We measure ellipticity of Rayleigh waves from polarization analysis (DOP-E) of oceanic microseismic noise at 23 broad-band seismic stations around the globe. We observe an unexpected, clear seasonal oscillation of Rayleigh ellipticity in specific geographical areas such as Northern Italy, Eastern Asia and Antarctica. We determine that these oscillations are not due to local changes of mechanical parameters in the crust beneath the stations, but they are linked to the seasonal variation of the ambient wavefield composition. In particular, we show that the superposition of overtones of Rayleigh waves impacts on ellipticity measurements, and infer that their fraction in the wavefield strongly increases during summers. This effect is not uniform around the globe, but it is stronger in specific regions relatively far from strong oceanic sources, hinting at generation of overtones during wave propagation. The geological structure beneath the stations can also play an important role, being the amplitude of overtones larger in sedimentary regions. This study highlights the importance of better understanding the composition of the oceanic microseismic field, and its variations during the year, in order to improve the ability of noise-based techniques to explore the Earth interior and its time-dependent variations.