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Subject: G4) Are we getting stronger and more frequent hurricanes, typhoons, and tropical cyclones in the last several years?

Contributed by Chris Landsea (NHC)

Globally, no. However, for the Atlantic basin we have seen an increase in the number of strong hurricanes since 1995. As can be seen in section E9, we have had a record 33 hurricanes in the four years of 1995 to 1999 (accurate records for the Atlantic are thought to begin around 1944). The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the United States and throughout the Caribbean attest to the high amounts of Atlantic hurricane activity lately.

As discussed in the previous section, it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes. We have not observed a long-term increase in the intensity or frequency of Atlantic hurricanes. Actually, 1991-1994 marked the four quietest years on record (back to the mid-1940s) with just less than 4 hurricanes per year. Instead of seeing a long-term trend up or down, we do see a quasi-cyclic multi-decade regime that alternates between active and quiet phases for major Atlantic hurricanes on the scale of 25-40 years each (Gray 1990; Landsea (NHC) 1993; Landsea et al. 1996). The quiet decades of the 1970s to the early 1990s for major Atlantic hurricanes were likely due to changes in the Atlantic Ocean sea surface temperature structure with cooler than usual waters in the North Atlantic. The reverse situation of a warm North Atlantic was present during the active late-1920s through the 1960s (Gray et al. 1997). It is quite possible that the extreme activity since 1995 marks the start of another active period that may last a total of 25-40 years. More research is needed to better understand these hurricane "cycles".

For the region near Australia (105-160E, south of the equator), Nicholls (1992) identified a downward trend in the numbers of tropical cyclones, primarily from the mid-1980s onward. However, a portion of this trend is likely artificial as the forecasters in the region no longer classify weak systems as "cyclones" if the systems do not possess the traditional tropical cyclone inner-core structure, but have the band of maximum winds well-removed from the center (Nicholls et al. 1998). These changes in methodology around the mid-1980s have been prompted by improved access to and interpretation of digital satellite data, the installation of coastal and off-shore radar, and an increased understanding of the differentiation of tropical cyclones from other type of tropical weather systems. By considering only the moderate and intense tropical cyclones, this artificial bias in the cyclone record can be overcome. Even with the removal of this bias in the weak Australian tropical cyclones that the frequency of the remaining moderate and strong tropical cyclones has been reduced substantially over the years 1969/70-1995/96. Nicholls et al. (1998) attribute the decrease in moderate cyclones to the occurrence of more frequent El Nino occurrences during the 1980s and 1990s.

For the Northwest Pacific basin, Chan and Shi (1996) found that both the frequency of typhoons and the total number of tropical storms and typhoons have been increasing since about 1980. However, the increase was preceded by a nearly identical magnitude of decrease from about 1960 to 1980. It is unknown currently what has caused these decadal-scale changes in the Northwest Pacific typhoons.

For the remaining basins based upon data from the late 1960s onwards, the Northeast Pacific has experienced a significant upward trend in tropical cyclone frequency, the North Indian a significant downward trend, and no appreciable long-term variation was observed in the Southwest Indian and Southwest Pacific (east of 160E) for the total number of tropical storm strength cyclones (from Neumann 1993). However, whether these represent longer term (> 30 years) or shorter term (on the scale of ten years) variability is completely unknown because of the lack of a long, reliable record.

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