Seasonality and trends in coastal water temperature from NOAA water level monitoring stations along US coasts

Coastal regions are complex environments. They lie at the confluence of physical oceanic, atmospheric, and land-based processes, and continue to undergo significant change due to both natural and human-driven factors. Impacts from the natural hazards affecting these regions are highly localized, such as infrastructure damage due to coastal flooding or ecosystem stress due to marine heatwaves. This paper introduces a new quality-controlled dataset of coastal water temperature collected at NOAA Center for Operational Oceanographic Products and Services (CO-OPS) water level monitoring stations. Water temperature (WT) data records were analyzed at 90 stations extending from the mid-1990s to 2023 and representing all major US coastal regions. A series of basic statistical metrics were computed and compared across regions, including daily and monthly WT climatologies, empirical extreme quantiles, and linear trend rates. WT seasonal cycles tracked well with solar heating and peak WTs consistently tracked behind peak air temperature by a few days to a few weeks. US East Coast and Great Lakes regions have much stronger WT seasonal cycles and larger ranges (up to 25°C) than West Coast and Island regions (as low as 3°C). The great majority of stations experienced positive trends over this time period, with statistically significant linear trend rates at 46 of 90 stations, averaging nearly 0.5 °C/decade. Largest trends were found along Mid-Atlantic, Southeast, and Gulf regions while smallest trends found in the Northwest. Lastly, two case studies were examined, a comparison to NOAA's Optimum Interpolation SST global dataset and frequency of marine heatwaves in South Florida, both of which highlight the need for local in situ monitoring. This new WT dataset from CO-OPS provides consistent, reliable data that can support continuous local environmental monitoring and helps inform future planning and decision-making in this dynamic environment.