS-CM2, the values appear to fall within a normal and expected range.
I would like to ask why the zos variable in ACCESS-ESM1-5 shows predominantly negative values, whereas ACCESS-CM2 does not. Any explanation or guidance would be greatly appreciated.
Hi @pkwon8423!
Thanks for opening this topic. It looks like these datasets are “ACCESS-XXX output prepared for CMIP6”. This preparation was likely completed by our colleagues at CSIRO, so I suspect that they’re better placed to clarify this. I’m going to ping a few folks here that might be able to better answer your question!
@chloemackallah / @Chloe_Mackallah, @tiloz, and @matthew.chamberlain – are you able to provide any insights into the differences in the zos variable between the ACCESS-ESM1.5 and ACCESs-CM2 models?
Cheers,
Lawrence
Hi @pkwon8423
Just quickly:
a) I get the same thing, so it’s probably not an error reading in the data
b) the long name seems to be “Sea surface height above geoid”, so it’s not physically problematic to have negative values, it just means that the ocean is below what the model seems to be defining as geoid
c) the ACCESS-ESM1-5 water cycle doesn’t close, and so global average sea level goes steadily down over time. I would guess that the sea level started out reasonable (ie, not negative), and slowly went down over multiple cycles of tuning, and running the pre-industrial control, etc. @sofarrell or @tiloz may be able to confirm this
Long story short, I think this is one of those model quirks rather than something to actually be concerned about.
(I’m going on leave tomorrow, and so probably won’t be able to respond again for a couple weeks, but I hope this helps)
@matthew.chamberlain has replied to @pkwon8423 , but in short @jemmajeffree was spot on with c. Here is Matt’s reply in full:
The negative sea level values in ESM1.5 are a result of a discrepancy in the balance of water forcing in the coupled system and the long spin up of the ocean.
While the mean values of sea level are different, I think you will find the relative spatial distribution and ocean dynamics to be otherwise similar.
Note that the ESM1.5 and CM2 have different atmospheric and sea ice models so the ocean dynamics are not going to be the same.
The ESM1.5 paper, Ziehn et al. 2020, https://doi.org/10.1071/ES19035,
discusses the drift in global salinity, the sea level is impacted in a similar way.
“The linear trend in global salinity indicates that ACCESS-ESM1.5 still has the same discrepancy in coupling water, related to use freshwater mass fluxes in a volume-conserving model, as discussed in Marsland et al. (2013).”
The relevant section of Marsland et al 2013, doi:10.22499/2.6301.007
“This is attributed to non-closure in the hydrological cycling between the various model sub-components.
As described in Bi et al. (2013b), the ACCESS-OM uses the Boussinesq approximation so the ocean interior is volume conserving.
However, the surface freshwater fluxes are real mass fluxes, which allows the ocean volume to change…”
@lawrenceabird @jemmajeffree @clairecarouge
Thank you to everyone who responded. I have another question. When comparing ZOS with CM2 and other CMIP6 models, I want to adjust the range to a similar level. How should I do this? Should I set the reference surface level similar to that of the other models? (For example, should I add about 3 meters?)
I havent looked at the actual data @pkwon8423 but yes I guess to do a comparison with CM2 and other CMIP6 models with positive values yes you would want to switch the ESM1.5 to have a positive mean value datam closer to the range of those models. Or you could just look at the relative change in sea level through the ESM1.5 runs as from say the scenario runs to the ESM1. 5 control run from to get the sea level component from thermal expansion.