Multivariate analysis of Kelvin wave seasonal variability in ECMWF L91 analyses

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.