By Sid Perkins
Warmer-than-normal sea-surface temperatures in the central Pacific lead to stronger, more frequent tropical storms and hurricanes in the North Atlantic, a new analysis suggests. Unlike the more familiar El Ni±o, or warming in the equatorial region of the eastern and central Pacific, trends in central Pacific warming alone are more predictable and may offer forecasters a more accurate method of anticipating hurricane activity during the upcoming year, scientists say.
The sea-surface warming characteristic of El Ni±o typically stretches along the equator from the coast of South America to the international date line, with the largest temperature anomalies in the eastern Pacific. During El Ni±o episodes, the number of tropical storms and hurricanes — both called cyclones — is lower than average across the North Atlantic, says Peter J. Webster, an atmospheric scientist at Georgia Tech in Atlanta. But when the equatorial sea-surface warming is concentrated only around the international date line, hurricane activity is much higher than normal, Webster and his colleagues report in the July 3 Science.
“This is a pattern that we [scientists] hadn’t really recognized before,” comments Chris Landsea, an atmospheric scientist at NOAA’s hurricane research division in Miami. He says the finding is “an advance in the field.”
Webster and his colleagues analyzed patterns in North Atlantic cyclone activity from 1950 through 2006 during August, September and October, the height of hurricane season. As many previous studies had noted, the number and strength of tropical cyclones were markedly lower in El Ni±o years than during La Ni±a episodes, when sea-surface temperatures in the eastern and central Pacific are substantially cooler than normal. Unlike previous research, says Webster, the new study reveals that when sea-surface warming is confined to the central Pacific, hurricane activity is higher, particularly in the Caribbean, the Gulf of Mexico and along the eastern coast of the United States.
Global weather models suggest how El Ni±o and other phenomena affect hurricanes thousands of kilometers away, Webster explains. During El Ni±o events, high-altitude wind shear over the North Atlantic is stronger than normal, which tends to disrupt the formation and strengthening of tropical storms there — and this explains the lower-than-average cyclone activity then, he notes. During La Ni±a years, wind shear is low, and storms more readily form and gain strength. When sea-surface warming is confined to the central Pacific, wind shear in the North Atlantic region where cyclones form is about average but is not large enough to totally disrupt cyclone formation.
Identifying the new central Pacific warming trend is important because the transition between El Ni±o warming and La Ni±a cooling isn’t always predictable, says Webster. Sometimes, just when it looks like a shift will occur from warm to cool, for example, temperatures will swing back to warm again, thwarting forecasters’ attempts to predict cyclone activity for the upcoming season. But shifts in central Pacific warming seem to follow a more predictable path, the new analysis suggests.
The new research “is a well-done analysis of the impact of sea-surface temperatures on Atlantic hurricanes,” Landsea says. Also, he notes, the new findings may help explain the 2004 hurricane season. That year, the Pacific experienced a weak El Ni±o, but sea-surface temperatures in the central Pacific were warmer than normal and cyclone activity in the North Atlantic was unexpectedly strong.
Data suggest that episodes of central Pacific warming have occurred more frequently since the 1960s, Webster notes. Because relatively few sea-surface temperature data were collected in that region before the 1950s, scientists can’t yet discern whether this increased frequency is a symptom of long-term global climate change or is merely part of a long-term climate cycle called the Pacific Decadal Oscillation, which typically lasts around two to three decades.
