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The Arctic Oscillation is a large-scale pattern of atmospheric variability involving pressure-related anomalies over the Arctic and middle latitudes. Its sign summarizes the projection of an observed field onto a chosen spatial pattern; it is not simply the mean temperature of the Arctic.

For the operational NOAA index, daily height anomalies on the $1000$-hPa pressure surface north of $20^{\circ }$N are projected onto a loading pattern and standardized against a stated reference period.^[ref] The resulting index helps describe changes in atmospheric circulation, especially in the cold season. Its relation to regional weather is statistical, and its definition differs from a regional North Atlantic Oscillation index.

The National Oceanic and Atmospheric Administration is a United States scientific agency whose observations and research cover the atmosphere and oceans. Its measurement products connect meteorology, climatology and ocean circulation, with a stated observing method and reference period needed to interpret each product.

For example, its Arctic Oscillation index projects a measured atmospheric field onto a specified spatial pattern, while a climatological normal summarizes a different kind of information: statistics over a defined reference interval.^[ref][ref] Neither product is a direct measurement of one universal scalar called climate.