PURPLE HAZE: The tiny particles that make up this purplish haze off the east coast of China actually serve to strengthen storms in the Pacific--potentially changing weather in North America. Image: NASA MODIS
Satellite images of Asia track clouds of haze that drift eastward with prevailing winds. Made up of the tiny particles created when fuel is burned, this haze drifts out to sea and slowly disappears—though not without exacting a toll. From shifting wind direction to suppressing the monsoon, aerosols' effects derive from the absorption of sunlight, cooling the surface and warming the surrounding air. And new research shows that this localized warming is strengthening storms over the Pacific Ocean—and impacting atmospheric circulation across the globe.
Atmospheric scientist Renyi Zhang of Texas A&M University and his colleagues used satellite imagery to assess the thickness and pressure of clouds over the Pacific. Every winter such clouds form and track east across the ocean, transferring the warmth of the equator toward the poles. The clouds coalesce out of water condensing on tiny impurities in the air. Eventually, enough cloud droplets form to begin colliding with one another; they combine to form raindrops that eventually fall from the clouds.
Adding aerosols changes this process by delaying the creation of raindrops; with more impurities in the air, condensation occurs more widely, creating smaller droplets and reducing the chances of collisions that produce raindrops. "The raindrop is going to fall out of the atmosphere and that produces a downdraft that kills the thunderstorm," Zhang says. "If you reduce the size of cloud droplets you make them hard to collide. You cannot make raindrops easily."
This gives the Pacific storms more time to build strength and reach higher into the atmosphere. In fact, such storms were as much as 50 percent stronger from 1994 to 2005 than similar storms in the previous decade thanks to the impact of Asian aerosols. This increase in strength does not correlate with overall cloud coverage, as would be expected if it were simply natural variability, Zhang and his co-authors write in the paper presenting the finding, published online in Proceedings of the National Academy of Sciences. And further testing with meteorological models showed that aerosols would be required to produce the observed storms.
Of course, intensifying Pacific storms means changes in North American weather; tiny particles of Asian pollution might impact continental weather an ocean away. "This storm track regulates the jet stream, which also controls the storms we have in North America," Zhang says. "If you change the storm track over the Pacific, it is very likely you are going to change the storms in the U.S. It's all connected."