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Sea ice sits atop Arctic waters. Credit: USGS
The Arctic is the pineapple upside-down cake of the oceans: To see the same temperature structure as in the other oceans, you’d have to flip it upside down.
Oceans and seas are made of layers. In most cases, those layers are based on temperature. The warmest layers are at the top, while the coldest are at the bottom. But the Arctic is just the opposite.
This topsy-turvy layering has a couple of causes. One is the water that flows into the Arctic, and the other is the sea ice that sits atop Arctic waters.
Water that enters from the Atlantic Ocean is warmer than other Arctic waters, but it’s also saltier. That makes it denser, so it sinks to the bottom.
The Arctic’s surface layers are fed by giant freshwater rivers and by the Pacific Ocean. Pacific waters quickly freeze, forming vast sheets of sea ice. As the water at the surface freezes, it squeezes out most of the “salts” that enter from the Pacific — the minerals that give saltwater its name. That makes the surface water even fresher.
The salt then sinks to a middle layer, which is cold, like the surface, but salty, like the bottom. This layer, known as the halocline, acts as an insulator between the other two. So even though the amount of Arctic sea ice keeps decreasing, the halocline prevents it from being worse. Without that layer, the warm Atlantic waters would prevent any ice at all from forming atop the Arctic — removing the “icing” from the upside-down-cake of the oceans.
