Decadal variability in the outflow from the Nordic seas to the deep Atlantic Ocean

The global thermohaline circulation is the oceanic overturning mode, which is manifested in the North Atlantic Ocean as northward-flowing surface waters which sink in the Nordic (Greenland, Iceland and Norwegian) seas and return southwards—after overflowing the Greenland–Scotland ridge—as deep water. This process has been termed the ‘conveyor belt’, and is believed to keep Europe 5–8 °C warmer than it would be if the conveyor were to shut down1. The variability of today's conveyor belt is therefore an important component of climate regulation. The Nordic seas are the only Northern Hemisphere source of deep water and a previous study3 has revealed no long-term variability in the outflow of deep water from the Nordic seas to the Atlantic Ocean. Here I use flows derived from hydrographic data to show that this outflow has approximately doubled, and then returned to previous values, over the past four decades. I present evidence which suggests that this variability is forced by variability in polar air temperature, which in turn may be connected to the recently reported Arctic warming4.