Abstract
The atmospheric Kelvin wave has been widely studied due to its importance in atmospheric dynamics. Since a long-term climatological study is absent in the literature, we have employed the two-dimensional fast Fourier transform (2D-FFT) method for the 40-year long-term reanalysis of the dataset, ERA-Interim, to investigate the properties of Kelvin waves with wavenumbers 1 (E1) and 2 (E2) at 6–24 days wave periods over the equatorial region of ±10° latitude between a 15 and 45 km altitude during the period 1979–2019. The spatio-temporal variations of the E1 and E2 wave amplitudes were compared to the information of stratospheric quasi-biennial oscillation (QBO), and the wave amplitudes were found to have an inter-QBO cycle variation that was related to sea surface temperature and convections, as well as an intra-QBO cycle variation that was caused by interactions between the waves and stratospheric mean flows. Also, the E1 waves with 6–10 day periods and the E2 waves with 6 days period were observed to penetrate the westerly regime of QBO, which has a thickness less than the vertical wavelengths of those waves, and the waves could further propagate upward to higher altitudes. In a case study of the period 2006–2013, the wave amplitudes showed a good correlation with the Niño 3.4 index, outgoing longwave radiation (OLR), and precipitation during 2006–2013, though this was not the case for the full time series. The present paper is the first report on the 40-year climatology of Kelvin waves, and the morphology of Kelvin waves will help us diagnose the anomalies of wave activity and QBO in the future.
Highlights
Atmospheric Kelvin waves are a branch of gravity waves with a horizontal wavelength on the planetary scale, and they propagate eastward and upward over equatorial (±10◦ ) latitudes
Das and Pan [7] used the Kelvin wave amplitude extracted from the FORMOSAT-3/COSMIC mission, and the Niño 3.4 index, which is a kind of sea surface temperature (SST) anomaly index in the Niño 3.4 region (5◦ N–5◦ S, 120◦ –170◦ W), to investigate a fast descending event of the quasi-biennial oscillation (QBO) westerly regime during the El Niño episode of 2009–2010
Kelvin wave activity was studied using some short-term datasets observed by radiosondes, rockets, radars, satellites, and so forth
Summary
Atmospheric Kelvin waves are a branch of gravity waves with a horizontal wavelength on the planetary scale, and they propagate eastward and upward over equatorial (±10◦ ) latitudes. There are free mode Kelvin waves, which result from high-frequency convective heating releases (usually in days to weeks), and do not propagate coherently with the convective centers in the middle troposphere These waves have a period of 10–20 days, a zonal wavenumber of 1 or 2, and a phase speed of 20–30 m/s [4,16]. Das and Pan [7] used the Kelvin wave amplitude extracted from the FORMOSAT-3/COSMIC mission, and the Niño 3.4 index ( known as oceanic Niño index, ONI), which is a kind of sea surface temperature (SST) anomaly index in the Niño 3.4 region (5◦ N–5◦ S, 120◦ –170◦ W), to investigate a fast descending event of the QBO westerly regime during the El Niño episode of 2009–2010 They reported significant enhancements of Kelvin wave amplitude during the. The adjectives faster/slower mean the wave(s) has a shorter/longer period compared to another one(s), though by definition it may not be classified as a fast/slow Kelvin wave
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