A dynamical downscaling approach for the improved representation of sub-annual coastal sea level variability: the importance of shelf and slope ocean dynamics

Using a recently developed dynamical downscaling framework, we build and assess an ensemble of year-long sea level hindcasts for the western North Atlantic. Downscaled high-resolution regional ocean model (NWA12) output of US East Coast sea level at long established tide gauge stations is compared to parent coarse-resolution global ocean model (SPEAR) output to quantify varied impacts of downscaling in a forecasting framework, particularly from a daily extremes perspective. Comparisons of modeled coastal sea level distributions and observations at tide gauge stations reveal downscaling enhances variability by a factor of two to ten across all resolved frequencies. As the downscaled simulations are forced at the surface with the same atmosphere felt by the SPEAR ocean, we attribute this enhanced variability to the improved representation of shelf and slope ocean dynamics. The magnitude of this enhancement, however, appears a strong function of latitude and suggests that the added value of downscaling varies geographically. Together, these results demonstrate how dynamical downscaling can offer application-relevant time varying statistics of higher frequency coastal sea level variability, like planning based on inundation return periods, and an improved understanding of the links between model resolution and the processes driving coastal sea level changes.