Revisiting the 2003-2018 deep-ocean warming through multi-platform analysis of the global energy budget

Abstract Recent estimates of the global warming rates suggest that approximately 9% of the Earth’s excess heat is cumulated in the deep and abyssal oceans (below 2000 m depth) during the last two decades. Such estimates assume stationary trends deducted as long-term rates. In order to reassess the deep ocean warming and potentially shed light on its inter-annual variability, we formulate the balance between the Earth’s Energy Imbalance (EEI), the steric sea level and the ocean heat content (OHC), at yearly time scales during the 2003-2018 period, as a variational problem. The solution is achieved through variational minimization, merging together observational data from top-of-atmosphere EEI, inferred from CERES, steric sea level estimates from altimetry minus gravimetry, and upper ocean heat content estimates from in-situ platforms (mostly Argo floats). Global ocean reanalyses provide background error covariances for the OHC analysis. The analysis indicates a 2000m-bottom warming of 0.08 ± 0.04 W m −2 for the period 2003-2018, equal to 13% of the total ocean warming (0.62 ± 0.08 W m −2 ), slightly larger than previous estimates but consistent within the error bars. The analysis provides a fully consistent optimized solution also for the steric sea level and EEI. Moreover, the simultaneous use of the different heat budget observing networks is able to decrease the analysis uncertainty with respect to the observational one, for all observation types and especially for the 0-700m OHC and steric sea level (more than 12% reduction). The sensitivity of the analysis to the choice of the background timeseries is proved insignificant.