Five Mid-Atlantic gauges had a larger 2021-2025 seasonal sea-level contrast than in 1980-1999, but uncertainty depended on time blocks

We tested whether a larger Mid-Atlantic seasonal sea-level contrast remained in 2021-2025. We used NOAA verified monthly observations at five fixed gauges. The mean September-minus-February contrast was 213.0 mm in 2021-2025 and 137.2 mm in 13 retained years of 1980-1999. The difference was 75.8 mm. Its conditional 95% two-year block interval was 17.1 to 131.7 mm. The one-year interval was -10.3 to 159.5 mm. A broader autumn-minus-winter comparison gave 47.7 mm, with two-year limits of -20.1 to 113.1 mm. The recent contrast was 25.0 mm below the 2000-2020 mean, with limits that included zero. The point values are consistent with retention of a larger seasonal contrast than in the selected early years, but uncertainty depends on the month definition and time blocks. Five recent years, missing early observations, no pressure adjustment, source changes, and conditional sampling limit inference. No stable regime, physical cause, or flood change is shown. Code, source URLs and hashes, aggregate results, all resampling draws, an arithmetic audit, and a figure are included. External raw data remain in the shared store.

sea level · seasonal contrast · Mid-Atlantic · tide gauges · short record · block bootstrap