The enhanced seasonal amplitude of atmospheric CO₂ has been viewed so far primarily as a Northern Hemisphere phenomenon. Yet, analyses of atmospheric CO₂ records from 49 stations between 1980 and 2018 reveal substantial trends and variations in this amplitude globally. While no significant trends can be discerned before 2000 in most places, strong positive trends emerge after 2000 in the southern high latitudes. Using factorial simulations with an atmospheric transport model and analyses of surface ocean <i>P</i>co₂ observations, we show that the increase is best explained by the onset of increasing seasonality of air-sea CO₂ exchange over the Southern Ocean around 2000. Underlying these changes is the long-term ocean acidification trend that tends to enhance the seasonality of the air-sea fluxes, but this trend is modified by the decadal variability of the Southern Ocean carbon sink. The seasonal variations of atmospheric CO₂ thus emerge as a sensitive recorder of the variations of the Southern Ocean carbon sink.