When there’s a big differential between the water temperature (around -1.9C) and the air temperature above the sea ice (below -20C). The warmer sea flows up to the air, freezing into new ice. According to the BBC, “the salt in this newly formed ice is concentrated and pushed into the brine channels. And because it is very cold and salty, it is denser than the water beneath.” This makes it fall down into the water, creating an ice plume that grows into the brinicle.