Stormy Weather, Again

Today Rita got her wings, and was upgraded to a Category 5 hurricane, with her sights set on Galveston, Houston, and whatever oil rigs are left in the Gulf of Mexico.

Nobody needed this, but since it's the second once-in-a-lifetime storm to hit the Gulf in the last 3 weeks, and the second year running with record numbers of tropical storms, it's only fair to ask, "what's going on here"?

Perhaps this is best described by the technical literature (Science, Vol 309, Issue 5742, 1844-1846 , 16 September 2005):


Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment
P. J. Webster, G. J. Holland, J. A. Curry, H.-R. Chang

During the hurricane season of 2004, there were 14 named storms in the North Atlantic, of which 9 achieved hurricane intensity. Four of these hurricanes struck the southeast United States in rapid succession, causing considerable damage and disruption. Analysis of hurricane characteristics in the North Atlantic (1, 2) has shown an increase in hurricane frequency and intensity since 1995. Recently, a causal relationship between increasing hurricane frequency and intensity and increasing sea surface temperature (SST) has been posited (3), assuming an acceleration of the hydrological cycle arising from the nonlinear relation between saturation vapor pressure and temperature (4). The issue of attribution of increased hurricane frequency to increasing SST has resulted in a vigorous debate in the press and in academic circles (5).

Numerous studies have addressed the issue of changes in the global frequency and intensity of hurricanes in the warming world. Our basic conceptual understanding of hurricanes suggests that there could be a relationship between hurricane activity and SST. It is well established that SST > 26°C is a requirement for tropical cyclone formation in the current climate (6, 7). There is also a hypothesized relationship between SST and the maximum potential hurricane intensity (8, 9). However, strong interannual variability in hurricane statistics (10-14) and the possible influence of interannual variability associated with El Niño and the North Atlantic Oscillation (11, 12) make it difficult to discern any trend relative to background SST increases with statistical veracity (8). Factors other than SST have been cited for their role in regulating hurricane characteristics, including vertical shear and mid-tropospheric moisture (15). Global modeling results for doubled CO2 scenarios are contradictory (15-20), with simulations showing a lack of consistency in projecting an increase or decrease in the total number of hurricanes, although most simulations project an increase in hurricane intensity.

Tropical ocean SSTs increased by approximately 0.5°C between 1970 and 2004 (21)... Here we examine the variations in hurricane characteristics for each ocean basin in the context of the basin SST variations. To this end, we conducted a comprehensive analysis of global tropical cyclone statistics for the satellite era (1970–2004). In each tropical ocean basin, we examined the numbers of tropical storms and hurricanes, the number of storm days, and the hurricane intensity distribution. The tropical cyclone data are derived from the best track archives of the Joint Typhoon Warning Center and of international warning centers, including special compilations and quality control (22).

...Tropical cyclonic systems attaining surface wind speeds between 18 and 33 m s–1 are referred to as tropical storms. Although storms of intensity >33 m s–1 have different regional names, we will refer to these storms as hurricanes for simplicity. Hurricanes in categories 1 to 5, according to the Saffir-Simpson scale (23), are defined as storms with wind speeds of 33 to 43 m s–1, 43 to 50 m s–1, 50 to 56 m s–1, 56 to 67 m s–1, and >67 m s–1, respectively. We define the ocean basins that support tropical cyclone development as follows: North Atlantic (90° to 20°W, 5° to 25°N), western North Pacific (120° to 180°E, 5° to 20°N), eastern North Pacific (90° to 120°W, 5° to 20°N), South Indian (50° to 115°E, 5°-20°S), North Indian (55° to 90°E, 5°-20°N), and Southwest Pacific (155° to 180°E, 5° to 20°S). Within these basins, total tropical storm days are defined as the total number of days of systems that only reached tropical storm intensity. Total hurricane days refer to systems that attained hurricane status, including the period when a system was at tropical storm intensity. Total tropical cyclone number or days refers to the sum of the statistics for both tropical storms and hurricanes.

...hurricanes in the strongest categories (4 + 5) have almost doubled in number (50 per pentad in the 1970s to near 90 per pentad during the past decade) and in proportion (from around 20% to around 35% during the same period). These changes occur in all of the ocean basins. A summary of the number and percent of storms by category is given in Table 1, binned for the years 1975–1989 and 1990–2004. This increase in category 4 and 5 hurricanes has not been accompanied by an increase in the actual intensity of the most intense hurricanes: The maximum intensity has remained remarkably static over the past 35 years...

Table 1. Change in the number and percentage of hurricanes in categories 4 and 5 for the 15-year periods 1975–1989 and 1990–2004 for the different ocean basins.
............................................Period
Basin...................1975–1989............................1990–2004
........................Number.......Percentage..............Number........Percentage
East Pacific Ocean......36...........25......................49............35
West Pacific Ocean......85...........25......................116...........41
North Atlantic..........16...........20......................25............25
Southwestern Pacific....10...........12......................22............28
North Indian............1............8.......................7.............25
South Indian............23...........18......................50............34

...There is evidence of a minimum of intense cyclones occurring in the 1970s (11), which could indicate that our observed trend toward more intense cyclones is a reflection of a long-period oscillation. However, the sustained increase over a period of 30 years in the proportion of category 4 and 5 hurricanes indicates that the related oscillation would have to be on a period substantially longer than that observed in previous studies.

We conclude that global data indicate a 30-year trend toward more frequent and intense hurricanes, corroborated by the results of the recent regional assessment (29). This trend is not inconsistent with recent climate model simulations that a doubling of CO2 may increase the frequency of the most intense cyclones (18, 30), although attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state.

References and Notes

1. S. B. Goldenberg, C. W. Landsea, A. M. Maestas-Nunez, W. M. Gray, Science 293, 474 (2001).[Abstract/Free Full Text]
2. J. B. Elsner, B. Kocher, Geophys. Res. Lett. 27, 129 (2000).[CrossRef][ISI]
3. K. E. Trenberth, Science 308, 1753 (2005).[Abstract/Free Full Text]
4. K. E. Trenberth et al., Bull. Am. Meteorol. Soc. 84, 1205 (2003).[CrossRef][ISI]
5. R. A. Pielke Jr. et al., Bull. Am. Meteorol. Soc., in press (available at http://sciencepolicy.colorado.edu/admin/publication_files/resourse-1762-hurricanes%20and_global_warming.pdf).
6. J. Lighthill et al., Bull. Am. Meterol. Soc. 75, 2147 (1994).
7. W. M. Gray, Mon. Weather Rev. 96, 669 (1968).[ISI]
8. K. A. Emanuel, Nature 326, 483 (1987).[CrossRef][ISI]
9. G. J. Holland, J. Atmos. Sci. 54, 2519 (1997).[CrossRef][ISI]
10. M. A. Lander, C. P. Guard, Mon. Weather Rev. 126, 1163 (1998).[CrossRef][ISI]
11. C. W. Landsea, R. A. Pielke Jr., A. M. Maestas-Nunez, J. A. Knaff, Clim. Change 42, 89 (1999).[CrossRef][ISI]
12. J. C. L. Chan, K. S. Liu, J. Clim. 17, 4590 (2004).[CrossRef][ISI]
13. W. M. Gray, Mon. Weather Rev. 112, 1649 (1984).[CrossRef][ISI]
14. C. K. Folland, D. E. Parker, A. Colman, R. Washington, in Beyond El Nino: Decadal and Interdecadal Climate Variability, A. Navarra, Ed. (Springer-Verlag, Berlin, 1999), pp. 73-102.
15. L. J. Shapiro, S. B. Goldenberg, J. Clim. 11, 578 (1998).[CrossRef][ISI]
16. H. G. Houghton et al., Climate Change—2001: The Scientific Basis (Cambridge Univ. Press, Cambridge, 2001).
17. A. Henderson-Sellers et al., Bull. Am. Meteorol. Soc. 79, 19 (1998).[CrossRef][ISI]
18. T. R. Knutson, R. E. Tuleya, J. Clim. 17, 3477 (2004).[CrossRef][ISI]
19. J. F. Royer, F. Chauvin, B. Timbal, P. Araspin, D. Grimal, Clim. Dyn. 38, 307 (1998).[CrossRef]
20. M. Sugi, A. Noda, N. Sato, J. Meteorol. Soc. Jpn. 80, 249 (2002).[CrossRef]
21. P. Agudelo, J. A. Curry, Geophys. Res. Lett. 31, Art. No. L22207 (2004).
22. C. J. Neumann, in Global Guide to Tropical Cyclone Forecasting, G. J. Holland, Ed. (WMO/TD-560, World Meteorological Organization, Geneva, Switzerland, 1993), chap. 1.
23. See www.aoml.noaa.gov/general/lib/laescae.html for a description of the Saffir-Simpson scale.
24. R. M. Hirsche, J. R. Slack, R. Smith, Water Resource Res. 18, 107 (1982).[ISI]
25. V. F. Dvorak, Mon. Weather Rev. 103, 420 (1975).[CrossRef][ISI]
26. C. S. Velden, T. L. Olander, R. M. Zehr, Weather and Forecasting 13, 172 (1998).[CrossRef][ISI]
27. J. P. Kossin, C. S. Velden, Mon. Weather Rev. 132, 165 (2004).[CrossRef][ISI]
28. G. J. Holland, Aust. Meteorol. Mag. 29, 169 (1981).
29. K. Emanuel, Nature 436, 686 (2005).[CrossRef][ISI][Medline]
30. See www.prime-intl.co.jp/kyosei-2nd/PDF/24/11_murakami.pdf.
31. This research was supported by the Climate Dynamics Division of NSF under award NSF-ATM 0328842 and by the National Center for Atmospheric Research, which is funded by NSF.

People, I'd get an umbrella, at least. A life jacket might be useful occasionally, too. Not to mention a government that paid attention to what its' scientists were trying to tell it.