Coastal Impacts, Adaptation, and Vulnerabilities - Climate ...

34 Coastal Impacts, Adaptation, and Vulnerabilitiesfor mean monthly values, at more extreme conditions (99 th - percentile), a clear statisticallysignificant trend can be seen of increasing wave height at high latitudes includingoff the U.S. West and East Coasts and more neutral conditions in equatorial regions.By analyzing monthly mean significant wave heights rather than extremes, Seymour(2011) also demonstrates a significant increase in wave energy affecting the West Coastof the U.S. during the interval of a 1984-2007. During the same period, a monotonic increasein the positive El Niño portion of the ENSO cycle and a monotonic decline in theatmospheric pressure in the Gulf of Alaska could be seen. Seymour (2011) speculates apossible connection between greenhouse gases and bigger waves in the North Pacificbecause both of these changes would be expected to produce higher wave energy levelsand both have been identified as resulting, at least in part, from global climate change.However, Seymour (2011) clearly points out that the wave height record is too noisy andtoo short to establish an estimate of a possible contribution from the changes to globalclimate.Research on trends in mid-latitude extra-tropical storms in the Eastern North Pacifichave confirmed that storm intensity has increased but other research has documented adecrease in frequency, possibly because the storm tracks have shifted poleward duringthe latter half of the 20th century. McCabe et al. (2001) showed a statistically significantdecrease in the frequency of storms over the years 1959-1997; however, Geng and Sugi(2003) found that the decrease in annual numbers of storms is typically of the weakmediumstrength variety while the stronger storms have actually increased in frequency.These documented changes in storm tracks are thought to be primarily due to changesin baroclinicity, which in turn is linked to changes in atmospheric temperature distributionsdue to increased greenhouse gas emissions. In other words, in the mid-latitudes ofthe Northern Hemisphere, poles are warming faster than lower latitudes , leading to adecrease in the meridional temperature gradient and a decrease in mid-latitude stormfrequency. Recognizing the trends in reanalysis data, Yin (2005) used the output of 15coupled general circulation models to relate the poleward shift of the storm track tochanges in baroclinicity in the 21st century. Though these studies concluded that thestorm track shifts poleward in the Northern Hemisphere with warmer temperatures,uncertainties remain regarding natural variability and model limitations.Tropical Cyclone Generated WavesIrish et al. (2011) have shown that historical observations of storm surges contain significantvariations at the scale of the storm size, typically on the order of one or two timesthe radius to maximum winds or about 25-40 kilometers. For this reason, Irish et al.(2011) argued that historical observations alone may not be not good predictors of longtermclimatological characteristics of surges in a coastal area. Wave fields tend to be abit more dispersed, but extreme waves in hurricanes and tropical storms tend to exhibitsimilar characteristic scales of variation. In this context, long-term Global Climate Modelsimulations should offer a much better estimate of the impacts of climatic variations onfuture wave climates than attempts to used local observations; however, although manypapers have discussed projections of future wave climates based on GCM simulations(Caires et al., 2006; Debenhard & Roed, 2008; Hemer et al., 2010; Mori et al., 2010; Sterles& Caires, 2005; Wang et al., 2004; Wang & Swail, 2006), the numerical models used