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. Plain Language Summary Most detectable atmospheric Rossby waves are associated with atmospheric intrinsic properties, including the free traveling Rossby wave normal modes and forced resonant stationary Rossby waves. The former is less understood than the latter because they are relatively weak and often distorted by the background atmosphere. In the current work, we customize a compact wave identifying technique to estimate the horizontal wavelength of Rossby wave-like predominant 10-and 16-day oscillations detected by two arctic radars during stratospheric sudden warming events. Results illustrate that five of the six most oscillations are westward traveling with zonal wave number 1. We explain the oscillations as free transient Rossby wave normal modes.

Quasi‐10‐Day Wave and Semidiurnal Tide Nonlinear Interactions During the Southern Hemispheric SSW 2019 Observed in the Northern Hemispheric Mesosphere

“…Particularly, in diagnosing RWs and tides, m could be assumed as a near‐zero integral number, which might relax the long‐wave assumption from half wavelength to one and a half wavelength. PDT has been implemented, through cross‐wavelet (CWL) analysis, to diagnose m of RW‐ and tide‐like oscillations in a few NH SSWs and validated by comparing results from different dual‐station configurations at the same latitude (e.g., He, Forbes, et al, 2020; He, Yamazaki, et al, 2020). The current work applies PDT to the SH SSW 2019, using three dual‐radar configurations, that is, M‐J, P‐A, and Y‐A.…”

“…A dual‐radar configuration with longitudinal separation λ Δ is associated with a Nyquist wave number . Then, for all integers Z and a wave number m 0 , the solutions m = m 0 + Zm N are aliases of each other, as implied in Equation A1 in He, Yamazaki, et al (2020). Particularly, in diagnosing RWs and tides, m could be assumed as a near‐zero integral number, which might relax the long‐wave assumption from half wavelength to one and a half wavelength.…”

“…Oscillations occurring around the periods of harmonics of the solar or lunar day are explained mostly as signatures of harmonics of solar or lunar tides. Oscillations of this nature are reported to be associated with or impacted by SSWs, such as the first six solar migrating tidal harmonics (He, Yamazaki, et al, 2020), and the second lunar migrating tidal harmonic (M2, e.g., He & Chau, 2019). Among these oscillations, the Sun‐synchronous (migrating tide‐like) components are typically explained in terms of SSW modulations of tidal heating (e.g., Goncharenko et al, 2012; Siddiqui et al, 2020) and of propagation conditions (e.g., Jin et al, 2012), whereas the non‐Sun‐synchronous (nonmigrating tide‐like) components are conventionally explained as arising from zonal asymmetries in heating or nonlinear interactions between stationary RWs and migrating tides (e.g., Liu et al, 2010).…”

Quasi-10-day wave and semi-diurnal tide nonlinear interactions during the southern hemispheric SSW 2019 observed in the northern hemispheric mesosphere

“…Then, for all integers Z and a wave number m 0 , the solutions 10.1029/2020GL091453 m = m 0 + Zm N are aliases of each other, as implied in Equation A1 in He, Yamazaki, et al (2020). Particularly, in diagnosing RWs and tides, m could be assumed as a near-zero integral number, which might relax the long-wave assumption from half wavelength to one and a half wavelength.…”

“…Oscillations occurring around the periods of harmonics of the solar or lunar day are explained mostly as signatures of harmonics of solar or lunar tides. Oscillations of this nature are reported to be associated with or impacted by SSWs, such as the first six solar migrating tidal harmonics (He, Yamazaki, et al, 2020), and the second lunar migrating tidal harmonic (M2, e.g., He & Chau, 2019). Among these oscillations, the Sun-synchronous (migrating tide-like) components are typically explained in terms of SSW modulations of tidal heating (e.g., Goncharenko et al, 2012;Siddiqui et al, 2020) and of propagation conditions (e.g., Jin et al, 2012), whereas the non-Sun-synchronous (nonmigrating tide-like) components are conventionally explained as arising from zonal asymmetries in heating or nonlinear interactions between stationary RWs and migrating tides (e.g., Liu et al, 2010).…”