Evidence for multidecadal variability in US extreme sea level records

Abstract

We analyze a set of 20 tide gauge records covering the contiguous United States (US) coastline and the period from 1929 to 2013 to identify long-term trends and multidecadal variations in extreme sea levels (ESLs) relative to changes in mean sea level (MSL). Different data sampling and analysis techniques are applied to test the robustness of the results against the selected methodology. Significant but small long-term trends in ESLs above/below MSL are found at individual sites along most coastline stretches, but are mostly confined to the southeast coast and the winter season when storm surges are primarily driven by extratropical cyclones. We identify six regions with broadly coherent and considerable multidecadal ESL variations unrelated to MSL changes. Using a quasi-nonstationary extreme value analysis, we show that the latter would have caused variations in design relevant return water levels (50-200 year return periods) ranging from 10 cm to as much as 110 cm across the six regions. The results raise questions as to the applicability of the ''MSL offset method,'' assuming that ESL changes are primarily driven by changes in MSL without allowing for distinct long-term trends or low-frequency variations. Identifying the coherent multidecadal ESL variability is crucial in order to understand the physical driving factors. Ultimately, this information must be included into coastal design and adaptation processes.

Spatial and Temporal Variability and Long-Term Trends in Skew Surges Globally

“…We also find more negative trends in NTR, but no statistically significant difference between the number of positive and negative trends in skew surge time-series. Our findings are consistent with those of Wahl and Chambers (2015) for the US, who found a greater number of sites had significant trends in NTR compared to skew surge. The number of sites with significant trends in skew surge and NTR may be generated from chance, but a formal assessment has not been made here, because of the spatially non-homogenous dataset.…”

“…The advantage of using skew surge is that it is an integrated and unambiguous measure of the storm surge . Changes in skew surges have only previously been assessed (to our knowledge) at sites around the north-west Europe (Batstone et al, 2013;Dangendorf et al, 2014) and the USA (Wahl and Chambers, 2015). Both of these regions generally display semi-diurnal tidal behavior, but our method works well in all tidal regimes.…”

“…Recently, several studies have used the parameter "skew surge, " rather than the traditional NTR, to assess ESL in north-west Europe (Batstone et al, 2013;Dangendorf et al, 2014), and in the USA (Wahl and Chambers, 2015). A skew surge is the difference between the maximum observed sea level and the maximum predicted tidal level regardless of their timing during the tidal cycle.…”

Climate and Weather Extremes

“…In stronger storms, these inundating surges can be over 3.0 m (10 ft). Often it is the combination of surge and rain that creates flooding in coastal areas (Wahl et al, 2015). In these scenarios, the surge covers drain pipes and the rainfall runoff backs up into the streets.…”

“…For Florida in particular, a strong relationship exists between extreme sea levels observed during the Atlantic tropical cyclone season and the Atlantic Multi-decadal Oscillation (AMO), which in turn is related to the size of the Atlantic Warm Pool that can favor the development of tropical cyclones; e.g., more landfalling cyclones were observed in AMO warm phases (Park et al, 2010a(Park et al, , 2010b. This leads to significant multidecadal variations in extreme sea levels, as shown in Wahl et al (2015Wahl et al ( , 2016, for the one in 100-year return water level (with a 1% probability of being exceeded in any given year) that is often used for design purposes by engineers or to define flood zones.…”