Seasonal Changes

The Earth's seasonal cycle is a direct consequence of its orbital characteristics around the Sun, primarily driven by its axial tilt and, to a lesser extent, orbital eccentricity. These factors combine to modulate the incident solar radiation (insolation) received at various latitudes over an annual period.

The primary driver of seasonal variation is the Earth's axial tilt, or obliquity, currently at approximately 23.44° relative to the invariable plane of its orbit. This tilt, combined with the Earth's revolution around the Sun, results in differential solar insolation—the amount of solar radiation received per unit area—across latitudes throughout the year.

Seasonality on Earth is primarily dictated by the planet's axial tilt relative to its orbital plane and its elliptical orbit around the Sun.

Seasonality, driven primarily by the Earth's axial tilt of $\approx 23.5^\circ$ relative to its orbital plane, dictates insolation patterns, which in turn propagate through atmospheric and oceanic circulation to establish thermal and hydrologic regimes. Biological systems exhibit remarkable plasticity and adaptation to these predictable, cyclical fluctuations.

2. Madden-Julian Oscillation (MJO): The largest source of intra-seasonal (30-90 day) tropical variability.
   • Characteristics: An eastward-propagating wave of convection and circulation anomalies originating in the western Indian Ocean, propagating into the Pacific.
   • Impact: Modulates tropical cyclone formation, monsoon breaks, and precipitation patterns on a sub-seasonal scale, influencing seasonal forecasts.
   • Phase Description (e.g., Wheeler-Hendon RMM Index): 8 phases describing the location and intensity of the convective anomaly.