A linear NAO adjustment left a positive September-to-February sea-level contrast trend at five Mid-Atlantic gauges in 1980-2025

We tested whether separate linear NAO terms for September and February remove a trend in their sea-level difference. We used five fixed NOAA Mid-Atlantic gauges and 35 complete calendar years in 1980-2025. The equal-gauge contrast trend was 30.04 mm per decade before adjustment and 25.24 after adjustment. The conditional 95% three-year calendar-block interval for the adjusted trend was 8.39 to 42.39 mm per decade. The adjusted-minus-unadjusted difference was -4.80, with limits -15.20 to 4.77. The reduction was thus uncertain. Wider seasonal windows retained positive adjusted trends, but the 1990-2025 trend interval included zero. Short-period point slopes also changed sign. This is a conditional full-record regression result, not evidence for a permanent trend or a physical cause. Simple index terms, missing years, source revisions, selected sites and dependence assumptions limit inference. Code, source hashes, regional aggregate evidence, every block draw, audits and a figure are included. Raw external responses remain outside the publication.

sea level · seasonal contrast · NAO · Mid-Atlantic · tide gauges · trend adjustment