A Case Study of the Wavenumber Transition between Westward Quasi-2 Day Wave s = 3 and s = 4 Modes in the Mesosphere

Abstract

We report observations of the wave number transition between two types of westward-propagating quasi-2 day wave (QTDW) with zonal wave number s = 3 and s = 4 in the mesosphere. The Aura/MLS temperature datasets from January and February 2011 are utilized in the analysis. A strong W4 was observed in the Southern Hemisphere in early February, following the fading away of a strong W3 in late January. The W3 and W4 were maximized on days 23 and 36 with maximum amplitudes of ~15 K and ~8 K, respectively. Both the amplitudes and periods were nearly equal on day 30, which coincided with the peak of the Sudden Stratospheric Warming (SSW) on day 31 in the polar region of the Northern Hemisphere. Our analysis shows that the equatorial barotropic/baroclinic and inertial instabilities, which are related with the winter planetary wave activities that lead to the SSW, may have triggered this wavenumber transition event and contributed significantly to the growth of the QTDW. Meanwhile, the relatively strong summer easterly jet at middle and high latitude regions in early February also provided favorable conditions for the amplification of W4 during 2011, and thus facilitated the wavenumber transition.