One of the major factors that shaped the autumn and winter of 2011/2012 was the La Niña in the Pacific. This pattern of wind and water in the Pacific helped to shape weather around the world.
Here in the US, it helped create the drought that eliminated so much of the snow in the Western mountains. It created a two-year drought in Texas and left large stretches of the South high and dry.
By the middle of March, 58% of the contiguous US was dry or in drought conditions.
La Niña SOURCE: NOAA
The La Niña is a large pool of unusually cool water in the Central and Tropical Pacific. It cools the air above it, altering not only the air’s temperature but also its ability of hold moisture. The air pressure changes and that, in turn, alter wind patterns. When over a million square miles of tropical air changes pressure, it changes wind and weather patterns around the globe, particularly in the tropics and the Pacific Rim.
The impact of a La Niña can be magnified or reduced by other climate factors. In the Northern Hemisphere, the wintertime behavior of the Arctic Oscillation (AO) can overwhelm the impact of a wintertime tropical oscillation. Normally a La Niña creates cold weather in Canada and the northern states. This year a positive Arctic Oscillation trapped the cold polar air north, leaving temperatures in most of the US positively toasty.
The Arctic Oscillation kept the La Niña from creating a cold winter. SOURCE: The Weathervane
This winter’s La Niña was weaker than the winter of 2010/2011, when the US froze and 49 of 50 states were covered with snow. It peaked in January and started to fade in February. At this point, most scientists expect La Niña to be gone by mid-to-late spring.
The good news is that to all intents and purposes, the La Niña is over. It is so weak that we are beginning to see a return to more normal winter. Rain has begun to return to the West and Texas. Storms lashed western states in late March, bringing near-normal snowpack to northern portions of the Pacific Northwest and welcome moisture, if not relief to central and southern portions of the West. Even parched Texas saw some relief, although 90% of the state remains in dry or drought conditions.
With La Niña fading, rains are returning to much of the drought-stricken USA. SOURCE: Wikipedia
Looking to the future, the majority of scientists expect the Pacific to be neutral this summer. Think of it – normal water and more normal weather.