Consensuses and discrepancies of basin-scale ocean heat content (OHC) changes in different ocean analyses – part III

It is apparent that all of the below regions are ocean heat sinks in the current decade, but there is no clear indication of which ocean basin dominates the global OHC change.

Figure 13 shows OHC changes based on the three datasets, including: – the upper 700-m OHC change in the Indian Ocean (as calculated in Lee et al. 2015); – OHC between 100-to-300-m depth in the Indo-Pacific basin (as calculated in Nieves et al. 2015); – OHC below 300-m in the Atlantic and Southern Ocean (as calculated in Balmaseda et al. 2013; Chen and Tung 2014; Drijfhout et al. 2014); – OHC between 100-to-300-m depth in the Pacific Ocean (as calculated in Meehl et al. 2011). OHC trends at different basins and depth intervals revealed by different datasets during 1998–2012. In700 is the upper 700-m OHC in the Indian Ocean, Indo-Pa100to300 is OHC between 100 and 300-m layer in total Indian Ocean and Pacific, Pa100to300 is the OHC between 100 and 300-m layer in Pacific, Atl300to1500 and So300to1500 are the OHC between 300 and 1500-m in Atlantic and the Southern Ocean respectively. The trends of global OHC (0–1500m) change during the same period are shown on the right end.

Substantial uncertainty in the decadal OHC redistribution globally among the three datasets prevents reaching a conclusion about the relative contribution of each basin to the global OHC changes.

To understand the relation between OHC changes and the global warming slowdown/hiatus, we need quantify the storage of heat not only during the hiatus period but also during the decades before the hiatus. So we recommend a comprehensive evaluation in the future to quantify the impact of insufficient ocean sampling on historical OHC calculations, i.e. the performance of different mapping methods. A recent study (Cheng et al. 2017) used data during Argo period as “truth”, and then subsampled the “truth” according to the historical sampling. By comparing the reconstructed field with the “truth”, the accuracy of the reconstruction can be assessed. This study showed a reliable reconstruction for IAP mapping for decadal and multi-decadal OHC variations in the major ocean basins. Similar tests are highly recommended in the future. Current observation system, covering mostly region of global ocean, increases the accuracy of OHC estimates and reduces the disagreement between different datasets compared with the pre-Argo era. But OHC estimates among oceans with more energetic oceanic phenomena such as ocean fronts and meso-scale eddies, still have large uncertainty in the Argo era. The current observation system should be maintained and extended in the future to be achieve a more complete ocean sampling.

Since one can never re-observe the ocean in the past, some synthetic data should be used, for instance high-resolution model outputs, sea level data, etc. Furthermore, data during the Argo period are still insufficient to observe the meso-scale eddies and OHC changes related to weather phenomena. Therefore, high-resolution models and sea level data will be helpful for the further evaluation of the current OHC analyses. Such a practice will significantly improve the understanding of the uncertainty in OHC estimates and facilitate more accurate reconstruction of historical OHC changes.

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