Global warming theory states that as the troposhere (surface to 10k) warms, because of atmospheric energy budget, this causes the stratoshere (10k to 50k) to cool.
I suppose this would apply seasonal as well, so summertime troposphere warms, stratosphere cools.
The web site claims that the data are global (except for some orbital coverage issues). Given that the seasonality should average out between the NH and SH there appears to be seasonality present (opposite to that of my understanding of the Earth’s orbital eccentricity).