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Potential high levels for southern during October–December 2017

Rising sea levels are predicted by some climate models over the next several months around southern in Micronesia, potentially enhancing seasonally high tides, which could cause minor coastal flooding.

Updated 21 August 2017 University of Sea Level Center (UHSLC)

Sea levels are near-normal now around most of Micronesia—a vast of the tropical northwestern Pacific , which includes thousands of islands—but forecasted to rise to as much as 20 cm above-normal by the end of 2017. The seasonal high sea level prediction comes from dynamical climate forecast models, which resolve how the upper ocean interacts with the atmosphere and ’s rotation. Less sophisticated, statistical models are not predicting above- normal sea levels; however, which lends more uncertainty than is typical to the forecast. High seasonal sea levels, combined with gradually rising because of greenhouse warming, and large astronomical tides (i.e., King Tides) potentially could impact islands with minor coastal flooding or salt water intrusions and increase vulnerability to flooding from storms or large waves.

Satellite-based measurements of the global sea surface height show a wide swath of above- normal sea levels in the central Pacific centered a few degrees north of the . The sea level anomaly has been propagating slowly westward this . These high sea levels were caused by the upper ocean’s response to wind anomalies together with the planet’s rotation. Such a combination produces a feature called a Rossby or planetary wave that, unlike ocean surface waves, stretch for hundreds of kilometers parallel to the equator. A similar high sea level anomaly impacted the southern during October 2016 and was actually predicted as early as August by some climate models.

So far this year, high sea levels associated with the equatorial Rossby wave have been diminishing as they propagate west toward Micronesia. But, two dynamical climate models are predicting that sea levels in the tropical northwestern Pacific will soon rise as planetary wave energy remains available and may actually amplify. Whereas, statistical models, which cannot represent such climate features, are not predicting such a sea level rise. The inter-model spread and the potential for amplifying or decaying sea level anomalies are two sources of uncertainty that forecasters will continue to monitor.

Based on the dynamical models and observations of past events, the highest sea levels are likely to be located between 4°N and 10°N, which would impact southern Micronesia (including , Kwajalein, , Chuuk, and Malakal) more than islands to the north ( and the Northern ). Sea levels will likely gradually rise by ~5 cm/month from east-to- west, beginning as soon as September in the southern Marshall Islands (NOAA’s tide gauge on Majuro could provide an early verification of the high sea level event), and peaking by November around islands further west such as Pohnpei (see forecast images on the next page). Above-normal sea levels (+10 to +20 cm) could persist in the tropical northwestern Pacific through early 2018, depending on how the ocean and atmosphere dynamics evolve.

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The Multi-model Ensemble Sea Level Forecasts for Tropical Pacific Islands is an experimental product developed by a partnership between scientists at the UHSLC, NOAA’s NCEI (Pacific Region) and Pacific ENSO Applications Climate Center, ’s Bureau of Meteorology seasonal forecasting operations, and ’s National Institute of Water and Atmospheric Research. For more information about this product, please visit http://uhslc.soest.hawaii.edu/sea- level-forecasts/ or contact Matthew Widlansky [email protected].

Sea level anomaly forecast for November 2017. The map shows an average of 120 forecast members from NOAA’s CFSv2 model (orange shadings indicate predicted above-normal sea levels). The upper-right plot shows the multi-model forecast for the next six months around Pohnpei. Note how the CFSv2 prediction (orange circles) compares similarly to Australia’s POAMA-2 model (blue circles) but is much higher than the statistical models (squares). Averaging the dynamical forecasts (CFSv2/POAMA-2) and adding recent decadal sea level rise gives a +15 cm prediction for November. Above-normal sea levels would enhance the occurrence and severity of extreme high tides (lower plot). Similar images for other islands can be accessed from http://uhslc.soest.hawaii.edu/sea-level-forecasts/.

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