Amplification of the quasi‐two day wave through nonlinear interaction with the migrating diurnal tide

Abstract

We present a case study of the non‐linear interaction between the quasi‐two day wave (Q2DW) and the migrating diurnal tide based on global synoptic meridional wind fields for January 2006 and January 2008 from a high‐altitude data assimilation/forecast system. We find large quasi‐two day wave amplitudes, small diurnal tide amplitudes, and phase locking of the Q2DW with the diurnal cycle during January 2006. In January 2008 the amplitudes of the Q2DW were much smaller, with no evidence of phase locking, while the tidal amplitudes were larger than in the 2006 case. Space‐time spectral analysis reveals an enhancement in a diurnal zonal wavenumber 6 feature in the January 2006 case, which can be attributed to a non‐linear interaction between the Q2DW and migrating diurnal tide. The relatively strong summer easterly jet in the extratropical upper mesosphere during early January 2006 appears to have created conditions favoring this interaction.