Evidence for link between modelled trends in Antarctic sea ice and underestimated westerly wind changes - PubMed (original) (raw)

Ariaan Purich et al. Nat Commun. 2016.

Abstract

Despite global warming, total Antarctic sea ice coverage increased over 1979-2013. However, the majority of Coupled Model Intercomparison Project phase 5 models simulate a decline. Mechanisms causing this discrepancy have so far remained elusive. Here we show that weaker trends in the intensification of the Southern Hemisphere westerly wind jet simulated by the models may contribute to this disparity. During austral summer, a strengthened jet leads to increased upwelling of cooler subsurface water and strengthened equatorward transport, conducive to increased sea ice. As the majority of models underestimate summer jet trends, this cooling process is underestimated compared with observations and is insufficient to offset warming in the models. Through the sea ice-albedo feedback, models produce a high-latitude surface ocean warming and sea ice decline, contrasting the observed net cooling and sea ice increase. A realistic simulation of observed wind changes may be crucial for reproducing the recent observed sea ice increase.

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Figures

Figure 1

Figure 1. Annual SIC and SST trends over 1979–2013.

(a) Observed SIC from the National Snow and Ice Data Center (NSIDC) Bootstrap algorithm, (b) CMIP5 multi-model mean SIC, (c) observed SST from Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) and (d) CMIP5 multi-model mean SST. Trends are expressed as a change per degree of global warming (°C−1 GW). Multi-model means are calculated using the first available ensemble member for each model. Stippling indicates significance: (a,c) above the 95% level as determined by a two-sided Student's _t_-test and (b,d) where 80% of models agree on the sign of the mean trend, which corresponds to 33 out of 41 models. The mean-state 15% SIC contour is shown in black. In (a) AS, Amundsen Sea; BS, Bellingshausen Sea; RS, Ross Sea; WS,Weddell Sea.

Figure 2

Figure 2. Trends in SIE versus trends in high-latitude SST over 1979–2013.

(a) DJF and (b) JJA. Trends are expressed as a change per degree of global warming. All available model ensemble members are shown (87 realizations). Observed SST from HadISST and SIE from NSIDC. Each model is shown by a marker with the number of runs per model indicated in the legend, the multi-model mean is shown by a black dot and observations are shown by a black asterisk. The inter-model correlation coefficient and _P_-value are shown above each panel. For P<0.05, the inter-model regression is shown by a black line.

Figure 3

Figure 3. Trends in jet strength versus trends in high-latitude SST over 1979–2013.

(a) DJF and (b) JJA. Trends are expressed as a change per degree of global warming. All available model ensemble members are shown. Observed jet strength from European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim). Figure details as per Fig. 2.

Figure 4

Figure 4. Potential temperature profiles and Ekman pumping trends over 1979–2013.

Seasonal zonal–mean potential temperature profiles averaged over 60–70°S for (a) DJF and (b) JJA. The first available ensemble member for each model is shown. The observed profile (black) is an average of Simple Ocean Data Assimilation (SODA) and Ishii reanalyses over 1979–2011. Trends in Ekman pumping versus trends in high-latitude SST for (c) DJF and (d) JJA. Trends are expressed as a change per degree of global warming. Ekman pumping trends are calculated as the trend in the Ekman pumping velocity principal component (PC) multiplied by the mean-state vertical temperature gradient near the surface. All available model ensemble members are shown. Details in (c,d) as per Fig. 2.

Figure 5

Figure 5. Schematic of decadal scale wind induced surface layer changes during summer.

Over recent decades, the observed westerly wind jet has strengthened and shifted poleward during austral summer (circles with dots). This has increased the upward Ekman pumping and equatorward Ekman transport (large arrows). During summer, increased upwelling at high latitudes brings cooler Winter Water to the surface. Combined with equatorward transport, this leads to SST cooling in observations. Multi-model mean (MMM) CMIP5 changes (red) are weaker than observed changes (blue). Under global warming, these weaker Ekman changes are insufficient to offset warming from other factors (curvy arrow). As such, multi-model mean CMIP5 high-latitude SST has warmed rather than cooled and Antarctic sea ice has declined rather than expanded (small arrows).

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