January 28, 2008

Warming Reduces Landfalling Hurricanes (Again)

Filed under: Climate Extremes, Hurricanes

World Climate Report could easily become World Hurricane Report given all of the articles we have featured in the last few years on the subject. Despite growing empirical and theoretical evidence to the contrary, the global warming advocates continue to insist that hurricanes will become worse in the coming decades. They argue that warmer seas will support more evaporation and more energy to spin-up and sustain more and/or stronger hurricanes. They rely heavily on a very few number of articles on this subject, and they disregard the bulk of material published in leading journals showing that global warming will likely not lead to any substantial changes in hurricane activity.

Yet another article has appeared in a leading journal of this subject. The piece is by Chunzai Wang and Sang-Ki Lee of NOAA’s Atlantic Oceanographic and Meteorological Laboratory in Miami and the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami; the research was funded by NOAA. The begin their article noting “the 2005 hurricane season is the most active year on record, with 28 named tropical storms in the Atlantic basin and 15 of them reaching hurricane intensity. The recent increase in Atlantic hurricane activity has fueled a debate on the role of global warming in the increase.” (There’s the “d-word” again that we thought wasn’t used anymore!). The last sentence of Wang and Lee’s first paragraph tips the world off that this piece is not headed to the top of the hurricane alarmists’ lists. “This paper uses observational data to demonstrate that the attribution of the recent increase in Atlantic hurricane activity to global warming is premature and that global warming may decrease the likelihood of hurricanes making landfall in the United States.”

The analyzed sea-surface temperatures from throughout the world and found the dominant mode of variance is one of widespread warming (Figure 1). While this overall warming trend may impact hurricane activity, the authors note “One important factor for controlling Atlantic hurricane activity is the vertical wind shear between the upper and lower troposphere in the main development region (MDR) within the 10°N–20°N latitude belt that stretches from West Africa to Central America. An enhancement in tropospheric vertical wind shear is associated with a quiet hurricane season in the Atlantic basin, as it inhibits the organization of deep convection, and vice versa for a reduction in vertical wind shear.”


Figure 1. (a) The spatial pattern and (b) temporal reconstruction for the dominant mode of variance, representing global ocean warming (from Wang and Lee, 2008).

They next calculated the actual wind shear around the globe based on the vector difference between winds at 200 millibars (~35,000 ft) and 850 millibars (~5,000 ft) during the Atlantic hurricane season of June to November. They determined the statistical relationship between wind shear and the temperature trend shown in Figure 1b and produced the map in Figure 2. They note “A positive regression appears in the MDR for Atlantic hurricanes and a negative regression is north of the MDR. The positive regression in the MDR indicates that global warming is associated with an increase of the vertical wind shear, thus inhibits atmospheric convection and disfavors the formation and development of Atlantic hurricanes.”


Figure 2. Regression coefficient (m/s per °C) of vertical wind shear onto the temporal variation of global warming of sea surface temperatures (from Wang and Lee, 2008).

They next showed the time series of landfalling hurricanes from 1851 to 2006 (Figure 3) and commented on the overall downward trend in the data. In reviewing the work of others, they note “Downward trends in Atlantic hurricanes or global hurricanes are also reported by using other hurricane datasets.” For instance, as we reported, a study published in Nature in 2007 showed a “record of major Atlantic hurricanes over the past 270 years using proxy data. They find that major hurricanes decrease gradually from the 1760s until the early 1990s and that the recent increase is not unusual compared with other periods of high hurricane activity.” Wang and Lee also state “The accumulated cyclone energy index, which has been used to measure tropical cyclone activity, is also observed to have a downward trend for global hurricanes over the past two decades when consistent satellite imagery has been available.”


Figure 3. The number of U.S. landfalling hurricanes from 1851 to 2006. The black straight line is the linear trend that is fitted to the U.S. landfalling hurricane time series, the blue line is the seven-year running mean of U.S. landfalling hurricanes, emphasizing longer (than interannual) timescale variations (from Wang and Lee, 2008).

The authors then examined the statistical relationship between the number of landfalling hurricanes and vertical wind shear in the main development region (MDR). Just as expected (Figure 4), the relationship is negative showing that as wind shear increases, the number of hurricanes striking the United States decreases.


Figure 4. Regression coefficient (m/s per number) of vertical wind shear during June–November onto U.S. landfalling hurricanes (from Wang and Lee, 2008).

So, from the results of Wand and Lee, it seems that global warming results in more wind shear in the main development region of the Atlantic ocean and more wind shear means fewer/weaker storms. We’ll let you extend this logic as to what a globally-warmed future may hold in store.

References:

Nyberg, J., B.A. Malmgren, A.Winter, M.R. Jury, K.H. Kilbourne, and T.M. Quinn. 2007. Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature, 447, 698-702.

Wang, C., and S.-K. Lee (2008), Global warming and United States landfalling hurricanes, Geophysical Research Letters, 35, L02708, doi:10.1029/2007GL032396.




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