Abstract
Abstract. This paper presents a multivariate analysis of the linear Kelvin waves (KWs) represented by the operational 91-level ECMWF analyses in the 2007–2013 period, with a focus on seasonal variability. The applied method simultaneously filters KW wind and temperature perturbations in the continuously stratified atmosphere on the spherical Earth. The spatial filtering of the three-dimensional KW structure in the upper troposphere and lower stratosphere is based on the Hough harmonics using several tens of linearized shallow-water equation systems on the spherical Earth with equivalent depths ranging from 10 km to a few metres. Results provide the global KW energy spectrum. It shows a clear seasonal cycle with the KW activity predominantly in zonal wavenumbers 1–2, where up to 50 % more energy is observed during the solstice seasons in comparison with boreal spring and autumn. Seasonal variability in KWs in the upper troposphere and lower stratosphere is examined in relation to the background wind and stability. A spectral bandpass filtering is used to decompose variability into three period ranges: seasonal, intra-seasonal and intra-monthly variability components. Results reveal a slow seasonal KW component with a robust dipole structure in the upper troposphere with its position determined by the location of the dominant convective outflow throughout the seasons. Its maximal strength occurs during boreal summer when easterlies in the eastern hemisphere are strongest. The two other components represent vertically propagating KWs and are observed throughout the year with seasonal variability mostly found in the wave amplitudes being dependent on the seasonality of the background easterly winds and static stability.
Highlights
Atmospheric equatorial Kelvin waves, first discovered in the stratosphere (Wallace and Kousky, 1968), are nowadays observed and studied over a broad range of spatial and temporal scales
During JJA, one can notice how the asymmetry in the tropical tropopause height over the Indian Ocean around 60◦ E coincides with increasing temperatures by the of the Kelvin waves (KWs) zonal wind along 0.7◦ N and the outgoing longwave radiation (OLR) averaged over the latitude belt 15◦ S–15◦ N
We showed that large-scale KWs readily persist in the data despite analysing selected processing times independently
Summary
Atmospheric equatorial Kelvin waves (hereafter KWs), first discovered in the stratosphere (Wallace and Kousky, 1968), are nowadays observed and studied over a broad range of spatial and temporal scales. By prescribing D, the horizontal structure of KW is defined by Eq (1) for any k and can be used to simultaneously analyse wind and geopotential height perturbations due to KWs on a single horizontal level Such analysis was carried out by Tindall et al (2006) for the lower stratosphere for the ERA-15 data in the 1981–1993 period. Their results suggested that KWs contribute approximately 1 K2 to the temperature variance on the equator with peak activity occurring during solstice seasons at 100 hPa, during December–February at 70 and at 50 hPa it occurs during the easterly to westerly QBO phase transition.
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