remember that scene where Benthomaar tells nya how nyad merged with the sea to stop wojira the first time? well, Kai overheard it. and he's not gonna let his baby sister sacrifice herself. so what does he do? he sacrifices himself instead. duh.
Finding the scroll used to depict nyad merging with the sea, he deduces that it must be possible for other elemental masters to merge with their element, too. When ninjago fully floods, and Jay is passed out from drowning, he merges with the sun to dry up the flooded city and defeat wojira. Cue angst, because he's one with the sun now. and where is the sun if not in the sky?