Dr. Jeff Masters' WunderBlog : Fewer but Stronger Global Tropical Cyclones Due to Ocean Warming | Weather Underground

Fewer but Stronger Global Tropical Cyclones Due to Ocean Warming

By: Dr. Jeff Masters , 03:36 PM GMT am 23. Juli 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 ledto 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.

Figure 1. One of the most spectacular images ever captured of a tropical cyclone from space: Category 5 Super Typhoon Maysak as seen from the International Space Station at approximately 6 pm EDT Tuesday March 31, 2015 (just after dawn local time.) At the time, Mayask had top winds of 160 mph as estimated by the Joint Typhoon Warning Center, and a central pressure of 905 mb, as estimated by the Japan Meteorological Agency. Image has been brightened and flipped 180 degrees. Image credit: Terry W. Virts.

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--for example, Knutson, T. R. et al., 2010, "Tropical cyclones and climate change", which projected intensity increases of 2 - 11% by 2100. The "official word" on climate, the 2013 IPCC report, predicts that there is a greater than 50% chance (more likely than not) that we will see a human-caused increase in intense hurricanes by 2100 in some regions. The 2014 U.S. National Climate Assessment found that "the intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.”  

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Dr. Jeff Masters' WunderBlog : Fewer but Stronger Global Tropical Cyclones Due to Ocean Warming | Weather Underground