Antarctic sea ice

[dabail10]

The Antarctic sea ice extent is too large in winter. This is for two historical runs (092 and 098b).

[duvivier]

This bias has continued into more of the test runs. It's present in PI and historical runs and current tests (e.g. #40 or #45) modifying properties have not impacted it substantially and we do not believe it will be something we can fix with sea ice albedo tuning. There has been some seasonality shift in recent tests that we also do not understand well (e.g. #48 & #49).

PCWG winter meeting discussed possibilities about what could be causing this showstopper bias. Slides about our investigation can be found here.

The CESM3 WINTER sea ice is the problem, while summer looks okay. But winter ice area is way outside historical bounds as well as both the CESM1-LE and CESM2-LE. There is also no paleo evidence that we're aware of to suggest the ice has ever been this extensive.

Other WG folks joined the discussion and we came up with the following ideas to test:

1. Do some model archeology to see when the bias started. I will do this with @dabail10 and look at old test simulations.
2. @dabail10 and I will check if this is primarily an ice edge issue or if it's also related to too high of concentrations within the pack. We know there is an ice edge bias, but we'll quantify this better.
3. Run a nudged CESM3 simulation to see if atmospheric SH jet location or strength biases might impact the sea ice. There were other reasons to run this and @islasimpson and @JulioTBacmeister are working on getting this set up.
4. Investigate if extensive (liquid?) cloud or precipitation biases exist in CAM. We've spoken with @adamrher about this and will look further.
5. Check if teleconnections of the SAM or ENSO might be impacting the bias here. There were some PCWG members who volunteered to help do some analysis on this.
6. Check if other MOM6 groups have cold biases in this area. @iangrooms and @gustavo-marques have been emailed and Ian suggested that tuning the GM parameter might help substantially. Ian pointed out that in Adcroft et. al 2019 (http://dx.doi.org/10.1029/2019MS001726) they showed that in MOM6 1/2deg there are biases in winter (September) similar to ours when GM is turned on (panel H) compared to when it's turned off (panel I). Ian is going to run some CESM3 tests where he tunes the GM parameter since he thinks that likely needs some adjustment. It does look like the GM tuning affects summer sea ice less (see bottom row), so hopefully this could improve winter ice without degrading summer ice too much.