Abstract/Summary

The formation of sea ice in polar regions is possible because a salinity gradient, or halocline, keeps the water column stable despite intense cooling. Here we demonstrate that a unique water property is central to the maintenance of the polar halocline, namely that the thermal expansion coefficient (TEC) of seawater increases by one order of magnitude between polar and tropical regions. Using a fully coupled climate model, it is shown that, even with excess precipitations, sea ice would not form at all if the near-freezing-temperature TEC was not well below its ocean-average value. The leading order dependence of the TEC on temperature is essential to the co-existence of the mid-/low-latitudes thermally stratified and the high-latitudes sea ice covered oceans that characterize our planet. A key implication is that non-linearities of water properties have a first order impact on the global climate of the Earth, and possibly exoplanets.