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. 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 filtersKW wind and temperature perturbations in the continuously stratifiedatmosphere on the spherical Earth. The spatial filtering of the three-dimensionalKW structure in the upper troposphere and lower stratosphere isbased on the Hough harmonics using several tens of linearized shallow-waterequation systems on the spherical Earth with equivalent depths ranging from 10 km to afew metres. Results provide the global KW energy spectrum. It shows a clear seasonal cycle with the KW activitypredominantly in zonal wavenumbers 1–2, where up to 50 % more energy is observed during the solstice seasonsin comparison with boreal spring and autumn. Seasonal variability in KWs in the upper troposphere and lower stratosphere is examined inrelation 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 positiondetermined by the location of the dominant convective outflow throughout the seasons. Its maximal strength occursduring boreal summer when easterlies in the eastern hemisphere are strongest. The two other componentsrepresent vertically propagating KWs and are observed throughout the year with seasonal variability mostlyfound in the wave amplitudes being dependent on the seasonality of the background easterly winds and static stability.

Multivariate analysis of Kelvin wave seasonal variability in ECMWF L91 analyses