|Above: One of the largest sunspots in a decade, labeled AR1944, was seen near the peak of the most recent solar cycle, in early January 2014, as captured by NASA's Solar Dynamics Observatory. An image of Earth has been added for scale. Image credit: NASA/SDO.
In our last post, we looked at seasonal forecasts that have long been calling for a cold late winter in the central and eastern United States based on various signals, including stratospheric warming and the El Niño/Southern Oscillation. A winter storm dubbed Harper is the first salvo in this transition. This weekend, Harper will bring very heavy snows and widespread ice across large parts of the Northeast. See weather.com for frequent updates on Harper.
There’s another player in the seasonal prediction game, and it’s one that many forecasters have long derided or dismissed: the 11-year cycle of solar activity. In recent months, sunspots have been nearly absent from the sun’s surface, indicating that a solar minimum—the end of one solar cycle and the start of the next one—is nearly at hand. We'll delve into the noteworthy weakness of the last two solar cycles (the one now ending was the weakest in a century) in a future post.
|Figure 1. Sunspot activity has declined more rapidly than expected from its peak in early 2014. Image credit: NOAA Space Weather Prediction Center.
Scientists have spent centuries trying to tease out connections between the solar cycle and earthly weather. Any such links aren’t at all obvious, and with good reason. For one thing, the sun’s total energy varies by only 0.1% from solar peak to solar minimum.
“I was a complete skeptic regarding any tangible impacts of the solar cycle, from when I started here in 2000 through about 2008 or 2009,” said Todd Crawford (@tcrawf_nh), who leads The Weather Company’s seasonal prediction activities. Eventually, Crawford found himself intrigued by the fact that ultraviolet radiation varies a great deal more across the solar cycle—by as much as 10%—and that some of this UV radiation is absorbed by ozone, thus heating up the equatorial stratosphere.
At that point, said Crawford, “Things became a little more interesting to me. Then I dug into the observations, and that's when I became a true believer in the ‘Church of Sol’.”
|Figure 2. These images from the Solar and Heliospheric Observatory (SOHO) spacecraft compare sunspots on the Sun’s surface (top row) and ultraviolet light radiating from the solar atmosphere (bottom row) at the solar maximum of 2000 (left) and the solar minimum (right.) On March 18, 2009, the face of the Sun was spotless. Image credit: NASA Earth Observatory.
When UV energy reaching Earth is diminished at solar minimum, Crawford reasoned, the north-south temperature contrasts in the stratosphere ought to be lessened because of less heating of ozone at lower latitudes. That, he reckoned, ought to favor a more variable polar vortex and a negative North Atlantic Oscillation (NAO).
A simple test helped convince Crawford: “If you pull the NAO values for the three winters centered on each of the last six solar minima (18 total values), and you compare them to the other winters, you find a statistically significant tendency for more negative NAO values during solar minimum.”
A negative NAO tends to increase the odds of cold, stormy weather over the eastern U.S. Another contributor is a strong upper-level ridge toward the west and a trough toward the east. Dubbed the positive Pacific North American (PNA) pattern, this setup is more frequent during El Niño, and also quite common overall during the last few years. “When you combine the negative NAO with the positive PNA, you get the sort of expectations that winter weather lovers in the East dream of,” Crawford said.
|Figure 3. A fallen tree on the 1700 block of T Street NW in Washington, D.C., following the Snowmaggeddon storm of February 2010, one of several intense winter storms that plastered the U.S. Northeast in the winter of 2009-10. A moderately strong El Niño event coincided that winter with a strong tendency toward negative NAO conditions. Image credit: dbking/Wikimedia Commons.
Third rail or paradigm shift?
Only a handful of weather and climate scientists are bringing the solar cycle into their work, although that may be just starting to change. When Crawford attended a conference in September on seasonal and subseasonal prediction, “there was no in-depth discussion of the impact of the solar cycle at all, outside of my own slide deck.”
At the December meeting of the American Geophysical Union, a poster led by Robert Tomas (National Center for Atmospheric Research, or NCAR) discussed a model simulation of the last 1000 years of climate that incorporated the 11-year solar cycle. The researchers found small but statistically significant effects of the solar cycle on near-surface air temperature, precipitation, and pressure.
At the same meeting, solar physicists Robert Leamon (NASA/University of Maryland) and Scott McIntosh (NCAR) gave a poster on their ongoing work involving the termination of solar cycles, which occurs as oppositely polarized bands of magnetism work their way toward the solar equator from opposite directions. Their findings, now in publication review, suggest that a major El Niño event becomes more likely around the transition from one cycle to another (i.e., at solar minimum).
“Forecasting the Sun's global behavior places the next solar termination in early 2020,” their poster noted. “Should a major oceanic swing follow, our challenge becomes: when does correlation become causation and how does the process work?”
Acknowledging the rocky history of attempts to link the solar cycle with earthly weather, Leamon and McIntosh have warned that “searching for the connection between the variability of the solar atmosphere and that of our troposphere has become ‘third-rail science’—not to be touched at any cost."
However, NCAR climate scientist Jerry Meehl is on a similar page. “I'm familiar with [Leamon’s] work, and it complements work that Harry van Loon, Julie Arblaster, and I did a few years ago,” Meehl told me. That paper was published in Science in 2009. “Namely, at peaks in the 11-year solar cycle, there tends to be a weak La Niña signal, followed a couple of years later by a weak El Niño. There's evidence for this connection in the observations, though the record is short, and from model simulations.”
As for the “third rail” concerns voiced by McIntosh and Leamon, “That was definitely the case until about seven or eight years ago,” Meehl said. “Since then, tangible mechanisms have been established in observations and models that connect solar variability to Earth's climate.”
For more on the connections between solar activity and long-term climate, and an update on the upcoming Solar Cycle 25, see our post from February 28, The Weirdly Quiet Sun May Get Even Quieter (and BTW, Earth is Still Warming).