One of the big questions in paleoclimate is how to warm polar regions
without significantly warming the equator. For example during parts of
the Cretaceous, dinosaurs were found in what is now the Arctic, but equatorial
regions were not significantly warmer. One line
of attempts to explain this phenomenon has focussed on changing
the transport of heat within the ocean. A number
of researchers have proposed that different continental configurations could
have caused the heat transport to increase.
In a paper recently submitted to the Journal of Advances in Modeling the Earth System Marie-Aude Pradal and Anand Gnanadesikan suggest a different pathway to warming. Instead of changing continental configurations, they increased the efficiency with which temperature and salinity are stirred by turbulence between the high and low latitudes in a computer model of the atmosphere, oceans and sea ice. They found that changes in this mixing efficiency could produce strong warming in the Southern Ocean of up to 7C locally. However, they found that such warming wasn't always accompanied by higher poleward ocean heat transport.
Instead, Pradal and Gnanadesikan found that changes in the sea surface temperature were accompanied by changes in the difference in salinity between deep and surface waters. If the deep waters are much saltier than the surface waters, the surface waters tend to get much colder. This relationship holds within individual simulations as a result of natural variability as well as across simulations with different levels of mixing. Higher mixing reduces the contrast in salinity between surface and deep waters, making it harder to form sea ice and allowing more absorption of sunlight.
This result implies that warming in high latitudes may be the result of increased export of freshwater and the resulting breakdown of stratification. This in turn suggests that modelers should potentially focus on opening pathways to the high latitudes by which freshwater can escape. Currently the distribution of land and ocean ridges around the Arctic limits this. Could changes- potentially forced by changes in the mantle- alter this connectivity? Future runs with climate models are planned to investigate this possibility.