Structure, Propagation and Variability of the 5‐Day Rossby‐Haurwitz Wave

Abstract

The 5‐day wave is the gravest free Rossby mode predicted by the Laplace shallow water equations, with a latitudinally symmetric, zonal wavenumber one theoretical structure in geopotential extending from pole to pole. It propagates westward along the equator at about 98 m and was first observed in surface pressure data during the 1950s. Since then, it has been shown to modulate tropical precipitation, sea level and bottom ocean pressure, stratospheric ozone and water vapor, and noctilucent clouds and water vapor in the polar mesosphere. There is still much to be learned about the 5‐day wave. We confirm previous observational results suggesting that most 5‐day wave events start near the earth's surface around Antarctica, where it initially has a strongly northwest‐to‐southeast tilted structure confined to the Southern Hemisphere. The wave then propagates northward to eventually encompass the entire globe in around a week, when its structure more closely resembles its freely propagating theoretical counterpart. At the same time, the wave's energy propagates upward into the middle atmosphere, reaching 80 km height in around 10 days with a height‐dependent upward group velocity inversely proportional to the static stability parameter . This evolution is dominant at all times of the year, with wave activity peaking at the equinoxes. Although the longitudinal position of baroclinic waves in the Southern Hemisphere storm track is weakly synchronized to the zonal phase of the 5‐day wave, episodes of enhanced 5‐day wave activity appear to be excited by anomalously strong baroclinic wave activity further south around Antarctica.