Averages and anomalies

At least as important as the mean (or average) rainfall is the departure from normal (from the mean). The word 'anomaly' is commonly used in atmospheric sciences. For instance, the central Amazon region (Para State) was rather wet compared to the Nordeste during December 1997 (Fig 1), but the rainfall anomalies show that Para was unusually dry (Fig 2), whereas the normally arid Nordeste was not. Amazon dry periods often occur during El Niño years, such as 1997. That this was an El Niño year is not so obvious in the SST field (Fig 3), but quite evident in the SST anomaly field (Fig 4). These figures illustrate the value of anomaly charts.

Fig 1. December '97 precipitation totals for Brazil.

Fig 2. December '97 precipitation anomalies for Brazil.

Fig 3. December '97 mean global sea surface temperature (ºC).

Fig 4. December '97 sea surface temperature anomalies (K).