chross22 · chross22 · Aug 7, 2026 · Aug 7, 2026
diff --git a/NEWS.md b/NEWS.md
@@ -173,6 +173,12 @@
 * **`LCR`**, the Labrador Current retroflection index, joins the climate index
   catalog. Published with Jutras et al. (2023) and cited in `index_dictionary()`.
 
+* **Every climate index now declares its `units`**, surfaced in
+  `index_dictionary()`. Most are standardized anomalies, in standard deviations
+  rather than anything physical; only `AMO` (degrees C) and `AMOC` (Sverdrups)
+  carry real units. A coefficient fitted to one of those is not comparable with
+  one fitted to `NAO`, and a table of model output gave no way to tell.
+
 * **`AMOC`**, the overturning transport measured by the RAPID array at 26.5°N,
   joins it too. Unlike the other indices this is a direct measurement rather than
   a pressure or SST pattern — which is also its limitation: it begins in April

diff --git a/R/climate_indices.R b/R/climate_indices.R
@@ -10,8 +10,8 @@
 #' year?") and useless for spatial ones. A model given only indices cannot
 #' produce a map.
 #'
-#' @return a named list, one entry per index, each with `label`, `source`,
-#'   `url`, and `description`
+#' @return a named list, one entry per index, each with `label`, `units`,
+#'   `source`, `url`, and `description`
 #' @examples
 #' names(climate_indices())
 #' @seealso [fetch_climate_index()], [attach_climate_index()]
@@ -20,6 +20,7 @@ climate_indices <- function() {
   list(
     NAO = list(
       label = "North Atlantic Oscillation",
+      units = "standardized anomaly",
       source = "NOAA CPC",
       url = "https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/norm.nao.monthly.b5001.current.ascii.table",
       format = "cpc_table",
@@ -31,6 +32,7 @@ climate_indices <- function() {
     ),
     AO = list(
       label = "Arctic Oscillation",
+      units = "standardized anomaly",
       source = "NOAA CPC",
       url = "https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/monthly.ao.index.b50.current.ascii.table",
       format = "cpc_table",
@@ -40,6 +42,7 @@ climate_indices <- function() {
     ),
     AMO = list(
       label = "Atlantic Multidecadal Oscillation",
+      units = "degrees C",
       source = "NOAA PSL",
       url = "https://psl.noaa.gov/data/correlation/amon.us.long.data",
       format = "psl_table",
@@ -50,6 +53,7 @@ climate_indices <- function() {
     ),
     PDO = list(
       label = "Pacific Decadal Oscillation",
+      units = "standardized anomaly",
       source = "NOAA PSL",
       url = "https://psl.noaa.gov/data/correlation/pdo.data",
       format = "psl_table",
@@ -60,6 +64,7 @@ climate_indices <- function() {
     ),
     LCR = list(
       label = "Labrador Current retroflection",
+      units = "fraction",
       source = "Jutras et al. 2023, Nature Communications",
       url = paste0("https://static-content.springer.com/esm/",
                    "art%3A10.1038%2Fs41467-023-38321-y/MediaObjects/",
@@ -83,6 +88,7 @@ climate_indices <- function() {
     ),
     AMOC = list(
       label = "Atlantic Meridional Overturning Circulation",
+      units = "Sv",
       source = "RAPID-MOCHA-WBTS array at 26.5N",
       url = "https://rapid.ac.uk/sites/default/files/rapid_data/moc_transports.nc",
       format = "rapid_netcdf",
@@ -118,8 +124,13 @@ index_dictionary <- function() {
   catalog <- climate_indices()
   dictionary <- do.call(rbind, lapply(names(catalog), function(name) {
     entry <- catalog[[name]]
-    data.frame(name = name, label = entry$label, source = entry$source,
-               url = entry$url,
+    data.frame(name = name, label = entry$label,
+               # Most of these are standardized anomalies, which is a unit in
+               # the sense that matters: it says the number is in standard
+               # deviations rather than in anything physical, so a coefficient
+               # fitted to it is not comparable with one fitted to AMOC.
+               units = entry$units %||% NA_character_,
+               source = entry$source, url = entry$url,
                # Indices published with a paper carry its citation. The
                # operational ones from NOAA have no single paper to point at,
                # so this is empty for them rather than invented.
@@ -139,7 +150,7 @@ print.datamatch_index_dictionary <- function(x, ...) {
   flat <- as.data.frame(x)
   cat("Climate indices available by name\n")
   cat(strrep("-", 62), "\n", sep = "")
-  print(flat[c("name", "label", "source")], row.names = FALSE, right = FALSE)
+  print(flat[c("name", "label", "units", "source")], row.names = FALSE, right = FALSE)
   cat("\nThese have no spatial dimension: one value per month, basin-wide.\n")
 
   cited <- flat[!is.na(flat$reference), ]

diff --git a/README.Rmd b/README.Rmd
@@ -701,14 +701,19 @@ useless for spatial ones. A model given only indices cannot produce a map.
 
 Six are available, and they are not interchangeable.
 
-| Name | What it measures | Timescale | Record |
-|---|---|---|---|
-| `NAO` | Pressure difference, Icelandic Low to Azores High | Year to year | 1950– |
-| `AO` | Strength of the polar vortex | Year to year | 1950– |
-| `AMO` | Detrended North Atlantic SST anomaly | Multidecadal | 1948– |
-| `PDO` | Leading mode of North Pacific SST | Decadal | 1948– |
-| `LCR` | Retroflection of the Labrador Current | Year to year | 1993–2014 |
-| `AMOC` | Overturning transport at 26.5°N | Year to year | 2004– |
+| Name | What it measures | Units | Timescale | Record |
+|---|---|---|---|---|
+| `NAO` | Pressure difference, Icelandic Low to Azores High | standardized anomaly | Year to year | 1950– |
+| `AO` | Strength of the polar vortex | standardized anomaly | Year to year | 1950– |
+| `AMO` | Detrended North Atlantic SST anomaly | degrees C | Multidecadal | 1948– |
+| `PDO` | Leading mode of North Pacific SST | standardized anomaly | Decadal | 1948– |
+| `LCR` | Retroflection of the Labrador Current | fraction | Year to year | 1993–2014 |
+| `AMOC` | Overturning transport at 26.5°N | Sv | Year to year | 2004– |
+
+Most are **standardized anomalies** — in standard deviations, not anything
+physical. Only `AMO` (degrees C) and `AMOC` (Sverdrups) carry real units, so a
+coefficient fitted to one of those is not comparable with one fitted to `NAO`.
+`index_dictionary()` carries the units at runtime.
 
 **`NAO`** is the usual first choice in the Northwest Atlantic. It sets the
 strength and track of the westerlies, and with them heat flux and mixing over the

diff --git a/README.md b/README.md
@@ -780,14 +780,19 @@ useless for spatial ones. A model given only indices cannot produce a map.
 
 Six are available, and they are not interchangeable.
 
-| Name | What it measures | Timescale | Record |
-|---|---|---|---|
-| `NAO` | Pressure difference, Icelandic Low to Azores High | Year to year | 1950– |
-| `AO` | Strength of the polar vortex | Year to year | 1950– |
-| `AMO` | Detrended North Atlantic SST anomaly | Multidecadal | 1948– |
-| `PDO` | Leading mode of North Pacific SST | Decadal | 1948– |
-| `LCR` | Retroflection of the Labrador Current | Year to year | 1993–2014 |
-| `AMOC` | Overturning transport at 26.5°N | Year to year | 2004– |
+| Name | What it measures | Units | Timescale | Record |
+|---|---|---|---|---|
+| `NAO` | Pressure difference, Icelandic Low to Azores High | standardized anomaly | Year to year | 1950– |
+| `AO` | Strength of the polar vortex | standardized anomaly | Year to year | 1950– |
+| `AMO` | Detrended North Atlantic SST anomaly | degrees C | Multidecadal | 1948– |
+| `PDO` | Leading mode of North Pacific SST | standardized anomaly | Decadal | 1948– |
+| `LCR` | Retroflection of the Labrador Current | fraction | Year to year | 1993–2014 |
+| `AMOC` | Overturning transport at 26.5°N | Sv | Year to year | 2004– |
+
+Most are **standardized anomalies** — in standard deviations, not anything
+physical. Only `AMO` (degrees C) and `AMOC` (Sverdrups) carry real units, so a
+coefficient fitted to one of those is not comparable with one fitted to `NAO`.
+`index_dictionary()` carries the units at runtime.
 
 **`NAO`** is the usual first choice in the Northwest Atlantic. It sets the
 strength and track of the westerlies, and with them heat flux and mixing over the

diff --git a/man/climate_indices.Rd b/man/climate_indices.Rd
diff --git a/tests/testthat/test-climate-indices.R b/tests/testthat/test-climate-indices.R
@@ -542,3 +542,33 @@ test_that("the overturning index carries its citation", {
   expect_true(any(grepl("Cite when used", output)))
   expect_true(any(grepl("AMOC", output)))
 })
+
+test_that("every index declares its units", {
+  # A coefficient fitted to AMOC is in Sverdrups; one fitted to NAO is per
+  # standard deviation. Without units recorded, the two look comparable in a
+  # table of model output and are not.
+  catalog <- climate_indices()
+
+  for (name in names(catalog)) {
+    expect_true(nzchar(catalog[[name]]$units %||% ""),
+                info = paste(name, "has no units"))
+  }
+
+  # The physical ones say what they are; the standardized ones say that they
+  # are standardized rather than pretending to be dimensionless.
+  expect_equal(catalog$AMOC$units, "Sv")
+  expect_equal(catalog$AMO$units, "degrees C")
+  expect_equal(catalog$NAO$units, "standardized anomaly")
+})
+
+test_that("units reach the dictionary and its printed output", {
+  dictionary <- as.data.frame(index_dictionary())
+
+  expect_true("units" %in% names(dictionary))
+  expect_false(anyNA(dictionary$units))
+  expect_equal(dictionary$units[dictionary$name == "AMOC"], "Sv")
+
+  output <- capture.output(print(index_dictionary()))
+  expect_true(any(grepl("units", output)))
+  expect_true(any(grepl("Sv", output)))
+})