Heightened Tropical Cyclone Activity in the North Atlantic: Natural Variability or Climate Trend?

• Main Authors: Greg J. Holl, Peter J. Webster
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: which 7 were major hurricanes; 3 category 5 hurricanes; and 2 category 4 hurricanes in July. The number of tropical cyclones and hurricanes was; respectively; 2.5 and 2.2 times the long-term seasonal means and the previous record for overall tropical cyclone; North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.177.6212; http://www.mmm.ucar.edu/people/holland/files/NaturalVariabilityOrClimateTrend.pdf

1 We find that long-period variations tropical cyclone and hurricane frequency over the past century in the North Atlantic Ocean 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 of sea surface temperatures (SSTs) in the eastern Atlantic Ocean. Overall, there has been a substantial 100-year trend leading to related increases of over 0.7 o C in SST and over 100 % in tropical cyclone and hurricane numbers. It is concluded 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 manifested in the proportions of major and minor hurricanes in comparison to the total number of tropical cyclones. This characteristic has no distinguishable net trend and appears to be associated with concomitant variations in the proportion of tropical and subtropical hurricane developments perhaps arising from internal oscillations of the climate system. The period of enhanced major hurricane activity during 1945-1964 is consistent with this oscillation. While there is no trend in the proportion of major hurricanes, the increasing cyclone numbers has lead to a distinct trend in the number of major hurricanes and one that is clearly associated with greenhouse warming.