Seasonality and Trends in Coastal Water Temperatures from NOAA Water Level Monitoring Stations along US Coasts

John A Callahan , Bailey Armos, Tigist Jima, Chris Zervas

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. Although it is well known that ocean sea surface temperatures (SSTs) have increased over the past several decades, extending these trends and patterns to coastal waters is nontrivial. We evaluated 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) records were analyzed at 95 stations extending from the mid-1990s to 2023 and representing all the major US coastal regions. Seasonal cycle metrics, linear trends, daily distributions, and extreme percentile thresholds were computed at each site. Seasonal cycles and peak times of year tracked well with solar heating and consistently tracked behind air temperature by a few days to a few weeks. US East Coast and Great Lakes regions have a much stronger seasonal cycle and larger range (up to 25℃) than West Coast and Island regions (as low as 3℃). The great majority of stations experience warming over this time period, with statistically significant trends at 56 of 95 stations, averaging approximately 0.5 ℃/decade. Many of these locations are also experiencing sea level rise, most notably in the Southeast and Gulf regions. Although geographic patterns exist, variations are found in extreme threshold temperatures and trends station to station and between bay systems and the open coast, highlighting the need for continuous in situ monitoring along the coast. This study provides baseline statistics of long-term near shore coastal WT data and supports informed future planning and decision-making in this dynamic environment.