Tropical Ocean Warming Drives Recent Northern Hemisphere Climate Change

"We believe the link between tropical ocean warming and the Northern Hemisphere climate trend may be a signal of human-induced climate change that has just begun to emerge in the last 50 years," say lead authors James Hurrell of the National Center for Atmospheric Research (NCAR) and Martin Hoerling of the Commerce Department's National Oceanic and Atmospheric Administration (NOAA).

The key player in this climate trend is the North Atlantic Oscillation (NAO), an atmospheric pressure seesaw between Iceland at one end and Spain and Portugal at the other.

Studies of predicted early impacts of increased greenhouse gases have shown a warming trend in the tropical oceans. Observations have revealed such a trend beginning around 1950. By analyzing results of a number of experiments using global climate models, Hoerling and Hurrell have found a correlation between these warmer sea-surface temperatures and climate changes in the Northern Hemisphere winter over the same period.

The experiment indicates that warmer waters, especially in the Pacific and Indian Oceans, produce more equatorial rain, which heats the tropical atmosphere. "It turns out that this exerts a strong control on the atmospheric pressure pattern and winds over the North Atlantic and North Pacific," says Hoerling, of NOAA's Climate Diagnostics Center. "In fact, it has forced the NAO to maintain a single phase in recent decades." Resulting changes in circulation have warmed land surfaces and shifted storm tracks farther north.

"The Northern Hemisphere surface temperature has shown a warming trend over the past several decades to values that are perhaps unprecedented over the past 1,000 years," write the authors, and the NAO change has been a key player in this.

Gradually, additional effects on climate have emerged. Winters in northern Europe and Scandinavia have grown wetter, while those in southern Europe and the Middle East have become dryer. European farmers have encountered an earlier and longer growing season. The habitats and life cycles of many marine and terrestrial species have changed.

Hoerling and Hurrell are now trying to find the physical mechanism that accounts for the tropical oceans' long-distance effects on northern atmospheric circulation. A critical component is the NAO, which controls winter weather in Europe and over much of the Northern Hemisphere.

"Until recently scientists believed the NAO was entirely chaotic, random, and unpredictable," says Hurrell. "No one paid much attention to it." All that changed in 1995 when Hurrell found that the NAO's winter-to-winter variations cloaked an underlying trend extending over several decades. That trend was soon correlated to changes in weather, agriculture, and wildlife from Canada to Siberia and from the Arctic to northern Africa.

Through the 1980s and 1990s, the NAO entered and maintained a largely "positive" phase characterized by stronger-than-average westerly winds across the middle latitudes of the Atlantic Ocean and into Europe, southerly flow over the eastern United States, and northerly flow across western Greenland, the Canadian Arctic, and the Mediterranean. Building on several attempts to explain the shift from its "negative" phase during the 1950s and 1960s, Hoerling and Hurrell have now found warming tropical oceans to be the driver.

NOAA's Office of Global Programs funded the study. NCAR's primary sponsor is the National Science Foundation.