INTERANNUAL VARIATION IN EXTREME WATER LEVEL PROBABILITIES IN THE GULF OF MAINE

Within the Gulf of Maine, accurately assessing flood risk requires separate statistical treatment of tides and storm surge. Tidal forcing generally exceeds meteorological surge during peak flood events, and there is significant interannual variation in the Gulf’s tides. The annual 90^th percentile of daily predicted higher high water (HHW₉₀, a measure of extreme water level) varies decadally with the 18.6-year nodal cycle and has been increasing over the past 100 years at a rate that exceeds mean sea level rise. Here, we adapt the Skew Surge Joint Probability Method from Batstone et al. (2013) to calculate annual extreme water level probabilities over the past century in the Gulf of Maine. We assume skew surge is stationary, and we perform all calculations relative to annual mean sea level; thus, interannual variation in water level probabilities is caused solely by tidal variation. We find that the 10 and 100-year storm tide have been increasing over the past century at a rate that exceeds mean sea level rise by 2 cm, 6 cm, and 8 cm per century in Boston, Portland, and Eastport, respectively. Over the 18.6-year nodal cycle, the 10 and 100-year storm tide also vary by 5 cm in Boston, 4 cm in Portland, and 11 cm in Eastport. These trends translate to dramatic year-to-year variability in the return period of a given storm tide. For example, we calculate that the return period of the Eastport, ME 100-year storm tide published by NOAA, which was determined by fitting a probability distribution to a century-long time series of annual maximum storm tide, varies between a 40 and 650 years when accounting for tidal variability with the adapted joint probability method.