Man I wish the ABC would give the doi when referencing scientific articles, or at least the title.

Anyway, I believe it is this one.
https://iopscience.iop.org/article/10.1088/1748-9326/ae5724

2. Data and methods

To characterise long-term changes in summer onset, length, withdrawal, transition speed, and heat accumulation, we identify the annual timing of summer onset and withdrawal by applying Fourier series analysis to daily mean air temperatures 2 m above the surface (T2 m). We focus on mid-latitude regions (23.5°–70° N, ‘NH’ and 23.5°–70° S, ‘SH’) and analyse inland areas (land excluding coasts), coastal margins, and oceans separately; we exclude the tropics and polar regions (23.5° S–23.5° N, >70° N/S) due to the lack of a clear division of four seasons .