Science 334, 655–659 (2011)

The climate of the North Atlantic Ocean is heavily influenced by a climate system called the North Atlantic Oscillation (NAO). The NAO in turn was thought to be governed over decadal timescales by changes in the flow of warm water, which normally sweeps north near the Atlantic Ocean's surface and travels south at depth. But now there is a new theory. It suggests that long-term shifts in the Atlantic Ocean climate are actually prompted when the prevailing westerly winds that cross the ocean become blocked — sometimes for just five days — by a large anticyclone over the Atlantic Ocean.

Sirpa Häkkinen of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues propose that such an atmospheric blockage can cause the high-altitude jet stream to develop large, near-stationary meanders. This reduces the wind-forcing that maintains the subpolar gyre to such an extent that a new pathway is opened up for warm, more saline water from the subtropics to flow north.

Among other evidence to support their idea, they show that winters with more blocking events between Greenland and Europe are also associated with a warmer and saltier subpolar ocean.