Thanks a lot @Aidan. I tried using iris.guess_bounds with conservative remapping, but that method does not sum. However, your comment prompted me to read about the other regridding methods properly, and it seems as though the following method will do what I want:
- Nearest destination to source: Each source point is mapped to the closest destination point. A destination point can be mapped to multiple source points, in which case the destination is the sum of the source values. Some destination points may not be mapped.
In short, the solution was to use xesmf with the method nearest_d2s.
In case others come across this, I’ve put my code on my GitHub, including the steps on how to convert the projection: https://github.com/dougrichardson/tutorials/blob/main/population_mask.ipynb