Climate of MTso vs LTso

[cecilehannay]

The goal of this thread is to assess differences between MTso and LTso.

Contacts: @cecilehannay @dlawrenncar @adamrher @PeterHjortLauritzen @JulioTBacmeister @swrneale @slevis-lmwg @wwieder @olyson @gustavo-marques @iangrooms @klindsay28 @dabail10 @duvivier @tilmes @mbramberger @gunterl

[islasimpson]

Here are some plots comparing HTso with LTso. I'm only using the first 20 years of LTso to be comparable to the 20 years that we have of HTso in terms of where it's at in the drift. I've randomly sampled 200 pairs of 20-year chunks from the CESM2 piControl and differenced them. Where it is stippled is where the difference between HTso and LTso lies within the 2.5th to 97.5th percentile range of differences obtained from the CESM2 sampling i.e., where there is not a detectable difference between HTso and LTso with this length of simulation. Note that we'd expect at least 5% of the planet to appear significant by change, probably quite a bit more because of the spatial autocorrelation in the patterns - I haven't done a field significance test to account for that, but basically we shouldn't be alarmed by small patches of significance.

I think the one thing that is potentially of concern is the difference in TAUX in the Southern Ocean in DJF. This is something that does appear different and also could intuitively be expected to be impact by the change in the model lid as the lid could affect the evolution of the springtime breakdown of the SH polar vortex with impacts on the Southern Hemisphere Jet into the summer. It might be nice to run MT for longer to see if this is robust.

TREFHT:

LHFLX:

SHFLX:

FSDS:

FLDS:

TAUX:

TAUY:

[swrneale]

Any way we could analysis if the Brewer Dobson circulations is having a substantially different impact in DJF?

[tilmes]

Shawn could run the WACCM diagnostics

[islasimpson]

Impact on what in DJF and in what hemisphere? It's possible there's something different in the North Atlantic in DJF as a result of stratospheric variability but I'm skeptical this will be a robust signal given how much variability there is there.

[cecilehannay]

Difference between namelists: MTso and LTso in 280:

[cecilehannay]

Updated differences in 308:

[islasimpson]

Here are the same plots as at the top of this discussion but for the 308 simulations

TREFHT: Nothing similar to the anomalies in 280

LHFLX: There is some similarity here in the North Atlantic with the signal in 280 in DJF. Could be some real difference in the winds there, but it looks like a pretty small difference.

SHFLX: Also similarity here in the North Atlantic in DJF to 280.

FSDS: Not much that's similar to 280 here.

FLDS: Not much that's similar to 280 here.

TAUX: Some similarity with 280 in the North Atlantic, but the Southern Ocean signal is not here.

TAUY: Also similarities in the North Atlantic during DJF with 280

So, it seems the only signal that might be robust between these two sets of simulations is differences in the North Atlantic during DJF between MT and LT.

[dlawrenncar]

Thanks Isla, I think it's worth summarizing this at the Project meeting next week, at least for general awareness. Dave

…

On Thu, Feb 19, 2026 at 9:13 PM islasimpson ***@***.***> wrote: Here are the same plots as at the top of this discussion but for the 308 simulations TREFHT: Nothing similar to the anomalies in 280 Screenshot.from.2026-02-19.20-42-22.png (view on web) <https://github.com/user-attachments/assets/bac0b3bf-407b-436b-83e0-ee65c42d568e> LHFLX: There is some similarity here in the North Atlantic with the signal in 280 in DJF. Could be some real difference in the winds there, but it looks like a pretty small difference. Screenshot.from.2026-02-19.20-46-16.png (view on web) <https://github.com/user-attachments/assets/f53ddbb2-bc53-46fb-875b-54aa38c1321f> SHFLX: Also similarity here in the North Atlantic in DJF to 280. Screenshot.from.2026-02-19.20-50-47.png (view on web) <https://github.com/user-attachments/assets/0f3e5062-edcb-4699-9db7-f93177a1fb7f> FSDS: Not much that's similar to 280 here. Screenshot.from.2026-02-19.20-54-09.png (view on web) <https://github.com/user-attachments/assets/38a1ef81-7eb5-4eab-bc94-2a78a96b9f53> FLDS: Not much that's similar to 280 here. Screenshot.from.2026-02-19.20-58-00.png (view on web) <https://github.com/user-attachments/assets/5a33060a-fd36-49ff-b8d0-92627b4eaf09> TAUX: Some similarity with 280 in the North Atlantic, but the Southern Ocean signal is not here. Screenshot.from.2026-02-19.21-02-10.png (view on web) <https://github.com/user-attachments/assets/f2dd08af-480c-4088-87a7-918090f91988> TAUY: Also similarities in the North Atlantic during DJF with 280 Screenshot.from.2026-02-19.21-09-09.png (view on web) <https://github.com/user-attachments/assets/34568d28-6589-4dab-b194-fd259390f4a3> So, it seems the only signal that might be robust between these two sets of simulations is differences in the North Atlantic during DJF between MT and LT. — Reply to this email directly, view it on GitHub <#291 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AFABYVF7NVZXKR4ROY7JQMT4M2CV7AVCNFSM6AAAAACU6IOJ6SVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKOBWG4ZDANQ> . You are receiving this because you were mentioned.Message ID: ***@***.***>