AbstractThe interannual and annual variations in the intra‐seasonal oscillation (ISO) are examined using upper air radiosonde data at Singapore. Spectral analysis of the zonal wind using the maximum entropy method (MEM) was made for each season, starting from the autumn of 1960 through to 1985. It is shown that the strongest signal of the ISO occurs during the Northern Hemisphere winter, while the weakest occurs in the Northern Hemisphere summer.The main finding of this study is a quasi‐biennial oscillation (QBO) in the intensity of the ISO. The cross‐spectral analyses indicate that the intensity of the ISO at 150 mbar is well correlated for a time‐scale of 2–3 years with the stratospheric QBO as well as with the strength of the zonal wind and the static stability of the upper troposphere.These results suggest that the stratospheric QBO may act as a trigger for the tropospheric QBO. The physical interpretation of this interaction requires further work, especially in the field of numerical modelling.