Fewer but Stronger Global Tropical Cyclones Due to Ocean Warming

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3052

By: Dr. Jeff Masters , 3:36 PM GMT on July 23, 2015

Global ocean temperatures hit their warmest levels in recorded history last month. Since hurricanes are heat engines which extract heat energy from the oceans and convert it to the kinetic energy of the storms' winds, we should be concerned about the potential for hurricanes to be stronger as a result of global warming. Indeed, the observed 0.3°C (0.5°F) warming of Earth's oceans over the past 30 years has made more energy available to hurricanes, says a new study published in May in Nature Climate Change by Florida State hurricane scientist James Elsner and the deputy director of the National Typhoon Center in South Korea, Namyoung Kang. The researchers found that this extra heat energy has led to a change in both the frequency and intensity of global tropical storms and hurricanes. Using a new mathematical framework to categorize all global tropical cyclones with wind speeds of at least 39 mph over the past 30 years, the authors showed that the observed warming of Earth's oceans during that time period has led to an average increase in wind speed of about 3 mph (1.3 m/s) for each storm--but there were 6.1 fewer named storms globally each year because of the warmer oceans. A typical year has about 85 named storms globally, so this represents about a 7% decrease in the number of storms.

Why fewer storms, but the strongest ones getting stronger?

More moisture is evaporated from a warmer ocean surface, resulting in water vapor concentrating in the lower atmosphere. Since moist air is less dense than dry air, this creates greater instability, leading to stronger updrafts in developing storms and more intense tropical storms and hurricanes. However, this low-level moisture is not effectively transported to high altitudes, resulting in warmer and drier conditions aloft and stronger high pressure in the middle and the upper troposphere. This high pressure aloft forces thunderstorm activity to concentrate in time and space, resulting in fewer tropical cyclones--but the increased moisture at low levels allows more efficient intensification once a tropical storm is spawned. "Thus tropical cyclone intensity increases at the expense of tropical cyclone frequency", the authors wrote. In an interview at sciencedaily.com, Kang said, "In a warmer year, stronger but fewer tropical cyclones are likely to occur. In a colder year, on the other hand, weaker but more tropical cyclones." Many climate models also predict fewer but more intense tropical cyclones in a future warmer climate

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While it is good news that warming of the oceans may potentially lead to fewer hurricanes, this will probably not decrease the total amount of hurricane damage if the strongest storms get stronger. Damage done by a hurricane increases by somewhere between the second and third power of the wind speed, so just a 10 mph increase in winds can cause a major escalation in damage. According to Pielke et al., 2008, over the past century, Category 3 - 5 hurricanes accounted for 85% of U.S. hurricane damage, despite representing only 24% of U.S. landfalling storms. Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth.) This study also found that hurricane damages in the U.S. were doubling every ten years without the effect of climate change, due to the increases in wealth and population. If we add in an increase in the frequency of the strongest storms, combined with storm surges that will be riding inland on top of ever-increasing sea levels due to global warming, the damage math for coastal regions gets very impressive for the coming decades.