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. 2023 Oct 28;14(1):6888.
doi: 10.1038/s41467-023-42468-z.

Recent acceleration in global ocean heat accumulation by mode and intermediate waters

Zhi Li  1   2   3 , Matthew H England  4   5 , Sjoerd Groeskamp  6
Affiliations

Recent acceleration in global ocean heat accumulation by mode and intermediate waters

Zhi Li et al. Nat Commun. .

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Multi-decadal acceleration in global ocean warming across the measurement era.
a Time series for the heat content (1022 J) 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 J) 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.
Fig. 2
Fig. 2. Regional intensification in ocean warming over the past two decades, 0–2000 m.
The ensemble mean of ocean heat content (OHC) changes averaged for years a 2000–2010 and b 2010–2020, relative to the 1980–2000 mean. Units of shadings in panels (a, b) are shown as 109 J m−2. The values over each basin indicate the OHC increase relative to the 1980–2000 mean over the Southern (S.O., south of 30°S, dark-red line), Atlantic (ATL), Pacific (PAC), and Indian (IND) Oceans, and are limited to 65°S–65°N. Units are shown as 1021 J. The values in parentheses in panel (b) indicate the basin-integrated OHC increase from 2000–2010 to 2010–2020. The basin mask used to distinguish ocean basins of the Southern, Atlantic, Pacific, and Indian Oceans is obtained from ref. . Superimposed gray contours represent the positions of wintertime isopycnals γn= 25, 26.45, 27.05, and 27.5 kg m−3 at 10 m depth from SIO RG-Argo. c, d Zonally integrated OHC change (1021 J per degree latitude) versus latitude for the period 2000–2010 (blue line), and 2010–2020 (red line), relative to the 1980–2000 mean. Lines in panels (c) and (d) represent the ensemble mean, and shadings indicate the ±2 ensemble standard deviation uncertainty range (±2σ) of OHC changes.
Fig. 3
Fig. 3. Maximum mixed layer depth (MLD) across the Argo era and locations of mode and intermediate waters in the global ocean.
MLD is color shaded while water masses are indicated by name and contours for thickness. The MLD is defined as the depth at which density is 0.03 kg m−3 greater than the value of density at 10-m depth from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean. Superimposed acronyms represent Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), North Atlantic STMW (NA-STMW), North Atlantic Madeira Mode Water (NA-MMW), North Atlantic SPMW (NA-SPMW), South Atlantic STMW (SA-STMW), North Pacific STMW (NP-STMW), North Pacific Eastern STMW (NP-ESTMW), North Pacific Central Mode Water (NP-CMW), North Pacific Intermediate Water (NP-IW), South Pacific Western STMW (SP-WSTMW), South Pacific Eastern STMW (SP-ESTMW), Indian Ocean STMW (IO-STMW). The density and geographic constraints for defining these mode and intermediate waters are detailed in “Methods” and Supplementary Table 3. Superimposed contours represent the 150-m, 200-m, and 250-m thickness of all STMWs (blue), 550-m thickness of NP-IW (red), 650-m thickness of SAMW and NA-SPMW (green), and 1000-m thickness of AAIW (yellow), to indicate their geographic locations. Note that the water-mass thickness is estimated based on the density constraints given in Supplementary Table 3, and for simplicity, only the core thickness of each water mass is shown. The geographic constraints for estimating heat content change of water masses are referred to in Supplementary Table 3. Further overview of the mode and intermediate water subduction sites can be found in refs. ,,–,–,.
Fig. 4
Fig. 4. Global ocean heat content (OHC) change relative to the 2016–2020 mean, shown both globally and also decomposed into key water mass layers.
Solid lines are the 13-month running means (1022 J). 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 γn=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 J 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.
Fig. 5
Fig. 5. Decomposition of ocean heat content (OHC) change into warming and volume change components.
Note that for clarity of this decomposition, the values are shown relative to the 2005–2009 ensemble mean. Lines are the 13-month running means (1022 J). a Upper 2000 m of the ocean between 65°S–65°N. The bold red line represents the ensemble mean time series from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean, and shading indicates the ±2 ensemble standard deviation uncertainty range (±2σ). Red and blue dashed lines indicate the warming and volumetric components of OHC changes in the global mode and intermediate water layers, respectively. b The tropical water layer (bold yellow line) and its components within the upper 100 m (gray line) and in the subsurface layer below 100 m (light blue line), as well as its warming (red dashed line) and volumetric change (blue dashed line) components. c The mode water layer, and its warming (red dashed line) and volumetric change (blue dashed line) components. d The intermediate water layer, and its warming (red dashed line) and volumetric change (blue dashed line) components. e, f The warming (e) and volumetric change (f) components of globally integrated OHC distributed by density and time (1021 J 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 proceeded to every layer. Colors indicate the ensemble average of OHC changes from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean.
Fig. 6
Fig. 6. Temporal linear trends in ocean heat content, sea-level anomaly, and thermal expansion and haline contraction coefficients over the Argo era 2005–2020.
a Heat content trend (108 J m−2 yr−1). b Sea-level anomaly trend (10−2 m yr−1). Trends in vertically averaged c ocean thermal expansion coefficient (10-7 K−1 yr−1) and d haline contraction coefficient (10−7 Kg g−1 yr−1) over the upper 2000 m of the ocean. For ease of comparison across panels, the haline contraction coefficient is shown with sign convention reversed: i.e., positive values in red indicate a salinity-related expansion of the water column. The results presented in panels (a, c, d) represent the ensemble means from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean. Gray lines represent the positions of wintertime isopycnals γn=25, 26.45, 27.05, and 27.5 kg m−3 at 10 m depth from SIO RG-Argo. Note that extending the analysis period to 2005–2022 shows overall robust trends over the subtropical Atlantic and Pacific Oceans in both hemispheres and the Southern Ocean, as in panels (a–d).
Fig. 7
Fig. 7. Linear trend in ocean heat content (OHC) zonally integrated over ocean basins.
Units are shown as 1012 J m−2 yr−1. a Western Pacific Ocean. b Eastern Pacific Ocean. c Atlantic Ocean. d Indian Ocean. The Western and Eastern Pacific Oceans are divided by the longitude of 170°W. The basin mask used to distinguish ocean basins of the Pacific, Atlantic, and Indian Oceans is obtained from ref. . The results presented in panels (a–d) represent the ensemble means from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean. Superimposed contours represent the wintertime isopycnals of γn=25, 26.45, 27.05 and 27.75 kg m−3 from SIO RG-Argo.
Fig. 8
Fig. 8. Heat content change in mode and intermediate waters.
a–d The ensemble mean ocean heat content (OHC) trends over 2005–2020 (108 J m−2 yr−1) for the a tropical water layer, b Subtropical Mode Water layer, c Subpolar Mode Water layer, and d intermediate water layer. Superimposed dark gray contours represent the wintertime isopycnals of γn=23, 25, 26.45, 27.05 and 27.75 kg m−3 from SIO RG-Argo. e Trends in OHC (1021 J yr−1) integrated within the specific water masses; from left to right are the 0–2000 m layer of the world ocean, SAMW, AAIW, NA-STMW, NA-MMW, NA-SPMW, SA-STMW, NP-STMW, NP-ESTMW, NP-CMW, NP-IW, SP-WSTMW, SP-ESTMW, IO-STMW, and finally the net of all these regionally defined mode and intermediate waters. The density and geographic constraints for defining these mode and intermediate waters are detailed in “Methods” and Supplementary Table 3, and depicted in Fig. 3. Bars in panel (e) represent the ensemble average of OHC trends from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean, and superimposed error bars indicate the ±2 ensemble standard deviation uncertainty range (±2σ).
Fig. 9
Fig. 9. Heat content change due to volumetric increase and warming for 2005–2020.
Trend in ocean heat content (OHC) due to thickness change for the a Subtropical Mode Water layer, b Subpolar Mode Water layer, and (c) intermediate water layer. Units are shown as 108 J m−2 yr−1. d–f Same as (a–c) but for the OHC trend due to temperature change. The results presented in panels (a–f) indicate the ensemble means from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean. g Same as Fig. 8e but for the OHC trend (1021 J yr−1, dark red) and its components by volumetric change (blue) and temperature change (orange-red) (detailed in Supplementary Table 4). The locations of the various water masses shown in panel (g) are depicted in Fig. 3. Bars in panel (g) represent the ensemble average of OHC trends from SIO RG-Argo, IAP data, and EN4.2.2 ensemble mean, and superimposed error bars indicate the ±2 ensemble standard deviation uncertainty range (±2σ).
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