Space‐time variability of equatorial Kelvin waves and intraseasonal oscillations around the tropical tropopause

Abstract

Using the ECMWF 40‐year reanalysis data, large‐scale variations of temperature and zonal wind around the tropical tropopause are investigated. From the space‐time spectral analysis, two dominant spectral regions in the eastward propagating domain are found: one for Kelvin waves and the other for intraseasonal oscillations (ISOs). To investigate activities of Kelvin waves and ISOs we reconstructed the grid data for the two spectral windows and calculated the square amplitude at each grid point. Kelvin wave activities in zonal wind are vigorous during two seasons around February and July in the upper troposphere up to 100 hPa. However, the longitudinal maxima are found in the Eastern Hemisphere at 100 hPa and in the Western Hemisphere at 150 hPa and below. Activities calculated from the outgoing longwave radiation data for the Kelvin wave spectral window are different from those in zonal wind about their maximum longitudes and seasons. As the climatological background zonal wind in the upper troposphere is easterly in the Eastern Hemisphere and westerly in the Western Hemisphere, it seems that the difference of Kelvin wave activities between 100 hPa and 150 hPa is closely related to the background wind field affecting Kelvin wave propagation and dissipation. ISO activities in zonal wind at 100 hPa are vigorous in the western Pacific during northern winter, and we may find relation to those in OLR. This indicates that the eastward moving disturbance with the ISO time scale is coupled with the organized convective system such as the Madden‐Julian oscillation.