Ocean Heat Content Estimates from Ocean Reanalyses since 1993

Abstract. Ocean heat content (OHC) is a key indicator of Earth’s energy imbalance and long-term climate change. Within the Marine Environment Reanalyses Evaluation Project (MER-EP), we use recent-generation ocean reanalysis products (ORAs) to assess long-term global OHC changes, spatial warming patterns, and inter-product consistency since 1993. We compare the results with those from objective analysis products (OAs) and earlier-generation ORAs. The results show sustained global ocean warming during 1993–2022, concentrated mainly in the upper 2000 m. The ORA ensemble gives a global 0–2000 m OHC trend of 0.73 [0.49, 0.97] W m-2 (1.65 std), with trend estimates from the OAs falling within the ORA uncertainty range. Warming is more pronounced at mid-to-high latitudes and extends to greater depths at high northern latitudes. Differences among ORAs are largest in the polar oceans, western boundary current regions, frontal systems, and below 2000 m. Inter-product consistency among ORAs improves markedly in the Argo era. In the upper 2000 m, the fraction of ocean area where the common ORA signal exceeds inter-product spread (signal-to-noise ratio > 1) increases by 20.8 percentage points for interannual variability and 21.3 percentage points for long-term trends after 2005. The largest improvement occurs in the 700–2000 m layer. Below 2000 m, inter-product consistency among ORAs remains low throughout the study period. Compared with earlier-generation ORAs, recent-generation ORAs also show a clear improvement in inter-product consistency for both interannual variability and long-term trends in upper-ocean OHC. These results increase confidence in using recent-generation ORAs to estimate global OHC changes and indicate that sustained ocean observations provide an essential foundation for reliably estimating global OHC changes.