### Monthly sa-ice extent from 1979-present

### Sea ice polygon monthly extent data downloaded from https://noaadata.apps.nsidc.org/NOAA/G02135/south/monthly/shapefiles/shp_extent/ on 2026-09-14
    Fetterer, F., Knowles, K., Meier, W. N., Savoie, M., Windnagel, A. K. & Stafford, T. (2025). Sea Ice Index. (G02135, Version 4). [Data Set]. Boulder, Colorado USA. National Snow and Ice Data Center. https://doi.org/10.7265/a98x-0f50
    
### Processing steps:

    1.  Run 02_download.py from cwd
        * Traverse the monthly NSIDC G02135 archive and download Antarctic sea-ice extent polygon ZIPs directly to cwd/input

    2.  Run 03_prepare_polygons.py from cwd
        * Read shapefiles from each ZIP and reproject from declared source CRS to EPSG:3031
        * Merge and clean the geometries, then dissolve to one feature per year and month
        * Derive MONTH, MONTH_SHOR and an end-of-month DATE field, and write unsmoothed master polygons to cwd/output

    3.  Run 04_smooth_polygons.py from cwd
        * Apply PAEK-like cartographic smoothing to the monthly polygon boundaries using 5 km boundary sampling
        * Apply topology-preserving post-smoothing simplification to reduce redundant vertices
        * Write the final smoothed polygon shapefile to cwd/output