Traveling 10-Day Waves at Mid-Latitudes in the Troposphere and Lower Stratosphere Revealed by Radiosonde Observations and MERRA-2 Data in 2020

Abstract

Although the characteristics of the traveling 10-day waves (10DWs) above the middle stratosphere have been well explored, little research has been performed on the counterpart in the troposphere and lower stratosphere (TLS). In the present study, we use radiosonde observations and MERRA-2 data from 2020 to characterize traveling 10DWs in mid-latitudes in the TLS. Single-site observations in both hemispheres show that strong 10DW activities are always accompanied by strong eastward jets (10–13 km). MERRA-2 data indicate that in the troposphere, the eastward-propagating modes with larger wavenumbers, i.e., E3, E4, E5 and E6 are dominant, while in the lower stratosphere, the eastward- and westward-propagating modes with small zonal wavenumbers, e.g., 1 and 2, are dominant. Further research on E3, E4, E5 and E6 modes in the troposphere of both hemispheres shows that all the wave activities are positively correlated to the background zonal wind. The refractive index squared reveals that a strong eastward jet is suitable for these four modes to propagate. However, just above the jet, the eastward wind decreases with altitude, and a thick evanescence region emerges above 15 km. E3, E4, E5 and E6 10DWs cannot propagate upward across the tropopause; as such, this can explain why these four modes are weak or even indiscernible in the stratosphere and above. In the troposphere, E5 10DW at 32° S was the most dominant mode in 2020. A case study of the anomalously strong E5 10DW activity on 12 May 2020 indicates that the wave amplification resulted from the upward and equatorward transmission of wave energy flows. Moreover, the tropopause and equatorial region can prevent the propagations of wave energy flows of E5 10DW. The upward-propagating wave energy is deposited near the tropopause, resulting in significant short-term variability in the tropopause height.