"We are witnessing a true reversal of ocean circulation in the Southern Hemisphere—something we’ve never seen before,” explains Antonio Turiel, ICM-CSIC researcher and co-author of the study. “While the world is debating the potential collapse of the AMOC in the North Atlantic, we’re seeing that the SMOC is not just weakening, but has reversed. This could have unprecedented global climate impacts.”
As water starts to freeze the molecules start to rearrange thrmselves from a free flowing random motion into a crystal lattice. As it does this the molecules push each other a little further apart and lock into place, but because they are further apart they now take up more ‘space’ and as such they are less dense. The water wround them now has more water molecules for the same volume that the ice does and that means the water sinks (as its heavier) and ice rises.
To really ELI5: Imagine water is like a crowd of people balanced on a REALLY giant see-saw, everybody is really squished together and when they get cold they decide to put their arms out wide and grab hands with the people around them, but making small circles with the other people also grabbing hands until there is a big group of people made of little rings of people arms out holding hands. Now imagine that the people with their arms out have to spead out and some fall off the edge of the see-saw. Now there are less people on one side of the see-saw and because the other side is heavier, that side drops and the people with their arms out rises.