Keep in mind these are mostly preprints, so adjust their value accordingly:

https://www.biorxiv.org/content/10.1101/2020.05.26.115832v1

https://www.biorxiv.org/content/10.1101/2020.05.18.100545v1

https://www.cell.com/cell/fulltext/S0092-8674(20)30610-3

https://www.medrxiv.org/content/10.1101/2020.04.17.20061440v...

https://hal-pasteur.archives-ouvertes.fr/pasteur-02569149

https://www.biorxiv.org/content/10.1101/2020.05.21.108308v1

https://www.medrxiv.org/content/10.1101/2020.06.02.20120477v...

https://www.medrxiv.org/content/10.1101/2020.04.27.20081893v...

There isn't much completed research about the seasonality of COVID-19 itself (understandably), the "cliff" is mostly an educated guess based on the strong seasonal pattern we see with every other respiratory virus. Here are a couple papers:

https://www.medrxiv.org/content/10.1101/2020.03.12.20034728v...

https://www.medrxiv.org/content/10.1101/2020.02.22.20025791v...