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Assessing CMIP6 combined projections of changing sea levels and enhanced extreme rainfall events for determining coastal flood risks in the U.S.-affiliated Pacific Islands

Global sea level rise requires that climate models produce realistic simulations of not only sea
level variability but also weather extremes such as heavy rainfall, to provide maximum utility
assessing future risk of coastal flooding. Prolonged high sea levels enhance the likelihood and
severity of rainfall-related floods along vulnerable coasts, such as in Hawaii and the U.S.-
affiliated Pacific Islands (USAPI). For example, high sea levels during heavy rainfall can slow
freshwater runoff into the ocean, causing compound flooding. Considering future projections of
increasing sea level variability, as well as likely wetter storms, motivates assessing the combined
effect of both sea level and weather changes on the risk of coastal flooding.

In the tropical Pacific, regional sea levels and rainfall patterns vary in response to well-observed
climate oscillations such as the El Niño-Southern Oscillation. Since the strong El Niño in 2015,
island communities in the Marshall Islands (2016) and Hawaii (2017) experienced minor coastal
flooding events. Whereas the Hawaii high sea levels occurred during several months of mostly
fair weather, the recurrent minor flooding could have been worse if heavy rainfall happened
around the times of highest tides. Assessing the temporally and regionally varying climate
processes responsible for sea level fluctuations is critical, but incomplete, to efforts projecting
the occurrence of future coastal flooding.

Improved coupled ocean-atmosphere models (i.e., CMIP6) provide the opportunity to assess the
combined occurrence of high sea levels and extreme rainfall, which is relevant to determining
coastal flooding risk. We will evaluate CMIP6 simulations of the climate processes that affect
sea levels (e.g., greenhouse warming and El Niño) and heavy rainfall (i.e., storms) associated
with coastal flooding in tropical Pacific islands. We will develop integrated projections of future
changes to sea levels and extreme rainfall events, focusing on Hawaii and the Marshall Islands as
well as Guam and American Samoa. Using a multi-model assessment approach, we will
constrain uncertainty ranges on the CMIP6 projections, such as by selecting well-performing
models compared to observations. Our objective is to project 21st-century changes in the joint
distribution of high sea levels and rainfall extremes associated with coastal flooding in Hawaii
and the USAPI.

Our proposed research is relevant to the MAPP competition “21st Century Integrated U.S.
Climate Predictions and Projections”, and specifically, the competition identified research
focus– “characterizing long-term changes in weather extremes and sea level”. Through our
proposed 21st-century projections and assessments of the model processes associated with
uncertainty, we will contribute to the competition’s Priority Areas A and B. By delivering
projections of the combined effect of changing sea levels and more extreme rainfall on the
likelihood of future coastal flooding, we will support NOAA’s long-term goals to increase
climate intelligence concerning “weather and climate extremes” as well as “coasts and climate
resilience” by building capabilities regarding “Earth system science and modeling”.

Climate Risk Area: Coastal Inundation

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