Anomaly

An anomaly is the departure of an observed value from a long-term climatological mean computed over a fixed reference period, typically 30 years. Working in anomalies removes the seasonal cycle and the spatial mean, so the residual signal (a warm or cool SST anomaly, a positive or negative pressure anomaly) becomes comparable across regions and seasons. Climate-mode indices like ENSO, NAO, and PDO are defined as standardized anomaly fields. The 1991 to 2020 normals are the current World Meteorological Organization reference period, and shifting that baseline changes the sign and size of every anomaly.