If last month's weather seemed a bit off to you, you're right. Depending on which half of North America you dwell, February was characterized by either bitter cold and winter storms, or unusually warm temperatures. This new map from NASA shows just how anomalous — and divided — our current winter has been.
This is a land surface temperature (LST) anomalies map (as opposed to an air temperature map). Land temperature is a measure of land surface heating, where solar energy is absorbed and then radiated back out. As a measure, it's often significantly warmer than air temperature.
Looking at the map, the color gradients show LST for February 2015 as compared to the 2001–2010 average for the same month. Warm anomalies are shown in red, near-normal in white, and cooler-than-normal temperatures in blue.
Last month, large areas of Arizona, Colorado, Nevada, Oregon, Utah, and Wyoming experienced temperatures more than 18 degrees F (10 degrees C) above average. Meanwhile, states in the Midwest, Mid-Atlantic, and New England experienced temperatures about 18 degrees F (10 degrees C) below normal. Those are all significant and wild variances.
At the same time, many states saw air temperature records fall, including Massachusetts, Maine, Michigan, and various parts of New York. But in San Francisco, Seattle, Portland, Las Vegas, and Salt Lake City, residents experienced the warmest winter on record.
NASA's Earth Observatory provides an explanation:
Most meteorologists attribute the sharply contrasting temperatures to the combination of a persistent ridge of high pressure—nicknamed the "Ridiculously Resilient Ridge"—over the northeastern Pacific Ocean, and an equally persistent trough that has funneled chilly air from the Arctic into central North America. But why that resilient ridge has appeared over the northeastern Pacific during the last few winters is a more complicated and controversial question among experts.
A new line of research suggests that the loss of Arctic sea ice associated with global warming may be causing the jet stream to slow down and become wavier, thus setting up the unusual pattern over North America. Other researchers think there could be a link between Siberian snowfall and mid-latitude weather extremes. Still others think changes in the Arctic have little to do with mid-latitude weather extremes; instead they see periods with an anomalously sharp gradient in sea surface temperature in the far western Pacific as the key factor.
Clearly, more research is needed.
Image: NASA Earth Observatory image by Jesse Allen, using data from the Level 1 and Atmospheres Active Distribution System (LAADS).