CoachA mode of climate variability is a recurrent pattern of natural variability of the climate system, with its own spatial pattern, seasonality and time scales, arising from the dynamics of the atmosphere or from the coupling of the ocean and the atmosphere. A climate index is a time series built from climate variables that summarises the state of such a pattern, for example the difference of sea-level pressure between Iceland and the Azores. The climatology of a variable is its mean over a reference period, computed for each calendar month or season; the standard reference period is 30 years long, currently 1991–2020.
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Example 23.3 (An anomaly)
The mean sea-surface temperature of the central equatorial Pacific between N and S, W and W was in September 2026. Its September climatology is , so the anomaly was . In September 1997 the same ocean was at against , an anomaly of . The two climatologies differ slightly because the reference period is moved every five years: the ocean itself has warmed.