El Nino Increases High-Tide Flooding in Tidal Wetlands Along the US Pacific Coast

Periodic oscillations between El Nino and La Nina conditions in the Pacific Basin affect oceanographic and meteorological phenomena globally, with impacts on the abundance and distribution of marine species. However, El Nino effects on estuarine hydrology and tidal wetland processes have seldom been examined rigorously. We used detailed wetland elevation and local inundation data from 10 tidal wetlands located along the Pacific coast of the United States to assess changes in flooding during the 2015-2016 El Nino and to determine decadal-scale relationships between estuarine sea-level anomalies and Pacific Basin climate indices for this region. During the 2015-2016 El Nino all sites experienced significant increases in high-tide water levels exceeding those predicted by astronomical tides, and increased flooding frequency during at least one of the El Nino subperiods relative to pre-El Nino conditions. The magnitude of positive sea-level anomalies varied by site (4-15cm), with local hot spots of high water in southern Oregon, northern California, and Pt. Mugu lagoon in the Southern California Bight. Furthermore, over the last three decades of historic tide records, there were positive relationships between high-tide sea-level anomalies and equatorial Pacific Basin sea surface temperature anomalies across the region, and negative relationships with the Northern Oscillation Index. Increases of 1 degrees C in equatorial sea surface temperature were associated with 3-5cm of increased high-tide flooding at the sites. Elevated estuarine flooding associated with future El Ninos could impact important tidal wetland processes and could be an additive stressor for wetlands facing accelerating sea-level rise. Periodic changes between El Nino and La Nina conditions in the Pacific Basin affect global weather and ocean patterns, causing changes in coastal waters that can impact plants and animals. However, few studies have focused on El Nino effects on tidal wetlands. This study identifies substantial changes to wetland flooding during the 2015-2016 El Nino event using detailed wetland elevation and local tide height data from 10 tidal wetlands located along the Pacific coast of the United States. During the 2015-2016 El Nino, all sites experienced significant increases in high-tide water levels exceeding those predicted by astronomical tides as well as increased flooding frequency during at least one part of the El Nino event. Flood height anomalies ranged from 4 to 15cm on average, with local hot spots of high water in southern Oregon, northern California, and Pt. Mugu lagoon in Southern California Bight. Tidal records show that over the last three decades, high-tide sea-level anomalies in coastal wetlands were related to two indicators of El Nino and La Nina conditions. Elevated estuarine flooding associated with future El Ninos could impact important tidal wetland ecosystems and could weaken their ability to keep pace with accelerating sea-level rise.