Tropical climate responses to projected Arctic and Antarctic sea-ice loss

Abstract

Arctic and Antarctic sea-ice extent are both projected to dramatically decline over the coming century. The effects of Arctic sea-ice loss are not limited to the northern high latitudes, and reach deep into the tropics. Yet little is known about the effects of future Antarctic sea-ice loss outside of the southern high latitudes. Here, using a fully coupled climate model, we investigate the tropical response to Antarctic sea-ice loss and compare it with the response to Arctic sea-ice loss. We show that Antarctic sea-ice loss, similar to Arctic sea-ice loss, causes enhanced warming in the eastern equatorial Pacific and an equatorward intensification of the Intertropical Convergence Zone. We demonstrate that Antarctic sea-ice loss causes a mini global warming signal comparable to the one caused by Arctic sea-ice loss, and reminiscent of the response to greenhouse gases. We also show that ocean dynamics are key to capturing the tropical response to sea-ice loss. In short, we find that future Antarctic sea-ice loss will exert a profound influence on the tropics. Combined Arctic and Antarctic sea-ice losses will account for 20–30% of the projected tropical warming and precipitation changes under the high-emissions scenario Representative Concentration Pathway 8.5.

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Fig. 1: Seasonal cycle of SIE in experiments and the zonally averaged temperature response to sea-ice loss.
Fig. 2: The tropical surface temperature (with tropical mean removed) and surface wind response to sea-ice loss.
Fig. 3: The tropical precipitation response to sea-ice loss.
Fig. 4: The response of northward atmospheric and oceanic heat transport to sea-ice loss.

Data availability

All model output analysed in the study is stored on the data servers at the National Center for Atmospheric Research in Boulder, Colorado, and can be made available upon request from the corresponding author.

Code availability

Where possible, pre-processed variables and the NCL code for reproducing the related figures are publicly available at https://figshare.com/projects/The_Tropical_Responses_to_Projected_Arctic_and_Antarctic_Sea_Ice_Loss/72518. All other code used to produce the figures can be made available upon request from the corresponding author.

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Acknowledgements

M.R.E. is funded by a collaborative agreement between NASA GISS and Columbia University and NSF grants OPP-1643445 and OPP-1744835. L.M.P. is funded by NSF award number 1745029. This material is based on work performed at the National Center for Atmospheric Research, a major facility sponsored by the National Science Foundation. We thank S. Gille, I. Eisenman, S. Sun, M. Mazloff and R. Abernathey for productive discussions.

Author information

M.R.E. performed the integrations, carried out the analysis and wrote the first draft of the manuscript. L.M.P. designed the integrations. L.S. provided assistance in setting up the integrations. C.D. designed the study and provided expertise in tropical climate. All authors were heavily involved in interpreting the results and the drafting of the final manuscript.

Correspondence to Mark R. England.

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The authors declare no competing interests.

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Peer review information Primary Handling Editor: Heike Langenberg.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Arctic and Antarctic sea ice extent climatology.

The seasonal cycle of (a) Arctic and (b) Antarctic SIE [x106 km2] averaged over the years 1979–2000 from six WACCM historical runs (grey) and observational data from the NSIDC Sea Ice database (NSIDC, 2019) (blue). The shading shows the + /-2σ envelope.

Extended Data Fig. 2 Additivity of the zonally average temperature response to sea ice loss.

(a) The zonally averaged temperature response [°C] to both Arctic and Antarctic sea ice loss as a function of latitude and height. (b) The linear sum of the response to Arctic sea ice loss and the response to Antarctic sea ice loss. (c) The difference between panels a, b. Contours show the climatological temperature structure with intervals of 15 °C.

Extended Data Fig. 3 Tropical surface temperature response to sea ice loss.

(Shading) The annual mean surface temperature response [°C] to (a) Arctic sea ice loss, (b) Antarctic sea ice loss and (c) both Arctic and Antarctic sea ice loss compared to (d) the projected changes under RCP8.5, the 15-year average of 2085-2099 minus the 15-year average of 1955-1969, (scaled by a factor of 1/5). The contours show the climatological surface temperature with contour intervals of 4 °C.

Extended Data Fig. 4 Additivity of the surface temperature response to sea ice loss.

(a) The surface temperature response [°C] after subtracting the mean tropical SST warming (shaded contours) and the surface wind response [m/s] (vectors) to combined Arctic and Antarctic sea ice loss. (b) As in panel (a) except for the linear sum of the response to Arctic sea ice loss and the response to Antarctic sea ice loss. (c) The difference between panels a and b.

Extended Data Fig. 5 The condensational heating rate response to sea ice loss.

The response of annual mean condensational heating rate [x10-2 °C/day] to (a) Arctic sea ice loss, (b) Antarctic sea ice loss and (c) both Arctic and Antarctic sea ice loss. (d) The projected change in annual mean condensational heating rate under RCP8.5, the 15-year average of 2085-2099 minus the 15-year average of 1955-1969. Note that the response in panel (d) is scaled by a factor of 1/5.

Extended Data Fig. 6 Zonally averaged precipitation response to sea ice loss.

(Shading) The response of the zonally averaged precipitation [mm/day] to (a) Arctic sea ice loss, (b) Antarctic sea ice loss and (c) both Arctic and Antarctic sea ice loss. (d) The projected change under RCP8.5, the 15-year average of 2085-2099 minus the 15-year average of 1955-1969. Note that the response in panel (d) is scaled by a factor of 1/5. Stippling shows a statistically significant response at 95% confidence. (Contours, black) The zonally averaged precipitation with contour intervals of 2 mm/day. Regions are highlighted (contours, red) in which the response is over 20% of the RCP8.5 response.

Extended Data Fig. 7 Zonally averaged (Pacific sector) precipitation response to sea ice loss.

As in Extended Data Fig. 6 but for the Pacific sector (130°E-100°W).

Extended Data Fig. 8 The response of the Pacific subtropical meridional overturning circulation to sea ice loss.

(Shading) The depth-latitude response of the Pacific subtropical meridional overturning circulation [Sv] to (a) Arctic sea ice loss, (b) Antarctic sea ice loss and (c) both Arctic and Antarctic sea ice loss. (Contours) The climatological meridional overturning circulation with contour intervals of 5 Sv. The solid lines indicate positive values (clockwise flow), the dashed lines indicate negative values (anti-clockwise flow) and the thick black contour indicates the 0 Sv contour.

Extended Data Fig. 9 Connecting the slowdown of the subtropical meridional overturning circulation to Antarctic sea ice loss.

(a) Average vertical velocity in the upper-100m of the ocean in the control run. The white contours enclose the subduction zones, which we define to be < -6cm/year. (b) The response of mean sea level pressure [hPa] (shaded) and surface wind [m/s] (vectors) to Antarctic sea ice loss. (c) The response of the vertical Ekman velocity to Antarctic sea ice loss. Positive values indicate anomalous upwelling (Ekman suction) and negative values indicate anomalous downwelling (Ekman pumping). As in panel (a) the black contours enclose the subduction zones. (d) Same as panel (b) but for the atmosphere-only experiments.

Extended Data Fig. 10 Vertical velocities in the subduction zones.

Averaged over the three subduction zones (the Indian, Pacific and Atlantic; as shown in Extended Data Fig. 9a), the response of the vertical velocity in the upper-100m to Antarctic sea ice loss (black bars) and the response of the vertical Ekman velocity (white bars) [cm/year].

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England, M.R., Polvani, L.M., Sun, L. et al. Tropical climate responses to projected Arctic and Antarctic sea-ice loss. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0546-9

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