Departure of a field from its long-term climatological average. A strong anomaly indicates unusual climate conditions. Because the long-term climatological average is needed to calculate anomalies, they are always defined relative to a base period.
Take, as an example, the sea-surface temperatures in the eastern equatorial Atlantic in July. Say you measured this quantity from 2000 to 2009 and the results are (25.4C, 25.8C, 26.1C, 25.6C, 25.1C, 24.7C, 26.7C, 26.5C, 26.8C, 26.3C). The average over these 10 years is 25.9C. So this would be our long-term climatological average (usually a time series should be at least 20-30 years long to establish a climatology but for simplicity’s sake we only look at 10 years). To get the anomaly for a given year, we take the temperature of that year and subtract the long-term average (25.9C). For our example this yields (-0.5C, -0.1C, +0.2C, -0.3C, -0.8C, -1.2C, +0.8C, +0.6C, +0.9C, +0.4C). The strongest negative anomaly occurs in 2005 (-1.2C), while the strongest positive anomaly occurs in 2008 (+0.9C). These pronounced anomalies suggest unusual climate conditions and, indeed, 2005 was marked by the occurrence of an Atlantic Niña, while 2008 featured an Atlantic Niño.
A spatial pattern of anomalous climate conditions (i.e. a departure from the long-term climatological average) that recurs at irregular intervals in a seesaw fashion. Probably the best known climate mode is the El Niño-Southern Oscillation (ENSO) phenomenon that seesaws between anomalously warm and cool sea-surface temperatures (SSTs) in the central and eastern tropical Pacific. Many climate modes are located in oceanic regions but have remote impacts on land areas. Again, ENSO, provides a textbook example because the actual phenomenon is limited to the tropical Pacific but has worldwide impacts on rainfall. The prediction of climate modes and their remote impacts is therefore the objective of many climate centers.
While the recurrence interval of climate modes is irregular it usually has a preferred time-scale. ENSO, for example, typically recurs every 2-7 years, and is therefore classified as an “interannual” phenomenon. Other modes, such as the Pacific Decadal Oscillation (PDO) in the North Pacific, recur about every 10 years and are therefore called “decadal”. Other prominent example of climate modes include the Indian Ocean Dipole (IOD), the Atlantic Niño or Atlantic Zonal Mode (AZM), the Madden-Julian Oscillation (MJO), the North Atlantic Oscillation (NAO), and the Quasi-Biennial Oscillation (QBO).
Typically climate modes are defined through one particular field. Often this field is SST (e.g. ENSO, PDO, IOD or AZM) but other variables are also used, such as sea-level pressure in the case of the NAO or equatorial zonal (east-west) winds in the case of the QBO. Even though the definition of climate modes often relies on a single variable, the actual phenomenon is more complex and involves an interplay of many physical quantities that are often not as readily observed. SST anomalies during ENSO, e.g., are, in a sense, just the tip of the iceberg because temperature anomalies are not limited to the surface but extend several hundred meters deep. Furthermore, ENSO involves not only ocean temperatures but also ocean currents, sea-surface height, near-surface winds, and atmospheric convection in the tropical Pacific.
Climatology (broad sense)
The study of climate. Also called climate science.
Climatology (narrow sense)
The average of a field such as temperature or precipitation over a certain base period (usually 20 years or longer). Often, the climatology is calculated for each season or calendar month. This is because many geographical regions experience very different conditions depending on the season. As an extreme example, the average temperature of Yakutsk, Russia, is about -40C in January but +20C in July. The deviation from climatology in any given year is called anomaly.