The Eastern U.S. Keeps Its Cool While the World Warms
Much of the Earth has warmed over the last half-century, but the eastern half of the United States has shown a cooling trend. NASA-funded research indicates cooler temperatures in the eastern U.S. are caused by an increase in sun-shielding clouds produced by warmer ocean temperatures in the Pacific.
Walter A. Robinson of the University of Illinois at Urbana-Champaign, James Hansen of NASA Goddard Institute for Space Studies, and Reto Reudy of Science Systems and Applications, Inc. will present these findings in a paper entitled "Where's the Heat? Insights From GCM Experiments into the Lack of Eastern U.S. Warming" at the American Meteorological Society Annual Meeting in Albuquerque, N.M. on January 15.
Eastern U.S. temperatures have displayed a cooling trend of 0.1°C per decade, while global temperatures warmed by that same amount from 1950 to 1997. The researchers used a computer climate model to see if this regional cooling could be caused by changes in sea surface temperature. Robinson said that in the GISS model, "Warmer sea surface temperatures in the tropical Pacific cause greater cloud cover over the eastern United States. This increased cloud cover is directly responsible for the cooling." The brightness of a cloud causes a large percentage of incoming solar radiation to be reflected back into space, thus keeping the atmosphere cooler than if the cloud wasn't there.
Using the climate simulations, Robinson found the amount of water vapor in the Gulf of Mexico follows closely the water vapor released by the warm sea surface temperatures in the Pacific Ocean. Water vapor from the Pacific moves east to the Gulf of Mexico and is then carried over the eastern U.S. by the clockwise circulation around an Atlantic subtropical high pressure system. When the water vapor arrives over the U.S. it condenses and generates more cloud cover, allowing less solar radiation to reach and warm the Earth's surface.
Robinson's research utilized the GISS "general circulation model," which simulates the circulation of the atmosphere around the world and used sea surface temperatures from around the globe.
In order to create a focus on sea-surface temperatures in the model runs, three components that can contribute to warming or climate forcing, were "fixed." These are aerosols (particles in the atmosphere), solar irradiance or brightness, and greenhouse gases (such as carbon dioxide). Because these factors were fixed, they can be ruled out as the cause of cooling in the model, leaving only sea surface temperatures as a variable.
The GISS model used ocean temperature data over a 47-year span, from 1950 to 1997 and looked at global sea surface temperatures in different areas. The model used temperatures from 20 degrees north to 20 degrees south, and from each of those endpoints to each pole. The only time the model showed significant cooling in the eastern United States was when the tropical Pacific waters warmed.
Robinson, W.A., R. Ruedy, and J.E. Hansen 2002. General circulation model simulations of recent cooling in the east-central United States. J. Geophys. Res. 107, no. D24, 4748, doi:10.1029/2001JD001577.
Leslie McCarthy, NASA Goddard Institute for Space Studies, New York, N.Y., 212-678-5507, firstname.lastname@example.org