Observations of the 5-day wave in the mesosphere and lower thermosphere

The 5-day wave is the gravest symmetric Hough mode of westward propagating zonal wavenumber 1. This wave is observed using the SABER instrument aboard the TIMED satellite during the first three years of the spacecraft mission (2002–2004). Supporting measurements were made with mesospheric radar systems. To better interpret the observations, the NCAR thermosphere–ionosphere–mesosphere–electrodynamics general circulation model (TIME-GCM) simulation of year 2003 is used for comparative analysis. For the simulation the lower boundary was specified using NCEP data. The climatology from SABER shows a May maximum in the amplitude of the 5-day wave, which is consistent with the seasonal dependence found in earlier studies. A particularly strong wave with a ∼6∼6 day period was observed in May 2003 and is studied in some detail. There is considerable evidence from both data and model in our study that a major source for this wave was in the southern (winter) hemisphere. Cross-equatorial ducting allowed the wave to propagate into the northern (summer) hemisphere, where it was amplified by baroclinic instability.