Year

Figure 2.3 Evolution of the sea surface temperature anomalies relative to the 1970-2004 period for the North Atlantic, Western Pacific, East Pacific, South Indian Ocean, Southwest Pacific and North Indian Ocean Basins

Source: Curry et al, 2006.

Atlantic Oscillation), and some scientists have argued that the increase in tropical cyclone intensity since 1970 can be explained by such natural variability. However, decadal-scale oscillations tend to be specific to each ocean basin and are often anti-correlated from one basin to another. In particular, there have been repeated assertions from the National Hurricane Center that the recent elevated hurricane activity is associated with natural variability, particularly the Atlantic Multidecadal Oscillation (AMO). Figures 2.2 and 2,4 suggest that natural modes of multidecadal variability, notably the AMO (~70 year cycle), do have an influence on North Atlantic hurricane activity. However, recent examination of the data by Mann and Emanuel (2006) suggests that the impact of the AMO on tropical SST and hurricane activity has been overestimated owing to the convolution of the AMO with the temperature changes resulting from global forcing (natural plus anthropogenic). They found that analyses that rely solely on SST to identify the AMO might have aliased the phase and amplitude of the AMO signal. If the AMO analyses are correct, the next peak of the AMO is anticipated in about 2020. The strength of the tropical cyclone activity during the period 1995-2005 (50 per cent greater than the previous peak period in about 1950; see Figure 2.2), which is at least a decade before the expected peak of the current AMO cycle, suggests that the AMO alone cannot explain the elevated tropical cyclone activity observed in the North Atlantic over the last decade.

What can we conclude from the above analysis regarding the global increase in hurricane intensity and the increase in the North Atlantic of the total number of tropical cyclones? The arguments that natural variability can explain the increase run counter to the known range of natural variability in the past record and the absence of a natural mechanism that would explain the global increase in both oceanic temperatures and the frequency of intense hurricanes. The best available evidence instead supports the hypothesis that greenhouse warming is contributing to the increase in hurricane activity, although the evidence is not yet conclusive.

E 12

North Atlantic named storms and SSTs e

North Atlantic named storms and SSTs

26.6

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