Recent water mass changes reveal mechanisms of ocean warming

Zika, Jan D.; Gregory, Jonathan M.; McDonagh, Elaine L.; Marzocchi, Alice ORCID: https://orcid.org/0000-0002-3430-3574; Clément, Louis ORCID: https://orcid.org/0000-0002-6935-9455. 2021 Recent water mass changes reveal mechanisms of ocean warming. Journal of Climate. 1-52. https://doi.org/10.1175/JCLI-D-20-0355.1

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Abstract/Summary

Over 90% of the build up of additional heat in the earth system over recent decades is contained in the ocean. Since 2006 new observational programs have revealed heterogeneous patterns of ocean heat content change. It is unclear how much of this heterogeneity is due to heat being added to and mixed within the ocean leading to material changes in water mass properties or due to changes in circulation which redistribute existing water masses. Here we present a novel diagnosis of the ‘material’ and ‘redistributed’ contributions to regional heat content change between 2006 and 2017 based on a new Minimum Transformation Method informed by both water mass transformation and optimal transportation theory. We show that material warming has large spatial coherence. The material change tends to be smaller than the redistributed change at any geographical location, however it sums globally to the net warming of the ocean, while the redistributed component sums, by design, to zero. Material warming is robust over the time period of this analysis, whereas the redistributed signal only emerges from the variability in a few regions. In the North Atlantic, water mass changes indicate substantial material warming while redistribution cools the subpolar region due to a slowdown in the Meridional Overturning Circulation. Warming in the Southern Ocean is explained by material warming and by anomalous southward heat transport of 118 ± 50 TWdue to redistribution. Our results suggest near termprojections of ocean heat content change and therefore sea level change will hinge on understanding and predicting changes in ocean redistribution.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1175/JCLI-D-20-0355.1
ISSN:	0894-8755
Date made live:	15 Feb 2021 11:50 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/529462

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