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There is a common misconception that because the global temperature has increased, hurricanes also must increase in number and intensity. The primary factor in the ability of a hurricane to strengthen or weaken after formation is the wind shear profile of the atmosphere - not ocean water temperature. Another major factor is the amount of moisture in the lower and middle atmosphere. It has been warm enough in the tropics to produce hurricanes for a very long time. The temperature for tropical cyclone formation equates to a sea surface temperature of about 80°F (26.5°C). If the behavior of hurricanes are studied, cases can be found where hurricanes have strengthened over cooler water and have weakened over warmer water. There maybe a small "boost" to a hurricane's strength as it moves over the Gulf Stream if upper air wind profiles do not change much. Some of the computer generated hurricane models have this bias built in and will rapidly strengthen a tropical cyclone if predicted to go over a warm pool of water.
The recent upturn in tropical cyclone activity was predicted long before Global Warming became a household name. Tropical cyclone activity in the tropical north Atlantic has been known to vary in cycles and an increase was anticipated. For the 20th century, there were nine tropical storms of which five became hurricanes on average. Since 1995, there has been a marked increase to fifteen tropical storms of which eight became hurricanes on average. The 2013 season showed a slow down in numbers of tropical cyclones. In fact, only two hurricanes formed. This is the lowest number since 1982. Looking at the past century, there were othere active periods such as the early to mid 1930's. In general the 1930's through the 1960's were an above average period.
Other considerations should take into account medium to long-term atmospheric - oceananic processes. ENSO is a well know process that has distinct effects on north Atlantic tropical cyclone activity. Kelvin waves and the Madden-Julian Oscillation are also currently being studied on their affects of tropical cyclone activity. The PDO or Pacific Decadal Oscillation also may be connected. There are many different types of waves or oscillations in the atmosphere and ocean. There is much research to be done on how these processes contribute to tropical cyclone formation and intensification.
