A New Era of Hurricanes—1995–2006
Hurricanes come in cycles. A casual glance at a list of active hurricane seasons shows clusters of activity in the 1880s–1890s and 1920s-1960s, while the early twentieth century and the 1970s, 1980s, and early 1990s look relatively calm. Another active period started in 1995 and continues today.
Dramatic economic growth, immigration, increasing longevity, and other factors coincided with the hurricane downturn of the late 1960s to mid-1990s, an unfortunate and dangerous confluence of events. The effect can be seen throughout the coastal zone. Would 18 million people be living in Florida today if hurricanes had continued coming at the rate they occurred in the late 1940s and 2004 and 2005? I wonder.
Unfortunately, the people in government with the responsibility for seeing that citizens are safe during disasters were (and are) not good students of history. Most coastal areas, including the state of Florida, were developed without regard to the hurricanes of the past and without protection from the hurricanes to come. Thus, future disasters are guaranteed.
Hurricane cycles correlate with the natural fluctuation in the temperature of the ocean.
This chart compares the Atlantic Ocean water temperature to normal. The match with hurricane activity is remarkable. Notice that the late 1800s and the period from the late 1920s to the late 1960s show warmer than normal temperatures.
They were active hurricane periods as well. The swing in temperature is only about plus or minus one degree Fahrenheit, but the amount of energy that slight difference makes, when spread out over the entire Atlantic Basin, is significant. See “Global Warming and Hurricanes,” page 112.
Hurricane Season 2006
Every hurricane has its own personality, and every hurricane season does as well. The contrast between hurricane activity in 2006 and in the mega year of 2005 could not be sharper. The number of storms was down by about two-thirds, and overall tropical-cyclone activity was down dramatically as well.
Four issues came into play:
• The temperature of the ocean water
• The upper-level winds
• The amount of moisture in the atmosphere over the tropical Atlantic
• Factors we don’t understand.
Ocean temperature. The sea-surface temperatures in the tropical Atlantic were somewhat cooler in 2006 than in the superheated year before, but still much warmer than the long-term average. The water was plenty warm for hurricanes to develop.
Upper-level winds. There was a lot of talk about the upper-level winds being unfavorable for hurricane formation, and at times that was the case. But, during the heart of the hurricane season, the wind regime was no less favorable than it was in 2005 over the prime development areas. Some systems were, no doubt, “sheared” by upper-level winds, but that happens to some systems every year. Even if some of the upper winds were driven by El Niño, clearly there were other factors involved.
Dry air. The air in the mid and upper levels of the tropical Atlantic was unusually dry in 2006. A tropical system is fueled by moist air from the ocean surface rising into the midlevels of the atmosphere. If the air there is dry, however, the fueling mechanism is disrupted.
The dryness was likely caused by two mechanisms. It seemed that an unusual amount of dry Saharan air lingered over the tropical Atlantic deep into the hurricane season. Also, the wind flow created by El Niño appears to have contributed.
The upper-level winds discussed above operate in a horizontal direction, shearing the tops off systems as they try to develop. There is also a vertical component to the wind, however. The El Niño-driven rising air in the Pacific spreads across the Atlantic, then descends, creating a “circulation cell.” The descending air warms and dries as it moves lower in the atmosphere where the barometric pressure is higher.
Everyday examples of this double-barreled physical phenomenon:
• A tire pump gets hot because air heats up when it is compressed, everything else being equal.
• A hair dryer warms the air going through it. Warmer air has a greater capacity to hold water. Therefore the air coming out the nozzle acts like a bigger sponge, holding more moisture. The amount of water in the air isn’t reduced, just its percentage of the air’s maximum capacity, making the air feel drier. (This percentage is known as the “relative humidity”)
Factors we don’t understand: Even with the somewhat cooler sea-surface, El Niño-affected upper-level winds, and drier-than-normal air, the atmospheric pattern over the hurricane-development areas was not all that bad. There were times during the 2006 hurricane season when the atmosphere appeared primed for tropical development, but it didn’t happen. Clearly there are other subtle factors involved that need more study to detect and understand.
Jet-stream dip. Another player in the 2006 hurricane season was a persistent dip in the jet stream over the east coast of the United States.
The net effect of this pattern was to drive most of the significant storms to the north toward Bermuda and eventually the Maritimes of eastern Canada. Again, whether this unusually persistent dip was caused by El Niño, exacerbated by El Niño, or mostly caused by the natural randomness of weather patterns is unclear.
The net effect, however, was that the Bermuda high was much farther east than in 2005, and the steering currents kept the storms well away from the east coast.
Tropical Storm Ernesto over the Florida straits, August 29, 2006. It appeared conditions were favorable for strengthening, but subtle factors kept the system’s top winds at only 45 mph.
Tropical Storm Ernesto. The only significant storm to affect the United States was Tropical Storm Ernesto, which came ashore in the Florida Keys the evening of August 29, 2006. (One year to the day after Katrina hit New Orleans.) It was mostly a nonevent at landfall.
Ernesto’s center spent nearly twenty-four hours over the Cuban landmass, which no doubt disrupted the storm’s low-level circulation. If it had emerged into the Straits of Florida sooner and had, therefore, twelve more hours over the deep, warm water, it could have been an entirely different story in South Florida. Ernesto continued to strengthen after it moved ashore over the Florida peninsula. (The central pressure at landfall was 1004mb when it came over the Upper Keys, 1,003 mb when it made landfall on the southern tip of the peninsula, and 1,000mb when the center moved offshore near Cape Canaveral a day later.)
Subjectively, the system had a reasonably well-defined circulation during its time over the Straits, and the water was plenty warm. It appears that some dry air and some unfavorable winds impacted the system during that time; the Hurricane Hunters reported that the system was slightly tilted. The computer models indicated that Ernesto would overcome the factors and be at least a strong tropical storm at landfall, but it was not to be.
The mechanisms involved were subtle. In the end, the computer models and the National Hurricane Center had the right idea. The track was accurately forecast for the sixty hours before landfall, although before that time the forecast cone was well west of the final track. The lesson here, for most people, is that little attention should be paid to the 5-Day forecast cone (I would prefer that the NHC didn’t offer it). The 3-Day Cone is the one to pay attention to.
The intensity forecast, on the other hand, never picked up the factors inhibiting intensification. But, based on the best risk analysis that modern science could offer, hurricane warnings were issued for the southern Florida peninsula. There was a good enough chance that Ernesto would intensify that full preparation was required.
Ernesto was a reminder that sometimes—quite often actually—things turn out for the good. Every storm that comes along doesn’t blow up in to a mega monster. That fact was easy to forget after the hurricane barrage of 2004 and 2005.
Hurricane Season 2005
Anytime there are a lot of landfalling storms, let alone a record-setting season such as 2005, lessons abound—big and small. In contrast to 2006, the biggest lesson of the hurricane season of 2005 was, the worst does happen!
Hurricane Katrina reaches category 5 strength, August 28, 2005.
Hurricane Wilma reaches category 5 strength with the lowest pressure ever measured in the Atlantic Basin, October 19, 2005. Courtesy NASA.
In case after case people suffered and property was destroyed because someone—a governmental body or official, a business owner or a private citizen—decided to ignore hurricane history, ignore hurricane research, and hope for the best. But, for hour after grueling hour on television we saw that hoping does not hold back wind and water.
The two most remarkable storms of 2005 were Hurricanes Katrina and Wilma. That’s not to ignore or discount the effects of Dennis, Emily, Ophelia, Rita, Stan, Gamma, and the rest. In a normal hurricane season, any one of those storms would have been memorable. But Katrina and Wilma rose above the pack in this extraordinary season of storms.
An easy-to-forget aspect of the 2005 season was that none of the record-setting hurricanes formed in the deep tropics. Through history, most of the “great” hurricanes have formed well east of the Lesser Antilles. From there they have time to organize and gain strength. That the traditional breeding ground for big, powerful hurricanes was not fertile in 2005, and yet fifteen hurricanes formed, gives us pause.
Traditional thinking, before 2005, was that a hurricane season when the storms developed mostly on the western side of the Atlantic or in the Caribbean would likely not be as bad because, on average, they wouldn’t have as much time...