Load packages

library(stringr)
library(readr)
library(tidyr)
library(dplyr)

Read the text from the NAO

nao <- readLines("https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/norm.nao.monthly.b5001.current.ascii.table")

nao

Format the text into a nice looking wide form tibble (one row per year)

wide_form <- nao  |>
  str_trim(side = "both") |>
  str_squish() |>
  str_replace_all(stringr::fixed(" "), stringr::fixed(",")) |>
  paste(collapse = "\n") |>
  read_csv(col_names = c("Year", month.abb),
           col_types = strrep("d", 13))

wide_form

Convert the tibble into long form (one row per month/year)

long_form <- wide_form |>
  pivot_longer(all_of(month.abb), 
               names_to = "Month",
               values_to = "AMO")

long_form

means <- long_form |> 
  group_by(Year) |> 
  summarise(mean = mean(AMO))

nao_w_mean <- wide_form |> 
  mutate(mean_nao = means$mean)

nao_w_mean