Surface pCO2 at HOT and BATS followed atmospheric CO2 in 2013-2025 forecasts frozen at 2012, but annual temperature anomalies did not pass to pCO2 and a shared alkalinity reversal stays unexplained

Question: after 2012, did surface ocean carbonate chemistry at the two longest open-ocean time series (HOT, North Pacific, 1988-2024, 346 cruises; BATS, Sargasso Sea, 1991-2025, 378 cruises) follow frozen trend models, an atmospheric-tracking model, or tracking plus the thermodynamic temperature effect? Approach: pre-registered forward test. We fitted models on data to 2012 (check: to 2007), recalculated the carbonate system from DIC and TA with PyCO2SYS, and scored test-period bias and RMSE with year-block bootstrap intervals. Results: seawater pCO2 rose 2.74 (HOT) and 3.14 (BATS) uatm/yr in 2013-2025, against 2.42 and 2.51 in the atmosphere. The extra sea-air trend depends on the split year. Atmospheric tracking cut aragonite saturation forecast RMSE by 19-31% at both stations and splits. For pCO2 it did not beat a linear trend, and for pH the result was mixed. Pre-registered H3 failed: adding the observed temperature anomaly (test years about 0.5 C warmer) made all pCO2, pH and saturation forecasts worse (HOT pCO2 RMSE 7.5 to 14.2 uatm). Exploratory: annual-mean temperature anomalies passed 0.00 [-0.23, 0.23] (BATS) and -0.14 [-0.33, 0.12] (HOT) of the thermodynamic effect to pCO2. Within-year deviations passed 0.58 and 0.37. At both stations, TA rose above the pre-2013 TA-salinity line until about 2013-2016 and then fell. This adds about 0.5-0.7 uatm/yr to calculated pCO2, the same size as the extra post-2012 pCO2 acceleration. At HOT, measured spectrophotometric pH fell at -0.00231/yr, as atmospheric tracking predicts (-0.00237), while pH calculated from DIC and TA fell at -0.00272/yr (difference trend -0.00041/yr [-0.00064, -0.00019]). Limitations: two subtropical stations; 12-13 test years with large interannual noise; pass-through is correlational; we cannot tell if the TA reversal is oceanographic or analytical (shared reference-material calibration). External data are not bundled; scripts download them.

ocean acidification · carbonate system · pCO2 · Hawaii Ocean Time-series · Bermuda Atlantic Time-series Study · air-sea CO2 disequilibrium · aragonite saturation · total alkalinity · temperature sensitivity · forecast verification · pre-registered test · out-of-sample test · PyCO2SYS · subtropical gyre