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

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Physical Climate Forces 29(low to moderate confidence). Models consistently projected increases of near-storm rainfallrates (moderate confidence) with projections averaged within 100 kilometers of stormcenter ranging from about 5 percent to over 20 percent. These precipitation results werebased on six studies reporting Atlantic basin results or multi-basin results that includethe Atlantic and based on storms of tropical storm intensity or greater. Similar resultsfor tropical cyclone frequency, intensity, and precipitation rates were reported for theNortheast Pacific basin, with tropical storm frequency projections ranging from about-70 percent to +80 percent (very low confidence in sign of change) and intensity changesranging from about -5 percent to +20 percent (low to moderate confidence in an increase).Specific northeast Pacific region results have not been reported for near-storm precipitationrates, although relevant multi-basin studies report increases (moderate confidence)ranging from about +5 percent to over +20 percent.Two dynamical modeling studies (Bender et al., 2010; Murakami et al., 2011) haveexplored present-day simulations and 21st century projections of intense Atlantic and/or Northeast Pacific hurricane frequency (category 4 and 5, with wind speeds exceeding131 miles per hour or 210 kilometers/hour; or category 5, with wind speeds exceeding155 miles per hour or 249 kilometers/hour). For the 21st century projections, both studiesuse an average projected climate trend calculated from 18 different CMIP3 globalclimate models under an assumed A1B future emission scenario (Table S2). The Benderet al. (2010) study, using a regional model/case study approach, projects a 10 percentincrease per decade (i.e., 100 percent increase over the 21st century) in Atlantic basin category4-5 frequency, based on the multi-model ensemble climate change; however, notall of the individual models used to derive this value indicate an increase in frequencyof Atlantic category 4-5 hurricanes. The Murakami et al. (2011) study (with auxiliary informationprovided by Dr. H. Murakami, personal communication, 2011) projects a nonsignificantincrease in category 4-5 storm days in the Atlantic basin (+15 percent) andglobally (+4 percent) but a significant (+180 percent) increase in the NE Pacific basin. Forcategory 5 storm days, their model projects significant increases (+56 percent globally,and +290 percent in the Atlantic basin). Several important caveats to these results shouldbe noted. First, the Bender et al. (2010) model has a substantial (~50 percent) low bias intheir simulation of Atlantic category 4-5 hurricane frequency under present climate conditions.Murakami et al. (2011) report a relatively small bias in their present-day simulationof Atlantic category 5 storm days but a large positive bias (almost a factor of 4) intheir simulation of Atlantic category 4-5 storm days and a substantial low bias in NEPacific category 4-5 storm days. In addition, the global model used by Murakami et al.does not include an interactive ocean component, in contrast to the regional case studymodel of Bender et al. In summary, we have low to moderate confidence in projections of anincrease in intense hurricane frequency in the Atlantic basin.A general caveat to available results reported here is the focus on basin-wide activitymeasures, which do not necessarily correlate in an obvious way to coastal eventsbecause landfalling activity is intimately related to storm track variability, which itself isalso linked to climate variability and change (Kossin et al., 2010). U.S. landfalling tropicalstorm or hurricane activity in particular are explored in limited studies (Knutsonet al., 2008; Villarini et al., 2011); thus, the above projections, and particularly those ofintense hurricane activity, may be considered as early attempts at scenario developmentwith respect to U.S. landfalling activity.
30 Coastal Impacts, Adaptation, and VulnerabilitiesExposure of coastal areas to devastating storm surge and waves will increase withthe ever- increasing population that seeks to reside along the coast and its accompanyinginfrastructure. The storm surge threat will be exacerbated by relative sea-level riseand potentially by climate induced changes in frequency and intensity of hurricanes. Ashurricanes approach the coast, four storm-related phenomena can occur to modify localwater levels: set up due to wind, low barometric pressure, set up due to wave forcing,and rainfall. Storm winds force water towards the coast and typically create the greatestchange in local water elevation. Low barometric pressure provides a secondary effect,creating a bulge in the water surface around the center of the storm. Wave forcing createsan increase in the mean water level due to breaking waves at the coast. Storm rainfallcan also increase the local water elevation. Additional factors not related to the stormitself are the astronomical tide and river flows at the time the storm reaches the coast.Storm surges are also greatly influenced by the geometry of the basin and continentalshelf leading up to the coastal floodplain. A mildly sloping continental shelf, such as inthe Gulf of Mexico, results in a higher storm surge as compared to a coast with a steeperbathymetry.Several recent studies (Irish et al., 2008; Irish & Resio, 2010; Resio et al., 2009) haveshown that the maximum surge can be estimated as a function of several storm parametersincluding storm intensity, size, and motion as well as the alongshore position ofthe point of interest relative to the landfall position. In general, the primary drivers ofsurge potential are storm intensity and size. Projected increases in storm intensity due toglobal warming will result in greater storm surge potential. Extensive studies with thenumerical model ADCIRC (Westerink et al., 1992, 2007) indicate that, for small to moderatelysized storms, peak alongshore surge increases about 30 centimeters per 10 mb ofintensification. For large storms, peak alongshore surge increases about 40 centimetersper 10 mb (Irish et al., 2009).Storm surge potential is also increased by global warming induced sea-level rise dueto the mean water level increase as well as the complex interaction of storms and thecoastal landscape. Data and numerical simulations have shown that landscape featuresand vegetation cover have the potential to reduce inland storm surge elevations alongthe coast by slowing the surge propagation (Wamsley et al., 2009, 2010). Elevations greaterthan the storm surge elevation provide a physical barrier to the surge. Even when inundated,landscape features and vegetation have the potential to create friction and slowthe forward speed of the storm surge, effectively reducing inland water levels. Higherwater levels resulting from sea-level rise can modify wetland vegetation type and mayalso lead to wetland loss, shoreline erosion, erosion of barrier islands through overwashand breaching, and an overall change in the local morphology such as islands transformingto submerged shoals and wetlands becoming open lakes or bays. Therefore,storm surge response does not increase linearly with sea-level rise in some coastal areasbecause of differences in slope of the coastal plain. Numerical simulations by Smith etal. (2010) indicate that for deltaic areas such as south Louisiana, the nonlinear responseto sea-level rise is greatest in areas of modest surge (2-3 m). These areas may experiencesurges two to three times the amount of sea-level rise. Thus, in a statistical analysis ofwater levels, the long return period water levels would increase modestly above the sealevelrise, but shorter return period water levels could increase significantly.
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National Climate Assessment Regiona
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© 2012 The National Oceanic and At
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Coastal Impacts, Adaptation,and Vul
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Page 16 and 17: ContentsKey TermsAcronymsCommunicat
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ReferencesAbuodha, P. & Woodroffe,
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Coastal Impacts, Adaptation, and Vu
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