Storm Surge

As

the tattered remains of Hurricane Isabel blew off over Canada last week, the once formidable Category 5 storm left in its wake not only flooded streets, downed power lines and grieving families but also a sense of rising menace. That's because a growing number of scientists believe that conditions favorable for brewing more and even bigger hurricanes in the Atlantic locked into place about eight years ago and will probably persist for at least a decade and maybe longer. "We're not talking about a minor little increase," says Stanley Goldenberg, a hurricane expert with the National Oceanic and Atmospheric Administration, "but an overall doubling of major hurricane activity."

Starting in 1995, Goldenberg notes, the corridor of warm water that lies between the Cape Verde Islands and Central America has been producing, on average, nearly four big storms a year, as compared with fewer than two in the preceding three decades. And that has caused him and others to snap to attention. Unlike the Pacific and Indian oceans, notes Colorado State University meteorologist William Gray, "the Atlantic is a marginal area for tropical storms. When global conditions are not right, it sees very few, and when they are, it sees quite a lot."

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One of the chief ways those conditions affect Atlantic hurricane formation is by increasing or decreasing vertical wind shear, the difference in wind speed and direction at different levels of the atmosphere. Too much shear can disrupt the structure of a hurricane's eyewall, whereas more uniform winds allow a hurricane to grow to maximum potential. When Isabel briefly exploded into a Category 5 storm, wind shear was low, and its eyewall formed a nearly flawless cone of clouds some 60,000 ft. high. In the eyewall itself, winds whirled at an epic 230 m.p.h. "When we got into the eye," says Colorado State University atmospheric scientist Michael Montgomery, who flew through the eerie stillness at the storm's core, "it was like being in the middle of a beautiful coliseum."

Among the most important factors affecting wind shear in the Atlantic, scientists have learned, are the swings between El Nino and La Nina that occur in the tropical Pacific. Best known for their impact on sea-surface temperatures — El Nino produces a pronounced warming along the equator, La Nina a distinct cooling — these swings also affect atmospheric patterns worldwide. El Nino, for example, promotes high-level westerlies that tear off the tops of Atlantic Ocean storms, while La Nina and La Nada — the name some have given to the neutral phase of the cycle that currently reigns — do the opposite.

But El Ninos and La Ninas occur once every two to seven years, whereas hurricane activity in the Atlantic appears to oscillate on time scales that extend over 20 years or more. There are, however, indications that the El Nino cycle may be embedded in a longer-term oscillation, widely known as the PDO, or Pacific decadal oscillation. Some scientists are convinced that the PDO has shifted into a mode in which El Ninos, which dominated the 1980s and '90s, have become muted, allowing La Ninas and La Nadas and their attendant hurricanes to become more prominent.

But there may be a lot more involved. Goldenberg and his colleagues, for example, are focusing on a multidecade oscillation in Atlantic sea-surface temperatures that closely tracks long-term patterns in hurricane activity. Sea-surface temperatures in the Atlantic rose, for example, between the 1920s and 1970, when hurricane activity was high. And sea-surface temperatures fell in the 1970s, '80s and early '90s, just as hurricane activity dampened.

The problem with that is that the oscillation in Atlantic sea-surface temperatures is not large — less than 2°F at maximum — and so it's not easy to explain how this would be so critical to hurricane formation. Yes, says Goldenberg, warm water is the energy source for hurricanes, and so any temperature rise represents an increase in available fuel. Even more important, however, may be the impact that higher sea-surface temperatures have on wind patterns. That's because there are two ways to change wind shear. One is by reducing or amplifying high-level winds, which is what the El Nino cycle does. The other is by tinkering with the winds that take a low-level course. And the rise and fall of Atlantic sea-surface temperatures appear to correlate with both.

It's a nice story, say other scientists, who nonetheless remain skeptical. As Kerry Emanuel, a hurricane expert at the Massachusetts Institute of Technology, puts it, "We really don't have the foggiest idea why hurricane formation in the Atlantic was inactive in the '70s, '80s and '90s and so active in the '40s, '50s and early '60s. So if someone says there'll be more hurricanes than normal over the next 10 years, and someone else says there'll be fewer, each has a 50% chance of being right." The variability of hurricane formation in the Atlantic is one of meteorology's biggest unsolved puzzles.