The Role of the Quasi 5-Day Wave on the Onset of Polar Mesospheric Cloud Seasons in the Northern Hemisphere

The quasi 5-day wave (Q5DW) with zonal wavenumber 1 is a dominant planetary wave (PW) oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. In this paper, the Q5DW signal derived from 16 years (2007-2022) of Microwave Limb Sounder temperature observations is used to investigate the role of this PW mode on the onset of PMC seasons in the northern hemisphere (NH). PMC data from the Cloud Imaging and Particle Size (CIPS) instrument during this time indicates that NH PMC season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Except 2013 and 2022, the other four earlier onsets were also characterized by enhanced Q5DW activity. The wave amplification appears to be driven by baroclinic instability arising from the negative meridional gradient of potential vorticity in the high-latitude summer mesosphere. CIPS data show that when the Q5DW was present at the beginning of the season, clouds formed preferentially in the cold troughs of the wave. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid-May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While the 11-year solar cycle, inter- and intra-hemispheric coupling due to gravity wave and PW activity have been shown to contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role.