Future sea-ice loss slows down the subtropical Pacific shallow overturning cell and enhances the tropical warming

Under the high emission scenario for the future (RCP 8.5), a nearly ice-free Arctic Ocean (i.e., sea-ice extent less than 10-6 km2 for at least five consecutive years) in northern summer is likely before 2050. A decrease in Antarctic sea-ice extent is also expected during the 21st century, but at a slower rate with large uncertainty. However, it is not entirely clear how the tropical atmosphere and ocean respond to the future Arctic and Antarctic sea-ice losses. A study recently published in Nature Geoscience (England et al., 2020) investigated this question by performing a series of  fully coupled climate model experiments with two sets of sea-ice concentration map, one for the mid-20th century (1955-1969) and the other for the late-21st century (2085-2099). According to the model experiments, Antarctic sea-ice loss, similar to Arctic sea-ice loss, causes enhanced warming in the eastern equatorial Pacific and an equatorward intensification of the Inter-Tropical Convergence Zone. Therefore, this study concluded that Arctic and Antarctic sea-ice losses enhance the tropical warming response to the increasing greenhouse gases (GHGs) by 20-30%. The tropical response to Antarctic sea-ice loss is largely mediated by a wind-driven reduction of subtropical subduction and subsequent slowdown of the subtropical Pacific shallow overturning cell, which results in a reduction of the equatorial Pacific upwelling.

Extended Data Fig. 8 from England et al. (2020). 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.

England, M.R., Polvani, L.M., Sun, L. et al. 2020: Tropical climate responses to projected Arctic and Antarctic sea-ice loss. Nat. Geosci. 13, 275–281. https://doi.org/10.1038/s41561-020-0546-9.

3 thoughts on “Future sea-ice loss slows down the subtropical Pacific shallow overturning cell and enhances the tropical warming

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  1. A message from Malte Stuecker: This is consistent also what we found in a recent study. In CESM1 this mechanism is even larger when the off-equatorial forcing is located in the subtropics and mid-latitudes compared to the polar regions:

    Stuecker, M.F., Timmermann, A., Jin, F. et al. Strong remote control of future equatorial warming by off-equatorial forcing. Nat. Clim. Chang. 10, 124–129 (2020). https://doi.org/10.1038/s41558-019-0667-6.

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