Opposite-season thermal sea-level anomaly trends had no clear hemispheric contrast in NOAA 1955-2025 estimates

We tested fixed opposite-season thermal sea-level anomaly contrasts in NOAA upper-700-m hemisphere means for 1955-2025. The northern contrast was July-September minus January-March; the southern contrast used the reverse order. Their trend difference was -0.469 mm per decade, with conditional 95% five-year block limits of -1.182 to 0.250. Three- and seven-year blocks also gave limits that included zero. The point difference changed sign with a 1980 start or with the other pair of seasons. Thus, this test did not resolve a stable hemisphere contrast. It does not estimate the full seasonal amplitude or identify a physical cause. One mapped product, changes in observing coverage, fixed seasons, and conditional uncertainty limit inference. Code, annual aggregate contrasts, bootstrap draws, source hashes, independent checks, and a figure are included.

thermosteric sea level · seasonality · hemisphere contrast · null result · NOAA · block bootstrap