Fix interior OBC indexing bug in tracer advection #1013
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Hallberg-NOAA
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Jan 5, 2026
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I have examined these changes and I agree with them. Thank you for tracking down and fixing this bug.
There is no bug fix flag needed here because this is in essence a parallelization bug and it will not change the single PE answers. There are no valid answers to recreate. Moreover, I do not think that there will be other production cases that are impacted by this.
Move nsten_halo out of iteration loop
Fix a bug that tracers in domain outside of the OBC is falsely updated when then the OBC is in the interior. The bug was due to an indexing error in routine advect_x.
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This PR has passed pipeline testing at https://gitlab.gfdl.noaa.gov/ogrp/mom6ci/MOM6/-/pipelines/29801. |
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This PR fixes a bug that can occur in some marginal cases with an interior OBC.
In subroutine
advect_x, the masking that avoids tracer points right outside of OBC to be updated,do_i, is not applied to the last valid point in the zonal direction. This indexing bug causes problems when the interior OBC is right on the edge of a processor's computational domain, or any of the the marching stencil boundaries during the iteration. This also means the answer is sensitive to PE layout.Two comments/questions:
I did not include a bug flag as I would imagine this bug rarely happens. Not sure how many people are actively using interior OBC. And the bugs is only triggered by interior OBC placed right on the boundary of a processor. Let me know if a bug flag is warranted.
The patch fixes my unlucky case and passes the TC tests. But I am not sure if it breaks or changes answers to any other OBC runs. So any help with additional tests is appreciated.