The interannual variation used for NOAA sea-level anomaly calculations is derived by removing both the long-term trend and the average seasonal cycle from the monthly mean sea level.
Notes on verification
Confirmed by multiple independent NOAA pages (anomaly map, station pages, raw data files) all consistently describing the same methodology; matches technical documentation exactly.
Sources
- Monthly Anomalies - Sea Level Trends - NOAA Tides & Currents (tidesandcurrents.noaa.gov)
- https://www.epa.gov/sites/default/files/2021-04/documents/sea-level_td.pdf (epa.gov)
- https://iridl.ldeo.columbia.edu/descriptions/.topexnoaa.html (iridl.ldeo.columbia.edu)