The Climatology of the 5-Day Wave in the Mesosphere

In the upper mesosphere, westward propagating waves are sometimes observed with periods between 5 and 7 days. These motions are usually attributed to the so-called 5-day wave, a wavenumber 1, Rossby planetary wave that has been extensively studied in the stratosphere. A survey was carried out of planetary wave signatures in SABER temperatures over the three years of the mission. We find that 5-day wave events are relatively infrequent in the upper mesosphere. However, during May of 2003, an exceptionally strong outbreak of 5-day wave activity was observed by SABER with an amplitude of 15 K at mesopause heights. The wave penetrated well into the thermosphere. Although the period of this wave was longer than 5 days ( ∼6 days), its global structure was consistent with that of the 5-day normal mode. The May 2003 event also showed up strongly in a year-long run of the NCAR TIME-GCM model that was forced at the 10 hPa lower boundary with NCEP reanalyses. The model output is used to study the relationship of mesospheric 5-day wave activity to perturbations in the lower atmosphere and diagnose the background conditions that determine wave propagation and amplification.