Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions

Increasing sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950 (refs 1–5), raising concerns that future greenhouse-gas-induced warming could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions. Here we assess, in our model system, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations. Tropical Atlantic sea surface temperatures (SSTs) have increased over the past century, and anthropogenic forcing has probably contributed to this warming. There is little question that Atlantic hurricane counts and the power dissipation index (PDI, a measure of the destructive potential of storms) have increased markedly since 1980, along with a recent increase in Atlantic SSTs (refs 4,5). Looking further back in time, observational analyses have produced intriguing but often conflicting results, depending in part on details of the adjustments for storm undercounts or intensity biases in the pre-satellite era. Some investigators have concluded that past greenhouse warming caused a substantial rise in Atlantic tropical storms and hurricanes during the twentieth century. It has been argued that the Atlantic PDI in the past decade has reached high levels (unprecedented since ∼1950) and that it correlates well with low-frequency tropical Atlantic SST variations from 1950 to the present. Given the much more pronounced Atlantic warming projected for the twenty-first century, extrapolation of these reported relationships would suggest a very large future increase in Atlantic hurricane frequency and PDI. Dynamical models bring important extra perspectives to this question. We have developed a new regional modelling framework designed specifically for downscaling Atlantic hurricane activity. For the present-day simulations, the model uses observed SSTs, and the interior solution is nudged towards observed atmospheric reanalyes on large spatial scales (see the Methods section) for each August–October season from 1980 to 2006. Hurricanes develop from weak disturbances that are generated spontaneously by the model, in ways that are dependent on the large-scale environment. To demonstrate the model’s hurricane simulation skill, Fig. 1a shows simulations obtained using two slightly different versions of the model (referred to here as Model1 andModel2). Model2 is our control for the climate warming experiments described below. The year-to-year variability in Atlantic hurricane counts (1980–2006) is well reproduced by the model ensemble mean (r= 0.84), as well as by Model1 and Model2 individually. El Niño-related interannual variations in hurricane activity are also well reproduced. The model simulates an increase in hurricane counts (1980–2006), although the simulated trend is ∼40% larger than the observed increase (Fig. 1a). A limitation of the model is that it does not simulate hurricanes as intense as observed (Fig. 1b, Supplementary Information, Fig. S1). Minimum central pressures of ∼937 hPa and maximum surface winds of ∼47m s are the most intense simulated in the control, compared with 882 hPa and ∼85m s maximum intensities in observations. Thus, model simulations of indices that are strongly influenced by intense hurricanes (such as PDI) should be treated with caution. The ensemble model produces an increasing trend in PDI (1980–2005), although it is∼25% smaller than the observed increase. For major hurricanes (using the central pressure criterion of <965 hPa (ref. 7)), both themean number and increasing trend (1980–2005) in the model are about half as large as observed, although the model’s variability correlates well (r = 0.7) with observations. Storm trajectories for contrasting active and inactive seasons (Fig. 2a–d) further illustrate the control model’s performance. The truncated tracks near the northern boundary of the model domain are an artefact of the boundary nudging. Animated fields from sample seasons are available in the Supplementary Information. We use Model2 to explore the influence of future climate warming on Atlantic hurricane activity. We re-run each August–October season (1980–2006), keeping the daily to multidecadal variations unchanged, but altering the August–October mean atmospheric state (used by the interior nudging) and SSTs according to late twenty-first-century changes