Sea Ice
Spring is the time of year when the Arctic comes out of its long, dark winter and the sea ice that covers most of the Arctic Ocean in winter begins its annual melt. This melting continues through the warm summer months until the sea ice reaches its minimum extent in September. Then, the ice begins to grow back throughout the winter until it reaches its maximum extent in late February or early March. Over the past few decades, the Arctic has warmed approximately twice as fast as the rest of the Northern hemisphere which makes ice harder to grow in the winter and easier to melt in the summer. Today, the Arctic’s average sea ice minimum is approximately 1.3 million square miles smaller than it was at the beginning of the 1980s – 1.3 million square miles is enough ice to blanket most of Europe. Larger areas of the Arctic Ocean are exposed to the sun’s energy for longer periods the year, which allows it to release its surplus energy into the atmosphere. This affects the general circulation of the atmosphere in two distinct ways.
- First, the temperature differences create a pressure gradient between the mid-latitudes and the poles. This pressure gradient creates strong upper-level eastward winds that during most of the year pull storm systems away from the United States at a rapid speed. The enhanced warming of the high northern latitudes relative to the rest of the Northern Hemisphere, known as Arctic Amplification, has caused the temperature and pressure gradient to weaken by about 10 percent since the late 1980s. This corresponds to a weakening of the upper-level easterlies by as much as 14 percent.
- Second, the rivers of air currents called Rossby waves that flow through the storm-steering peaks (ridges) and valleys (troughs) of air pressure have stretched out and shifted pole-ward. For storms crossing the United States, this means a more circuitous route. Both of these atmospheric conditions which result from a warmer Arctic, favor slower-moving storm systems that hang around longer to increase flooding, prolong droughts, and break records.