A fresh look at the Gulf Stream’s Arctic limb
A recent Bjerknes study shows that the Gulf Stream’s Arctic limb is constrained by its heat transfer from the south.
Tor Eldevik at the Geophysical Institute, University of Bergen, and Jan Even Ø. Nilsen at the Nansen Environmental and Remote Sensing Center have constructed a simple mathematical model rooted in 60 years of observations. The scientists, who are both associated with the Bjerknes Centre for Climate Research, find that the Norwegian Atlantic Current is constrained by the amount of excess heat that it brings from lower latitudes. Northern freshwater input, on the other hand, explains to what extent the return flow from to the Norwegian Sea to the North Atlantic takes place in the surface or in the deep ocean.
As a consequence, Eldevik and Nilsen in particular infer that much of the observed decrease in Arctic winter sea ice cover can be explained by increased poleward heat transport with the Norwegian Atlantic Current. The results are published in a recent number of Journal of Climate.
Critical for the earth’s climate
The sun provides most heat at the Equator, causing evaporation and consequent ocean salinification – heat, evaporation, and salt that would accumulate unless continuously transported poleward with winds and ocean currents. The Gulf Stream’s Arctic limb, the Norwegian Atlantic Current, gives up its excess heat to the colder atmosphere as it travels north. Concurrently, the atmospheric moisture transport is drained by precipitation. The result is a cold and less saline oceanic return-flow – a flow that is subsequently subject to warming and evaporation in the equatorial region, completing the cycle.
The Norwegian Atlantic Current is thus an important component in Earth’s balanced distribution of heat and freshwater, and a main contributor to regional climate. It is therefore a major concern how and to what extent this ocean current changes with climate, and vice versa. Is it for example so that the vigour of the Norwegian Atlantic Current is predominantly constrained by the heat it carries? Or does its strength reside with the regional freshwater input, largely contributed by river runoff in the Arctic? The latter is often described as the most critical factor, particularly when considering climate change in the high northern latitudes. Eldevik and Nilsen’s paper shows that it is the heat that is decisive.