Intraseasonal and interannual variability of the quasi 2 day wave in the Northern Hemisphere summer mesosphere

Abstract

This study uses global synoptic meteorological fields from a high-altitude data assimilation system to investigate the quasi 2 day wave (Q2DW) and migrating diurnal tide during the Northern Hemisphere (NH) summers of 2007–2009. By applying a two-dimensional fast Fourier transform to meridional wind and temperature fields, we identify Q2DW source regions and diagnose propagation of Q2DW activity into the upper mesosphere and lower thermosphere. We find that the Q2DW in NH summer is composed primarily of westward propagating zonal wave number 3 and wave number 4 components that originate within baroclinically unstable regions along the equatorward flank of the summer midlatitude easterly jet. The amplitude of the wave number 3 Q2DW tends to peak in July while the amplitude of the wave number 4 Q2DW tends to peak in late June and again in early August. The seasonal mean Q2DW amplitudes are largest in 2009, when the amplitude of the migrating diurnal tide in the upper mesosphere near 30° N was relatively weak. However, there is no evidence of rapid amplification of the Q2DW via nonlinear interaction with the diurnal tide. Instead, variations of Q2DW amplitudes during NH summer appear to be linked to variations in the strength and location of the mesospheric easterly jet from one summer to the next, with a stronger jet producing larger Q2DW amplitudes. Linear instability model calculations based on the assimilated wind fields indicate that the fastest-growing modes are zonal wave numbers 3 and 4 with periods near 2 days that originate in the vicinity of the easterly jet.