SK1:
I infer you are surprised by Norwegian's 'Summer Only' temporary contract. Don't be.
Unless you have a direct employment relationship with Norwegian, ALL pilots may be considered as temporary when rented from an agency by Norwegian. Your employment is with the agency, not Norwegian. Rented pilots have no entitlement to labor rights nor labor principles with Norwegian whatsoever (including seniority).
As a rented pilot your services may be terminated by Norwegian at any time, without recourse. Norwegian simply notifies the agency your particular services are no longer required. The agency may or may not find you alternative work. Your notice period is with the agency, not Norwegian.
You may not even get paid during your notice period. You will not be protected by Employment Law because you have entered into an employment contract/agreement with the agency, which almost certainly indemnifies the agency from their obligations due to any action taken by the airline.
Read my previous posts.
Good luck.