Tropical wave

A tropical wave is an inverted trough of low pressure or a region of cyclonic curvature that generally moves from east to west through the tropical easterlies. The U.S. National Weather Service also calls it an easterly wave. These disturbances are features of the atmosphere, not already-formed tropical cyclones: a wave may be accompanied by clouds and thunderstorms, but it does not have to meet the criteria for a tropical cyclone.

Tropical waves can provide a pre-existing disturbance from which a tropical cyclone may develop. NOAA notes that most tropical cyclones form from easterly waves, but a wave alone is not sufficient. Formation also depends on a favorable environment, including sufficiently warm ocean water, moist air, atmospheric instability, adequate distance from the equator for rotation, and relatively weak vertical wind shear. Many waves fail to develop into cyclones.

A tropical wave describes a moving atmospheric disturbance, while a tropical disturbance is a broader weather system with organized convection that maintains its identity for at least 24 hours. A tropical cyclone is a warm-core, non-frontal system over tropical or subtropical waters with organized deep convection and a closed surface circulation. These distinctions matter in forecasting: observing a wave does not by itself mean a storm has formed or that one is certain to form.

Forecasters monitor waves as they move across tropical regions, looking for changes in their circulation, thunderstorms, and surrounding environmental conditions. NOAA's tropical-cyclone guidance identifies waves among several possible precursor disturbances, alongside other systems such as West African disturbance lines, upper-level troughs, and old frontal boundaries. Development is assessed from the complete weather pattern rather than from the presence of a wave alone.