[1] Tide gauge data are used to relate low frequency sea level changes over the past 60 years in the western tropical Pacific, including a significant positive trend over the past two decades, to Pacific climate indices.
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Coastal areas epitomize the notion of 'at-risk' territory in the context of climate change and sea level rise (SLR). Knowledge of the water level changes at the coast resulting from the mean sea level variability, tide, atmospheric surge and wave setup is critical for coastal flooding assessment. This study investigates how coastal water level can be altered by interactions between SLR, tides, storm surges, waves and flooding. The main mechanisms of interaction are identified, mainly by analyzing the shallow water equations. Based on a literature review, the orders of magnitude of these interactions are estimated in different environments. The investigated interactions exhibit a strong spatiotemporal variability. Depending on the type of environments (e.g., morphology, hydrometeorological context), they can reach several tens of centimeters (positive or negative). As a consequence, probabilistic projections of future coastal water levels and flooding should identify whether interaction processes are of leading order, and, where appropriate, projections should account for these interactions through modeling or statistical methods.
Coastal locations around the United States (US), particularly along the Atlantic coast, are experiencing recurrent flooding at high tide. Continued sea-level rise (SLR) will exacerbate the issue where present, and many more locations will begin to experience recurrent high-tide flooding (HTF) in coming decades. Here we use established SLR scenarios and flooding thresholds to demonstrate how the combined effects of SLR and nodal cycle modulations of tidal amplitude lead to acute inflections in projections of future HTF. The mid-2030s, in particular, may see the onset of rapid increases in the frequency of HTF in multiple US coastal regions. We also show how annual cycles and sea-level anomalies lead to extreme seasons or months during which many days of HTF cluster together. Clustering can lead to critical frequencies of HTF occurring during monthly or seasonal periods 1-2 decades prior to being expected on an annual basis.The impact of HTF accumulates over numerous seemingly minor occurrences, which can exceed the impact of rare extremes over time [1][2][3] . These impacts are subtle-e.g., loss of revenue due to recurrent road and business closures 4 -compared to the physical damage of property and infrastructure associated with extreme storm-driven events. As SLR increases the frequency of HTF in the US [5][6][7][8][9][10][11] , coastal communities will need to adapt. However, developing adaptation pathways for recurrent coastal flooding is challenging and requires knowledge of environmental and social tipping points for which current actions and policies become ineffective [12][13][14] .Here we characterize projected increases in US HTF-including the impact of the 18.6-year nodal cycle in tidal amplitude [15][16][17] -in a way that can be used to establish planning horizons and develop adaptation pathways. First, we focus on the rate of flooding-frequency increase, which is not well understood despite being critical to establishing SLR impact timelines 18 .More specifically, we examine acute inflections, or tipping points, in the rate of increase that mark transitions from periods of gradual (and potentially imperceptible) change to rapid increase in HTF frequency. Second, we focus on the tendency for HTF episodes to cluster in time 19 . Scientists, engineers, and decision-makers are accustomed to the statistics and impacts of isolated extreme events [20][21][22][23] , but given the cumulative nature of HTF impacts 1-3 , we describe extreme months or seasons during which the number of flooding episodes, rather than the magnitude, is exceptional. Projections of high-tide flooding frequencyEnsemble projections of twenty-first century HTF frequency (Methods) are generated for 89 tide-gauge locations across the contiguous United States (US) and US-affiliated Pacific and Caribbean islands (Supplementary Data). HTF frequencies are represented as counts of days in monthly and annual windows for which at least one hourly sea-level value exceeds the flooding threshold of interest. NOAA SLR Scenarios 24 and derived HTF t...
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