Winter wave heights at 47 NDBC buoys fell 3.1% per decade near Hawaii in 1985-2025 and rose in no US region; the few instrument steps found by neighbour tests do not cause the decrease

We examined winter (November to March) significant wave height (Hs) at 47 long-record NDBC moored buoys for 1985-2025. We added quality control, built regional fixed-effects composites, and fitted trends with the December-February Oceanic Nino Index (ONI) as a covariate. A pairwise neighbour test found breaks and adjusted them. Synthetic steps measured its detection power. NCEI NetCDF metadata gave a 2011-2025 hull and payload history for a network model with one offset for each deployment. Results: Hawaii winter mean Hs decreased by 3.1% per decade (95% CI 2.2 to 3.9), and the 95th percentile decreased by 3.9% per decade. All four Hawaii buoys show the decrease, and it continues in 2003-2025. The Southeast Atlantic decreased by 1.4% per decade. The Gulf of Mexico 95th percentile decreased by 1.6% per decade. The US West Coast trend depends on the start year (-1.5% from 1985, not clear; -5.0% from 1995). Alaska and the Northeast Atlantic shelf have no clear 1985-2025 trend. No region increased. The ONI coefficient is positive in all regions (+0.1% to +2.8% per degree C in the 95th percentile). The test assigned 4 breaks (11-31%, 1995-2002) to single buoys. Removal of these breaks changed regional trends by at most 0.54 points per decade. The test finds 41% of 10% steps and 93% of 22% steps. In 2011-2025, a new deployment shifts Hs by a median absolute 2.3%. Foam hulls read 1.7% higher than discus hulls (n = 37). This has the wrong sign to explain the decreases. Limitations: the method cannot see steps that occur at the same time on all buoys of a region, hull metadata starts in 2011, and Hawaii has few neighbours. Satellite or hindcast data must confirm the Hawaii decrease.

significant wave height · NDBC buoys · wave climate trends · homogenization · pairwise break detection · ENSO · Hawaii · instrument change · buoy hull · NAO · PDO