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. 2020 Jun 26;6(26):eaaz4876.
doi: 10.1126/sciadv.aaz4876. eCollection 2020 Jun.

Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

Wei Liu  1 , Alexey V Fedorov  2   3 , Shang-Ping Xie  4 , Shineng Hu  5
Affiliations

Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

Wei Liu et al. Sci Adv. .

Abstract

While the Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down under anthropogenic warming, the exact role of the AMOC in future climate change has not been fully quantified. Here, we present a method to stabilize the AMOC intensity in anthropogenic warming experiments by removing fresh water from the subpolar North Atlantic. This method enables us to isolate the AMOC climatic impacts in experiments with a full-physics climate model. Our results show that a weakened AMOC can explain ocean cooling south of Greenland that resembles the North Atlantic warming hole and a reduced Arctic sea ice loss in all seasons with a delay of about 6 years in the emergence of an ice-free Arctic in boreal summer. In the troposphere, a weakened AMOC causes an anomalous cooling band stretching from the lower levels in high latitudes to the upper levels in the tropics and displaces the Northern Hemisphere midlatitude jets poleward.

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Figures

Fig. 1
Fig. 1. AMOC strength and global mean surface air temperature in CCSM4 historical and RCP8.5 simulations and sensitivity experiment AMOC_fx.
(A) From 1850 to 1980, the AMOC strength is adopted from CCSM4 historical simulation (purple, ensemble mean; light purple, ensemble spread). After 1980, the AMOC strength from CCSM4 historical and RCP8.5 simulations (AMOC_fx) is shown as green (purple) curve for ensemble mean and light green (light purple) shading for ensemble spread. The AMOC strength is defined as the maximum of the annual mean stream function below 500 m in the North Atlantic. (B) Similar to (A) but for annual and global mean surface air temperature (GMST) anomalies relative to 1961–1980.
Fig. 2
Fig. 2. Surface temperature and precipitation projections and AMOC impacts.
Left column: Relative to 1961–1980, annual mean surface air temperature changes (shading in K) during 2061–2080 based on the ensemble means of (A) CCSM4 RCP8.5 simulation and (C) AMOC_fx. (E) shows (A) minus (C). Right column: Similar to left column but for annual mean precipitation changes (shading in mm/day). (F) shows (B) minus (D). In all the panels, stippling indicates that the response is statistically significant at the 95% confidence level of Student’s t test. AMOC impacts on surface temperature and precipitation are revealed in (E) and (F).
Fig. 3
Fig. 3. AMOC impacts on Atlantic oceanic temperature projection.
Difference of zonal mean ocean temperature in the Atlantic (shading in K) during 2061–2080 between the ensemble means of CCSM4 RCP8.5 simulation and AMOC_fx (RCP8.5 minus AMOC_fx). The black lines at 48°N and 60°N denote the southern and northern borders of the water column over the NAWH region used for the heat budget analysis.
Fig. 4
Fig. 4. The AMOC and NAWH in CESM large ensemble simulations.
(A) The AMOC strength during 1920–2100 from CESM large ensemble simulations (blue, ensemble mean; light blue, ensemble spread). The AMOC strength is defined as the maximum of the annual mean stream function below 500 m in the North Atlantic. (B) Ensemble mean SST change (years 2061–2080 minus 1961–1980) in the North Atlantic in CESM large ensemble simulations. (C) The scatter plot of SST changes over the NAWH region [46°N to 56°N and 22°W to 35°W, indicated in (B)] and AMOC strength changes in CESM large ensemble simulations (blue dots for individual members). The best-fit line (black) is calculated as the first empirical orthogonal function mode in the SST AMOC space. The correlation coefficient between SST and AMOC changes is 0.545, which is statistically significant at the 95% confidence level.
Fig. 5
Fig. 5. Arctic sea ice projections and AMOC impacts.
Top row: (A) September and (B) March Arctic sea ice extent (SIE) in CCSM4 historical and RCP8.5 simulations (green, ensemble mean; light green, ensemble spread) and AMOC_fx (purple, ensemble mean; light purple, ensemble spread) with 11-year running mean adopted. SIE is defined as the total ocean area that has an ice concentration of 15% or more. The horizontal line in (A) denotes the common threshold for an ice-free Arctic (1 × 106 km2). Middle row: Relative to 1961–1980, September Arctic sea ice concentration (SIC) changes during 2061–2080 based on the ensemble means of (C) CCSM4 RCP8.5 simulation and (D) AMOC_fx. (E) shows (C) minus (D). Bottom row: Similar to middle row but for March Arctic SIC. (H) shows (F) minus (G). AMOC impacts on Arctic sea ice are shown in (A), (B), (E), and (H).
Fig. 6
Fig. 6. Atmosphere temperature and zonal wind projections and AMOC impacts.
Left column: Relative to 1961–1980, annual and zonal mean atmosphere temperature changes (shading in K) during 2061–2080 based on the ensemble means of (A) CCSM4 RCP8.5 simulation and (C) AMOC_fx. (E) shows (A) minus (C). Contours in three panels show annual and zonal mean atmosphere temperature during 1961–1980 (contour interval of 10 K). Right column: Similar to left column but for boreal wintertime [December-January-February (DJF)] zonal mean zonal wind changes (shading in m/s). Contours in three panels show DJF zonal mean zonal winds during 1961–1980 (contour interval of 5 m/s and zero contours thickened). (F) shows (B) minus (D). In all the panels, stippling indicates that the response is statistically significant at the 95% confidence level of Student’s t test. AMOC impacts on atmosphere temperature and zonal winds are revealed in (E) and (F).
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