quasibiennial

Quasibiennial is an adjective meaning occurring or recurring approximately every two years. In meteorology and atmospheric science, the term most often denotes the quasi-biennial oscillation, or QBO, a prominent pattern in the tropical stratosphere.

The QBO is characterized by alternating easterly and westerly zonal winds in the equatorial stratosphere, which propagate downward over roughly two years. The cycle comprises two phases: an easterly (eastward) phase followed by a westerly (westward) phase, each lasting about 12 to 14 months, yielding an overall period near 28 to 29 months. The signal is strongest between about 15 and 50 hPa, but its influence can extend to adjacent altitudes and latitudes. The downward progression of the wind regime results from the interaction of upward-propagating atmospheric waves that deposit momentum at different levels.

Mechanism and implications: The oscillation arises from wave-mean flow interactions in the tropical stratosphere. The QBO modulates stratospheric temperatures and ozone distribution and can influence tropopause structure, tropical precipitation patterns, and the development or tracks of tropical cyclones in certain seasons. It is modulated by external forcings and interacts with other climate modes, including the El Niño–Southern Oscillation and solar variability, though the strength and character of these interactions vary over time.

Etymology and usage: The term combines quasi- meaning "almost" with biennial, reflecting its approximate two-year cadence rather than a strict biennial schedule. The organized oscillation is the best-known example of a quasibiennial phenomenon in Earth’s atmosphere.