Recent Water Mass Changes Reveal Mechanisms of Ocean Warming

Jan D Zika,Jonathan M Gregory,Alice Marzocchi,Elaine L Mcdonagh,Louis Clément

doi:10.1175/jcli-d-20-0355.1

Abstract

AbstractOver 90% of the buildup 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 is due to changes in circulation that 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 that is 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, whereas 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 Ocean, water mass changes indicate substantial material warming while redistribution cools the subpolar region as a result of 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 TW through redistribution. Our results suggest that near-term projections of ocean heat content change and therefore sea level change will hinge on understanding and predicting changes in ocean redistribution.

Highlights

Over the past 50 years, as atmospheric greenhouse gas concentrations have increased, the ocean has absorbed more than 10 times as much heat as all other components of the climate system combined (Rhein et al 2013)
The large apparent meridional heat transport we have identified in the Southern Ocean was previously identified by Roberts et al (2017) based on the residual of observed ocean heat content (OHC) change and estimates of air–sea heat fluxes
In summary we have shown the following: d Water mass changes between 2006–11 and 2012–17 can be interpreted in terms of material warming across the globe and with the highest concentrations in the tropical and subtropical North Atlantic Ocean, consistent with simulations of the addition of heat into the ocean due to greenhouse forcing. d The majority of the variance in ocean heat content change at scales of 18 3 18 over that period can be explained by a redistribution of existing water masses within the ocean. d The inferred redistribution indicates a downturn in northward meridional heat transport into the subpolar North Atlantic of 40 6 13 TW and an anomalous southward heat transport into the Southern Ocean of 118 6 50 TW

Summary

Introduction

Over the past 50 years, as atmospheric greenhouse gas concentrations have increased, the ocean has absorbed more than 10 times as much heat as all other components of the climate system combined (Rhein et al 2013). This warming showed substantial spatial variability between 1993 and 2005, being up to 10 times as much in some regions as the global average (Zhang and Church 2012). It is unclear whether this variability is due to geographical variation in the interior propagation of surface warming versus redistribution of existing heat within the ocean. Understanding how heat has been taken up and redistributed by the ocean is essential for predicting future changes in sea level

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