Projections of future hurricane activity in the North Atlantic

While scientists that support the natural variability explanation and those that support the global warming contribution both predict that hurricane activity in the North Atlantic will remain elevated for some years, their projections for hurricane activity for the rest of the century are quite different. Analyses based on the hypothesis that natural variability has been the cause of the high level of hurricane activity in the North Atlantic since 1995 project a forthcoming downturn in hurricane activity. For example, the analysis by Goldenberg et al (2001) implies a downturn in 10-40 years, and Gray (2006) anticipates a downturn in 3-8 years associated with a global cooling that he predicts will also result from ongoing natural variability.

If our hypothesis is correct that greenhouse warming is causing an increase in hurricane intensity globally and also an increased number of storms in the North Atlantic, what does this imply for future hurricane activity as SSTs continue to rise as a result of greenhouse warming? The following analysis addresses specifically the projection of North Atlantic tropical cyclone activity. We develop a range of projections using two different approaches: climate model responses to the increasing concentrations of greenhouse gases, and linear projections based upon the historical data record. Our projections are for average conditions in the years around 2025 (such that high frequency fluctuations from short-term oscillations such as El Niño are ignored), corresponding to an increase in tropical SST of 1°F (0.56°C) that would result from greenhouse gas-induced warming.

The Webster et al (2005) observations scale to a 6 per cent increase in maximum wind speeds for a 1°F SST increase. By contrast, high-resolution climate model simulations (Knutson and Tuleya, 2004; Oouchi et al, 2006) have found a 2 per cent increase in intensity when scaled for a 1°F SST increase, which is a factor of three smaller than that determined from the observations. Oouchi et al (2006) also found that the number of North Atlantic tropical cyclones increased by 30 per cent for a 4.5°F (2.5°C) increase in SST, which scales to an increase of one tropical cyclone per 1°F increase in SST. By contrast, based upon the historical data record in the North Atlantic (see Figure 2.4), an increase of 1oF in tropical SST implies an additional five tropical storms per season, which is a factor of five greater than the number inferred from climate model simulations.

Projections of future changes in hurricane variability must include both natural variability and greenhouse warming. Estimates of the magnitude of the impact of the AMO on the total number of tropical cyclones per year range from zero (no effect) to four to six (the AMO explains the entire magnitude of the trough to peak variability in Figure 2.1). Assuming that the AMO continues with a 70-year periodicity, the peak of the next cycle would be expected in 2020 (70 years after the previous 1950 peak). So, 2025 would be very near the peak of the AMO cycle. Proponents of the natural variability explanation refer to active and quiet phases rather than actual cyclic behavior; their analyses indicate that we are currently in an active phase that will last another 10 to 40 years, and that the level of activity in 2025 will be similar to the activity of the past decade.

Based upon these assumptions, consider the following simple statistical model. The average annual number of North Atlantic tropical cyclones for the past decade has been 14.4. Assuming that the effects of greenhouse warming and the AMO are separable and additive, Table 2.1 compares simple statistical projections based on the projected effects of both anthropogenic greenhouse warming (AGW) and AMO, of AGW only, and of AMO only. The range of projections given in Table 2.1 provides some broad constraints on the conceivable elevation of North Atlantic tropical cyclone activity in coming decades. The combination of AGW+AMO would result in the greatest elevation in the number of named storms, and an unprecedented level of tropical cyclone activity. The different assumptions lead to estimates of an increase of 0 to 6.5 named storms per year. In terms of the intensity of the storms, Figure 2.1 suggests that the distribution of storm intensity is changing with warming, such that the increase in the number of tropical storms is in the number of category 4-5 storms (NCAT45) rather than in the number of weaker hurricanes. Even if, as some suggest, the AMO begins its descending mode about 2020, continued warming makes it doubtful that we will ever again see the low levels of hurricane activity of the 1980s; instead, it seems

Table 2.1 Projectionsfor the average total number of North Atlantic tropical cyclones (named storms) for 2025

Situation

AGW+AMO

AGW only

AMO only

Average last decade

14.4

14.4

14.4

Global warming increases SST 1oF

+1 to +5

+1 to +5

0

Continued increase of AMO

+1.5

0

0

Total

16.9 to 20.9

15.4 to 19.4

14.4

Note: AGW refers to anthropogenic greenhouse warming; AMO refers to Atlantic Multidecadal Oscillation.

Note: AGW refers to anthropogenic greenhouse warming; AMO refers to Atlantic Multidecadal Oscillation.

likely that we can expect a leveling off rather than a significant decrease in activity until the next ascending phase of the AMO.

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