Interannual and Interhemispheric Comparisons of Q2DW Bimodal and Unimodal Structures During the 2003–2020 Summer Period

Abstract

By analyzing Sounding of the Atmosphere using Broadband Emission Radiometry and the Modern‐Era Retrospective analysis for Research Applications Version 2 reanalysis observations from 2003 to 2020, we found that the structural variation of the westward quasi‐2‐day wave (Q2DW) is related to the mean zonal wind in the background atmosphere, and the zonal wavenumber 3 (W3) is more affected by the background atmosphere than the zonal wavenumber 4 (W4). Our study focuses on the spatial structure of ∼50–95 km and ∼50°S–50°N. The spatial structure of W3 and W4 in the Northern and Southern Hemispheres is unimodal and bimodal, while the bimodal structure of W3 in the Southern Hemisphere is more obvious in some years. In the unimodal structure, W3 has a higher (altitude) peak in the Southern Hemisphere (∼82 km) than in the Northern Hemisphere (∼70 km), while W4 peaks at ∼70 km in both hemispheres. In the bimodal structure, W3 in the Southern Hemisphere fluctuates mostly at ∼82 km, followed by ∼68 km, while W3 in the Northern Hemisphere fluctuates mostly at ∼70 km, followed by ∼82 km. The spatial structure of W4 in both hemispheres fluctuates mainly at ∼70 km, followed by ∼82 km. In addition, W3 is stronger in the Southern Hemisphere than the Northern Hemisphere, W4 is stronger in the Northern Hemisphere than the Southern Hemisphere, and peak amplitudes of both W3 and W4 are larger in a bimodal structure than unimodal structure. The diagnostic analysis shows that the interannual and interhemispheric structure changes of Q2DWs may be due to the larger mean flow instabilities, background winds, and refractive index in the mesosphere, which make the bimodal structure of W3 and W4 obtain larger energy for propagation and amplification, resulting in higher (altitude) and larger amplitudes. This indicates that the background atmosphere of the mesosphere can affect the spatial structure of Q2DWs.