Unlike the gridded variables, these have no spatial dimension — one value per month describes the whole basin. They enter a model as a shared state every observation in a month experiences, which is a different kind of covariate from local temperature: they carry no information about where within a region conditions are better, only about what year and season it was.
Details
That makes them useful for interannual questions ("was this a warm-regime year?") and useless for spatial ones. A model given only indices cannot produce a map.
Examples
names(climate_indices())
#> [1] "NAO" "AO" "AMO" "PDO" "LCR" "AMOC"