Diagnosing and communicating the effect of climate variability on frequency of coastal inundation

Philip Thompson, University of Hawai‘i at Mānoa

alt textPago Pago, the territorial capitol of American Samoa. Backed by steep slopes, communities and infrastructure are highly vulnerable to coastal inundation. (Photo: Alan Grey/CC BY2.0)

All islands are surrounded by the sea, but exactly where the ocean and the land meet varies significantly over time. Daily and monthly tide height changes interact with seasonal ocean swells and storms, mean sea level due to climate change, and interannual to decadal climate modes such as the Pacific Decadal Oscillation and El Niño–Southern Oscillation (ENSO), resulting in extreme sea level events. Extreme high sea levels cause coastal flooding that may damage infrastructure and ecosystems. Extreme low sea levels can result in coral die off on shallow reefs that fringe many Pacific islands. It is important to try to understand how all these sea level states will be affected by climate variability and long-term change, so that local communities can prepare more detailed short- and long-term flood risk plans

alt textSea level around Kwajalein Atoll may well rise 0.4 m by 2050, at which point the islet of Roi-Namur will experience severe flooding (as pictured) more than 10 days in a typical year, and could experience a year per decade with over 30 days of severe flooding.

We used wave and water level data from Hawai‘i, Guam, American Samoa, and the Kwajalein and Majuro atolls of the Republic of the Marshall Islands to model how the seasonal swells that affect each island originate, and to assess how possible it is to forecast these swells well in advance. We combined these results with tidal and sea level data to produce seasonal inundation outlooks and are now working to integrate these outlooks with future climate predictions to estimate the effects of climate change on the frequency and timing of coastal inundations.

Our work suggests that the long-term predictability of large swells is fairly low, much like that of the storms that drive them. However, even though it is difficult to predict specific timing of swells, seasonal outlooks will help to inform local managers about the times of the year that wave inundation is more likely, and these can be considered together with short-term weather forecasts.

Extreme sea level events in the tropical Pacific are expected to become more frequent due to sea level rise, so actionable information regarding timing and location will be valuable for adaptation planning. Our outreach materials are localized by island, and will provide information for island residents, natural resource managers, and local decisionmakers. In addition, we will be integrating our inundation frequency information into existing forecast and projection products such as those produced by the National Weather Service.

Quick Summary

      The potential for increasing wave inundation of coastal areas due to climate change is a serious threat to the culture, habitat, and essential infrastructure of the Hawaiian and U.S. Affiliated Pacific Islands. Better understanding and communication about local seasonal inundation risks can help island communities plan for the future.
      We developed maps for Guam, Hawai‘i, American Samoa, and parts of the Republic of the Marshall Islands showing how local and large­scale factors may affect seasonal inundation risk, and are providing this information to local stakeholders.
      Information about seasonal coastal inundation frequency can help improve both short and long-term adaptation and resilience planning in coastal communities.

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This project was supported by the DOI Pacific Islands Climate Adaptation Science Center, the USGS Coastal and Marine Geology Program, and the NOAA/National Geodetic Survey. For more information, please contact Jeffrey Danielson (daniels@usgs.gov). To learn more about climate science at PI-CASC, contact David Helweg at dhelweg@usgs.gov or visit: https://nccwsc.usgs.gov/pacificislandscsc.