Evidence for Non-Stationarity in US Extreme Sea Level Records

Abstract:

Mean sea level (MSL) research has generally focussed on quantifying and understanding long-term trends and more recently multidecadal fluctuations in order to provide stakeholders and decision makers with scenarios to be considered for developing sustainable coastal adaptation strategies. However, with the exception of several small island states, extreme sea level events associated with storm surges mostly endanger coastal communities instead of the immediate impact of MSL rise. In this context several studies conducted at global, regional, and local scales suggested that changes in extreme sea levels broadly follow changes in MSL. Many of these studies (especially when relying on global data sets) only focussed on relatively short time periods (~ 40 years), used only one out of a whole range of different approaches available to compare MSL and extreme sea level changes, or concentrated on long-term trends ignoring multidecadal variations.

Here we use a set of 20 tide gauge records covering the continuous US coastline and the period from 1929 to 2013 to identify long-term trends and multidecadal variations in extreme sea levels relative to changes in MSL. We apply a range of different approaches to assure that the results are robust against the selected methodology. We find significant long-term trends in extreme sea levels above MSL trends for several sites in the southeast and on the west coast and identify six regions with coherent multidecadal fluctuations unrelated to MSL changes. Using non-stationary extreme value analysis we show that the latter caused variations in the 100 year return water levels ranging from ~10 cm to as much as 80 cm across the regions, significantly deviating from the stationary assumption. Identifying and understanding these changes is crucial in order to include the information into coastal design and adaptation processes in addition to long-term changes in MSL.