Large‐Amplitude Quasi‐10‐Day Waves in the Middle Atmosphere During Final Warmings

Y Yamazaki,V Matthias

doi:10.1029/2019jd030634

Abstract

AbstractGeopotential height measurements from the Aura Microwave Limb Sounder between 9‐ and 97‐km altitudes during 2004–2018 are used to examine long‐period (3–20 days) wave activity during the Northern Hemisphere winter and spring, with the primary focus on the response of normal mode Rossby waves in the middle atmosphere to sudden stratospheric warmings (SSWs). Unusually large westward propagating waves with Zonal Wave Number 1 and period ∼10 days are observed at 55° latitude at the stratopause height (∼48 km) and above following final warmings of 2016, 2015, and 2005. In each case, large‐amplitude waves are observed for the duration of two to three wave cycles. Characteristics of the waves are in conformity with the second antisymmetric Rossby normal mode of Zonal Wave Number 1, or the quasi‐10‐day wave. The growth rate of the waves is significantly greater than the classical normal mode in the upper stratosphere (approximately 30–50 km) where instability conditions are met, indicating the amplification or excitation of the waves in that region. The response of the quasi‐10‐day wave during midwinter SSWs, and also during the spring transition without an SSW, is not as obvious as the wave response during final warmings. The results suggest that not only the occurrence of SSW but also the seasonal timing of SSW is an important factor for the transient variability of the quasi‐10‐day wave in the middle atmosphere.

Highlights

Classical wave theory predicts a series of normal mode oscillations that the Earth's atmosphere can support
The present study utilizes over 14 years of geopotential height measurements from the Aura satellite to investigate the stratospheric warmings (SSWs) effect on long-period waves (τ = 3–20 days), in particular, those propagate westward with zonal wave number s = 1, including the quasi-6-day wave, quasi-10-day wave, and quasi-16-day wave
Enhanced wave activity is observed near wave period τ = 10 days at 55◦ latitude between 48- and 97-km altitudes, following the final breakdown of the stratospheric polar vortex of 2016, 2015, and 2005 (Figures 3 and 4)

Summary

Introduction

Classical wave theory predicts a series of normal mode (or resonant) oscillations that the Earth's atmosphere can support (see, e.g., reviews by Forbes, 1995; Kasahara, 1976; Madden, 1979, 2007; Salby, 1984). The 5and 16-day waves are symmetric modes, for which perturbations in the zonal wind and geopotential height are symmetric about the equator, while the 2-day wave (s = 3) and 10-day wave are antisymmetric modes, for which the perturbations change the sign at the equator (see Figure 1). The energy of these waves decays with height from the surface, but the amplitude of perturbations in winds and geopotential height grows in the vertical due to the density decrease

Results

Discussion

Conclusion