
The regular annual variation in a climate variable, often removed to study other signals.The seasonal cycle refers to the regular, repeating pattern of change in a climate variable—such as temperature, precipitation, or wind—that occurs over the course of a year. This cycle is driven primarily by the tilt of Earth's axis and its orbit around the Sun, leading to variations in solar radiation received at different latitudes and times of year. In climate science, the seasonal cycle is often considered a baseline signal that must be understood and removed to reveal other, less obvious variations, such as those due to El Niño, volcanic eruptions, or long-term climate change.
The recognition of seasonal patterns in weather dates back to ancient civilizations, but the systematic removal of the seasonal cycle in climate analysis became standard practice in the 20th century with the advent of statistical climatology.
The seasonal cycle is typically modeled as a periodic function with a period of one year, often represented by a sum of sine and cosine terms (a Fourier series). By fitting this model to observed data, scientists can estimate the average seasonal pattern for a given location and variable. Subtracting this fitted cycle from the raw data yields anomalies, which highlight deviations from the expected seasonal behavior and are used to study climate variability and change.
Seasonal cycle removal is essential in climate monitoring, seasonal forecasting, and the detection of climate change signals, as it allows scientists to isolate non-seasonal trends and events.
The seasonal cycle in atmospheric carbon dioxide is so pronounced that it creates a noticeable 'breathing' pattern in global CO2 measurements, with levels dropping in the Northern Hemisphere summer as plants absorb CO2.

















