Government weather forecasters love El Nino. For years they have used this fluctuating current in the Pacific Ocean (in El Nino years, sea surface temperatures in the western tropical Pacific rise, while in La Nina years they fall) to forecast winter weather months in advance. In El Nino years, they basically expect the northeast to be a little warmer than the historical average, the southern tier to be colder, and the southeast to be snowier. La Nina years are supposed to bring balmier winters to almost all the continental U.S.
In fact, government forecasters love El Nino so much that even when a better basis for seasonal prediction comes along, they stick with the kid.
Since 1999, scientists led by meteorologist Judah Cohen, director of seasonal forecasting at AER Inc., of Lexington, Mass., have been gathering more and more evidence that there’s a better way. As he and physicist Christopher Fletcher of the University of Toronto report in the August issue of the Journal of Climate, the predictive power of El Nino, at least outside the tropics where its effects are directly felt, can’t hold a candle to an alternative: using the amount of snowcover in October in Eurasia and, especially, Siberia to predict upcoming winter temperatures and snowfall for the high- and mid-latitudes of the Northern Hemisphere—basically, the eastern U.S. and northern Europe and Asia. They call their predictive model sCast, where the "s" stands for snow.
If you blink, you’ll miss summer in Siberia. The snow starts accumulating in October, which is also when a high-pressure system called the Siberian High forms. In Octobers with above-average Siberian snow cover, the Siberian High is stronger and Northern Eurasia experiences a chillier-than-normal fall. That chill is balanced by a warming in the stratosphere, the atmosphere’s upper layer, in January, said Cohen. This warmth reaches Earth’s surface in the arctic and sub-arctic, bringing a warmer-than-normal winter to those regions.
But the see-sawing atmosphere (high pressure in one place balanced by low pressure somewhere else) means that when Siberia is especially snowy, the northeastern U.S. and much of Europe are likely to be colder and snowier than normal.
When sCast was pitted against El Nino-based forecasts for seven winters starting in 1999, and in “hindcasts” (seeing if October snowcover in the past correctly predicted winter temperatures and snowfall), sCast beat El Nino hands down.
Climate forecasters have grumbled for years that the eastern United States and northern Eurasia have eluded seasonal forecasts. As any homeowners and city managers who have based their fall purchases of snow shovels and salt on these forecasts can attest, the El Nino-based forecasts blow it more often than they nail it. Turns out the forecasters were using a faulty premise for their predictions.
Stay tuned for what’s happening with the white stuff in Siberia this October.