Meyssignac et al. Measuring OHC to Estimate the EEI efficient approach to estimate EEI. In this community paper we review the current four state-of-the-art methods to estimate global OHC changes and evaluate their relevance to derive EEI estimates on different time scales. These four methods make use of: (1) direct observations of in situ temperature; (2) satellite-based measurements of the ocean surface net heat fluxes; (3) satellite-based estimates of the thermal expansion of the ocean and (4) ocean reanalyses that assimilate observations from both satellite and in situ instruments. For each method we review the potential and the uncertainty of the method to estimate global OHC changes. We also analyze gaps in the current capability of each method and identify ways of progress for the future to fulfill the requirements of EEI monitoring. Achieving the observation of EEI with sufficient accuracy will depend on merging the remote sensing techniques with in situ measurements of key variables as an integral part of the Ocean Observing System.

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Combining the dynamical surface‐trapped mode derived from the Surface Quasi‐Geostrophic (SQG) function with the statistical mode calculated from multivariate empirical orthogonal function (EOF) reconstruction (mEOF‐R) method, this paper proposes a new method, SQG‐mEOF‐R, to estimate the interior density from the sea surface density and sea surface height. This method is applied to the eddy‐resolving Ocean General Circulation Model For the Earth simulator simulation and compared with the conventional SQG, isQG (interior plus SQG), and mEOF‐R methods. Two different dynamical regimes were considered: the NorthWest Pacific, dominated by surface‐intensified eddies, and the Southeast Pacific, characterized by the occurrence of subsurface‐intensified eddies. In both regions, the proposed method proves to be robust in reconstructing mesoscale features, with reduced root‐mean square error and relatively high correlation coefficient. The superiority of SQG‐mEOF‐R is highlighted in those subsurface‐intensified eddies that are little or nothing reconstructed by SQG or isQG method.

In addition to the sea level (SL) change, or anomaly (SLA), due to ocean thermal expansion, total steric SLA (SSLA, all change to the existing volume of ocean water) is also affected by ocean salinity variation. Less attention, however, has been paid to this halosteric effect, due to the global dominance of thermosteric SLA (TSLA) and the scarcity of salinity measurements. Here, we analyze halosteric SLA (HSLA) since 2005, when Argo data reached near-global ocean coverage, based on several observational products. We find that, on global average, the halosteric component contributes negatively by~5. Our analysis suggests that local salinity changes cannot be neglected, and can even play a more important role in SSLA than the thermosteric component in some regions, such as the Tropical/North Pacific Ocean, the Southern Ocean, and the North Atlantic Ocean. This study highlights the need to better reconstruct historical salinity datasets, to better monitor the past SSLA changes. Also, it is important to understand the mechanisms (ocean dynamics vs. surface flux) related to regional ocean salinity changes.

Ocean heat content (OHC) is the major component of the earth’s energy imbalance. Its decadal scale variability has been heavily debated in the research interest of the so-called “surface warming slowdown” (SWS) that occurred during the 1998–2013 period. Here, we first clarify that OHC has accelerated since the late 1990s. This finding refutes the concept of a slowdown of the human-induced global warming. This study also addresses the question of how heat is redistributed within the global ocean and provides some explanation of the underlying physical phenomena. Previous efforts to answer this question end with contradictory conclusions; we show that the systematic errors in some OHC datasets are partly responsible for these contradictions. Using an improved OHC product, the three-dimensional OHC changes during the SWS period are depicted, related to a reference period of 1982–1997. Several “hot spots” and “cold spots” are identified, showing a significant decadal-scale redistribution of ocean heat, which is distinct from the long-term ocean-warming pattern. To provide clues for the potential drivers of the OHC changes during the SWS period, we examine the OHC changes related to the key climate modes by regressing the Pacific Decadal Oscillation (PDO), El Niño-Southern Oscillation (ENSO), and Atlantic Multi-decadal Oscillation (AMO) indices onto the de-trended gridded OHC anomalies. We find that no single mode can fully explain the OHC change patterns during the SWS period, suggesting that there is not a single “pacemaker” for the recent SWS. Our observation-based analyses provide a basis for further understanding the mechanisms of the decadal ocean heat uptake and evaluating the climate models.