AMS Forum: Climate Change Manifested by Changes in Weather


Heightened tropical cyclone activity in the North Atlantic: Natural variability or climate trend?

Greg Holland, NCAR, Boulder, CO; and P. Webster

We find that long-period variations tropical cyclone and hurricane frequency over the past century has occurred as three, relatively stable regimes separated by sharp transitions. Each regime has seen 50% more cyclones and hurricanes than the previous one and is associated with a distinct range in eastern Atlantic sea surface temperatures (SSTs). The end result has been a substantial, 100-year trend leading to related increases of over 0.7oC in SST and over 100% in tropical cyclone and hurricane numbers. The compelling conclusion is that the overall trend in SSTs and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming. Superimposed on the evolving tropical cyclone and hurricane climatology is a completely independent oscillation in the proportions of major and minor hurricanes (compared to all tropical cyclones). This has no distinguishable net trend, is associated with concomitant variations in the proportion of tropical and subtropical hurricane developments and may arise largely from internal oscillations of the climate system. The period of enhanced major hurricane activity during 1945-1964 arose entirely from this oscillation. However, while there is no trend in the proportion of major hurricanes, the combination of this with increasing cyclone numbers has lead to a distinct trend in the number of major hurricanes, one that is clearly associated with greenhouse warming.

Session 1, Climate and Extreme Weather Events
Tuesday, 16 January 2007, 1:30 PM-5:30 PM, 214D

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