Why do these irregularities appear in the trench?

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Hello all. I'm checking the trench smoothing in a nested grid. I don't know way I obtain, consistently at different latitudes and only in the nested grid, some irregularities in the trench as seen in the figure attached (croco_grd line). Do you know why this happens?, and how could I fix this problem? Thanks in advance. C:\fakepath\irregularities_at_the_trench.png