Abstract

The ocean absorbs >90% of anthropogenic heat in the Earth system, moderating global atmospheric warming. However, it remains unclear how this heat uptake is distributed by basin and across water masses. Here we analyze historical 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, spanning 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 volumetric 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.


Introduction

The ocean directly impacts the Earth’s climate by absorbing and redistributing large amounts of heat, freshwater, and carbon, and by exchanging these properties with the atmosphere1. About 91% of the excess heat trapped by greenhouse gases and 31% of human emissions of carbon dioxide are stored in the ocean, shielding humans from even more rapid changes in climate. However, warmer oceans result in sea-level rise, ice-shelf melt, intensified storms, tropical cyclones, and marine heatwaves, as well as more severe marine species and ecosystem damage. These effects depend on the pattern of ocean warming; it is thus critical to quantify the dynamics and distribution of ocean warming to better understand its consequences and predict its implications.

The observed distribution of ocean warming is not uniform. About 90% of total ocean warming is found in the upper 2000m, with over two-thirds concentrated in the upper 700m since the 1950s, and an increase of warming rates at both intermediate depths of 700–2000m, and in the deeper ocean below 2000m. The Southern Ocean south of 30°S has been estimated to account for 35–43% of global ocean warming from 1970 to 2017, and an even greater proportion in recent years, while Northern Hemisphere ocean warming appears to be concentrated in the Atlantic Ocean. Due to the accumulated excess heat in ocean basins, an acceleration of total ocean warming has become more evident from recent observational-based studies. While much past work has focused on the distribution of ocean warming as a function of depth and basin, relatively little analysis has been undertaken of the distribution as a function of water-mass layers and within specific water masses. This is the focus of the present study.

Ocean warming is often measured by tracking changes in ocean heat content (OHC). A useful coordinate to analyze OHC changes is a density- or temperature-based framework because this is a more natural coordinate to explore the thermodynamics of ocean warming. Deepening of isopycnals is associated with ocean heat uptake, with past work suggesting the maximum deepening can be found in the mode water density range over the Southern Hemisphere extratropics and in the North Atlantic, implying a volumetric increase of mode waters and associated subduction and lateral spreading of heat from high-latitude well-ventilated regions. A temperature-based framework further suggests that about half of the surface heat uptake during 1970–2014 is confined to about a quarter of the ocean’s surface area in the subpolar regions, which is in turn capable of exchanging heat with the coldest 90% of the global ocean volume.

The above studies have provided insights and identified where we lack observational-based constraints in understanding OHC variability. However, most past studies of OHC changes are depth- or basin-integrated, obscuring the OHC evolution in mode and intermediate waters and the related processes at play. Mode and intermediate waters are distinguished by their properties of density, salinity and low stratification by wintertime ventilation, and their importance in storing and redistributing heat, carbon and oxygen, yet their heat content changes associated with global ocean warming are yet to be quantified. Furthermore, relatively little attention has been paid to their temperature and volumetric evolution and the associated role of subduction, spreading, and mixing in accumulating oceanic heat, despite observations highlighting a significant upper ocean volumetric increase of mode waters and showing striking local warming signals at depths of 700–2000 m in the North Atlantic and Southern Oceans.

a Time series for the heat content (1022
) of the upper 2000 m of the ocean relative to 2016–2020 mean, using various observational products. Red lines in panels (a) and (b) represent the ensemble mean time series of global ocean heat content (OHC) during 1955–2020, and shading in panel (a) indicates the ± 2 ensemble standard deviation uncertainty range (±2σ) for the global OHC time series. b Blue rectangle bars indicate the ensemble-averaged global ocean heat uptake (1022) for every 11-year period across the measurement era (“Methods”). Superimposed error bars indicate the ±2 ensemble standard deviation uncertainty range (±2σ) of global ocean heat uptake across various datasets. Pre-Argo period represents the period 1955–2004, WOCE era represents the Hydrographic Program of the World Ocean Circulation Experiment during the 1990s, and Argo era indicates the period with Argo record since 2005.

The increased ocean warming is non-uniformly distributed across ocean basins. Overall, in each ocean basin, an increase in OHC is observed (values indicated in Fig. 2a, b), with stronger warming in the mid-latitude Atlantic Ocean and the Southern Ocean compared with other basins. Total warming in the Southern Ocean is estimated to account for ~31% of the global upper 2000m OHC increase from 1980–2000 to 2000–2010 (Fig. 2a), and almost half of the global OHC increase from 2000–2010 to 2010–2020 (values indicated in parentheses of Fig. 2b). Hence the Southern Ocean has seen the largest increase in heat storage over the past two decades, holding almost the same excess anthropogenic heat as the Atlantic, Pacific, and Indian Oceans north of 30°S combined (Fig. 2d). The most striking warming in the Southern Ocean is concentrated on the northern flank of the Antarctic Circumpolar Current, the location of deep mixed layers and subduction hotspots for Subantarctic Mode Water and Antarctic Intermediate Water, as well as the location of subtropical mode waters formation further equatorward (Fig. 3). The well-ventilated regions near western boundary current extensions in the North Atlantic and North Pacific also reveal large warming over the past two decades. These hotspots of ocean warming are likely linked to enhanced uptake, subduction, and lateral spreading of heat associated with mode and intermediate waters that warrant further investigation.

Heat accumulation in mode and intermediate water layers
The global OHC change of the upper 2000 m is next decomposed into the tropical water layer, the mode water layer, and the intermediate water layer to study the distribution of ocean warming by global water-mass layers (Fig. 4). The most remarkable OHC change over the Argo era is the increase within the mode water layer (Fig. 4c, f). The mode water layer occupies only 20% of the upper 2000 m of the ocean, yet plays a dominant role in total ocean heat content increase (Fig. 4a, Supplementary Table 1). The intermediate water layer occupies 42% of the upper 2000 m, but exhibits a decreasing heat content trend (Fig. 4d). These opposing signals are mainly due to the mode water layer accumulating heat content via increasing its volume, while the intermediate water layer loses heat content due to a volume decrease (Fig. 5c, f). After factoring out volumetric effects, both the global mode and intermediate water layers show a near-continuous and monotonic warming across the Argo era (Fig. 5, Supplementary Fig. 2), revealing a robust footprint of global warming penetrating into the ocean interior. Taken together, the total heat accumulated in the mode and intermediate water layers is 89% of the net global ocean warming during the Argo era, despite their total volume just occupying 62% of the upper 2000 m. The net warming of these two layers, separate from volumetric effects, accounts for ~76% of global ocean warming, while ~12% is due to their combined volume change (Fig. 5a, Supplementary Table 2).

Solid lines are the 13-month running means (1022
). a Upper 2000 m of the ocean, 65°S–65°N. Thin lines indicate the OHC change from SIO RG-Argo (yellow), IAP data (blue), and EN4.2.2 ensemble mean (light blue), respectively. The thick line indicates the ensemble mean time series. b The tropical water layer. c The mode water layer. d The intermediate water layer. e Zonally averaged density to indicate the geographic locations of the tropical water layer (TW, yellow shading), the mode water layer (MW; red shading) and its components within the Subtropical Mode Water layer (STMW) and the Subpolar Mode Water layer (SPMW), and the intermediate water layer (IW, blue shading) over the upper 2000 m of the ocean (“Methods”). Superimposed contours represent the wintertime isopycnals of
25, 26.45, 27.05 and 27.75 kg m−3 from SIO RG-Argo. f Globally integrated annual mean OHC distributed by density and time (1021
per 0.1 kg m−3 density bin), with the averaged OHC during the Argo era being removed and then the 3-year running mean being applied to every layer. Colors indicate the ensemble mean estimate of OHC changes from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean.