Keeping Cool

While surface temperatures across most of the globe are on the rise, the eastern U.S. appears to be slowly cooling. Scientists say the trend could be a result of increasing cloud cover triggered by warming Pacific waters.
January 18, 2001 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. Eastern U.S. temperatures have displayed a cooling trend of 0.1 degrees Celsius 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.
Right: Clouds appear bright white when viewed from above because they reflect sunlight back into space. The ground and atmosphere below these clouds absorb less solar radiation and therefore release less heat into the environment. Image taken by NASA's SeaWiFS satellite
In the computer 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," said researcher Walter A. Robinson, an associate professor of atmospheric sciences at the University of Illinois at Urbana-Champaign.
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 NASA Goddard Institute for Space Studies (GISS) "general circulation model" (GCM), which simulates the circulation of the atmosphere around the world, and used sea surface temperatures from around the globe.

Above: Models in Walter Robinson's research showed that an increase of almost 1 degree Celsius in tropical Pacific Ocean (TPac) temperatures (black curve) correspond to an increase in Eastern U.S. (EUS) low cloud cover (red and blue curves) by almost 5 percent. The blue curve represents increased cloud cover using only tropical (TO) Pacific water temperatures, while the red curve shows the increase in Eastern U.S. clouds using global (GO) water temperatures. In the model, this increase in cloud cover keeps the Eastern U.S. from warming along with the rest of 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, solar irradiance, and greenhouse gases such as carbon dioxide. Because these factors were held constant, they could 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 considered global sea surface temperatures in different areas. The only time the model showed significant cooling in the eastern United States was when the tropical Pacific waters warmed.