# North fold bug? Artefact along north fold in sea ice variables **URL:** \ **Category:** Versions\ **Tags:** orca, SI3\ **Created:** [15 December 2025 10:14 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972 "2025-12-15T10:14:35Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [15 December 2025 10:14 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/1 "2025-12-15T10:14:35Z")
I run `ORCA2-ICE` simulations and often see artefacts in output variables along the grid cells where north-folding occurs. It sometimes takes several years before it appears, depending on the forcing and namelist settings. An example: ![fig01_0055-08](https://global.discourse-cdn.com/free1/uploads/nemo_ocean/original/1X/c75a025bc9ce135372734253e0220b5af8e2f1e1.png) This is sea ice concentration, August year 0055, in a configuration of `ORCA2-ICE` (`PISCES` removed during build), main branch ([#0bb9add6](https://forge.nemo-ocean.eu/nemo/nemo/-/tree/0bb9add612109e7503bb2fd135cce9518ab553e1)), with the standard input climatological forcing from SETTE v5.0. For the record, the following `cfg` namelist parameters are changed in this run but I don’t think these matter to the root problem: - Form drag activated (`ln_Cx_ice_cst = .false.; ln_Cx_ice_frm = .true.`) - Reduce ice/snow layers (`nlay_i = 4; nlay_s = 1`) - `ln_leadhfx = .false.` - Change melt pond scheme (`ln_pnd_LEV = .false.; ln_pnd_TOPO = .true`) **The artefact is** _ **not** _: - A plotting issue (it can be seen with, e.g., `ncview`) - Related to above custom namelist changes, since it can also be seen in a run without any of them activated (though it is less obvious; see below, top-left) - To do with input forcing (in this post I just show it occurring with SETTE forcing to keep the examples as reproducible as possible, but I have also seen this with other custom forcings, e.g., JRA-55-do) **Further testing indicates (see _Fig._ below):** 1. It might not be occurring in 5.0.1 ([#6f645ba8](https://forge.nemo-ocean.eu/nemo/nemo/-/tree/6f645ba8c1d97fb6c73c4deb3760148f5c5d1b9d)) 2. In main ([#0bb9add6](https://forge.nemo-ocean.eu/nemo/nemo/-/tree/0bb9add612109e7503bb2fd135cce9518ab553e1)), it seems to disappear with `ln_nnogather = .false.` To emphasise this occurs in the reference configuration (i.e., minimum reproducible example), here is a comparison of (left) main vs. (right) 5.0.1, with and without `ln_nnogather` (top vs bottom), in `ORCA2-ICE`, no other namelist changes. Each panel is June year 45 and I have zoomed in on the colour scale so that is clear. Note how `ln_nnogather` makes no difference in 5.0.1 (as expected, if I understand it correctly): ![fig02_no-nml-changes_0045-06](https://global.discourse-cdn.com/free1/uploads/nemo_ocean/original/1X/115b3edf20ee69256b18458cde937d81c99db501.png) I think this suggests the newer north-folding code (or whatever is activated by `ln_nnogather`) is somehow not working correctly in main, but it’s beyond me to find out any more than this… Jake ---
**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\ **Post date:** [15 December 2025 17:28 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/2 "2025-12-15T17:28:13Z")
Could you try the main + `ln_nogather = T` + in `mpp_nfd_generic.h90` you replace: ```fortran llfull = .FALSE. ``` by ```fortran llfull = .TRUE. ``` ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [16 December 2025 06:51 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/3 "2025-12-16T06:51:34Z")
**Yes, that does seem to resolve it.** Repeat of above second figure: year 0045 June, only top-left panel changed after changing `llfull = .TRUE.` in `mpp_nfd_generic.h90`. The artefact also does not appear for at least a few years more (went up to year 0066 with no sign of it). Left panels now look the same: ![fig02_no-nml-changes_0045-06_llfull=T](https://global.discourse-cdn.com/free1/uploads/nemo_ocean/original/1X/730f9fbd9df2da41757dd319ad4db45befe9ca21.png) (I’m still running the first simulation with our various other custom namelist changes, plus `llfull = .TRUE.`, to confirm). Jake ---
**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\ **Post date:** [17 December 2025 09:35 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/4 "2025-12-17T09:35:01Z")
Thank you for these tests. We have tests within our continuous integration process which are supposed to detect this kind of errors… 😩 I will do additional tests on my side but could you compare the restart files of 2 experiments which differs only by the definition of `llfull = .TRUE.` or `.FALSE.`. ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [18 December 2025 09:36 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/5 "2025-12-18T09:36:29Z")
No problem, thanks for looking into it. 🙂 The restarts for the above tests are end of December so it’s not really visible (actually might not be present) in either case, although the variables are different between the two restart files (well, I only looked at `a_i`). So I ran these simulations again but ending after 30 June year 0046 so we get restarts in summer, and below the artefact can be seen in the default run with `llfull = .FALSE.` (left) but not in the run where that alone is changed to `.TRUE.` (right): ![fig_00132848_restart_ice_0006_llfull_F_vs_T](https://global.discourse-cdn.com/free1/uploads/nemo_ocean/original/1X/e23908fcdec49f4d0856777df172ead240e60a9a.png) Here I calculated `at_i` from `a_i` in the restart files, but it can also be seen in the individual thickness categories (most obviously in the smallest category). Also, just to confirm changing `llfull` does remove the artefact in our custom configuration (various namelist changes; first Fig. in original post) as well. By the way, I note `llfull = .FALSE.` in the 5.0.1 version of `mpp_nfd_generic.h90` as well (the whole file is identical), even though the problem doesn’t seem to arise there (I assume you’re aware of this but just noting anyway as I’m curious why it only seems to matter in main). ---
**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\ **Post date:** [18 December 2025 10:50 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/6 "2025-12-18T10:50:04Z")
Thank you for these additional tests, which confirm the problem in the main. Setting `llfull = .TRUE.` is only a way to correct (or hide) a problem appearing somewhere else… ☹ If the code was properly working, setting `llfull` to `.TRUE.` or `.FALSE.` should give you exactly the same results. We will check why it is no more the case in the main and why we were not able to detect this in our sanity tests on GitLab. ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [7 January 2026 13:29 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/7 "2026-01-07T13:29:31Z")
Hello Jake, we are not able to reproduce the problem (same revision, same ORCA2-LIM3 config). Assuming that you didn’t modify the code itself, we guess that you are using slightly different namelists. Would you like to share your namelist\_cfg and namelist\_ice\_cfg with us ? Best regards, Eric Maisonnave (at [locean.ipsl.fr](http://locean.ipsl.fr)) ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [7 January 2026 15:18 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/8 "2026-01-07T15:18:26Z")
Hi Eric, In the runs I’ve shown in this thread we have not included any modified code (except the `mpp_nfd_generic.h90` as discussed). In the top-left panel of my earlier figure we also have no changes to `namelist_cfg`, **except** I did forget to mention: `ln_icebergs` is set to `.false.`., and some `cn_dir` are changed to point to the data on our system. I attach the `namelist_cfg` below anyway, as it is entirely possible I’ve missed/forgotten about something (`namelist_ice_cfg` is clearly not changed except for the restart directory paths, so I haven’t attached it here). [\>\> Link to copy of namelist\_cfg (Google Drive) \<\<](https://drive.google.com/file/d/1VOOcQQX1PceMOnwVVk49j3HO1eAG0t41/view?usp=sharing) Jake ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [12 January 2026 13:55 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/9 "2026-01-12T13:55:19Z")
Hi Jake, still not possible to reproduce your bug. I forgot to ask you which machine you use and your compiling options. Could you send your arch/\*fcm file ? ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [12 January 2026 21:06 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/10 "2026-01-12T21:06:40Z")
I link to our `*.fcm` files below. We compile with default/prod flags. I thought maybe optimisation could be the culprit, but `-O3` is also set in the template [arch-linux\_gfortran.fcm](https://forge.nemo-ocean.eu/nemo/nemo/-/blob/main/arch/arch-linux_gfortran.fcm?ref_type=heads) which is what ours is based on. Also, if helpful we have GCC 8.5.0 20210514 (Red Hat 8.5.0-28), OpenMPI 4.1.1, netcdf 4.7.0. Is it also worth trying your end the namelist changes from the very first figure/post in this thread where the artefact is more obvious? (`ln_Cx_ice_frm = .true.`; `nlay_i = 4`; `nlay_s = 1`; `ln_leadhfx = .false.`; `ln_pnd_TOPO = .true.`). [\>\> Link to our .fcm file for NEMO (GitHub) \<\<](https://github.com/jraylmer/NEMO-SI3-UoR/blob/main/resources/nemo/arch/arch-RACC2.fcm) [\>\> Link to our .fcm file for XIOS2 (GitHub) \<\<](https://github.com/jraylmer/NEMO-SI3-UoR/blob/main/resources/xios/arch/arch-RACC2.fcm) ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [21 January 2026 10:19 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/11 "2026-01-21T10:19:35Z")
No change with different compile options, or nothing more obvious with your namelist parameters. May I propose you to recompile NEMO with these modified routines ? [lbc\_lnk\_pt2pt\_generic.h90 - Cloud Fédération IPSL](https://cloud.ipsl.fr/index.php/s/JoD5K3zaAomGWR5) , [lbc\_lnk\_neicoll\_generic.h90 - Cloud Fédération IPSL](https://cloud.ipsl.fr/index.php/s/MjfiPxsMw5tRsmd) , [mpp\_nfd\_generic.h90 - Cloud Fédération IPSL](https://cloud.ipsl.fr/index.php/s/LcA4kkKMDKWMpf9) A new file should appear in your working directory (fort.71), identifying places and variables which North Pole folding update may cause problems. Could you send me a fort.71 file produced by (the last chunk of) your problematic simulation ? ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [21 January 2026 16:01 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/12 "2026-01-21T16:01:53Z")
I just set up the original run with those source changes and the `fort.71` output is filling up very fast (almost 1MB after a few seconds of the run) so I might not have the disk space to complete the whole simulation until the artefact appears in the outputs. So I’m just attaching what I have here in case this is enough to go on, which it looks like it might be, but I’ll let you interpret… [\>\> fort.71 output [Google Drive] \<\<](https://drive.google.com/file/d/1ZP3wVFwC3xqmA2-FhMWHKuqJ2oV1LhGU/view?usp=sharing) Let me know if the full run (until artefact appears) would be useful and I can workaround for the file size. ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [4 February 2026 10:52 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/13 "2026-02-04T10:52:58Z")
Thank you for the checking file. I realised that it takes some times for the artifact been visible on this diagnostic (basically 299 time step here). I modified the lbc\_lnk routines to limit the output to the error messages (and stop the simulation there). However, the diagnostic reveals a North Pole mismatch in the model prognostic variables, which doesn‘t really help us to find the precise origin of the problem. More work needed from my side … ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [16 March 2026 11:35 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/14 "2026-03-16T11:35:15Z")
Hi Jake, after investigations, we found a wrong update of North Pole boundary conditions in one ice model routines. The detail is here: [Redefine lbc\_lnk strategy at the end of routine to ensure NPF duplicate grid point identity (14466146) · Commits · NEMO Workspace / Nemo · GitLab](https://forge.nemo-ocean.eu/nemo/nemo/-/commit/14466146b739186a933cf9bdd8f0605c1b7bcb34) If you have time and CPU resources, I propose you to include these modifications on your code and check whether the North Pole Folding effect on your sea ice disappear or not. I am sorry not to be able to give you more guarantees that this is the solution to your issue. At least, for sure, it is a part of it Best, Eric ---
**Author:** ![jraylmer](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jraylmer](https://nemo-ocean.discourse.group/u/jraylmer)\ **Post date:** [18 March 2026 12:17 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/15 "2026-03-18T12:17:46Z")
Thanks, Eric, for the ongoing efforts with this. I can confirm this does make it disappear. Comparison of simulation shown at the top of the original post repeated with the updated `icedyn_adv_pra.F90` at [#14466146](https://forge.nemo-ocean.eu/nemo/nemo/-/commit/14466146b739186a933cf9bdd8f0605c1b7bcb34) as the only difference. This is at year 55 (with repeat-year forcing), and it is still not visible (on the right) up to year 66, which is far as I went for now: ![image](https://global.discourse-cdn.com/free1/uploads/nemo_ocean/original/1X/2e47e4b4d02aacd7baa66a76626ba5dd8db87625.jpeg) ---
**Author:** ![emalod](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@emalod](https://nemo-ocean.discourse.group/u/emalod)\ **Post date:** [17 April 2026 10:12 UTC](https://nemo-ocean.discourse.group/t/north-fold-bug-artefact-along-north-fold-in-sea-ice-variables/972/16 "2026-04-17T10:12:01Z")
Issue fixed in 5.0.x [!1112](https://forge.nemo-ocean.eu/nemo/nemo/-/merge_requests/1112) and main [!1072](https://forge.nemo-ocean.eu/nemo/nemo/-/merge_requests/1072)