Recent summer melts have left lots of the ocean exposed to sunlight.
In early 2011, the US and Europe froze, even as Greenland and Alaska experienced unusual periods of warmth. This year, the US and Europe were baking as the winter drew to an end, even as cold air hovered over Central Europe and Asia. In the Northern Hemisphere, extreme winter weather tends to be associated with the negative phase of the Arctic Oscillation, a wind pattern that dominates the polar region. And a consensus is building that changes in the Arctic may have permanently placed the Oscillation in the negative mode, leading to stable changes in the winters of the Northern Hemisphere. Cornell professor Charles H. Greene
has just published a review of this idea, and we talked with him about what the warming Arctic might mean for the US and Europe.
Greene's paper describes a key determinant of the Northern Hemisphere's winter weather: the Arctic Oscillation. When that is in its positive phase, a strong set of winds called the Polar Vortex forms. These winds help trap Arctic air masses at the pole, keeping the cold out of the mid-latitudes. This also allows the jet stream to take a more direct route around the globe, moderating the weather.
But over the last few years, the Oscillation has been strongly negative; in fact, in 2010, we saw a record for the most strongly negative period we'd ever recorded. During this phase, the winds of the Polar Vortex weaken, allowing the cold Arctic air to intrude or mix into the air at lower latitudes. As a result of this, Greene told Ars two things happen to the jet stream: it gets substantially weaker, and it tends to meander widely from north to south as it traverses the globe. This can lead to the severe chills the US and Europe have experienced over the past several winters, but the meandering jet stream can also draw warmer southern air north, as happened in the US this spring.