September, 2006, Sep2006, Vol. 37, Issue 6
by Madeleine J. Nash
...A systematic effort to understand these storms dates to 1898, when President William McKinley directed what was then the U.S. Weather Bureau to expand its rudimentary network for hurricane warnings. The impetus was the outbreak of the Spanish-American War. "I am more afraid of a . . . hurricane than I am of the entire Spanish Navy," McKinley reportedly said. In 1886, a record seven hurricanes hit the U.S. coast; one completely destroyed the thriving port city of Indianola, Texas. The year 1893 was almost as bad; six hurricanes hit the United States. One came ashore near Savannah, Georgia, overwhelming the low-lying Sea Islands off the South Carolina coast; another devastated the island of Cheniere Caminanda off the Louisiana coast. In those two storms alone, 4,500 lives were lost.
Over the next half century, forecasters relying on observations of winds and pressure taken by an expanding network of ship and ground-based weather stations struggled to provide hurricane warnings to vulnerable populations. They often failed. In 1900, a hurricane burst upon the unsuspecting citizens of Galveston, Texas, killing 8,000 to 12,000. In 1938, people stood along Long Island's Westhampton Beach marveling at what they thought was an approaching fog bank, only to realize, too late, that it was the storm-seized ocean heaving up. Twenty-nine people died.
World War II propelled hurricane science into the modern era. In July 1943, Army Air Forces pilot Joseph B. Duckworth--on a dare, it is said--flew through the eye of a hurricane as it neared the Texas coast; he did it again a couple of hours later as weather officer First Lt. William Jones-Burdick took measurements at 7,000 feet, inside the storm's eye. In February 1944, the Joint Chiefs of Staff approved the first of a series of hurricane missions by Army and Navy aircraft. Later that year, military planes gave chase to a storm that came to be known as the Great Atlantic Hurricane, following it as it roared up the East Coast, taking aim at New England. All along the storm's path, radio newscasters blared out warnings. Of 390 deaths, all but 46 occurred at sea.
After the war, the U.S. Weather Bureau--renamed the National Weather Service in 1970--established a formal program of hurricane research. To study these formidable whirlwinds, flights continued to transport scientists through turbulent eye walls and the eerie stillness of the eye itself. In the 1960s, earth-orbiting satellites began providing even higher observational platforms. Since then, forecasters have progressively narrowed "the cone of uncertainty," the teardrop-shaped blob that surrounds their best predictions of where a hurricane is likely to go. At 48 hours, track forecasts are now "off" on average by just 118 miles; at 24 hours, by less than 65 miles, both significant improvements over 15 years ago. Despite these advances, hurricanes undergo sudden surges in power that are easy to spot once they start but dauntingly hard to predict.
LIKE A GIANT BUMBLEBEE, the P-3 Orion buzzes in from Biscayne Bay, dipping a wing as it passes the compact concrete building that houses the National Oceanic and Atmospheric Administration's Miami-based Hurricane Research Division. The plane, a modification of the submarine hunters built in the 1960s for the U.S. Navy, is one of two that fly scientists in and out of some of the planet's mightiest storms, including Hurricane Katrina as its engorged eye neared landfall.
Among those on that flight was research meteorologist Stanley Goldenberg, whose third-floor office looks, appropriately enough, as if a hurricane just blew through it. Goldenberg is well acquainted with hurricanes blowing though. In 1992 Hurricane Andrew demolished his family's rented house in Perrine, Florida. A computer-enhanced satellite image of the hurricane, with its monstrous circular eye wall, now hangs on his wall. "The bagel that ate Miami," he quips.
Hurricanes belong to a broad class of storms known as tropical cyclones, which also occur in the Indian and Pacific oceans. They do not develop spontaneously but grow out of other disturbances. In the Atlantic, most evolve out of "African waves," unstable kinks in the atmosphere that spiral off the West African coast and head toward Central America. Along the way, these atmospheric waves generate ephemeral clusters of thunderstorm-producing clouds that can seed hurricanes.
At the same time, hurricanes are much more than collections of thunderstorms writ large; they stand out amid the general chaos of the atmosphere as coherent, long-lasting structures, with cloud towers that soar up to the stratosphere, ten miles above the earth's surface. The rise of warm, moist air through the chimney-like eye pumps energy into the developing storm.
Ocean warmth is essential--hurricanes do not readily form over waters cooler than about 79 degrees Fahrenheit-but the right temperature is not enough. Atmospheric conditions, such as dry air wafting off the Sahara, can cause hurricanes--along with their weaker cousins, tropical storms and depressions--to falter, weaken and die. Vertical wind shear-the difference between wind speed and direction near the ocean's surface and at 40,000 feet--is another formidable foe. Among the known regulators of vertical wind shear is El Niño, the climate upheaval that alters weather patterns around the globe every two to seven years. During El Niño years, as Colorado State University tropical meteorologist William Gray was first to appreciate, high-level westerlies over the tropical North Atlantic increase in strength, ripping developing storms apart. In 1992 and 1997, both El Niño years, only six and seven tropical storms formed, respectively, or a quarter of the number in 2005. (Then again, Goldenberg observes, the devastating Hurricane Andrew was one of the 1992 storms.)
For years, Goldenberg notes, scientists have been pondering why the number of Atlantic hurricanes varies from year to year, even though roughly the same number of African waves move out over the ocean each year. What accounts for the difference? El Niño explains some, but not all, of the variance. By combing through the historical record and more recent recordings from scientific instruments, Gray, along with Goldenberg's colleague Christopher Landsea, has found another pattern: hurricanes in the Atlantic march to a slowly alternating rhythm, with the 1880s and 1890s very active, the early 1900s comparatively quiescent, the 1930s through 1960s again active, 1970 through 1994 quiescent again.
Five years ago, a possible explanation for this pattern emerged. Goldenberg shows me a graph that plots the number of major hurricanes--Category 3 or higher--that spin up each year in the Atlantic's main hurricane development region, a 3,500-mile-long band of balmy water between the coast of Senegal and the Caribbean basin. Between 1970 and 1994, this region produced, on average, less than half the number of major hurricanes that it did in the decades before and after. Goldenberg then hands me a second graph. It shows a series of jagged humps representing the Atlantic multi-decadal oscillation, a swing of sea surface temperatures in the North Atlantic that occurs every 20 to 40 years. The two graphs seem to coincide, with the number of major hurricanes falling as waters cooled around 1970 and rising as they began warming about 1995...
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