Recent acceleration in global ocean heat accumulation by mode andintermediate waters

The ocean absorbs >90% of anthropogenic heat in the Earth system, moder

ating global atmospheric warming. However, it remains unclear how this heat

uptake is distributed by basin and across water masses. Here we analyze his

torical and recent observations to show that ocean heat uptake has accelerated

dramatically since the 1990s, nearly doubling during 2010–2020 relative to

1990–2000. Of the total ocean heat uptake over the Argo era 2005–2020,

about 89% can be found in global mode and intermediate water layers, span

ning both hemispheres and both subtropical and subpolar mode waters. Due

to anthropogenic warming, there are significant changes in the volume of

these water-mass layers as they warm and freshen. After factoring out volu

metric changes, the combined warming of these layers accounts for ~76% of

global ocean warming. We further decompose these water-mass layers into

regional water masses over the subtropical Pacific and Atlantic Oceans and in

the Southern Ocean. This shows that regional mode and intermediate waters

are responsible for a disproportionate fraction of total heat uptake compared

to their volume, with important implications for understanding ongoing ocean

warming, sea-level rise, and climate impacts.