Coastal communities of the eastern United States are currently experiencing the effects of accelerated sea level rise (SLR). Increased frequency and severity of extreme sea level (ESL) events are observed along the U.S. east coast (USEC) in recent decades, with high-tide flooding occurring on regular basis. Continued SLR will further increase the frequency, extent, and severity of ESLs, disrupting the coastal society. Climate change, compounded by modes of climate variability, will cause changes in location and strength of USEC ESLs over the coming decades. Recent studies emphasized the importance of the El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) in affecting USEC sea level, and showed the impacts of the tropical Indian Ocean (TIO) sea surface temperature anomalies (SSTA) and the Madden-Julian Oscillation (MJO) on the NAO. However, how these climate modes intertwin in a changing climate to determine the location, frequency, and severity of ESLs along USEC is not well understood. Yet, this understanding is essential for developing effective coastal adaptation strategies.

This proposal aims to investigate the impacts of climate modes and external forcing on the spatial and temporal evolutions of ESLs along the USEC in the recent past (1950-present) and in the near-term projections (present-2050), using observational analysis combined with model experiments using the high-resolution (HR) Energy Exascale Earth System Model (E3SM) of the Department of Energy and other models that participated in the High Resolution Model Intercomparison Project (HighResMIP) for CMIP6. The specific objectives are to: (1) Explore the spatial and temporal evolutions of the frequency and intensity of ESL events along the USEC, using tide gauge data, astronomical tides and reanalysis products from 1950-present;  assess the effects of climate modes on ESLs, and understand the relevant processes (e.g., dynamic sea level by a weakening Atlantic Meridional Overturning Circulation (AMOC), barotropic versus baroclinic effects); (2) Examine the capabilities of HR E3SM and other HighResMIP models in simulating the observed ESL statistics (e.g., frequency, intensity), investigate the impacts of climate modes and external forcing on ESLs, quantify uncertainties and biases of simulated ESLs, and identify the sources of uncertainties and biases; (3) Assess the ESL statistics in the near-term projections of HR E3SM and other HighResMIP models; use the knowledge gained from understanding the recent past (objective 2) to inform and hopefully reduce uncertainties in the near-term projections of USEC ESLs.

Policy makers need cutting-edge science for understanding and projecting the regional extreme statistics along the USEC, to ensure informed decision making for developing coastal zone management strategies and energy-resource planning. Accomplishment of the proposed research will strengthen the science base and thus contribute to more accurate predictions and reliable projections of USEC ESLs. The proposed research will contribute to the larger goals of the program by improving the predictive understanding of the roles played by modes of climate variability and change in affecting the US coastal sea level extremes. We will develop metrics to quantify uncertainties and biases of ESLs simulated by HR E3SM and other HighResMIP models. Through innovative use of a hierarchy of experiments with HR E3SM and multi-models, the proposed project will advance our scientific understanding of the factors that determine the USEC sea level extremes in the recent past, which will be used to inform the analysis of near-term projections of ESL statistics.