Zonal Wave Number Diagnosis of Rossby Wave-Like Oscillations Using Paired Ground-Based Radars

Abstract

Free traveling Rossby wave normal modes (RNMs) are often investigated through large-scale space-time spectral analyses, which therefore is subject to observational availability, especially in the mesosphere. Ground-based mesospheric observations were broadly used to identify RNMs mostly according to the periods of RNMs without resolving their horizontal scales. The current study diagnoses zonal wave numbers of RNM-like oscillations occurring in mesospheric winds observed by two meteor radars at about 79◦N. We explore four winters comprising the major stratospheric sudden warming events (SSWs) 2009, 2010, and 2013. Diagnosed are predominant oscillations at the periods of 10 and 16 days lasting mostly for three to five whole cycles. All dominant oscillations are associated with westward zonal wave number m = 1, excepting one 16-day oscillation associated with m = 2. We discuss the m = 1 oscillations as transient RNMs and the m = 2 oscillation as a secondary wave of nonlinear interaction between an RNM and a stationary Rossby wave. All the oscillations occur around onsets of the three SSWs, suggesting associations between RNMs and SSWs. For comparison, we also explore the wind collected by a similar network at 54◦N during 2012–2016. Explored is a manifestation of 5-day wave, namely, an oscillation at 5–7 days with m = 1), around the onset of SSW 2013, supporting the associations between RNMs and SSWs.